Cells on the MCAT

It is crazy to think about, but everything is made up of this small unit called a cell. That means even you and I are composed of cells, over 75 trillion cells, to be exact!

Cells refer to a mass of cytoplasm that is bound externally by a cell membrane. Usually microscopic in size, can have one or more nuclei and other organelles that carry out a variety of tasks.

This guide will introduce you to the fascinating world of cells, along with some key terms and definitions you should remember as you prepare for the MCAT. 

Let’s get started!

Cells on the MCAT: What Do You Need to Know?

Cells are covered in the Biology section of the MCAT. 

Introductory biology accounts for 65% of the Biological and Biochemical Foundations of Living Systems section (Bio/Biochem) and 5% of the Chemical and Physical Foundations of Biological Systems section (Chem/Phys).

It’s hard to predict the exact number of questions about cells that will appear on the MCAT. However, you can expect it to appear in both the Bio/Biochem and Chem/Phys sections.

Important Sub-Topics – Cells

We cannot stress enough the importance of understanding cells during MCAT prep since this is closely related to biochemistry, and many other important MCAT topics! 

Cells can be complicated, so let’s break them down into important sub-topics you can concentrate on.

1. Tissues Formed From Eukaryotic Cells

Tissues are defined as a group of structurally-related cells that come together to perform a function. Four types of eukaryotic cell tissues exist, but here are the most important ones:

2. Mitosis: The Cell Cycle

Our cell cycle is an organized process containing differentiating phases that are regulated via checkpoints! The cell cycle is divided into G1, S, G2, and M phases with a quiescent G0 state while being regulated for possible errors by many checkpoints.

A. G1 Phase

In this phase, the cell grows in both size and number of organelles to eventually reach an even distribution of each into the two daughter cells.

B. S (synthesis) Phase

This phase involves the synthesis and replication of DNA. This replication of DNA contains 2 genetically identical sister chromatids held together by a centromere.

C. G2 Phase

In this phase, the cell continues to grow and synthesizes many mitotic proteins. At this stage if any DNA damage is detected, entrance to M phase is halted until endonucleases and exonucleases repair the damage.

D. M (mitotic) Phase

This phase results in the formation of two genetically identical daughter cells from one parent cell. Has four phases – prophase, metaphase, anaphase, and telophase.

E. G0 (Growth arrest) Phase

Sometimes, cells enter this phase when not actively pursuing to divide, such as neuronal cells.

There are also two checkpoints – G1/S and G2/M checkpoints to regulate the cell cycle pathway, ensuring progression through the cell cycle is functioning properly.

3. Mitosis: Structures and Processes

There are four phases of mitosis – prophase, metaphase, anaphase and telophase.

Phase 1: Prophase

This involves the degradation of the nuclear envelope.

Phase 2: Metaphase

In metaphase, the condense chromosomes begin to migrate towards the midline of the cell.

Phase 3: Anaphase  

Separation of sister chromatids via retraction of spindle fibers.

Phase 4: Telophase

Nuclear envelope reforms and chromosomes decondense back to chromatin.

4. Meiosis: Significance and General Overview

Meiosis is the basis for how we reproduce as species! Meiosis results in gametes or sex cells that fuse together during fertilization, generating a genetically unique zygote.

Human gametes are diploid, meaning we have two sets of chromosomes. In total, we have 23 pairs of homologous chromosomes or 46 chromosomes.

Meiosis differs from mitosis since meisosis produces 4 gamete sex cells each with half the amount of genetic information of the parent cell. Whereas mitosis produces two genetically identical daughter cells. Meiosis also occurs in two phases, whereas mitosis occurs in only one.

5. Meiosis: Processes

As mentioned previously, meiosis has two phases – Meiosis I and Meiosis II.

Let's begin with meiosis I: segregation of homologous chromosomes

Following meiosis I, is meiosis II: segregation of sister chromatids and genes

6. Prokaryotic cells: General Structural and Physiological Characteristics

Prokaryotic cells are somewhat similar to eukaryotic cells but have a list of characteristics unique to them:

1. Lack of nucleus and mitotic features such as spindle fibers and centrioles since prokaryotic cells carry out binary fission
2. Lack of membrane-bound organelles
3. Presence of a bacterial cell wall and composition
4. Presence of flagella allow for bacterial locomotion
5. Have symbiotic relationships between the bacteria and the host (mutualism, commensalism, and parasitism)

7. Prokaryotic Cells: Cell Theory and Major Classifications

Cell theory was discovered by Robert Hooke and has 4 main tenets:

1. Cells are the basic structural units of life. 
2. All living organisms are composed of cells. 
3. Cells can only arise from pre-existing cells and do so independently. 
4. DNA is used to transmit the necessary genetic codes, which contain the information needed for cell growth and survival. 

