Learn essential MCAT concepts about cells and viruses, practice key questions, and review answers to strengthen your understanding and performance.
(Note: This guide is part of our MCAT Biology series.)
Table of Contents
Part 1: Introduction to cells
Part 2: Cell theory
Part 3: Cell Division
a) Mitosis
b) Meiosis
c) Dysfunctional Cell Growth
Part 4: Prokaryotic Cell Properties and Structure
a) Classifications by shape
b) Anaerobic versus aerobic
c) Parasitic versus symbiotic
d) Properties of prokaryotes
Part 5: Prokaryotic Reproduction and Cell Growth
a) Binary fission
b) Growth
c) Antibiotic resistance
d) The Jacob-Monond model
Part 6: Prokaryote Genetics
a) Transduction
b) Transformation
c) Conjugation
d) Transposons
Part 7: Eukaryotes
a) Organelles
b) Eukaryotes versus prokaryotes
c) Higher structure in multicellular organisms
d) Stem cells
Part 8: Viruses
a) Viral structure
b) Viral life cycle
c) Prions and viroids
Part 9: High-Yield Terms
Part 10: Passage-Based Questions and Answers
Part 11: Standalone Questions and Answers
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Part 1: Introduction to cells
Cells are the building blocks of organisms, and similarly, they are an integral component of biology on the MCAT. Cells are incredibly high yield because they can both be tested directly and make up the basis for many of the concepts talked about in biology passages and experiments. As a result, it is important to have a strong foundation in cellular biology.
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Part 2: Cell Theory
In 1655, English scientist Robert Hooke examined cork under a microscope, developed the concept of cells, and wrote the 3 tenets of cell theory:
All living organisms are composed of one or more cells
Cells are the most basic unit of life
All new cells are products of pre-existing, living cells
Throughout the years, cell theory has been expanded upon to include new findings, most notably that DNA is the genetic information of each cell, and that DNA is transmitted from cell to cell.
There are 2 types of cells that we will discuss in this section: prokaryotes and eukaryotes, and all living organisms can be classified into one of these two. Viruses, on the other hand, are not living and will be discussed in Part 7.
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Part 3: Cell division
a) Mitosis
Mitosis is the process through which a somatic cell divides to form two genetically identical daughter cells. It is composed of four steps: prophase, metaphase, anaphase, and telophase.
During prophase, the nuclear membrane dissolves, the chromatin condenses into chromosomes, and the spindle apparatus forms. The spindle fibers attach at a chromosome’s kinetochore, a protein located on the centromere where sister chromatids are held together. The centrosome and its fibers make up the aster. Next, the chromosomes align along the metaphase plate during metaphase. During anaphase, the centrosomes then pull sister chromatids to opposite sides of the cell. Finally, during telophase and cytokinesis, the nuclear membrane begins to re-form and the single-cell separates into two daughter cells.
Figure 1 Steps of mitosis
It’s important to note that mitosis only refers to the replication of the nucleus. The division of the cytoplasm into two daughter cells occurs during cytokinesis.
b) Meiosis
Meiosis is the division of germ cells that gives rise to four non-identical gametes. It is composed of two phases: meiosis I and meiosis II. Both of these have prophase, metaphase, anaphase, and telophase steps. There are three key differences you should know between mitosis and meiosis.
Cells that undergo meiosis have a ploidy of 2n but produce daughter cells that have a ploidy of n. Cells undergoing mitosis have a ploidy of 2n and their daughter cells have a ploidy of 2n.
Meiosis only occurs in germ cells, while mitosis occurs in somatic cells.
Homologous chromosomes pair up and result in crossing over events in meiosis. During mitosis, however, homologous chromosomes are not paired up. Instead, our focus is on sister chromatids.
Additionally, we can count the number of chromosomes in a cell by counting the number of functional centromeres. Recall that the centromere is a special region on the chromosome that links together two sister chromatids.
The first step of meiosis is prophase I, during which the nuclear membrane dissolves, the chromosomes form from chromatin condensing, and the spindle apparatus forms. Homologous chromosomes group together during this step via the process of synapsis. These pairs of homologous chromosomes contain one chromosome that is inherited from the father and one from the mother. Unlike sister chromatids, which are identical DNA strands, these are not identical.
Homologous chromosomes pair together by the synaptonemal complex. Since each pair contains a total of four chromatids, we refer to them as tetrads. Chromatids of homologous chromosomes can undergo crossing over where overlapping equivalent segments of DNA are exchanged. If this occurs once, we refer to it as a single crossover event. As you might expect, if this occurs twice, it is referred to as a double crossover event. The spot at which the crossing over occurs is referred to as the chiasma. This recombination of genes promotes genetic diversity.
During the second step of meiosis, metaphase I, our homologous chromosomes align along the metaphase plate. They are then sent to opposite ends of the cell during anaphase I. Finally, during telophase I and cytokinesis, our cell splits into two daughter cells.
Figure 2 Meiosis I. From left to right: Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis
In the second phase of meiosis, meiosis II, the focus shifts from recombining homologous chromosomes to separating sister chromatids. At this point, the process is virtually identical to mitosis.
Prophase II involves the migration of the centrioles to opposite ends of the cell to form the spindle apparatus. During metaphase II, the chromosomes align along the metaphase plate. The sister chromatids are separated to opposite sides of the cell by our spindle fibers during anaphase II. Finally, during telophase II and cytokinesis, the nuclear membrane reforms and splits into two daughter cells.
Thus, meiotic division results in the formation of four haploid daughter cells.
Figure 3 Steps of meiosis II: same as mitosis
Figure 3 Steps of meiosis II: same as mitosis
c) Dysfunctional cell growth
Cells carry out highly regulated processes that facilitate their growth. The cell cycle is composed of four key stages: G1, S, G2, and M. Interphase refers to the first three stages (G1 - G2).
During G1, a cell will produce organelles to prep for division. To complete this stage and move onto the next one, the cell checks to make sure that it has the correct complement of DNA.
During the S stage (“synthesis”), the cell undergoes DNA replication to produce the DNA for the daughter cells.
