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Mitosis &
Chromosome Dynamics

Master the stages of mitotic cell division, understand chromosome structure, and learn how cells faithfully duplicate their genetic material for the MCAT.

MCAT Biology Foundational Concept 2C

What Is Mitosis?

Mitosis is a type of cell division that produces two genetically identical daughter cells from a single parent cell. It is essential for growth, tissue repair, and asexual reproduction in eukaryotic organisms.

The result: each daughter cell receives an exact copy of every chromosome, maintaining the diploid number \(2n\).

Chromosome Terminology

Chromatin
Loosely packed DNA-histone complex found during interphase; allows gene transcription
Chromosome
Tightly condensed chromatin visible during division; each consists of two sister chromatids
Centromere
Constricted region joining sister chromatids; attachment site for kinetochore proteins

The Cell Cycle Overview

G1 S G2 M Cell Cycle Growth & prep DNA synthesis Prep for mitosis Mitosis + cytokinesis

DNA Content: \(2c\) to \(4c\) and Back

2c 4c G1 S G2 M G1 DNA Content

The Five Stages of Mitosis

1
Prophase: Chromatin condenses into visible chromosomes. Centrosomes migrate to opposite poles and begin forming the mitotic spindle.
2
Prometaphase: Nuclear envelope breaks down. Kinetochore microtubules attach to centromeres of chromosomes.
3
Metaphase: Chromosomes align at the metaphase plate (cell equator). Spindle assembly checkpoint ensures all kinetochores are attached.
4
Anaphase: Sister chromatids separate at centromeres and are pulled to opposite poles by shortening kinetochore microtubules.
5
Telophase: Chromosomes decondense. Nuclear envelopes re-form around each set. Cytokinesis begins, dividing the cytoplasm.

Prophase & Prometaphase

Prophase Prometaphase

Metaphase & Anaphase

Metaphase Anaphase

Telophase & Cytokinesis

Daughter Cell 1 Daughter Cell 2 Cleavage Furrow

Mitosis vs Meiosis

Feature Mitosis Meiosis
Divisions 1 2
Daughter Cells 2 identical 4 unique
Ploidy Result \(2n\) (diploid) \(n\) (haploid)
DNA Content \(2c\) per cell \(c\) per cell
Crossing Over No Yes (prophase I)
Homolog Pairing No Yes (synapsis)
Function Growth, repair Gamete production

Cell Cycle Checkpoints

G1 Checkpoint
Checks cell size, nutrients, and DNA damage. Governed by p53 and Rb. If conditions fail, cell enters G0.
G2 Checkpoint
Verifies DNA replication is complete and error-free. CDK1-cyclin B complex must be activated to proceed to M phase.
Spindle Assembly
At metaphase: ensures all kinetochores are attached to spindle fibers. Prevents premature anaphase and aneuploidy.

How to Identify Each Stage

1
Look at chromatin state. Diffuse = interphase. Condensed and visible = mitosis has begun (prophase or later).
2
Check the nuclear envelope. Present = prophase or interphase. Absent = prometaphase through telophase.
3
Find chromosome position. Scattered = prometaphase. Aligned at center = metaphase. At poles = anaphase/telophase.
4
Count chromatid arms per chromosome. X-shaped (joined sisters) = pre-anaphase. V-shaped (individual) = post-anaphase.
5
Look for cleavage furrow or cell plate. If visible, the cell is in telophase/cytokinesis.

Example: Chromosome Counting

A human somatic cell enters mitosis. How many chromosomes and chromatids are present at metaphase vs after anaphase?

Metaphase 46 chromosomes 92 chromatids DNA content = 4c Each chromosome = 2 sisters After Anaphase 92 chromosomes 92 chromatids DNA content = 4c (total) Each chromatid = 1 chromosome

Example: Cancer & Lost Checkpoints

A mutation in the p53 gene prevents a cell from arresting at the G1 checkpoint. What is the consequence?

1
Damaged DNA is not repaired before replication in S phase. Mutations accumulate with each division.
2
Apoptosis is not triggered. p53 normally activates pro-apoptotic genes (Bax, PUMA). Without it, damaged cells survive.
3
Uncontrolled proliferation. Cells bypass the restriction point and divide indefinitely, a hallmark of cancer.

Example: Ploidy Problem

An organism has a diploid number of \(2n = 8\). Complete the table for DNA content and chromosome count at each stage.

Stage Chromosomes Chromatids DNA (c)
G1 8 8 2c
After S phase 8 16 4c
Metaphase 8 16 4c
After Anaphase 16 16 4c
Daughter Cells 8 8 2c

Common Mistakes

✕ Confusing \(n\) and \(c\)
\(n\) = number of unique chromosomes (ploidy). \(c\) = amount of DNA. A cell can be \(2n\) but \(4c\) after S phase. They are independent concepts.
✕ Mitosis = 2 divisions
Mitosis involves only one division producing 2 cells. Meiosis has two divisions producing 4 cells. This is a frequent trap on MCQs.
✕ Ignoring the SAC
The spindle assembly checkpoint operates at metaphase, not anaphase. It prevents anaphase from starting until all kinetochores are attached.

Question 1 of 5

A human cell at metaphase of mitosis contains how many chromatids?

Question 2 of 5

During which phase of mitosis does the nuclear envelope break down?

Question 3 of 5

The spindle assembly checkpoint ensures that:

Question 4 of 5

An organism has \(2n = 12\). Immediately after anaphase of mitosis (before cytokinesis), how many chromosomes are in the cell?

Question 5 of 5

Which of the following is true about mitosis but NOT meiosis?