Lecture 7: Meiosis Fall 2024 PDF

Lecture 7: Meiosis Fall 2024 Textbook sections: 9.3, 10.1, 11.1, 12.1, 12.2, 12.3 Mitosis recap Mitosis is the division of somatic cells A somatic cell is a non-reproductive cell Mitosis Mitosis is generally used in multicellular organisms for growth. If that is the case,...

Lecture 7: Meiosis Fall 2024 Textbook sections: 9.3, 10.1, 11.1, 12.1, 12.2, 12.3 Mitosis recap Mitosis is the division of somatic cells A somatic cell is a non-reproductive cell Mitosis Mitosis is generally used in multicellular organisms for growth. If that is the case, how to they preproduce? Reproduction In some cases, multicellular organisms reproduce asexually. Examples: copperhead snakes use parthenogenesis Hydras use budding  Reproduction Other organisms reproduce sexually using gametes (reproductive/sex cells: sperm and egg) Sexual reproduction requires the fusion of two gametes to produce a zygote, this process is called fertilization Hold on… If parental cells are diploid, and you need to fuse two of them to get functioning offspring, how is it that the offspring are also diploid????  Sexual reproductive cycles contain haploid and diploid stages Gametes are haploid cells (1n, in humans n=23) When two gametes fuse, a diploid zygote is produced (2n=46) Gametes arise from a group of cells called germ-line cells, that will undergo meiosis to eventually produce haploid cells (gametes)  This seems complicated Sexual reproduction is complex, but has one big evolutionary advantage…it increases variation within a population… So, how do diploid cells produce haploid cells? Through meiosis! Meiosis consists of two rounds of cell division for one round of DNA replication Fundamentally, meiosis and mitosis are similar processes, but meiosis presents certain characteristics that increase variability 1. In meiosis, homologous chromosomes physically associate During a process called synapsis, two homologous chromosomes, through the formation of a synaptonemal complex, form tetrads 2. Recombination (AKA crossing over) occurs between homologous chromosomes Homologous chromosomes, when associated in tetrad formation, can exchange pieces of DNA between sister chromatids to form entirely new chromosomes This increases variation and is known as recombination or crossing over It occurs at a location called the chiasmata  Meiosis overview As previously mentioned, meiosis consists of two rounds of cell division for only one round of DNA replication It is organizes into two rounds of prophase, metaphase ,anaphase and telophase  Prophase 1 After interphase, chromosomes condense and become visible in the form of sister chromatids Nuclear envelope breaks down Synapsis occurs to form tetrads Crossing-over occurs between non sister chromatids Centrioles migrate to poles, and the mitotic spindle appears  Metaphase 1 Because of the formation of tetrads, when chromosomes align along the metaphase plate, paired homologues are aligned The orientation of paired homologues is random ( increases variability) The kinetochores of opposite homologues attach to microtubules from different poles of the cell. How does this compare to mitosis?  Anaphase 1 Homologous chromosomes are pulled towards the poles of the cells. The tetrad becomes separated. Compare this to mitosis. Which structures are migrating? Sister chromatids remain attached at centromere Because of the random orientation of chromosomes in metaphase 1, paternal and maternal chromosomes are separated independently, and randomly between the new cells, this is known as independent assortment In other words a pole could receive either a maternal or paternal chromosome Metaphase and anaphase 1  Telophase 1 Chromosomes migrate to poles, nuclear envelope reforms Each cell is now haploid Sister chromatids are no longer identical due to crossing over Cytokinesis occurs Meiosis 1  Meiosis 2 After meiosis 1, a brief interphase known as interkinesis occurs. There is no DNA replication in interkinesis. Think of it as a rest between cell divisions After interkinesis, meiosis 2 proceeds with both daughter cells of meiosis 1 Prophase 2 Nuclear envelope breaks down Chromosomes condense into sister chromatids New spindle forms, and attaches to kinetochore Metaphase 2 Chromosomes align on the metaphase plate Spindle fibres attach to opposite sister chromatids at the kinetochore Anaphase 2 Sister chromatids are pulled to opposite poles of the cell Very similar to mitotic anaphase Each sister chromatid is now a chromosome Telophase 2 Nuclear envelope reforms Cytokinesis occurs Meiosis 2 Results in the formation of four haploid cells Depending on the organism, this may lead directly to the production of gametes Meiosis recap 2n n Four big ideas in meiosis Meiosis is characterized by 4 features: Synapsis and crossing over Sister chromatids remain joined at their centromeres throughout meiosis I Kinetochores of sister chromatids attach to the same pole in meiosis I DNA replication is suppressed between meiosis I and meiosis II. Meiosis increases variability through crossing over and independent assortment  Meiotic errors Non-disjunction: event whereby chromosomes are not properly separated during anaphase 1 or 2 Leads to aneuploid gametes: gametes with the wrong number of chromosomes, gives rise to individuals with abnormal chromosome numbers. Ex. Trisomy 21 Recap https://www.youtube.com/watch?v=VzDMG7ke69g&vl=en https://www.youtube.com/watch?v=zrKdz93WlVk https://www.youtube.com/watch?v=f-ldPgEfAHI Homework Lab report due Wednesday No online quiz this week Test on Monday Ask review questions before Wednesday 8pm!!!!