Meiosis and Oogenesis Overview

Overview

This lecture explains the steps of meiosis and applies them to oogenesis, detailing how germ cells become mature oocytes through cell division.

Mitosis is cell division in somatic cells, resulting in two diploid (2n) cells with 46 chromosomes each.
Meiosis occurs in germ cells, producing four haploid (n) gametes, each with half the chromosome number (23).
Mitosis involves one division; meiosis involves two sequential divisions (meiosis I and II).
DNA replicates during interphase before mitotic or meiotic division.

Meiosis I: includes prophase I, metaphase I, anaphase I, and telophase I.
Prophase I has sub-stages: leptotene (chromatid condensation), zygotene (homologous chromosome pairing/synapsis), pachytene (crossing over), diplotene (synaptonemal complex disintegration, chiasmata form), diakinesis (chromatids separate).
Crossing over occurs in pachytene, promoting genetic variation.
Meiosis I is a reduction division, cutting chromosome number in half.
Meiosis II resembles mitosis but without prior DNA replication, resulting in four haploid cells.

Primordial germ cells migrate to the developing ovary and undergo mitosis to increase their number.
These cells differentiate into oogonia (diploid), which also divide by mitosis until 5 months gestation.
Surviving oogonia begin meiosis I, becoming primary oocytes arrested in prophase I (diplotene stage) until puberty.
At puberty, luteinizing hormone allows a primary oocyte to complete meiosis I, forming a secondary oocyte and a first polar body.
The secondary oocyte starts meiosis II but halts at metaphase unless fertilization occurs.
Only upon fertilization does meiosis II complete, producing a mature ovum and a second polar body.
Three polar bodies degenerate, leaving one functional mature oocyte as the gamete.

Gametogenesis — process where germ cells form gametes (egg or sperm).
Oogenesis — process by which an oogonium becomes a mature oocyte.
Mitosis — cell division creating two identical diploid cells.
Meiosis — cell division that reduces chromosome number by half to form gametes.
Diploid (2n) — cell with two sets of chromosomes (46 in humans).
Haploid (n) — cell with one set of chromosomes (23 in humans).
Synapsis — pairing of homologous chromosomes during meiosis.
Crossing Over — exchange of genetic material between chromatids during prophase I.
Chiasmata — points where chromatids remain attached after crossing over.
Polar Body — small cell formed during oogenesis that usually degenerates.