Crossing-Over (Meiosis) — Definition & Overview

Result of Crossing-Over

During prophase I of meiosis, homologous chromosomes align in a process called synapsis to form a tetrad. In this formation, parts of non-sister chromatids can touch to form a chiasma. Here, parts of homologous chromosomes can be exchanged.

There is an equal chance of crossing-over at any point along the chromosome. As a general rule, if two genes are very far apart on a chromosome, it is more likely that crossing-over will occur somewhere between them.

After crossing-over occurs, the homologous chromosomes separate to form two daughter cells. These cells go through meiosis II, during which sister chromatids separate. In the end, there are four possible gametes. Two of these are called parental because they contain the same alleles as one of the parents. The other two are called recombinant because they have unique allele combinations that are not found in either of the parental chromosomes.

Crossing-over is an important contributor to genetic diversity by creating new combinations of DNA sequences.

(Video) Meiosis: Where the Sex Starts

by CrashCourse

Crossing-over is an important part of meiosis where new genetic combinations are created. In this video by Crash Course, Hank gives a great description of this process and shows an example of what it looks like by comparing himself to his brother.

What Is Crossing-Over?

Crossing-over is when similar pieces of homologous chromosomes switch places. This is a very important process during meiosis.

Crossing-over happens during prophase I of meiosis. Remember, at this point in the cell cycle there are four copies of each chromosome (two copies from each parent). This happens because the goal is to make four new cells. So each X, or tetrad, contains two copies of the chromosome. Each half of the X is a sister chromatid and the Xs are homologous chromosomes.

The homologous chromosomes will line up and exchange parts of their genetic information with each other. This ensures that when one of the four new sex cells eventually fertilizes or is fertilized, it will contain information from both parents.

That might be confusing, so think of it like this: two parents have a kid. This kid now has one set of chromosomes from each of its parents (maternal and paternal DNA). When it is time for this kid to have children, crossing-over makes sure that the grandchild inherits DNA from both of the grandparents. Because a parent can only pass on half of their DNA, crossing-over ensures that the half that is passed on contains DNA from both sets of chromosomes.

Why Is Crossing-Over Important?

During meiosis, homologous chromosomes are paired up and form tetrads. Homologous chromosomes have the same shape, size, and carry the same type of genetic information. So in other words, sister chromosomes pair up and get close to each other. They trade parts of their chromatids with each other in a process called crossing-over.

Crossing-over allows for the exchange of genetic material and produces unique combinations of alleles. This creates genetic variation. It's why you are not a clone of your parents, but rather a unique mixture of their traits.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Meiosis}}$ : The process of dividing one nucleus into four unique haploid cells. Only occurs in sex cells

$\color{#8925AE}{\text{Crossing-Over}}$ : Homologous chromosomes trade pieces of genetic information

(Video) Chromosomal Crossover

by A3Academy

Crossing-over happens when chromosomes stick together and exchange DNA with each other. As a result, no two zygotes will be the same. This is a big contributor to genetic diversity. Check out this video to see a diagram of what this process actually looks like.