AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/5/5-1)
Unit 5 · Topic 5.1
5.1 Meiosis
Meiosis is the two-round cell division that turns one diploid cell into four haploid gametes. It keeps the chromosome number steady from generation to generation, because when two gametes fuse at fertilization the diploid number comes back.
Key terms
- haploid
- diploid
- gamete
- homologous chromosomes
- sister chromatids
- synapsis
Why meiosis exists
Most of your body cells are diploid (2n): they carry two full sets of chromosomes, one set from each parent. In humans that's 46 chromosomes, or 23 pairs. If eggs and sperm were diploid too, every generation would double the chromosome count. Meiosis prevents that by making haploid (n) cells, called gametes, with just one set. A human egg or sperm has 23 chromosomes.
The two chromosomes in a pair are homologous chromosomes. They carry the same genes in the same order, but they can carry different versions (alleles) of those genes, because one came from your mother and one from your father. Don't confuse them with sister chromatids, which are the two identical copies of one chromosome made during S phase and held together at the centromere.
Meiosis I: homologs separate
Before meiosis starts, the cell copies its DNA in S phase, so every chromosome has two sister chromatids. Then the cell divides twice. The first division, meiosis I, is the one that halves the chromosome number.
- Prophase I: chromosomes condense and homologous chromosomes pair up side by side, which is called synapsis. Non-sister chromatids can swap segments at crossing points called chiasmata (this is crossing over, covered in 5.2). The spindle starts forming and the nuclear envelope breaks down.
- Metaphase I: the spindle lines up homologous pairs along the middle of the cell. Pairs line up, not single chromosomes.
- Anaphase I: homologous chromosomes are pulled to opposite poles. Sister chromatids stay attached to each other.
- Telophase I and cytokinesis: the cell splits into two haploid cells. Each still has chromosomes made of two chromatids.
Meiosis II: sister chromatids separate
Meiosis II looks a lot like mitosis, but it starts with haploid cells and there's no DNA copying beforehand. In prophase II a spindle forms again. In metaphase II single chromosomes line up in the middle, with each sister chromatid attached to spindle fibers from opposite poles. In anaphase II the centromeres come apart and the sister chromatids move to opposite poles. After telophase II and cytokinesis you have four haploid cells, each with one unduplicated copy of each chromosome.
The key idea: meiosis I separates homologs (that's the reduction from 2n to n), and meiosis II separates sister chromatids (just like mitosis).
Mitosis vs. meiosis
Both processes use a spindle made of microtubules to move chromosomes, and both start after one round of DNA replication. They differ in what they're for and what they produce.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Cells produced | 2 | 4 |
| Chromosome number of products | Same as parent (2n → 2n) | Half of parent (2n → n) |
| Genetic content of products | Identical to parent and to each other | Different from parent and from each other |
| Homologs pair up (synapsis)? | No | Yes, in prophase I |
| Crossing over? | No | Yes, in prophase I |
| What lines up at metaphase | Single chromosomes | Homologous pairs (meiosis I), then single chromosomes (meiosis II) |
| Used for | Growth, repair, asexual reproduction | Making gametes for sexual reproduction |
Counting chromosomes and chromatids
A lot of exam questions ask you to track numbers through the stages. Count chromosomes by counting centromeres. A replicated chromosome (two chromatids joined at one centromere) still counts as one chromosome. The DNA amount doubles in S phase, halves at the end of meiosis I, and halves again at the end of meiosis II, so each gamete ends up with half the DNA of an unreplicated body cell.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Tracking numbers in a human cell
Human body cells have 46 chromosomes. For a cell going through meiosis, give the number of chromosomes and chromatids (a) in metaphase I and (b) in each cell in metaphase II, and (c) give the number of chromosomes in each gamete.
Show the solutionHide the solution
- Step 1: Start from the replicated cell. After S phase the cell still has 46 chromosomes (46 centromeres), but each has two sister chromatids, so there are 92 chromatids.
- Step 2: (a) Metaphase I happens before any separation, so the cell has 46 chromosomes and 92 chromatids, arranged as 23 homologous pairs.
- Step 3: (b) Anaphase I sends one homolog from each pair to each pole, so each daughter cell has 23 chromosomes. Sister chromatids haven't separated yet, so each of those 23 chromosomes still has 2 chromatids: 46 chromatids per cell.
- Step 4: (c) Anaphase II separates the sister chromatids, and each one becomes its own chromosome. Each gamete gets 23 chromosomes, each made of a single chromatid.
Answer: (a) 46 chromosomes, 92 chromatids; (b) 23 chromosomes, 46 chromatids per cell; (c) 23 chromosomes per gamete.
- Example 2
Mitosis or meiosis II? (classic trap)
An organism has a diploid number of 6 (2n = 6). A diagram shows a cell with 3 chromosomes lined up single file in the middle of the cell. Each chromosome has two chromatids, and no two of the chromosomes look alike. Is this cell in metaphase of mitosis, metaphase I or metaphase II?
Show the solutionHide the solution
- Step 1: Count the chromosomes: 3. A diploid cell of this organism would have 6, so this cell is haploid (n = 3).
- Step 2: Check for homologs: no two chromosomes match, which confirms there's only one set.
- Step 3: Rule out mitosis: a cell in mitotic metaphase of this organism would have all 6 chromosomes lined up.
- Step 4: Rule out metaphase I: in metaphase I, homologous chromosomes line up in pairs, and there would be 3 pairs (6 chromosomes).
- Step 5: A haploid cell with replicated chromosomes lined up single file is in metaphase II.
Answer: Metaphase II of meiosis. It's haploid (3 chromosomes, no homologs), but each chromosome still has two chromatids.
Common mistakes
- Counting chromatids as chromosomes. A replicated chromosome with two sister chromatids is still one chromosome; count centromeres.
- Saying the chromosome number is halved in meiosis II. It's halved in meiosis I, when homologs separate. Meiosis II separates sister chromatids and keeps the number at n.
- Mixing up homologous chromosomes and sister chromatids. Homologs come from different parents and can carry different alleles; sister chromatids are copies of one chromosome and start out identical.
- Thinking DNA replicates again between meiosis I and meiosis II. It doesn't, which is why four haploid cells come out of one round of copying.
On the exam
- Expect diagrams of cells at a stage of division where you must name the stage and justify it. Count chromosomes, check whether homologs are paired, and check whether chromosomes have one or two chromatids.
- When asked to compare mitosis and meiosis, give a specific difference in number of cells and in genetic content, and name one similarity (both use a spindle to move chromosomes).
Connected topics
Videos
Check yourself
4 questions on 5.1 Meiosis. Pick an answer to see if you got it, and why.
The body cells of a fruit fly have a diploid number of 8 (2n = 8). How many chromosomes are in each of its gametes?
Which of the following occurs in meiosis I but not in mitosis?
A cell from an organism with 2n = 6 is examined. It contains three chromosomes, each made of two sister chromatids, and no homologous pairs. This cell is most likely
A diploid cell in G₁ contains an amount X of DNA. How much DNA does each cell contain at the end of meiosis I, before meiosis II begins?
0 of 4 answered