MCAT Mitosis vs. Meiosis Chromosome Numbers

This guide explains exactly how to count chromosomes and chromatids during mitosis and meiosis—especially through the S phase—so you won’t get confused on test day.

Quick Definitions You Need Before Counting Anything

Before you can count anything correctly on the MCAT, you need to know what you’re actually counting. Most confusion on chromosome number comes down to this: people mix up chromosomes, chromatids, and pairs. Let’s clear it up:

Key Terms:

• Chromosome: A single piece of DNA with a centromere. After DNA replication, each chromosome consists of two sister chromatids, but it's still one chromosome because there’s only one centromere.
  

• Chromatid: One strand of DNA. When chromosomes replicate during S phase, they form two identical sister chromatids joined at a centromere. The number of chromatids doubles—but the number of chromosomes stays the same.
  

• Homologous chromosomes: A pair of chromosomes (one from mom, one from dad) that are similar in size and gene content. These are what get separated in Meiosis I, not in mitosis.
  
  
• Centromere rule: This is your cheat code: 🧠 Count centromeres = count chromosomes (No matter how many chromatids are attached)

DNA Replication in S Phase:

• What changes: The amount of DNA (chromatids) doubles
  
• What stays the same: The chromosome number (still 46 in human somatic cells)

High-Yield Callouts:

• G1: 46 chromosomes, 46 chromatids
  
• After S phase: 46 chromosomes, 92 chromatids
  
• Chromosome number changes only when centromeres are pulled apart, like in anaphase or meiosis I

What Happens in Meiosis (And Why It’s Trickier)

Most students mix up mitosis and meiosis because the numbers look similar at the start—but meiosis throws in a ploidy shift and two rounds of division. Here’s how to keep it all straight.

A. Starting Point: Before Meiosis Begins (G1 & S Phase)

Germ cells (precursors to sperm or egg) start just like somatic cells:

• G1: 46 chromosomes, 46 chromatids, diploid (2n)
• After S phase: 46 chromosomes, 92 chromatids, still diploid

Key Misconception to Avoid: Meiosis does NOT start with haploid cells. It starts with diploid cells, just like mitosis.

B. Meiosis I: Homologous Chromosomes Separate

Homologous pairs line up and separate. That’s the key event that cuts the chromosome number in half.

Each new cell gets:

• 23 chromosomes, but each chromosome still has 2 chromatids → 46 chromatids total
• Ploidy: Now haploid (1n)—because there's only one set of chromosomes
  

At the end of Meiosis I:

Two haploid cells, each with:

• 23 chromosomes
• 46 chromatids

C. Meiosis II: Sister Chromatids Separate

• No additional DNA replication before this phase
  
• Sister chromatids of each chromosome are pulled apart (just like in mitosis)
  

At the end of Meiosis II:

Four haploid gametes, each with:

• 23 chromosomes
• 23 chromatids

High-Yield Summary:

Common MCAT-Style Question Types Involving Chromosome and Chromatid Counts

The MCAT won’t just ask you to recite chromosome numbers—it’ll test whether you understand the logic of DNA replication, ploidy shifts, and cell division stages. Here are the most common formats you’ll see:

1. "At this point in the cell cycle..." Style Questions

These ask you to identify the number of chromosomes or chromatids at a particular phase (G1, S, G2, metaphase, anaphase, etc.).

Example:
A somatic cell is in G2 phase. How many chromosomes and chromatids does it contain?
→ 46 chromosomes, 92 chromatids

What it’s really testing:
Your ability to apply the "S phase doubles chromatids, not chromosomes" rule.

2. Diagram Labeling or Interpretation

These questions show a simplified cell with chromosomes aligned at the metaphase plate, or chromatids separating in anaphase.

Example:
A diagram shows homologous chromosomes being pulled to opposite poles. What phase is this, and how many chromosomes are in each resulting cell?
→ Anaphase I of meiosis; each resulting cell will have 23 chromosomes

What it’s really testing:
Whether you can distinguish between mitosis and meiosis visually.

3. Ploidy Change Questions

These ask if a cell is haploid or diploid at a certain phase—and often try to trip you up during meiosis.

Example:
After Meiosis I, a cell has 23 chromosomes, each with two sister chromatids. What is its ploidy?
→ Haploid (1n), because homologous pairs have been separated

What it’s really testing:
Your understanding of what makes a cell diploid vs. haploid—not just chromosome numbers, but where they came from (homologous or not).

4. Experimental or Mutation Scenarios

These wrap chromosome-counting concepts in a passage about lab techniques, such as flow cytometry, gene deletions, or nondisjunction.

Example:
Researchers observe a gamete with 24 chromosomes. At what meiotic stage did nondisjunction most likely occur?
→ Meiosis I or II; requires an understanding of normal counts and where errors happen

What it’s really testing:
Application of chromosome logic under unfamiliar, data-heavy conditions.

5. Comparative Tables or “Which of the following is true?” Options

These questions may give a list of chromosome/chromatid numbers and ask which applies to mitosis, meiosis I, or meiosis II.

Example:
Which of the following best describes a cell after completing S phase but before mitosis?
A) 46 chromosomes, 46 chromatids
B) 92 chromosomes, 92 chromatids
C) 46 chromosomes, 92 chromatids
D) 23 chromosomes, 23 chromatids
→ Correct answer: C

What it’s really testing:
Precision with terminology and an ability to spot misstatements (especially the classic trap: "92 chromosomes").