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Crossing Over

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 Over

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...

Conservative Site-specific Recombination and Phase Variation

Conservative Site-specific Recombination and Phase Variation

Because the DNA segments are cut and reorganized in a direction-specific manner, site-specific recombination has emerged as an efficient genetic engineering technique. Flippase and Cyclization recombinases or Flp and Cre, respectively, are two members of the tyrosine recombinase family derived from bacteriophages, that are used to mediate site-specific DNA insertions, deletions, and targeted expression of proteins in mammalian cell lines.
The recognition sites for Cre recombinase called LoxP...

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Related Experiment Video

Updated: May 17, 2026

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

Studying recombination in mouse oocytes.

Xianfei Sun¹, Paula E Cohen

¹Department of Biomedical Sciences, Cornell University, Ithaca, NY, USA.

Methods in Molecular Biology (Clifton, N.J.)

|November 10, 2012

Summary

This summary is machine-generated.

Meiosis is crucial for producing gametes, but errors in female meiosis I prophase can cause birth defects. New techniques enable better analysis of these critical events in mouse and human oocytes.

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Area of Science:

Cell Biology
Genetics
Reproductive Biology

Background:

Meiosis is essential for sexual reproduction, producing haploid gametes.
Meiotic prophase I involves homologous chromosome pairing, synapsis, and recombination (crossing over).
Disruptions in prophase I can lead to aneuploidy and severe birth defects, particularly noted in human oocytes.

Purpose of the Study:

To compare meiotic mechanisms between sexes.
To address challenges in studying female meiosis, such as early developmental timing and limited tissue availability.
To present novel techniques for analyzing meiotic prophase I in oocytes.

Main Methods:

Description of three distinct techniques for meiotic prophase I analysis.
Application of these techniques to mouse and human oocytes.
Analysis spanning early prophase I events to crossing over resolution.

Main Results:

The presented techniques facilitate detailed examination of meiotic prophase I.
These methods allow tracking of key events from synapsis to recombination resolution.
The study highlights differences in meiotic control stringency between sexes.

Conclusions:

Effective methods are now available for studying female meiotic prophase I.
Understanding these mechanisms is critical for addressing reproductive health issues.
Comparative studies across sexes are vital for a complete picture of meiosis.