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Meiosis I

Meiosis I

Meiosis is a carefully orchestrated set of cell divisions, the goal of which—in humans—is to produce haploid sperm or eggs, each containing half the number of chromosomes present in somatic cells elsewhere in the body. Meiosis I is the first such division, and involves several key steps, among them: condensation of replicated chromosomes in diploid cells; the pairing of homologous chromosomes and their exchange of information; and finally, the separation of homologous chromosomes by a...

Meiosis II

Meiosis II

Meiosis II is the second and final stage of meiosis. It relies on the haploid cells produced during meiosis I, each of which contain only 23 chromosomes—one from each homologous initial pair. Importantly, each chromosome in these cells is composed of two joined copies, and when these cells enter meiosis II, the goal is to separate such sister chromatids using the same microtubule-based network employed in other division processes. The result of meiosis II is two haploid cells, each containing...

Oogenesis

Oogenesis

In human women, oogenesis produces one mature egg cell or ovum for every precursor cell that enters meiosis. This process differs in two unique ways from the equivalent procedure of spermatogenesis in males. First, meiotic divisions during oogenesis are asymmetric, meaning that a large oocyte (containing most of the cytoplasm) and minor polar body are produced as a result of meiosis I, and again following meiosis II. Since only oocytes will go on to form embryos if fertilized, this unequal...

Meiosis I

Meiosis I

Meiosis is the division of a diploid cell into haploid cells forming sperm and eggs in animals through differentiation. Meiosis I is the first stage of meiosis, where the genetic recombination of homologous chromosomes and the reduction of the ploidy level by half occurs.
Prophase I is the most extended and complex step of meiosis I characterized by synapsis, chromosome pairing, and recombination of the homologous chromosomes. This process is facilitated by a proteinaceous structure called the...

Meiosis II

Meiosis II

Meiosis II entails cell division and segregation of the sister chromatids, resulting in the production of four unique haploid gametes. The steps for meiosis II are similar to mitosis, except that meiosis II occurs in haploid cells, whereas mitosis occurs in diploid cells.
The timing and cell division patterns of meiosis differ between males and females. In male meiosis, the centrosomes are part of the formation of the meiotic spindle. However, in oocytes, including that of humans, Drosophila,...

Meiosis vs. Mitosis

Meiosis vs. Mitosis

Cell division is necessary for growth and reproduction in organisms. Mitosis aids cell growth and development by dividing somatic cells. In contrast, meiosis causes the division of germ cells and plays an essential role in sexual reproduction. Due to their unique functional requirements, mitosis and meiosis differ from each other in multiple aspects.
Before the start of mitosis and meiosis I, the cell synthesizes DNA, resulting in two homologous copies of each chromosome. DNA synthesis is...

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Related Experiment Video

Updated: Jul 28, 2026

Chromatin Spread Preparations for the Analysis of Mouse Oocyte Progression from Prophase to Metaphase II

Chromatin Spread Preparations for the Analysis of Mouse Oocyte Progression from Prophase to Metaphase II

Published on: February 26, 2018

DNA synthesis studies in pre-meiotic mouse oogenesis.

W K Sung, J Van't Hof, G Jagiello

Experimental Cell Research

|April 1, 1986

Summary

Mouse oogenesis DNA replication shows faster chain growth and larger replicons, contrary to Callan

Area of Science:

Molecular Biology
Developmental Biology
Genetics

Background:

DNA replication timing is crucial for cell cycle progression.
Callan's hypothesis proposed that extended S phase duration is linked to fewer active DNA initiation sites.

Purpose of the Study:

To investigate the mechanism of the extended S phase during mouse oogenesis.
To test Callan's hypothesis regarding DNA initiation sites in oocyte development.

Main Methods:

DNA fiber autoradiography was employed to analyze DNA replication in mouse oocytes.
Replication characteristics were compared between oogenic and somatic cells.

Main Results:

Mouse oogenesis exhibited a rapid initial rate of DNA chain growth.

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Detection of DNA Double-Stranded Breaks in Mouse Oocytes

Detection of DNA Double-Stranded Breaks in Mouse Oocytes

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Preparation of Meiotic Chromosome Spreads from Mouse Oocytes for Assessment of Synapsis and Recombination

Preparation of Meiotic Chromosome Spreads from Mouse Oocytes for Assessment of Synapsis and Recombination

Published on: July 18, 2025

Related Experiment Videos

Last Updated: Jul 28, 2026

Chromatin Spread Preparations for the Analysis of Mouse Oocyte Progression from Prophase to Metaphase II

Chromatin Spread Preparations for the Analysis of Mouse Oocyte Progression from Prophase to Metaphase II

Published on: February 26, 2018

Detection of DNA Double-Stranded Breaks in Mouse Oocytes

Detection of DNA Double-Stranded Breaks in Mouse Oocytes

Published on: June 23, 2023

Preparation of Meiotic Chromosome Spreads from Mouse Oocytes for Assessment of Synapsis and Recombination

Preparation of Meiotic Chromosome Spreads from Mouse Oocytes for Assessment of Synapsis and Recombination

Published on: July 18, 2025

Replicons in oocytes were larger compared to those in somatic cells.

Findings contradicted Callan's hypothesis on activation site repression.

Conclusions:

The extended S phase in mouse oogenesis is not due to a reduction in active initiation sites.
Replication dynamics in oogenesis differ significantly from somatic cells and spermatogenesis.