Crossing over (genetic recombination during meiosis)
Crossing over is the exchange of chromosome segments between homologous chromatids during meiosis, creating recombinant gametes, promoting genetic diversity, and aiding correct chromosome segregation.
Overview
Crossing over, commonly called recombination, is a cellular process in which segments of DNA are exchanged between homologous chromosomes. It is a central concept in genetics and cell biology, and it occurs during meiosis. Through reciprocal exchange between non-sister chromatids, crossing over reshuffles alleles and produces gametes with new combinations of parental traits.
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Mechanism and visible features
Crossing over takes place when paired homologous chromosomes align and form a synaptonemal complex during prophase I. Recombination typically follows a pathway in which one or more double-strand DNA breaks are processed and repaired using the homologous chromatid as a template. The result can be a physical exchange of chromosome arms between chromatids of the paired homologues.
- Participants: homologous chromosomes and non-sister chromatids.
- Outcome: exchange of genetic material in cells that will become gametes.
- Visible sign: chiasmata (singular: chiasma), the X-shaped points where chromatids remain connected and crossing over occurred; the term comes from Greek and was interpreted early by cytologists such as F. A. Janssens (chiasma).
Biological consequences
By swapping segments, crossing over changes the arrangement of alleles along chromosomes and therefore the combinations of genes passed to offspring. This produces recombinant genotypes and increases the variety of phenotypes available to natural selection. Recombination also helps ensure proper segregation of homologous chromosomes; failure to recombine can lead to mis-segregation and aneuploidy.
Historical observation and detection
Early geneticists inferred recombination from patterns of trait inheritance; cytologists later observed physical correlates in stained preparations. Crossovers can be directly visualized in prepared meiotic cells as chiasmata under a microscope (stained cells). Genetic crosses and molecular methods now allow precise detection of crossover events at the DNA sequence level.
Uses and scientific importance
Recombination is a tool and subject in many fields: genetic mapping relies on recombination frequency to estimate distances between loci (map units or centimorgans); plant and animal breeders exploit crossing over to combine desirable alleles; evolutionary biologists study how recombination shapes genetic variation and adaptation. Modern genomics also examines patterns of crossover hotspots and the biochemical machinery that controls where and how often recombination happens.
Distinctions and notable facts
Not every homologous interaction results in a visible crossover—some interactions lead to noncrossover gene conversion events. Crossing over is distinct from other rearrangements like translocations or deletions that are not part of normal meiotic recombination. While it promotes diversity, misregulated recombination can cause chromosomal abnormalities. For accessible introductions and deeper technical treatments, see resources on genetics, meiosis and recombination (cell biology, meiosis), and cytological or molecular assays (chromosomes, chromatids, gametes, alleles, genes, phenotypes, stained cells, chiasma).
Expiration
Before meiosis, a normal duplication of the DNA occurs, so that all chromosomes are present with two chromatids. During meiosis, in prophase I, the two homologous chromosomes, i.e. the corresponding maternal and paternal chromosomes, attach to each other: The synaptonemal complex forms between them. The phase of attachment is called zygotene, the subsequent phase of pairing is called pachytene. The resulting structure is called bivalent (because there are two chromosomes) or tetrad (because there are four chromatids).
In some eukaryotes, the formation of the synaptonemal complex is only possible if recombination has already begun; in others, it can be formed without recombination having begun. However, the recombination process is always completed within the synaptonemal complex.
The breaks in the chromosomes are put together "crosswise" (crossing over). Therefore, whole chromosome regions are exchanged between two chromosomes. The DNA single strands are separated and so-called Holliday structures are formed.
As meiosis progresses, the newly combined homologous two-chromatid chromosomes shorten and diverge towards the cell poles as they migrate there along the spindle apparatus. However, if crossing-over has occurred, the chromatids remain slightly longer attached to each other at the sites of the still-fused region, which can be observed in the light microscope as a chiasma - a figure similar to the Greek chi "χ". Mosaic chromatids are formed, which contain both paternal and maternal genetic material.
Crossing-over is the prerequisite for intrachromosomal recombination and is one of the reasons why new trait combinations arise in sexually reproducing organisms.
Inequivalent crossing-over
If, during tetrad formation, two very similar but not homologous sequences are located next to each other - for example in the case of paralogous genes or transposons or satellite DNA - a crossing-over can occur, as a result of which sections are exchanged that do not correspond to each other: non-homologous or inequivalent crossover. After such an unequal exchange, corresponding parts are missing on one strand (deletion), which are represented twice on the other strand (insertion or duplication).
Inequivalent crossing-over can occur in paired chromatids during meiosis, occasionally also in asexual reproduction with mitotic nuclear divisions. On the one hand, inadequate crossover events are accidents; both deletions and insertions can lead to diseases, such as Huntington's chorea. On the other hand, cases of inaequal crossover with gene duplication represent a significant source for the evolution of gene families.
See also XX male and testis-determining factor: crossover of X and Y chromosomes.
Questions and answers
Q: What is crossing over?
A: Crossing over, also known as recombination, is the exchange of chromosome segments between non-sister chromatids that occur during meiosis.
Q: When does crossing over occur?
A: Crossing over occurs during meiosis.
Q: What is the effect of crossing over?
A: The effect of crossing over is to shuffle the alleles on parental chromosomes, which results in the gametes carrying different combinations of genes from either parent, and ultimately increases the variety of phenotypes in a population.
Q: What are chiasma?
A: Chiasma, also known as chiasmata, are visible crossovers that occur during crossing over, and they are Greek for a cross.
Q: Who suggested what chiasmata meant?
A: F.A. Janssens was the first to suggest what chiasmata meant.
Q: What is the main result of sexual reproduction compared to non-sexual modes of reproduction?
A: The main result of sexual reproduction compared to non-sexual modes of reproduction is the spread of variation through a population.
Q: What is the main advantage to parents in sexual reproduction?
A: The main advantage to the parents in sexual reproduction is the greater variety in their offspring, which increases the chances that some offspring will survive and reproduce.
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AlegsaOnline.com Crossing over (genetic recombination during meiosis) Leandro Alegsa
URL: https://en.alegsaonline.com/art/24347