Cells and Viruses for the MCAT: Everything You Need to Know
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.
