8.5
交叉互换
与有丝分裂不同，减数分裂在产生单倍体配子的过程中旨在实现遗传多样性。正在分裂的生殖细胞首先在减数第一次分裂前期开始这一过程；此时，每条染色体——已在S期完成复制——均由两条在着丝粒处连接的姐妹染色单体（相同拷贝）组成。
随后，同源染色体——一条来自母方，另一条来自父方基因组——开始沿纵向彼此并排排列…
Humans produce genetically distinct egg and sperm cells, and thus unique offspring, as a result of the meiotic process of crossing over.
In such organs, crossing over occurs within the nuclei of diploid precursor cells during the first stage of Meiosis I called Prophase I. Previously, all of the cell's chromosomes replicated and condensed, yielding X-shaped structures.
Two sets of Xs are visible in a cell, one maternally derived and the other, paternal. Importantly, each arm of an X is a copy of the same parental chromosome and such duplicate pairs are termed sister chromatids.
Maternal and paternal versions of the same chromosome then begin to pair up and become linked as a protein framework manifests between them called the synaptonemal complex.
The result is connected pairs of homologous chromosomes, aligned so that the same maternal and paternal genes match up that begin to intertwine. The genetic material at the sites where non-sister chromatids intersect breaks off and the disconnected segments reattach to opposite chromosomes.
After this crossing over, the synaptonemal complex dissipates, but the homologous pairs stay fastened at points of genetic transfer, individually called chiasma, during most of Meiosis I, thus crossing over ends in chromatids with new, unique blends of parental information and as a result, is an example of genetic recombination.
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.