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Related Concept Videos

Crossing Over01:30

Crossing Over

Crossing over is the exchange of genetic information between homologous chromosomes during prophase I of meiosis I. Genetic recombination gives rise to allelic diversity in the newly formed daughter cells. In humans, crossing over produces genetically distinct haploid egg and sperm cells that undergo fertilization to produce unique offspring. Before cell division starts, the germ cell’s chromosome(s) undergo duplication in the S phase of the cell cycle. As the cells enter prophase I, duplicated...
Crossing Over01:34

Crossing Over

Unlike mitosis, meiosis aims for genetic diversity in its creation of haploid gametes. Dividing germ cells first begin this process in prophase I, where each chromosome—replicated in S phase—is now composed of two sister chromatids (identical copies) joined centrally.
The homologous pairs of sister chromosomes—one from the maternal and one from the paternal genome—then begin to align alongside each other lengthwise, matching corresponding DNA positions in a process called synapsis.
In order to...
Oogenesis02:07

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...
Oogenesis01:22

Oogenesis

Oogenesis,  the process of developing egg cells (female gametes), occurs within the ovaries and is fundamental to female fertility. This sequence begins during fetal development when diploid oogonia in the developing ovaries undergo mitotic divisions to produce primary oocytes. By birth, these primary oocytes enter prophase I of meiosis but become arrested in this stage, remaining suspended until puberty.
Each primary oocyte is surrounded by a layer of pre-granulosa cells, forming what is known...
Meiosis II01:57

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...
Meiosis II02:02

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,...

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Related Experiment Video

Updated: Jun 20, 2026

Preparation of Meiotic Chromosome Spreads from Mouse Oocytes for Assessment of Synapsis and Recombination
09:24

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

Published on: July 18, 2025

Meiotic recombination in human oocytes.

Edith Y Cheng1, Patricia A Hunt, Theresa A Naluai-Cecchini

  • 1Department of Obstetrics and Gynecology, University of Washington, Seattle, Washington, United States of America.

Plos Genetics
|September 19, 2009
PubMed
Summary

Abnormal meiotic recombination, specifically concerning crossover locations, is linked to human trisomies. This study examined MLH1 protein in fetal oocytes, identifying vulnerable configurations that may lead to nondisjunction errors.

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Preparation of Meiotic Chromosome Spreads from Mouse Oocytes for Assessment of Synapsis and Recombination
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Published on: July 18, 2025

Using Mouse Oocytes to Assess Human Gene Function During Meiosis I
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Detection of DNA Double-Stranded Breaks in Mouse Oocytes

Published on: June 23, 2023

Area of Science:

  • Reproductive biology
  • Human genetics
  • Cellular biology

Background:

  • Human trisomies, such as Down syndrome, are associated with errors in meiosis, particularly maternal nondisjunction.
  • Abnormal meiotic recombination, including issues with crossover number or positioning relative to the centromere, is implicated in trisomy formation.

Purpose of the Study:

  • To investigate the role of meiotic recombination in human trisomies by examining crossover-associated proteins in fetal oocytes.
  • To analyze the number and distribution of MLH1 foci as a marker for recombination events in human oocytes.
  • To identify specific recombination configurations that may predispose chromosomes to nondisjunction.

Main Methods:

  • Immunofluorescence microscopy was used to visualize and quantify MLH1 foci in human fetal oocytes.
  • MLH1 foci, indicative of crossover sites, were analyzed for their number and chromosomal localization.
  • Genome-wide recombination levels and specific chromosome vulnerabilities were assessed.

Main Results:

  • The observed number of MLH1 foci was lower than predicted by genetic linkage analyses.
  • The localization pattern of MLH1 foci was consistent with their role as crossover-associated proteins.
  • Evidence suggests the presence of "vulnerable" crossover configurations in fetal oocytes, correlating with later nondisjunction events.

Conclusions:

  • The study provides direct evidence of abnormal meiotic recombination configurations in human fetal oocytes.
  • These identified configurations are consistent with mechanisms leading to maternal nondisjunction and trisomies.
  • Further research into recombination patterns may offer insights into preventing trisomic conditions.