8.5
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: What is crossing over in meiosis?
Crossing over is the exchange of genetic material between homologous chromosomes during meiosis and crossing over. This process occurs when paired chromosomes physically align and swap DNA segments, creating new combinations of alleles. The result is increased genetic diversity in gametes, ensuring offspring inherit unique genetic combinations from their parents.
Q2: When does crossing over occur during meiosis?
Crossing over occurs during prophase I of meiosis I, when homologous chromosomes pair up in a process called synapsis. At this stage, the chromosomes are closely aligned, allowing segments to be exchanged. This timing is critical because it happens before the chromosomes separate, ensuring the recombined genetic material is properly distributed to daughter cells.
Q3: How does crossing over increase genetic variation?
Crossing over shuffles alleles between homologous chromosomes, creating new genetic combinations that differ from either parent. Since each gamete receives a unique mix of maternal and paternal DNA segments, no two gametes are genetically identical. This variation is essential for evolution and ensures genetic diversity within populations and offspring.
Q4: What are homologous chromosomes and why do they cross over?
Homologous chromosomes are paired chromosomes, one inherited from each parent, that carry genes for the same traits at corresponding locations. They cross over to exchange genetic material, combining different alleles on a single chromosome. This recombination produces genetic diversity, allowing organisms to adapt to environmental changes and improving population fitness.
Q5: What happens if crossing over does not occur?
Without crossing over, gametes would contain only parental combinations of alleles with no new genetic arrangements. This would severely limit genetic diversity in offspring, reducing their ability to adapt to environmental changes. Populations would lose evolutionary flexibility, making them more vulnerable to disease and environmental stress.
Q6: How does crossing over relate to gamete formation?
Crossing over during meiosis I produces recombined chromosomes that are then distributed during meiosis II and gamete formation. The genetic shuffling ensures each gamete carries a unique set of alleles before fertilization occurs. This process is fundamental to sexual reproduction, generating the genetic diversity necessary for species survival and evolution.
Q7: Where do crossing over events occur on chromosomes?
Crossing over can occur at multiple points along homologous chromosomes, creating segments called tetrads or bivalents. The exchange points, called chiasmata, are visible under a microscope as X-shaped structures. Multiple crossing over events on a single chromosome pair increase genetic recombination, further enhancing the diversity of allele combinations in gametes.