Cells can also be classified in many different ways, such as cell domains, anaerobic vs. aerobic metabolism, and shape.

8. Prokaryotic Cells: Cell Growth

In order for cell growth to occur, we must undergo mitosis. However, in prokaryotes, mitosis does not occur. Instead, there is a similar cellular mechanism called binary fission. This process allows a bacterial cell to divide into 2 genetically identical daughter cells.

Due to the simplicity of binary fission, bacterial growth can actually be modeled as an exponential function, which has four phases:

1. Lag Phase: Represents when bacteria are not yet dividing and acclimating to the new environment. 
2. Exponential (Log) Phase: Represents the stage of rapid, exponential bacterial cell growth. 
3. Stationary Phase: Represents the slowing of reproduction due to the depletion of nutrients and/or resources. 
4. Death (Decay) Phase: Represents bacterial death due to the environment no longer being able to support cell growth. 

9. Prokaryotic Cells: Genetic Exchange and Recombination

Bacterium can have extragenomic DNA plasmids outside of their typical circular DNA chromosome.

These extragenomic DNA plasmids have genes that code for specialized functions such as antibiotic resistance!

Gene transfer can occur both horizontally and vertically. Meaning the transfer is occurring within the same generation or different generations, respectively.

For an example, we’ll look at the steps for a horizontal gene transfer (within the same generation of bacterium):

Step 1 - Transformation:

This is described as the uptake of extragenomic DNA from the environment.

Step 2 - Conjugation:

This is a gene transfer that occurs through the use of a sex pilus structure.

Step 3 - Transduction:

The use of a bacteriophage to insert foreign bacterial DNA into a bacterium.

10. Prokaryotic Cells: Transposons

Transposons are DNA sequences that can switch/translocate between different positions in the genome.

The main structure of a transposon is a transposase gene sandwiched between two inverted terminal repeat (ITR) ends.

Transposase utilizes the ITRs as binding recognition sites to cut out the transposon. Afterward, transposase cuts at another DNA sequence site which allows for the insertion of the transposon at the new location.

11. Viruses: General Structural Characteristics

Viruses have two main components – a capsid protein shell and a nucleic acid genome. Additionally, depending on the presence of a lipid membrane enclosing the virus, the virus can be determined as enveloped or nonenveloped.

Viruses break the 4th tenet of cell theory, as they can have both DNA and RNA genomes that can be either single or double-stranded. 

Single-stranded RNA genomes can either be (+) or (-) sense. If the RNA genome can be translated immediately, it is a (+)-sense genome.  A (-)-sense genome must first utilize an RNA-dependent RNA polymerase to transcribe a complementary (+)-sense strand that can undergo translation.

Another key characteristic of viruses is their lack of nuclei and organelles. Additionally, viruses are extremely small, much smaller than prokaryotes and eukaryotes.

12. Steps of the Viral Cycle

The viral cycle occurs in a series of steps. Both the phage and animal viruses share general steps:

A. Entry

Attachment occurs via protein-receptor binding for animal viruses and through tail fiber anchoring for general phages. Animal viruses infiltrate the cell through either membrane fusion, endocytosis, or genome insertion.

B. Transcription, translation, and genome replication

Transcription and translation come first to create necessary proteins for viral infection such as capsid proteins and restriction enzymes. Following this, genome replication occurs.

C. Assembly: self-replicating biological units

The replicated viral components then self-assemble to form virions.

D. Release

As all the virions come together, they are then released via cell lysis, killing the cell.

13. Subviral Particles: Prions and Viroids

Prions and viroids are a relatively new discovery and, therefore, a somewhat low-yield topic on the MCAT:

Key Terms and Definitions – Cells

Here are some of the more important key terms and definitions to remember for this general guide to cells!

Additional FAQs – Cells on the MCAT

Additional Reading Links – Study Notes for Cells on the MCAT

For more in-depth content review about cells on the MCAT, check out these detailed lesson notes created by top MCAT scorers!

Additional Reading: MCAT Biology Topics:

• Cardiovascular Systems on the MCAT
• Digestive Systems on the MCAT
• Embryogenesis and Development on the MCAT
• Endocrine Systems on the MCAT
• Excretory Systems on the MCAT
• Genetics and Evolutions on the MCAT
• Immune Systems on the MCAT
• Nervous Systems on the MCAT
• Musculoskeletal Systems on the MCAT
• Reproduction on the MCAT
• Respiratory Systems on the MCAT