Next, during the G2 stage, the cell again checks to see if it is ready for cell division. Specifically, this checkpoint ensures that DNA synthesis occurred properly and that there is enough cytoplasm and organelles present to divide into two daughter cells.
Finally, during the M phase, mitosis and cytokinesis occur.
When cellular control checkpoints fail, cancer, or unregulated cellular division, can occur. This can often occur as a result of genetic causes. Mutagens, or agents that cause mutations in the DNA, can give rise to cancer by disrupting genes responsible for regulating cell division. Oncogenes are genes that can promote cell division and cancer when they are overexpressed. These are the result of mutations in proto-oncogenes, which have normal functions. Since they are the result of overactivation, they require a mutation in only one allele to lead to dysfunctional cell growth. Other genes known as tumor suppressor genes can give rise to cancer when a mutation causes them to become inactivated. These typically require mutations at both alleles.
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Part 4: Prokaryotic cell properties and structure
Prokaryotes are defined as having no nucleus or membrane bound organelles, and they can be classified into one of two domains of life: Bacteria and Archaea. Archaea are single cell organisms that are often found in extreme environments, such as high temperatures or high salt concentrations. They are sometimes thought of as being in between bacteria and eukaryotes as they have a structure resembling bacterium but certain enzymes and metabolic pathways resembling eukaryotes. Bacteria, on the other hand, are the most prevalent cell type, with an estimated 1030 bacteria on earth.
a) Classifications by shape
Bacteria are often classified based on their shape and fall into one of 3 categories: round (cocci), rod-shaped (bacilli), and spiral-shaped (spirilli).
Figure 4 Different bacterial shapes: Cocci are spherical, Bacilli are rods and Spirilli are spiral organisms
The concept of shape is important to remember as the name of the shape is often reflected in the name of the bacteria in an MCAT passage. For example, when looking at Streptococcus, Lactobacillus, and Leptospira, we can infer that their shapes are spherical, rod-like, and spiral-shaped, respectively.
b) Anaerobic versus aerobic
Another important classification of bacteria is based on their use of oxygen for metabolism. Anaerobes are defined as bacteria that do not need oxygen for metabolism while aerobes do require oxygen for metabolism. Among aerobes and anaerobes, there are several different types of organisms depending on their relationship with oxygen.
Obligate anaerobes are organisms that require the absence of oxygen as oxygen is toxic for them. Aerotolerant anaerobes are similar to obligate anaerobes in that they can only engage in anaerobic metabolism, but the presence of oxygen is not toxic to them. On the other side of the spectrum are obligate aerobes, which require oxygen for metabolism. Finally, there are facultative anaerobes and facultative aerobes. These are bacteria that have a preferred method of metabolism (e.g., oxygen present) but are capable of using the other method (e.g., oxygen absent) in certain conditions. The figure below shows where different types of bacteria would likely be found in a culture medium that is exposed to oxygen at the surface.
Figure 5 Anaerobic versus aerobic bacteria. Aerobes cluster at the top of the beaker as that has the greatest concentration of oxygen, and vice versa for anaerobes avoiding oxygen.
c) Parasitic versus symbiotic
The final classification method of bacteria that we will go over is the type of relationship that the bacteria have with other organisms. Parasitic bacteria gain benefit from themselves while harming the host. For example, think of mosquitos that bite you! On the other hand, symbiotic bacteria have a relationship with the host organism that is either harmless or mutually beneficial. For example, we have gut bacteria that aid in digestion and help the immune system.
d) Properties of prokaryotes
Prokaryotes are defined by an absence of a nucleus or other membrane-bound organelles, and they have a structure much more basic than that of eukaryotes. Additionally, there are other structural differences between prokaryotes and eukaryotes that help antibiotics target bacteria specifically. The main components of prokaryotic cells that we’ll discuss are the cell membrane and cell wall, the propulsion mechanism, ribosomes, and DNA.
Bacteria have a cell wall that surrounds the cell membrane. Eukaryotic cells, on the other hand, have only a cell membrane. Some bacteria also have a layer surrounding the cell wall called a capsule, which is a sticky layer of a polysaccharide “goo” that sometimes surrounds an entire colony of bacteria. While the cell wall is used to provide structural support for the cell, the capsule allows the bacteria or the colony to survive better in animals.
The most important component of the cell wall is a polysaccharide known as peptidoglycan, which helps provide the rigid support for the cell. Peptidoglycan contains cross-linked chains of sugars and amino acids. Since peptidoglycan is unique to prokaryotes, it is often the target of many antibiotics that use enzymes to destroy the peptidoglycan and render the bacterial cell fragile.
The quantity of peptidoglycan in the cell wall is an important classification of bacterial cell walls, and peptidoglycan levels are differentiated in a process known as Gram staining. This process involves applying a crystal violet dye to heat-fixed bacteria, adding iodide to trap the dye to the peptidoglycan layer, washing the culture with ethanol or acetone to remove the lipopolysaccharide membrane in Gram-negative bacteria, and finally applying a counterstain to visualize the Gram-negative bacteria. This process results in Gram-positive cells appearing dark purple and Gram-negative cells appearing pink.
The difference between these two types of cell wall is that Gram-positive cells have a thick layer of peptidoglycan outside the cell membrane, and Gram-negative cells have only a thin layer of peptidoglycan in the cell wall. Importantly, Gram-negative cells also have an additional outer membrane with lipopolysaccharide. The main clinical difference between Gram-positive and Gram-negative bacteria is the resistance of Gram-negative bacteria to antibiotics.
Figure 6 Gram-positive versus gram-negative cells
The cell membrane in prokaryotes functions similar to that of a eukaryotic cell as it is an important regulator of transport in and out of the cell. However, the cell membrane also plays an important role in prokaryotic aerobic respiration. Aerobic respiration relies on the electron transport chain and ATP synthase through the mitochondrial membrane, but bacteria don’t have mitochondria! As a result, aerobes use the cell membrane for aerobic respiration.
