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Lecture 10

Biology 1001A Lecture 10.pdf

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Department
Biology
Course
Biology 1001A
Professor
Beth Mac Dougall- Shackleton
Semester
Fall

Description
Biology 1001A | 2012 LECTURE NOTES Lecture 10: Meiosis –––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– Mechanisms to Generate Genetic Diversity - unequal crossing over gives rise to CNVs - recombination in general - we are still talking about ways to make copy number variations - mating and sexual reproduction seems wasteful of resources, but it is needed to generate copy number variations - making zygotes brings DNA from different cells into the same cell in new combinations - a 2n zygote is formed from two n haploid gametes - without sexual recombination, diversity in the species very difficult to achieve - homologous chromosomes have the same size and shape, they have the same genes in the same places, and one comes from each parent - alleles are from different parents unlikely to be identical, therefore they are similar yet not the same - parent organisms have to make gametes via meiosis - in meiosis I, the chromosome number is reduced from diploid to haploid, therefore is called the reductional phase - in meiosis II, cells divide to give products with 1 x C amount of DNA, therefore this is called the equational phase Meiosis in Different Species (its different) - meiosis is only used to make gametes in animals - in animal life cycles, the zygotes divide by mitosis - in plant life cycles, the zygotes and the spores divide by mitosis - the products of meiosis in plants are spores, which in turn produce gameotophytes, which produce gametes via mitosis - in some fungi/algae, the zygotes divide by meiosis Meiosis - in prophase of meiosis I, replicated homologues pair and recombine - recombination during meiosis is a method of generating copy number variations - the red and blue chromosomes have very similar base sequences - in order to recombine: recombination enzymes make nicks in the backbone, then the backbones are switched and attached - the end result is that the chromosome segments are switched between homologous chromosomes - this double stranded change is technically a mutation, but it is a very controlled and precise mutation! - the genes are still in the same places, though the alleles may be different - homologous chromosomes don’t pair side by side, they pair back to back so that all chromatids can participate Biology 1001A | 2012 - ultimately: sex is not about reproduction, it is about recombination - many organisms are content to replicate asexually, they just miss out on the genetic variation due to recombination - males experience prophase of meiosis after puberty - female chromosomes undergo all sexual recombination before birth as fetuses - gonadal tissue begins meiosis in - cells are in meiotic arrest until they are fertilized by sperm - males begin genetic recombination at puberty - crossing over gives new arrangements of alleles, not new genes - homologous pairs can cross over multiple times - recombination during meiosis creates new combinations of alleles - unequal crossing over can generate copy number variations (CNVs) - different alleles, but in the same places - repetitive DNA means that there can be a slip, causing change in the copy number of sequences, and hence CNVs - gene duplications are a type of mutation that can be good - homologues are held together by protein complex called synaptonemal complex - independent assortment - on the metaphase plate, the two homologues orient independent of each other - heterozygotes have different alleles at the same location - an individual is homozygous if they have the same alleles on both homologous chromosomes - the CCR5 ∆32 allele makes you resistant to HIV but more susceptible for West Nile Virus - meiosis one might result in having a preferred combination of alleles which can confer specific traits such as HIV resistance, cancer risk or cholesterol - you can inherit unfavourable alleles yet give preferable alleles to offspring because of the random arrangement of h
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