Cells and Viruses for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about cell biology and viruses, plus practice questions and answers
(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: Practice passage and answers
Part 11: Practice 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.
At the end of this section, there is a practice passage and standalone questions to test your knowledge and show you how cells can make up an MCAT passage. In addition, we’ve bolded terms throughout this guide and provide definitions at the end. We encourage you to come up with your own definitions as you read through the guide that make sense to you!
(Suggested Reading: MCAT Biology Practice Questions)
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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.
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: 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.
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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