While there are no membrane-bound organelles in prokaryotes, there are still ribosomes. Prokaryotic ribosomes are made of a 30S and 50S subunit that combine to make a 70S ribosome. In eukaryotic cells, on the other hand, a 40S and 60S subunit combine to make an 80S ribosome. The unit S is known as a Svedberg unit, and it quantifies the sedimentation rate of the ribosomes, which describes how long it takes for the ribosome to sink to the bottom of a test tube under a high-intensity centrifugation. This means that the sedimentation is related to the ribosome’s mass, and a larger mass particle taking less time to sediment.
Another important property of bacteria is their mechanism for movement. Flagella are long filaments that spin in a whipping motion and propel the cell, similar to a boat motor. The flagella are incredibly important for chemotaxis, which is the movement of a bacterium based on chemical signals. There are chemoreceptors on the surface of the cell that bind attractants or repellents, and these molecules dictate which way the flagella rotates and thus which direction the bacteria moves. To picture this, think of the game you play when your friend hides an object. As you get closer, the signal “warmer” means that the direction you’re moving in is correct. On the other hand, the signal “colder” acts as a repellent and would drive you in the opposite direction.
Another component of bacterial movement is pili, which are long projections on the surface of the bacteria that help the bacteria attach to different surfaces. There is a sex pilus that helps the male bacteria attach to the female bacteria and form conjugation bridges.
The last property of prokaryotes we will discuss is their DNA. Since prokaryotes don’t have membrane-bound organelles, there is no nucleus to house the DNA. This means DNA is stored in a single circular chromosome that stays in the nucleoid region of the cell. The fact that prokaryote DNA is circular is an important distinguishing factor between prokaryotes and eukaryotes, and this is an important distinction to remember when reading MCAT passages. Along with the main chromosome, some bacteria also have small circular DNA pieces called plasmids. Plasmids are not essential components of DNA, but they often code for advantageous traits in bacteria. This high genetic variability and adaptability can result in traits, such as antibiotic resistance and increased virulence, which increase the harm to humans. (For more information on this, be sure to refer to our guide on genetics and evolution.)
Once transcribed, prokaryotic mRNA is also translated in a unique manner. Prokaryotic transcripts are polycistronic, meaning that a single mRNA strand has multiple translation start sites and can generate a number of proteins. This differs from eukaryotic transcripts which are monocistronic and yield one specific protein per transcript. Polycistronic mRNA allows prokaryotes to both save energy when manufacturing proteins and express genes in a coordinated manner.
Figure 7 Monocistronic versus polycistronic mRNA. UTR indicates an untranslated region in the transcript.
Table 1 Summary of the differences between prokaryotes and eukaryotes
| Prokaryotes | Eukaryotes |
|---|---|
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Part 5: Prokaryotic reproduction and cell growth
a) Binary fission
Reproduction in bacteria occurs through a process known as binary fission. The end result is similar to mitosis in that binary fission also results in the formation of two identical cells, but the process of this duplication is very different. In binary fission, the first step is replication, and the chromosome and plasmid are duplicated while the cell grows. The next step is segregation, and each chromosome is pulled to one side of the cell. In addition, a new cell wall begins to form in the middle. The last step is separation, in which the two resulting cells have a fully formed cell wall in between them and begin to pull away from each other.
Figure 8 Binary fission components
b) Growth
Binary fission occurs rapidly, and as a result, bacteria tend to grow exponentially. This growth, however, doesn’t come until after the bacteria adapt to a new environment during a period known as the lag phase. While the bacteria are growing during the log phase, there are only a certain number of resources, which eventually causes the number of bacteria to level off in a period known as the stationary phase. The last stage of the cycle is the death phase, which is caused because there are no more resources left in the environment. This growth cycle shows the rise and fall in the number of bacteria over time. Bacterial growth is an example of a positive feedback loop, where more bacteria formation causes even more bacteria to spawn until the resources run out and the growth stops.
Figure 9 Bacterial growth cycle
When looking at the graph, it is important to keep in mind that the y-axis is logarithmic and not linear. This means that the seemingly linear increase during the log phase is actually exponential!
c) Antibiotic resistance
An important factor in the survival of bacteria is their ability to develop resistance to antibiotics. Some of the properties of bacteria that help them develop this resistance were discussed already, such as the capsule and plasmid DNA. Another mechanism that we did not touch on is the possibility of mutations during binary fission that make antibiotics less effective at eradicating the bacterial infection.
There are four types of mutations that could help a bacterium resist antibiotics. The first is the formation of new enzymes in the bacteria that deactivate the antibiotic before it is effective. The second is a mechanism that allows the bacteria to eject the antibiotics from the bacteria, thus rendering the antibiotics useless. The third mechanism changes the cell wall and prevents the antibiotics from entering in the first place. The last mutation alters a certain function of the bacteria that an antibiotic targets. For example, let’s look at a bacterial mechanism to process energy. If a certain antibiotic interrupts the energy pathway, a mutation that causes the bacteria to use a different energy pathway could render the antibiotic ineffective yet again.
d) The Jacob-Monod model
The Jacob-Monod model explains the function of operons in bacterial cells. From upstream to downstream, an operon contains a regulator gene, a promoter site, an operator site, and a structural gene.
The model is easiest to understand if we begin with what’s furthest downstream. The structural gene codes for a specific protein. The operator site preceding it serves as a binding site for repressor protein, while the promoter site provides a binding site for RNA polymerase. Finally, the regulator gene codes for the repressor protein.
Operons can be inducible or repressible systems. If the operon is inducible, a repressor is bound to the operator site. Removal of the repressor activates the system. If the operon is repressible, the system is activated, and the genes are transcribed. However, the system is turned off when the repressor occupying the operator site binds with a corepressor.
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Part 6: Prokaryote genetics
Another important concept on the MCAT is gene transfer among prokaryotes. Since binary fission creates two daughter cells identical to the parent, there is little genetic variability outside of genetic mutations. As a result, there is a concept known as horizontal gene transfer that introduces genetic diversity to bacterial species, and there are 3 main mechanisms by which this occurs: transformation, transduction, and conjugation.
