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Crossing Over in Meiosis
AP Biology
Ch. 13
Ms. Haut
Crossing Over
• Process by which parts of homologous chromosomes are interchanged
• Crossing over and independent assortment are mechanisms that produce new combinations of genes
Important Features
• Loci (position of genes) on chromosome are arranged in a linear sequence
• 2 alleles of a gene occupy corresponding positions in homologous chromosomes
Homologous chromosomes prior
to meiosis
Homologous chromosomes after
DNA replication
Chiasma
• Occurs after synapsis of homologues has occurred in Prophase I of meiosis
• Crossover involves the breakage of each of the 2 homologues (actually chromatids) and the exchange of parts
Notice the crossing over.
x + +
+ y +
x y z
+ + +
x y z
+ + +
x y +
+ + +
A
B
C
D
Where did the cross-over event take place?
Stop
Crossing Over and
Genetic Linkage
Genetic recombination
• The order of loci remains the same, but the alleles that reside along the chromosome are now re-shuffled.
• The “crossing-over” of the DNA between the two chromosomes has been observed microscopically, as well as inferred genetically.
• As long as the two genes being monitored are far enough apart on a chromosome so that a cross-over is likely to occur, independent assortment is observed.
• Chromosomes with recombinant combinations of linked genes are formed by the occurrence of crossover in the region between the 2 loci
• The probability that crossover will occur between 2 loci increases proportionally as the distance between those loci increases
The mean numbers of crossovers per meiosis depends on the distance separating genes
Genetic map distance = mean number of crossovers between two genes
Genetic linkage
• If no breakage occurs between them, the 2 loci will appear to be linked together.
• The frequency of recombination can be determined experimentally by counting the number of recombinants and “non-recombinants”.
• Larger numbers of observations improve the resolution of the frequency value.
• Unlinked genes will segregate independently and have a recombination frequency equal to or greater than 50%.
• Linked genes (on the same chromosome and close together) will have recombination frequencies of less than 50%.
Genetic linkage
• Accumulation of inheritance patterns of many gene pairs will lead (eventually) to gene maps of each chromosome.
• Pair-wise and three-locus linkage associations can be formed.
• The frequencies of recombination can also be used to estimate the physical distance between loci along a chromosome.
• The values for recombination frequency can be considered as “map distances” on the genetic map
• One unit of genetic map distance is defined as the length of chromosome in which, on average, one crossover is formed in every 50 meiotic events.
Genetic linkage
• Genetic map order of genes is usually conserved when the DNA is examined directly (physical order).
• Genetic map distance between genes varies considerably from the physical DNA distance.
Recombination Distance
• Distance between two genes =# Recombinants / Total
Crossing Over during Meiosis in Sordaria
• Sordaria fimicola – ascomycetes fungus
Strains of Sordaria that produce black and tan spores can be purchased. The strains can be used to inoculate a Petri plate containing an agar. At first, the strain grows as a mycelial mat and then it produces fruiting (reproductive) bodies.
Hyphae growing from the points of the inoculation are seen in the magnified view of the agar surface to the right.
If a strain producing tan spores is inoculated on one half of the plate and a strain producing black spores is placed on the other half, hyphae grow from both points and eventually meet at the center of the plate where they fuse in the equivalent of mating. Since the hyphae of both strains are haploid, the fusion product is diploid. The diploid hyphae start to differentiate into a fruiting body called a perithecium as seen to the right. You can see the perithecia forming in first picture of the Petri plate; they are the dark line down the center of the plate.
Perithecium –sac structure that contains ascospores
In the perithecium, diploid cells divide first by meiosis and then by mitosis to produce 8 haploid spores. The spores are contained in a translucent saclike structure called an ascus (pl. asci). To the right , you can see a ruptured mature perithecium releasing several asci. Normally the asci would break open and release haploid spores (seen below) which would be air-carried to new locations where they would germinate and divide by mitosis to produce new hyphae.
You can view the asci by taking a perithecium from a culture and placing it in a drop of water on a slide. If you gently press on a coverslip covering the drop, the perithecium will burst open and release the asci. Too much pressure and you will burst the asci as well, ruining the preparation.
To the right is a photo of asci that resulted from a cross between two black strains. All of the spores are black.
To the right is a photo of asci that resulted from a cross between two tan strains. All of the spores are tan.
To the right is a photo of asci that resulted from a cross between black and tan strains. Look at a single ascus and note that it contains both black and tan spores. Those on which the pattern of spore distribution in the ascus is 4 tan to 4 black were produced from cells in which no crossing over occurred. Such asci are called non-recombinants. Other asci contain black and tan spores that are distributed in 2:4:2 patterns or 2:2:2:2 patterns. These asci only result from cells in which crossing over has occurred and are called recombinants. Because the recombinant patterns result only from crossing over, the frequency of occurrence of recombinants is a measure of how often crossing over occurs.
