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Cytology · Year 1 · Medical University of Sofia

35

Cell division. Meiosis

Free notes for topic 35 of the Cytology syllabus, open without an account. Written by a senior student against the syllabus question and checked line by line by a second student before publishing. How content is made

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Summary

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Meiosis is a process involving 2 unique cell divisions: 1 cell becoming 4 cells with half the genetic information. It occurs only in cells that form gametes, the sperm and egg cells.

Its key features are synapsis, the pairing of 2 homologous chromosomes, one from each parent; crossing over, the exchange of genetic material between them; and the haploid result, a diploid mother cell producing 4 haploid daughter cells.

Meiosis I is a reduction in chromosome number. Meiosis II is a reduction in the amount of genetic material, each cell containing 23 chromosomes, and it runs much more rapidly than meiosis I, without S phase.

The comparison with mitosis is the examinable part: mitosis is one division, two diploid cells, genetically identical; meiosis is two divisions, four haploid cells, each genetically unique.

1. What meiosis is

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Meiosis is a process involving 2 unique cell divisions: 1 cell becoming 4 cells with half the genetic information.

It occurs only in cells that form gametes, that is sperm and egg cells.

Key features

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Synapsis: the pairing of 2 homologous chromosomes, one from the mother and one from the father.

Crossing over: the exchange of genetic material between parts of two homologous chromosomes. It leads to new allele combinations in the daughter cells.

Haploid: the diploid mother cell produces 4 haploid daughter cells.

Crossing over is the reason meiosis exists at all. Without it the four products would be reshuffled copies; with it, each one carries a chromosome no parent had.

2. Meiosis I

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Meiosis I is a reduction in chromosome number.

Prophase I is the same as mitotic prophase, plus crossing over, that is synapsis.

Metaphase I is the same as mitotic metaphase, but with alignment of chromosome pairs in the equatorial plane.

Anaphase I is the same as mitotic anaphase, but one chromosome pair is pulled to opposite poles.

Telophase I is the same as mitotic telophase, but haploid daughter cells are produced.

Diagram of the stages of meiosis showing prophase 1, metaphase 1, anaphase 1 and telophase 1, then prophase 2, metaphase 2, anaphase 2 and telophase 2
Diagram of the stages of meiosis showing prophase 1, metaphase 1, anaphase 1 and telophase 1, then prophase 2, metaphase 2, anaphase 2 and telophase 2

3. Meiosis II

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Meiosis II is a reduction in the amount of genetic material, that is DNA. Each cell contains 23 chromosomes.

  • It is much more rapid than meiosis I, and runs without S phase.
  • It is similar to mitosis, the only difference being that there are 2 cells instead of one.

Prophase II is the same as mitotic prophase.

Metaphase II is the same as mitotic metaphase.

Anaphase II: chromosomes separate, chromatids pulled to opposite poles.

Telophase II is the same as mitotic telophase, but 4 haploid cells with new genetic combinations are produced.

The absence of an S phase between the two divisions is what makes meiosis halving rather than copying: the DNA is replicated once and divided twice.

4. Mitosis and meiosis compared

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MitosisMeiosis
DivisionsOne cell divisionTwo cell divisions
ProductsTwo diploid cellsFour haploid cells
RecombinationLacks synapsis and DNA recombinationContains crossing over
Daughter cellsGenetically the sameEach genetically unique

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