(Note: these techniques are also used in the lab for various applications!)
a) Transduction
In transduction, bacterial DNA is transferred through viruses. Since the DNA of bacteria exists as the nucleoid, viruses can easily infect the bacteria and incorporate viral DNA into the bacterial DNA, creating what is known as a bacteriophage. These bacteriophages can then infect other bacteria and incorporate the DNA from the original bacterial cell into the DNA of the new bacteria.
b) Transformation
The simplest of these gene transfer mechanisms is transformation, which involves bacteria absorbing genetic material directly from the environment. This mechanism does not involve an external carrier of foreign DNA.
c) Conjugation
Conjugation in bacterial DNA is similar to sexual reproduction in animals. A sex plasmid is transferred through a bridge between two bacteria. The pilus extends from one bacterium (often referred to as F+) to another bacterium (known as F-) and creates a conjugation bridge. In MCAT passages, it is important to know that the F+ and F- are the fertility factors of the bacterium, and F+ bacteria are male while F- bacteria are female. Another important thing to note about the F factor is that the F+ factor itself is transferred to the new bacterium as well, creating a new F+ cell.
Figure 10 Bacterial transduction, transformation, and conjugation
d) Transposons
Transposons, sometimes called jumping genes, are a specific class of DNA segments found in both prokaryotic and eukaryotic DNA capable of moving around within the genome. This shuffling of the genome can cause or reverse mutations that may both help the organism survive and contribute to genetic variance.
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Part 7: Eukaryotes
Eukaryotes are the other main cell type that we’ll discuss, and they are virtually guaranteed to show up in some form on the MCAT. The main defining characteristics of eukaryotes are the presence of membrane-bound organelles and mitotic division. While mitotic division is covered in our Reproduction and Development guide, we’ll cover each of the organelles in detail. In addition, we’ll compare prokaryotes and eukaryotes to understand the important differences that you should know for the MCAT.
a) Organelles
Organelles are structures within eukaryotic cells that have a specific function, and they are typically surrounded by a membrane. These membranes protect the organelles from the contents of the cytoplasm and control what molecules are able to enter and exit the organelle. The table below gives a brief overview of each organelle and its primary function. The organelles are described in greater detail in the remainder of this section.
Table 2 Important organelles for the MCAT
| Organelle | Function |
|---|---|
Nucleus
The most important organelle of the cell is the nucleus, which houses the genetic information for the cell. The nucleus is surrounded by a double membrane known as the nuclear envelope, and transit across the nuclear envelope is regulated by nuclear pores. Similar to the cell membrane, nuclear pores are protein complexes that selectively allow molecules to cross the membrane. Within the nucleus is the nucleolus, which is a specific region where ribosomes are produced.
As the site of genetic information, the nucleus contains the various structures in which DNA is stored. Chromatin is formed by strings of DNA wrapping around a special type of protein called histones. One unit of histone wrapped in DNA is referred to as a nucleosome. A collection of these bead-like structures from the chromatin.
Chromatin can be in a transcriptionally active or inactive form. Euchromatin is an open and transcriptionally active form of chromatin. It is characterized by extensive acetylation of the histone proteins. Heterochromatin is a closed and transcriptionally silent form of chromatin. It is characterized by extensive methylation of the DNA bases.
Mitochondria
You've probably heard the phrase "the mitochondria is the powerhouse of the cell," but we need to know a little more information for the MCAT. The most common form of energy within cells is ATP, and both the Krebs cycle and the Electron Transport Chain, which produce a large amount of the cell's ATP, occur within the mitochondria.
The structure of the mitochondrion is essential to its function. The mitochondrion contains two membranes, an inner and outer mitochondrial membrane. The space at the center of the mitochondrion is referred to as the matrix, while the space between the two membranes is known as the intermembrane space. To better understand the vital chemical processes that take place in these structures, be sure to refer to our guide on carbohydrate metabolism.
Another interesting fact about mitochondrion is that they replicate using binary fission, similar to prokaryotes, and they even have their own DNA (commonly known as mtDNA). The DNA found in mitochondria is considered to be "extragenomic": that is, it exists outside of the nuclear genome. This finding has led to the endosymbiotic origin hypothesis, which suggests that mitochondria derived from an aerobic prokaryotic cell that was engulfed by another cell and formed an endosymbiotic relationship.
Endoplasmic Reticulum
The endoplasmic reticulum is a series of pockets that extends from the nuclear membrane. The pockets are made of cisternae, and the space they surround is known as the lumen. There are two parts of the endoplasmic reticulum: the rough endoplasmic reticulum and the smooth endoplasmic reticulum.
The rough endoplasmic reticulum (RER) gets its name because it is studded with ribosomes on its membrane. As a result, the RER is the site of most protein synthesis, particularly the synthesis of membrane and secretory proteins. After proteins are synthesized by the ribosomes, they are passed through the RER to begin post-translational modifications and then sent off to the Golgi apparatus.
The smooth endoplasmic reticulum (SER) does not have ribosomes, and thus it is not involved in protein synthesis. Instead, its main function is the synthesis of lipids, particularly phospholipids that are part of the cell membrane. In certain parts of the body, such as the liver, the SER can have other specialized functions, such as detoxification of drugs or catabolism of glycogen in the liver.
Ribosomes
As mentioned earlier, ribosomes are the site of protein synthesis, and they are mostly found on the RER. Eukaryotic ribosomes are structurally different from prokaryotic ribosomes, as was discussed in Cell division.
Golgi Apparatus
After proteins are synthesized in the RER, they are sent to the Golgi apparatus in vesicles for modifications. There are 3 regions of the Golgi: the cis region, which faces the nucleus and ER, the trans region, which faces the cell membrane, and the medial region that sits between the other two regions. The proteins move from the cis to the trans regions, and along the way, they undergo further modifications. After the proteins are modified in the trans region, they are packaged in vesicles and distributed to different regions of the cell or outside the cell. The process of vesicle export outside of the cell is referred to as exocytosis, while bringing extracellular vesicles into the cell is referred to as endocytosis.
