When is the DNA replicated during meiosis?
As in mitosis, DNA is replicated during the S phase of the cell, between the G1 and the G2 phases of the cell. The only difference between mitosis and meiosis is that the cell that is dividing for mitosis is a regular somatic cell, while the cell used for meiosis is a specialized germ cell located in the sexual organs whose sole purpose is to grow and divide into 4 gametes.
Are homologous pairs of chromosomes exact copies of each other?
Homologous pairs of chromosomes are not exact copies of one another because one of the homologs is given to the cell from its father, and the other is from its mother. When DNA is replicated, the entire chromosomes are not replicated, only the sister chromatids from each of the parent's genes. There are many genetic variations between the chromosome belonging to the father and the chromosome belonging to the mother, and this in turn promotes genetic variation.
What is crossing over?
Crossing over is a process by which homologous pairs of chromosome form crossing points, called synapse, and exchange sets of DNA between each other. This means that the father's chromosome could trade its portion of DNA that codes for hair color, for example, with the mother's chromosomes gene for hair color. The resulting chromosomes are called recombinant chromosomes. This increases genetic variation because then it means that gametes can have random combinations of their parents DNA, not just be forced to have one or the other parents's genetic information.
What physical constraints control crossover frequency?
The physical constraint for crossing over are where the gene is on the chromosome. Because crossing over could lead to huge problems in a gametes DNA if a gene is spliced in half incorrectly and would therefore become defective, crossing over can only happen when two complete genes are switched. Therefore, only certain sections of the chromosome can swith, and if they do switch, the entire gene is moved.
What is meant by independent assortment?
Independent assortment is the process by which homologous chromosome pairs line up along the metaphase plate during meiosis I. When the chromosomes line up, they are not forced to have all of the maternal DNA on one side and all the paternal DNA on the other. Instead, because the homologs assort independently, there is a random mixture of paternal and maternal DNA on each side of the metaphase plate. Thus, in turn, when the homologs separate, a random mixture of DNA is put into each resulting gamete.
What happens if a homologous pair of chromosomes fails to separate, and how might this contribute to genetic disorders such as Down syndrome and cri du chat syndrome?
When a homologous pair of chromosomes fails to separate, the gametes resulting from that meiosis will either have one less or one extra chromosome. Without the correct number of chromosomes, if this gamete becomes a zygote, the child will face genetic disorders such as downs syndrome, which is having an extra chromosome 20, 21, or 22; or cri du chat syndrome, which is the deletion of the 5th chromosome. The child's DNA is messed up, and therefore their mental capabilities and many of their body's structures are abnormal.
How are mitosis and meiosis fundamentally different?
Mitosis and meiosis are fundamentally different in both their purpose and their products. For mitosis, the purpose is just to produce two identical, diploid cells to either increase the size of an organism as in a teenager going through puberty, or replace dead cells such as on a skinned knee after a bicycle accident. Meiosis, on the other hand, produces 4 haploid cells used for sexual reproduction. Each cell produced is different from the rest of the haploid cells produced because of processes that promote genetic diversity like crossing over and independent assortment.
Link to meiosis video:
https://www.youtube.com/watch?v=OZLBftmBsow
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