Meiosis: Study Guide (Biology 5.1)

Why sexual reproduction needs a halving division

Your body cells are diploid (2n): they carry two full sets of chromosomes, one set from your mother and one from your father. In humans, 2n = 46. If an egg and a sperm were each diploid, the child would have 92 chromosomes, and the number would double again every generation.

Meiosis prevents this. It is a pair of nuclear divisions that turns one diploid cell into four haploid (n) cells, each with one set of chromosomes. In humans, n = 23. In animals these haploid cells become gametes (eggs or sperm). At fertilization, two gametes fuse to form a diploid zygote, and the chromosome number is back to 2n. Meiosis and fertilization together keep the chromosome number constant from one generation to the next.

You will often see the four products of meiosis called daughter cells. Whatever the name, the key outcome is the same: one set of chromosomes per cell, with one member of every homologous pair.

The vocabulary you need first

Most mistakes in this topic come from mixing up four words. Get these clear before you learn the stages.

• Chromosome: one DNA molecule with its proteins. Count chromosomes by counting centromeres.
• Sister chromatids: the two identical copies of one chromosome made in S phase. They are joined at the centromere. A chromosome with two sister chromatids still counts as one chromosome.
• Homologous chromosomes (homologues): a pair of chromosomes, one inherited from each parent. They are the same length, have the centromere in the same place and carry the same genes in the same order. They may carry different alleles (versions) of those genes, so they are not identical.
• Ploidy: the number of chromosome sets. A cell with both members of each pair is diploid (2n); a cell with one member of each pair is haploid (n).

One more point: DNA is copied once, in the S phase of interphase before meiosis begins. The cell then divides twice. One copying step and two divisions is exactly why each final cell ends up with half the original chromosome number.

Meiosis I: separating homologous chromosomes

Meiosis I is the reduction division. It halves the chromosome number by sending one member of each homologous pair to each new cell.

• Prophase I. The duplicated chromosomes condense and become visible. Homologous chromosomes find each other and pair up along their length. This close pairing is called synapsis, and the paired homologues (four chromatids in all) are often called a bivalent or tetrad. Points called chiasmata may form where chromatids of the two homologues are linked (what happens there is the subject of Topic 5.2). Meanwhile the spindle starts to form, the centrosomes move to opposite poles and the nuclear envelope breaks down.
• Metaphase I. Spindle fibres line up the homologous pairs at the equator of the cell, the metaphase plate. The two homologues of each pair face opposite poles.
• Anaphase I. The homologues of each pair separate and are pulled to opposite poles. Sister chromatids stay attached at their centromeres. Whole duplicated chromosomes move, not single chromatids.
• Telophase I and cytokinesis. The spindle breaks down and a nuclear envelope re-forms around each group of chromosomes. The cytoplasm divides: a cleavage furrow pinches an animal cell in two, and a cell plate forms across a plant cell. The result is two haploid cells. Each chromosome in them still has two chromatids.

Meiosis II: separating sister chromatids

Meiosis II follows without another round of DNA replication. Each of the two haploid cells divides again, in a way that looks very like mitosis.

• Prophase II. A new spindle forms in each cell. The chromosomes, each still made of two sister chromatids joined at the centromere, attach to the spindle.
• Metaphase II. The chromosomes line up in single file at the metaphase plate. Each sister chromatid has its own kinetochore, and the two sisters are attached to microtubules from opposite poles.
• Anaphase II. Proteins holding the sister chromatids together at the centromere break down. The sister chromatids separate and are pulled to opposite poles. From this moment each chromatid counts as a chromosome in its own right.
• Telophase II and cytokinesis. The spindle breaks down, nuclear envelopes re-form, the chromosomes start to decondense and the cytoplasm divides (cleavage furrow or cell plate again). The end result is four haploid cells, and each chromosome in them is a single, unduplicated chromatid.

Reading Figure 1. In stage A the cell has four chromosomes (count the centromeres) and eight chromatids. In stage B the long homologues sit on opposite sides of the equator, and so do the short ones. After meiosis I (stage C), each cell has one long and one short chromosome: one of each pair, so n = 2. Notice that the cells in C still have duplicated chromosomes. Meiosis II then splits each chromosome into its two chromatids, giving four cells (stage D), each with one long and one short single-chromatid chromosome.

Tracking chromosomes and DNA

Exam questions often ask you to count. Use this rule: chromosomes = centromeres; DNA content = number of chromatids. A chromatid is one DNA molecule.

Reading Figure 2. The DNA content doubles in S phase but the chromosome number does not change (12 chromosomes, now 24 chromatids). The first drop, at cytokinesis I, halves both the chromosome number and the DNA content. The second drop, at cytokinesis II, halves the DNA content again, but the chromosome number per cell stays at 6 because the chromatids that separated in anaphase II each became a chromosome.

Mitosis and meiosis compared

Both processes move chromosomes with a spindle apparatus, both have prophase, metaphase, anaphase and telophase stages, and both end with cytokinesis by a cleavage furrow or cell plate. They differ in how many cells they make and in what those cells contain.