Lysosomes
Lysosomes are membrane-bound vesicles with many different enzymes that help break down various polymers. These enzymes include hydrolytic enzymes, which work to break down the chained structure of old carbohydrates, proteins, nucleic acids, and more. Lysosomes can be thought of as a garbage disposal machine, which takes in unwanted items and breaks them down in a way that makes the molecules easier to dispose.
Peroxisomes
As their name suggests, peroxisomes are full of peroxides, which produce free radicals that aid in polymer degradation. For example, peroxides allow the peroxisome to metabolize long chain lipids by shortening them into medium chain lipids that go to the mitochondria for further breakdown.
Cytoskeleton
The cytoskeleton of the cell has a similar function to the human skeleton as it provides structural support to the cell and helps it move. There are 3 main components of the cytoskeleton that you should be familiar with: microfilaments, microtubules, and intermediate filaments.
Table 3 Cytoskeleton components
| Component | Structure | Function |
|---|---|---|
During normal cell function, cytoskeletal elements originate from one region close to the nucleus: the microtubule organizing center. This region serves as an area in which alpha- and beta-tubulin subunits polymerize to form long microtubule structures. It additionally contains centrioles, organelles which serve as additional sites for cilia formation and the polymerization of the mitotic spindle during cell division.
One function of the cytoskeleton is movement, and there are two structures on the cell that are primarily responsible for this movement: flagella and cilia. You should remember flagella from the discussion of prokaryotic cells, but eukaryotic flagella are significantly different in their structure. In eukaryotes, flagella are a tail-like structure that extends from the cell and helps it move. While prokaryotic flagella use a spinning motion to propel the cell, eukaryotic flagella simply flap back and forth to propel the cell. In addition, eukaryotic flagella are powered by ATP. Cilia are much smaller projections on the cell surface that move substances along a surface. One important fact to know about eukaryotic cilia and flagella is the 9+2 structure. This refers to the fact that they are made up of an outer ring of 9 pairs of microtubules around an inner ring of 2 microtubules
b) Eukaryotes versus prokaryotes
On the MCAT, it is important to be able to distinguish between prokaryotic and eukaryotic cells while reading passages and answering questions. This table provides a brief overview of the main differences that you should remember.
Table 4 Summary table differentiating Prokaryotes and Eukaryotes
| Prokaryotes | Eukaryotes |
|---|---|
c) Higher structure in multicellular organisms
In multicellular organisms such as ourselves, individual cells are able to specialize in specific functions. Further, associations formed between individual cells can create additional functions.
These associations will become especially important as we begin to discuss organ systems. Cells are able to first organize themselves into tissues. Multiple tissue types are able to associate together into organs. Finally, multiple organs and organ systems compose the entire organism.
Based on the function of a specific tissue or organ, its constituent cells may adopt increasingly specialized functions and connections. A major difference between specialized cell types may be its shape. Epithelial cells tend to be cube-shaped, while squamous cells tend to be flat and thin. Epithelial cells tend to be found in the linings of body cavities and vessels, while squamous cells are found in areas of the body that encounter wear (such as the outer surface of the skin).
The intercellular junctions, or connections between cells, can also vary depending on their function. For the MCAT, there are several types of intercellular junctions you should be familiar with:
Tight junctions hold cells together very ‘tightly’, preventing water and solutes from moving between them via paracellular pathways. They are present in our skin and digestive epithelium, where strict regulation of molecular transport is very important.
Adheren junctions are formed by transmembrane proteins called cadherins, which link to actin filaments in the cytoplasm
Desmosomes are exceptionally strong junctions that provide stability by anchoring the intermediate filaments of the cells’ cytoskeletons. This strength helps skin and muscle cells endure greater mechanical stress.
Gap junctions utilize special protein channels called connexons to allow for rapid communication between cells. Connexons allow for the exchange of ions and small molecules, enabling coordinated activity between cells, like the contraction of cardiac muscle.
Hemidesmosomes are similar to desmosomes, but anchor cells to their extracellular matrix rather than other cells. They are prominent in the basal layer of the epidermis, where they attach the basal cells to the underlying basement membrane. This attachment is crucial for the integrity and function of the skin
d) Stem cells
Stem cells are unique to eukaryotes and can be thought of as cells that are a “blank slate.” They have not yet been differentiated and so can undergo significant structural and biochemical changes to carry out a specific set of functions.
Multipotent stem cells are able to differentiate into cells of a particular group. For example, hematopoietic stem cells can give rise to platelets, red blood cells, and white blood cells.
Pluripotent stem cells are able to differentiate into an embryonic germ layer but not extra-embryonic cells, such as those making up the placenta. For example, a pluripotent cell may give rise to any cells that develop from the mesoderm, such as the bones, musculature, and gonads.
Finally, totipotent stem cells can differentiate into any cell derived from the embryonic germ layers or into any extra-embryonic cell. Stem cell research has been hard at work since the start of the twenty-first century. These cells may be the key to regenerating tissues and entire organs, thus providing a remedy to an ever-growing organ transplant list, neurodegenerative diseases, and more.
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Part 8: Viruses
a) Viral structure
Viruses are the third type of cell that we’ll discuss in this chapter. Unlike the first two, however, viruses are not considered to be living. Viruses are parasites, meaning they need to harm a host to replicate. As a result, based on the tenets of cell theory mentioned earlier, they are not considered to be living on their own.
Viral structure varies a lot depending on the specific virus, but they all have genetic material and a capsid, which is a protein coat surrounding the virus. Compared to other cell types, viruses lack a nucleus and organelles.
In addition, viruses are much smaller than eukaryotes and prokaryotes. Viruses can range from 20-400 nm, and their genome also has a lot of size variance. Some genomes code for only a few genes, while other viruses can code for thousands. Additionally, the viral genetic material can exist in the form of either single-stranded or double-stranded DNA or RNA. Single-strand viral genomes can be broken down even further into positive-sense or negative-sense viruses. Negative-sense RNA means that the RNA strand is complementary to the mRNA to be translated, while positive-sense RNA is already in the correct form of mRNA to be translated into proteins.
b) Viral life cycle
Viruses need a host to replicate their DNA and reproduce. The general steps for a virus to reproduce first involve the virus attaching to the host cell by penetrating through the host membrane or cell wall and injecting viral DNA/RNA into the host cell. After this, the genetic material is replicated using the proteins and other components already present within the host cell. The viral components then assemble, and the new viral particles are released to start the chain over again.
