11.3
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, whe…
In sexually reproducing organisms, such as humans, offspring inherit one set of chromosomes from each parent.
During the formation of germ cells, such as sperm or eggs, homologous chromosomes inherited from each parent exchange DNA segments. This process is called crossing over.
Before cell division, in the S phase of the cell cycle, the cell copies its DNA. As meiosis I begins, during the leptotene stage of prophase I, the duplicated chromosomes condense.
During the zygotene stage of prophase I, homologous chromosomes pair closely along their lengths. This process is called synapsis. A protein structure called the synaptonemal complex forms between them, creating paired structures called tetrads.
During the pachytene stage of prophase I, non-sister chromatids of homologous chromosomes exchange matching DNA segments. This creates new combinations of genes through genetic recombination.
After crossing over, the crossover sites become visible as chiasmata. At these sites, homologous chromosomes remain connected.
As the synaptonemal complex breaks down during the diplotene stage of prophase I, the chiasmata remain visible. Together with cohesin proteins, which hold sister chromatids together, chiasmata help keep homologous chromosomes paired.
This genetic recombination through crossing over increases genetic variation in gametes and helps produce genetically unique offspring.
View the full transcript and gain access to JoVE Core videos
Q1: When does crossing over occur during meiosis?
Crossing over occurs during Prophase I, the first stage of Meiosis I. At this point, chromosomes have already replicated into X-shaped structures called sister chromatids. Homologous chromosomes—one maternal and one paternal—pair up and align alongside each other in a process called synapsis, facilitated by the synaptonemal complex protein framework.
Q2: What are sister chromatids and how do they relate to crossing over?
Sister chromatids are identical copies of the same chromosome joined at the center, created during chromosome replication before Prophase I. During crossing over, genetic material exchanges occur between non-sister chromatids—copies from different homologous chromosomes—not between sister chromatids. This exchange creates new combinations of alleles on each chromatid.
Q3: What is the synaptonemal complex and what does it do?
The synaptonemal complex is a protein framework that forms between paired homologous chromosomes during Prophase I. It holds maternal and paternal chromosome versions together in precise alignment, matching corresponding DNA positions. This structure facilitates the exchange of genetic material between non-sister chromatids, enabling homologous recombination and genetic diversity.
Q4: How does crossing over create genetic diversity?
Crossing over exchanges segments of DNA between non-sister chromatids of homologous chromosomes, creating new combinations of alleles. This process, called genetic recombination, produces chromatids with unique blends of maternal and paternal genetic information. The result is genetically distinct egg and sperm cells, ensuring unique offspring through meiosis and gamete formation.
Q5: What are chiasmata and when do they form?
Chiasmata are X-shaped structures that mark the physical points where crossing over occurred between homologous chromosomes. They form after the synaptonemal complex begins to dissolve but remain visible, holding the homologous chromosome pairs together until recombination is completed. Each chiasma represents one site of genetic transfer between non-sister chromatids.
Q6: Why is crossing over important for producing unique offspring?
Crossing over shuffles parental genetic information by exchanging DNA segments between homologous chromosomes, creating new allele combinations on each chromatid. This genetic recombination ensures that each egg and sperm cell produced is genetically distinct. Without crossing over, offspring would inherit unchanged chromosome sets from each parent, reducing genetic variation.
Q7: What happens to homologous chromosomes after crossing over ends?
After crossing over is complete, the synaptonemal complex dissolves, but homologous chromosome pairs remain connected at the chiasma sites throughout most of Meiosis I. These connection points hold the paired chromosomes together until they separate during the later stages of meiosis. The chromatids now carry recombined genetic material from both parental sources.