Feature	Mitosis	Meiosis
Purpose	growth, repair, asexual reproduction	haploid cells for sexual reproduction (gametes in animals)
DNA replications before division	one	one
Number of divisions	one	two
Homologues pair up (synapsis)	no	yes, in prophase I
What lines up at metaphase	single chromosomes (metaphase)	homologous pairs (metaphase I); single chromosomes (metaphase II)
What separates at anaphase	sister chromatids	homologues (anaphase I); sister chromatids (anaphase II)
Cells produced	2	4
Chromosome number of daughter cells	same as parent (2n → 2n)	half the parent (2n → n)
Genetic content of daughter cells	identical to each other and to the parent	different from the parent and from each other

The last row needs a short explanation. Each cell from meiosis receives a different mix of maternal and paternal chromosomes, and chromatids can swap pieces before they separate. Topic 5.2 explains these sources of variation.

Worked example 1: counting through meiosis

Question. A plant species has 2n = 12. A cell in G1, just before it enters meiosis, contains 6.0 pg of DNA. Complete the table for the stages listed, then state how the end of meiosis would differ if the cell had divided by mitosis instead.

Step 1. Before division. In G1 each of the 12 chromosomes is a single chromatid: 12 chromosomes, 12 chromatids, 6.0 pg.

Step 2. After S phase. Replication copies every DNA molecule. The chromosome count stays 12 (still 12 centromeres), chromatids become 24 and the DNA doubles to 12.0 pg. This holds through prophase I and metaphase I.

Step 3. After meiosis I. Homologues went to opposite poles, so each new cell has 12 ÷ 2 = 6 chromosomes, each still with two chromatids: 12 chromatids and 6.0 pg per cell.

Step 4. Anaphase II. Sister chromatids separate. Within one cell there are briefly 12 separate chromatids, 6 moving to each pole.

Step 5. After meiosis II. Each of the four cells has 6 single-chromatid chromosomes and 6.0 ÷ 2 = 3.0 pg of DNA.

Stage (per cell)	Cells	Chromosomes	Chromatids	DNA / pg
G1	1	12	12	6.0
Metaphase I	1	12	24	12.0
End of meiosis I	2	6	12	6.0
Metaphase II	2	6	12	6.0
End of meiosis II	4	6	6	3.0

Mitosis instead. Mitosis would give two cells, each with 12 chromosomes and 6.0 pg, identical to the G1 parent.

Check. The gametes carry half the G1 DNA (3.0 pg is half of 6.0 pg), and two gametes fusing at fertilization give 12 chromosomes and 6.0 pg: the parent’s values. Notice that a cell at the end of meiosis I has the same DNA content as a G1 cell (6.0 pg) but only half the chromosomes. DNA content alone cannot tell them apart.

Worked example 2: which division is it?

Question. A student looks at dividing cells from an animal with 2n = 8 and records these observations. Identify the stage of each cell and justify your answer.

Cell	Observation
W	8 chromosomes, each with two chromatids, arranged as 4 pairs side by side across the equator
X	4 chromosomes, each with two chromatids, in a single line at the equator
Y	8 chromosomes, each with two chromatids, in a single line at the equator; no pairing
Z	8 single chromatids, 4 moving toward each pole

Method. Ask three questions in order. Are homologues paired? How many chromosomes are there compared with 2n? Are the chromosomes duplicated?

1. Cell W: metaphase I. Homologues are paired at the equator. Pairing only happens in meiosis I. The count is 2n = 8, as expected before the first division.
2. Cell X: metaphase II. There are only 4 chromosomes, which is n, so meiosis I has already happened. The chromosomes are still duplicated and are lined up singly, which fits metaphase II.
3. Cell Y: metaphase of mitosis. The count is 2n = 8 and the chromosomes are duplicated, but they are lined up singly with no pairing. A cell in metaphase I would show pairs.
4. Cell Z: anaphase II. Single chromatids are moving apart, so sister chromatids have separated. Only 4 go to each pole, which is n. In anaphase of mitosis, 8 would go to each pole (16 chromatids in all). In anaphase I, the chromosomes moving to the poles would still have two chromatids.

Interpretation. The two most reliable clues are pairing (only in meiosis I) and chromosome number relative to 2n. Counting chromatids alone can mislead: cells W and Y both have 16 chromatids.

Common misconceptions

• “Meiosis I separates sister chromatids.” No. Meiosis I separates homologous chromosomes. Sister chromatids separate in meiosis II.
• “Cells at the end of meiosis I are diploid because each chromosome has two chromatids.” Ploidy counts chromosome sets, not chromatids. Each cell has one member of each pair, so it is haploid.
• “DNA is copied again before meiosis II.” There is no S phase between the two divisions. That is how the DNA content falls to half the G1 value.
• “Homologous chromosomes are identical.” They carry the same genes, but often different alleles. Sister chromatids are the identical copies (before any crossing over).
• “Meiosis makes two cells, like mitosis.” Meiosis makes four cells from one, because there are two divisions.
• “A chromosome with two chromatids counts as two chromosomes.” Count centromeres. Two joined chromatids are one chromosome until they separate in anaphase.
• “Mitosis and meiosis are unrelated.” Both use a spindle and have similar stage names. Meiosis II in particular works much like mitosis, but in a haploid cell.

Where this leads

Meiosis builds directly on the cell cycle and mitosis from Topic 4.5. Next, Topic 5.2, meiosis and genetic diversity, explains why the four cells differ from each other and what happens when chromosomes fail to separate. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.