Once a virus attaches to the host cell, the genetic material is inserted into that cell in order to replicate. When the host cell is a bacterium, the combination of the virus and its host is known as a bacteriophage. In this scenario, the virus attaches to the outside of the cell and a simple tail injects into the bacteria and releases the genetic material inside. On the other hand, when the host is an animal cell, the entire virus is brought into the cell either via endocytosis or fusion with the plasma membrane.
After the genetic material of the virus is inside the cell, the genetic machinery of the host cell is used to replicate the viral genome and express the genes necessary to make a new virus. Using the similar mechanisms of DNA transcription and translation outlined in a separate chapter, the viral genetic material is copied, and the proteins are built and then released into the environment as replicated viruses.
Figure 12 General viral life cycle
Overall, there are two types of life cycles for bacteriophages. The first one, known as the lytic cycle, involves replicating the genome at full speed to make many new viruses inside the bacteria until finally the bacteria bursts and the new viruses spill into the environment. The other life cycle, known as the lysogenic cycle, involves the viral genetic material becoming a part of the bacteria’s genome and laying silent. Then, the lytic cycle is induced when certain environmental triggers are observed. In the lysogenic cycle, the bacteria with viral genetic material is known as a prophage or a provirus.
Figure 13 Lysogenic and lytic viral life cycles
A specific type of virus, known as a retrovirus, has single-stranded RNA and a slightly different method of replication. Once these viruses inject their genetic material into the host, an enzyme known as reverse transcriptase creates new DNA from their RNA genome. This DNA is then incorporated into the host DNA. Since the viral genome is now incorporated into the host cell, the DNA is replicated as the cell replicates and becomes incredibly difficult to remove. The most common type of retrovirus is HIV.
c) Prions and viroids
Subviral particles, which consist of prions and viroids, are also important for the MCAT. A prion is a protein that is folded incorrectly and causes correctly folded proteins to also become misfolded. In this way, they are considered infectious. These misfolded proteins are incredibly dangerous and can interrupt cell function and cause a wide range of fatal diseases. Examples of some prion diseases include mad cow disease in cows, fatal familial insomnia, and Creutzfeldt-Jakob disease.
The other subviral particle, viroids, are short pieces of single-stranded RNA that are capable of silencing gene expression. They do this by binding to specific RNA sequences in the host cell and not allowing them to be translated. Most of the diseases caused by viroids are found in plants, with the only human case being hepatitis D.
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Part 9: Cells and viruses high-yield terms
Cell theory: three tenets that describe properties of cells: living organisms are composed of cells, they are the most basic unit of life, and all new cells are products of pre-existing, living cells
Archaea: specific single cell prokaryote that lives in extreme environments
Bacteria: most common type of prokaryotic cell; classified based on shape (bacilli, spirilli, and cocci)
Anaerobes: bacteria that do not use oxygen for metabolism; for some oxygen is toxic, while for others it is tolerable but not necessary.
Aerobes: bacteria that use oxygen for metabolism; some need oxygen while some are able to use oxygen when available.
Parasitic: relationship between bacteria and host that benefits the bacteria, but harms the host
Symbiotic: relationship between bacteria and host that does no harm, and helps one or both parties
Bacterial cell wall: protective structure around the cell membrane of bacterial cells
Capsule: a sticky polysaccharide layer around some bacteria that enhances survival of individual bacteria or bacterial colonies
Peptidoglycan: a polysaccharide that is an important component of bacterial cell walls
Gram-positive: type of bacterial cell wall that has thick layer of peptidoglycan outside the cell wall
Gram-negative: type of bacterial cell that has a thin layer of peptidoglycan in between the cell wall and the outer lipopolysaccharide membrane
Bacterial flagella: long filaments that spin like a propeller to help the bacteria move
Chemotaxis: movement of bacteria in response to chemical signals in the environment
Pili: long projections on surface of bacteria that help with attachment and movement, as well as formation of conjugate bridges
Plasmid: small circular pieces of DNA in some bacteria in addition to primary DNA; often code for advantageous abilities in bacteria
Binary fission: process of reproduction in bacteria; results in two identical daughter cells
Lag phase: before cells start growing and reproducing, they have to start the process, and little growth happens
Log phase: exponential growth of bacteria due to quick nature of binary fission
Stationary phase: peak of exponential growth, population size levels off due to absence of certain resources needed to reproduce further
Death phase: after resources are diminished, bacteria begin to die off because they can’t support further growth
Transduction: transfer of bacterial DNA through viruses
Transformation: absorption of genetic material directly from the environment without a specific carrier
Conjugation: similar to sexual reproduction in bacteria; transfer of plasmid through a conjugate bridge between F+ and F- bacteria
transposons: “jumping genes,” DNA segments that move around in the genome providing genetic variation
Nucleus: houses genetic information and is involved in partial formation of ribosomes
Mitochondria: production of energy in the form of ATP through Krebs cycle and electron transport chain
Rough endoplasmic reticulum: studded with ribosomes and thus the site of protein synthesis, begins post-translational modification of some proteins, sends them to Golgi apparatus
Smooth endoplasmic reticulum: synthesis of lipids, mostly phospholipids; specialized functions in various organs in the body
Ribosomes: site of protein synthesis; 80S in eukaryotes, 70S in prokaryotes
Golgi apparatus: further post-translational modifications of proteins; packages proteins into vesicles to distribute to various regions of the cell
Lysosomes: garbage disposal of the cell; breaks down unwanted molecules into safer molecules easy to dispose of
Peroxisomes: metabolization of long chain lipids into medium chain lipids to go to mitochondria for further metabolization
Microfilaments: two strings of actin polymers for cell movement and cleavage of the cell during mitosis
Microtubules: polymers of tubulin dimers for cell structure and flagella/cilia composition
Intermediate filaments: polymers of filament monomer for structural support and cellular adhesion; keratin is a prominent example
Eukaryotic flagella: tail-like structure extending from eukaryotes to help the cell move; made of microtubules in 9+2 structure
Cilia: multiple small projections on the outside of eukaryotic cells that help move substances along the surface of the cell; made of microtubules in 9+2 structure
Capsid: a protein coat around all viruses that is used to classify viruses
Bacteriophage: a virus that is attached specifically to a bacterial cell
Lytic cycle: replicating genome of virus constantly to make new virus cells until bacterium bursts and releases new viruses
Lysogenic cycle: virus genome incorporating into bacterial DNA and beginning the lytic cycle only when environmental triggers are observed
Prophage/provirus: bacteria with viral genetic material in the lysogenic cycle
Retrovirus: a specific virus that uses incorporates genome into host DNA and becomes very difficult to get rid of
Prion: a misfolded protein that acts as a template to misfold other proteins; cause of some fatal diseases
Viroid: single strand RNA capable of silencing gene expression
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Part 10: Practice passage and answers
Flaviviruses, including Zika virus (ZIKV), utilize host mRNA degradation machinery to produce subgenomic flaviviral RNA (sfRNA). In mammalian hosts, this noncoding RNA facilitates replication and pathogenesis of flaviviruses by inhibiting IFN-signaling, whereas the function of sfRNA in mosquitoes remains largely elusive.
