Download Lecture 10: Meiosis Products of meiosis in animals vs. plants, fungi

Lecture 10: Meiosis
Products of meiosis in animals vs. plants, fungi, algae
- After meiosis I, cells are already haploid (contain one chromosome from each homologous pair)
- Produces of meiosis in animals are haploid gametes
- In plants, gametes are formed from mitotic division of specific sporophyte tissues
Timing of meiosis in vertebrate life cycles (humans)
- Spermatogenesis: cell produces 4 equal-sized sperm
o Testes have millions of cells continuously producing sperm through meiosis
- Oogenesis: cytokinesis divides cytoplasm unevenly
o Only one daughter cell will have a large volume
o Others will are minimal and non-functional (cannot develop into viable offspring)
Main differences between meiosis and mitosis
Mitosis
Meiosis
Produces genetically identical offspring
Produces daughter cells that are genetically distinct from,
and contain half as much DNA as, their parent cells
Somatic cells
Gametes
One cell division (2 daughter cells)
Two cell division (4 daughter cells)
Produces cells with 2x as many
Produces cells with half as many chromosomes as mitosis
chromosomes as meiosis
Characteristics of homologous chromosomes
- Same genes but possibly different alleles (DNA sequence) than homologous partner
o Homozygous for a gene when both have same allele
o Heterozygous for a gene when chromosomes have differing alleles
- Same size and shape
- Toward beginning of meiosis I, homologous chromosomes move together and are held together
by protein complex called the synaptonemal complex
Meiosis I is “reductional” because cell is going from diploid to haploid
- Meiosis II is “equational” because the daughter cells stay haploid
Changes in C and n during meiosis
Mechanism of recombination during prophase
- Recombination during meiosis is what causes CNVs
- Double stranded change in the sequence of the chromosome
- Sexual recombination happens in meiosis I
o Homologues do not pair side-by-side; they overlap!
- Women: recombination is done before they are done (as fetus) and cells sit in meiotic arrest
o Once they are released monthly, they can complete meiosis if fertilized
- Men: recombination is constantly being done
Roles of cohesion and synaptonemal complex
- Cohesin: holds sister chromatids together
- Beginning of meiosis I, homologous chromosomes move together and are held together by
protein complex called the synaptonemal complex
How homologues pair in order for all non-sister chromatids to participate in recombination
- Crossing over of homologous chromosomes occurs during prophase I
Mechanism by which recombination creates new combination of alleles
- Cutting, pasting, and crossing over of DNA backbones yields new combination of alleles
Mechanism by which recombination creates copy number variation (CNV)
- During meiosis, a mistake can occur during recombination
- A slip occurs and crossing over becomes unequal
- Unequal crossing-over can generate CNVs
Randomness of alignment by homologous pairs at metaphase I
- Alignment of one homologous pair is independent of others
- Mendel’s Law of Independent Assortment
- Hence different alleles can be given to offspring
Relationship between distance separating genes and the likelihood of recombination between them
- Likelihood of recombination between some genes are low when there isn’t enough space along
the length of the chromosome
- Genes close together on the same chromosome tend not to suffer recombination events
between them
- Thus genes that are far apart have a high likelihood of recombination
Ways in which meiosis can be thought of as a kind of DNA “repair”
- You can inherit mutations on both homologues of chromosome 6, but you can give a
chromosome 6 with no mutations to your offspring
- Recombination allows for double stranded repair
Mechanism by which errors in MI or MII give rise to aneuploid products of meiosis
- Nondisjunction results when homologues fail to separate in meiosis I
o Spindles do not attach peroperly or break down
o Causes aneuploid gametes at the end of meiosis II
- Misdivision results when chromatids fail to separate In MII
o Also creates an unbalance
- Problems with spindle is believed to be associated with mom’s (and dad’s) age