Meiosis and Its Significance: Quick Revision

A brief note on Meiosis and Its Significance. Read it before a test. Try each question at the end before you open its answer.

Meiosis in short

• Where: special diploid cells (meiocytes) that make gametes or spores.
• Features: two divisions, one copying of the DNA, four haploid cells.
• Meiosis I is the reduction division. Meiosis II is the equational division.

Prophase I

• Leptotene: chromosomes become visible as thin threads.
• Zygotene: synapsis, the pairing of homologues, with a synaptonemal complex.
• Pachytene: bivalent (tetrad) with four clear chromatids, recombination nodules, crossing over (recombinase).
• Diplotene: the complex dissolves, and X-shaped chiasmata are seen. It can last for months or years in some oocytes.
• Diakinesis: terminalisation of chiasmata, the nucleolus and the nuclear envelope go.

The rest of meiosis

• Metaphase I: bivalents on the equator. Anaphase I: homologues separate, and the centromeres do not split.
• Telophase I: two cells (a dyad). Interkinesis: short, with no DNA copying.
• Anaphase II: the centromeres split, and the sister chromatids separate. The cell has 2n chromosomes for a short time.
• Telophase II: four haploid cells (a tetrad of cells).
• DNA of a human cell: 2C, then 4C, then 2C after meiosis I, then 1C in each gamete.

Mitosis against meiosis

• Mitosis: one division, two identical diploid cells, for growth and repair.
• Meiosis: two divisions, four haploid cells that differ, for gametes and spores.
• Only meiosis has synapsis and crossing over.

Why meiosis matters

• It keeps the chromosome number of a species constant across generations.
• Variation: crossing over, independent assortment ($2^n$ kinds of gamete), random fertilisation.
• Non-disjunction gives $n + 1$ and $n - 1$ gametes.

A worked example

A plant cell has 2n = 14. How many bivalents and chromatids are there in prophase I?

1. The 14 chromosomes form $14 \div 2 = 7$ bivalents.
2. Each chromosome has two chromatids: $2 \times 14 = 28$ chromatids.

Practice with new numbers

A program to try

The program counts the chromosomes of a human cell at three points of meiosis.

Mistakes to avoid

• Placing crossing over in zygotene. It takes place in pachytene.
• Splitting the centromeres in anaphase I. They split in anaphase II.
• Counting n chromosomes in a cell at anaphase II. It has 2n.
• Saying that mitosis makes variation. It makes copies.

Check yourself

1. In which stage does synapsis begin?

2. What separates in anaphase I?

3. How many cells are formed at the end of meiosis, and what is their chromosome number?

4. Name one source of variation in meiosis.

Sources

• NCERT (2023). Biology, Textbook for Class XI, chapter 10.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 11.1.