Flaviviruses utilize multiple cellular processes to enable their replication. In particular, scientists previously discovered that flaviviruses exploit the cellular mRNA decay pathway and utilize the host 5ʹ–3ʹ exoribonuclease XRN-1 to produce flaviviral subgenomic RNAs (sfRNAs). While digesting genomic RNA of flaviviruses, XRN-1 stalls at the structured XRN-1-resistant RNA elements (xrRNAs) in the 3ʹ untranslated region (3ʹUTRs), which results in the generation and accumulation of incompletely degraded viral RNA. ZIKV contains two experimentally validated xrRNAs (xrRNA1 and xrRNA2), and it generates two sfRNA species; the predominant longer isoform sfRNA1, and less abundant shorter sfRNA2.
Researchers first generated mutant viruses deficient in the production of sfRNA isoforms by introducing point mutations that abolish XRN-1 resistance into each or both xrRNAs (xrRNA1 and xrRNA2) of ZIKV. To investigate the role of sfRNA in mosquito transmission of ZIKV, female Ae. aegypti mosquitoes were exposed to WT, xrRNA1ʹ and xrRNA2ʹ ZIKV. Viral titers were then determined in bodies, legs and wings, and saliva of mosquitoes exposed to infectious blood meals at 7- and 14-days post-infection (dpi) to examine viral replication, dissemination, and transmission.
Figure 1 At 7 days post infection (dpi), the percentage of infected mosquitoes (a) and viral titers in the bodies (b) were determined to serve as indicators of the infection rate and of initial viral replication. At 14 dpi (c), the infection, dissemination and transmission rates were determined as percentage of ZIKV-positive bodies, legs and wings, and saliva samples, respectively. ZIKV titers at 14 dpi (d) indicate viral replication efficiency and viral loads in saliva.
Deep sequencing of ZIKV 3ʹUTR amplicons showed that the C to G mutation in xrRNA2 had almost completely disappeared in all mosquitoes tested (G frequency in viral populations 1–8%). Instead, either the wild-type variant, C (42–62%), or A (31–54%) were found to be present in the corresponding position of viral genome. In contrast, in the group exposed to the xrRNA1ʹ infection, all five analyzed mosquitoes contained viruses that retained C to G mutation in xrRNA1. Four samples had a G frequency of 90–93%.
Question 1: Which xrRNA point mutation showed to be more resilient in preventing the formation of sfRNA isoforms long term?
A) xrRNA1’ because the infection rate at 7 dpi was higher than the infection rate of xrRNA2’ at 7dpi.
B) xrRNA2’ because the C to G mutation almost completely disappeared in the 3’UTR.
C) xrRNA1’ because the mosquitoes had viruses with C to G mutations in xrRNA1.
D) They both showed similar resilience in preventing formation of sfRNA isoforms long term.
Question 2: Which of the following is the most likely mechanism ZIKV uses to insert its genetic material?
A) Transduction
B) Endocytosis
C) Injection through tail sheath
D) Transfer through sex pilus
Question 3: According to Figure 1, which titer shows the most effective transmission rate among all xrRNA variants?
A) xrRNA1’ in saliva 14 dpi
B) WT in legs and wings 14 dpi
C) xrRNA2’ in legs and wings 14 dpi
D) WT in saliva 14 dpi
Question 4: Suppose that a separate study found that shorter sfRNAs are more important than longer ones for effective adaptation to replication in mosquitoes. Would this support the results of the study?
A) Yes, because the mutation to xrRNA2’ had higher attenuation than the mutation to xrRNA1’
B) Yes, because the mutation to xrRNA2’ had lower attenuation than the mutation to xrRNA1’
C) No, because the mutation to xrRNA1’ had higher attenuation than the mutation to xrRNA2’
D) No, because the mutation to xrRNA1’ had lower attenuation than the mutation to xrRNA2’
Question 5: For ZIKV to replicate inside animal cells, it requires the use of host machinery. Where would be the most likely location within the cell for protein translation to occur?
A) Nucleus
B) Golgi ribosomes
C) Lysosomes
D) Rough endoplasmic reticulum
Answer key for cells and viruses practice passage
1. Answer choice C is correct. The question is asking which mutation is more effective at preventing formation of the sfRNAs, which caused the virus to be more effective (choice C is correct). While a higher infection rate means that there were more sfRNAs present, the causal relationship is in the wrong direction (choice A is incorrect). An absence of C to G mutations means that the mutation was ineffective in the long term (choice B is incorrect). C to G mutations in xrRNA1’ were still present, and thus the mutations were effective (choice D is incorrect).
2. Answer choice B is correct. The question is asking which mechanism is used for the virus to enter the cells. From the passage, we can gather that the host cell is an animal cell, which is a eukaryote. In eukaryotes, viruses enter through endocytosis (choice B is correct). Transduction is virus-mediated gene transfer in bacteria (choice A is incorrect), injection through the tail sheath is how viruses inject genetic material into bacteria (choice C is incorrect), and transfer across the sex pilus is how bacteria exchange DNA in conjugation (choice D is incorrect).
3. Answer choice D is correct. This question is asking specifically about the transmission rate among all the tested species. The passage said that “the infection, dissemination, and transmission rates were determined as percentage of ZIKV-positive bodies, legs and wings, and saliva samples, respectively.” This means that the transmission rate was based on saliva samples (choices B and C are incorrect). Looking at the saliva samples in the figure, we see that the wild type had higher titers (choice D is correct).
4. Answer choice A is correct. This question asks about the introduction of new information and whether the new information agrees with or contradicts this article. The new information said smaller sfRNA segments were more important than longer ones, which does agree with our study (choices C and D are incorrect). We know this is true because when xrRNA2’, which stopped production of the sfRNA2 that was smaller than sfRNA1, was mutated, there was a more significant drop off than when xrRNA1’ was mutated (choice B is incorrect; choice A is correct).
5. Answer choice D is correct. This question also relies on your knowledge of eukaryotic cells and what the functions of each organelle are. Protein translation does not occur in the nucleus or lysosomes (choices A and C are incorrect). Golgi ribosomes are not a real organelle (choice B is incorrect). The correct answer is the rough endoplasmic reticulum (choice D is correct) because it is studded with ribosomes where protein translation occurs.
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Part 11: Practice standalone questions and answers
Question 1: When a vesicle leaves the rough endoplasmic reticulum for further packaging, where in the cell is it most likely to go next?
A) Smooth endoplasmic reticulum
B) Cis Golgi apparatus
C) Mitochondria
D) Trans Golgi apparatus
Question 2: Which of the following is not a tenet of cell theory?
A) Living organisms are composed of cell(s) and their products
B) All cells come from older cells
C) All cells have a nucleus containing their genetic material
D) Cells are the basic unit for any organism
Question 3: In the graph below showing the growth of bacteria over time, which segment is known as the stationary phase?
A) I
B) II
C) III
D) IV
Question 4: A disease that impairs the function of eukaryotic flagella would most likely target which cellular structure?
A) ATP
B) Microfilaments
C) Intermediate filaments
D) Microtubules
Question 5: Suppose researchers were performing a series of experiments on a bacterial colony. They performed a Gram staining and found that the cells looked a dark purple color, and when a simple antibiotic was placed in the middle of a cell culture, there was a clear spot on the plate. Which of the following are likely characteristics of the bacterial colony?
I. The bacteria are Gram-positive
II. The bacteria are rod-shaped
III. There is no capsule surrounding the bacteria
A) I only
B) I and III
C) II only
D) I, II, and III
Question 6: In a test tube with a culture medium, where would you expect to find facultative aerobes?
A) All at the bottom of the tube
B) All at the top of the tube
C) Half at the top of the tube, and half at the bottom of the tube
D) Spread throughout the tube, with a larger number near the top
Question 7: Which of the following cell structures does not have a membrane surrounding it?
A) Lysosome
B) Ribosome
C) Mitochondria
D) Golgi apparatus
Question 8: What type of cell infects eukaryotic cells by incorporating its DNA into the host DNA and replicating along with the cell?
A) Retrovirus
B) Prion
C) Bacteriophage
D) Viroid
Answer key to cells and viruses practice questions (standalone)
1. Answer choice B is correct. Once the ribosomes assemble the proteins, they are packaged by the RER and sent off to the Golgi apparatus (choices A and C are incorrect). The cis Golgi is the side facing the RER, and the trans Golgi faces the cell membrane (choice D is incorrect; choice B is correct).
2. Answer choice C is correct. This question asks which choice is NOT a tenet of cell theory, and while eukaryotic cells have nuclei, prokaryotic cells do not (choice C is correct).
3. Answer choice C is correct. The stationary phase describes the part of the bacterial life cycle after log growth when resources are depleted, and this is represented by III (choice C is correct). Choice A is known as the lag phase, choice B is the log phase, and choice D is the death phase.
4. Answer choice D is correct. This question asks about flagella in eukaryotes. Based on the content provided in this guide, microtubules are the building blocks of flagella (choice D is correct).
5. Answer choice B is correct. The question gives you a lot of information, and you have to select which of the possible choices are applicable. Since the Gram stain turned out to be dark purple, we know that the bacteria are Gram +, meaning I is correct (choice C is incorrect). There was nothing in the question suggesting anything about the shape of the bacteria, so we can eliminate option II (choice D is incorrect). Looking at option III, since there was no resistance to the antibiotic, there is likely no capsule to protect it, meaning that III is applicable (Choice A is incorrect; choice B is correct).
6. Answer choice D is correct. Facultative aerobes prefer using oxygen for metabolization, but they are capable of not using it as well. This means that they will be spread throughout the test tube, with a higher concentration near the top. Choice A would be true of obligate anaerobes, and choice B true of obligate aerobes. Choice C wouldn’t apply to any type of bacteria.
7. Answer choice B is correct. Among the options listed, the only structures that lack membranes are ribosomes. Lysosomes have a single membrane to contain the hydrolases (choice A is incorrect), Mitochondria have a double membrane crucial for metabolism (choice C is incorrect), and the Golgi apparatus is a group of membranous sacs stacked together (choice D is incorrect).
8. Answer choice A is correct. Retroviruses use reverse transcriptase in the cell to incorporate their genome into the cell DNA. Prions are misfolded proteins (choice B is incorrect), bacteriophages are bacteria that have been infected with a virus (choice C is incorrect), and viroids are single-stranded RNA that silences gene expression (choice D is incorrect).
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