Meiosis Lecture 20 (Spring 2024) PDF

Meiosis Lecture 20 Spring 2024 DNA - Eukaryotes DNA is found in the nucleus It is wound around histone proteins to help with compacting into the nucleus Chromatin is DNA in a loose uncoiled form - active Chromosomes are DNA tightly coiled and condensed - during cell division Chr...

Meiosis Lecture 20 Spring 2024 DNA - Eukaryotes DNA is found in the nucleus It is wound around histone proteins to help with compacting into the nucleus Chromatin is DNA in a loose uncoiled form - active Chromosomes are DNA tightly coiled and condensed - during cell division Chromosomes Chromatid - single strand Chromosomes - two chromatids - condensed Centromere - location on chromosome where chromatids are attached Chromosome Chromatin: DNA + Histones (Proteins) Allele: variation of a gene Homologous Chromosomes A homologous chromosome pertains to one of a pair of chromosomes with the same gene sequence, chromosomal length, and centromere location. A homologous pair consists of one paternal and one maternal chromosome. In humans, there are a total of 46 chromosomes in the nucleus of a somatic (body) cell. Half of them (22 autosomes + X or Y chromosome) are inherited from the father and the other half (22 autosomes + X chromosome), from the mother. Autosomes = Any chromosome not considered as a sex chromosome. Sex Chromosomes = X or Y chromosomes Chromosomes Mitosis Mitosis: sister chromatids separate Remember: sister chromatids are identical in mitosis Sister Chromatids Sister chromatids are two identical copies of the same chromosome formed by DNA replication, attached to each other by a structure called the centromere. During cell division, they are separated from each other, and each daughter cell receives one copy of the chromosome. Mitosis Eukaryotes Asexual DNA replicated Produces 2 diploid cells Meiosis Eukaryotes Sexual 2 cell divisions Produces 4 haploid cells (different DNA)  Eukaryotes same # chromosomes (diploid) (2n) After fertilization - how does the zygote (2n) grow/develop? Mitosis or Meiosis? ½ # chromosomes (haploid) (n) Meiosis Meiosis I & II  Meiosis I Prophase I - Synapsis to form tetrad (crossing over) Variation Metaphase I - Independent Assortment Variation Anaphase I Telophase I Meiosis I - Start 2n Meiosis I - End 1n Meiosis II Prophase II Metaphase II Anaphase II Telophase II Start Meiosis II - 1n End Meiosis II - 1n Homologues Meiosis I: Separate the homologues (homologous chromosomes) Sister Chromatids Meiosis II: Separate the sister chromatids (similar to Mitosis) Crossing Over Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis, which results in new allelic combinations in the daughter cells. This occurs in prophase of Meiosis I Why would only the non-sister chromatids participate in crossing over? Synaptonemal Complex Variation This diversity of possible gametes reflects two factors: crossing over and the random orientation of homologue pairs during metaphase of meiosis I. Crossing over. The points where homologues cross over and exchange genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis. This produces a wide variety of recombinant chromosomes, chromosomes where fragments of DNA have been exchanged between homologues. Random orientation of homologue pairs. The random orientation of homologue pairs during metaphase of meiosis I is another important source of gamete diversity. Random fertilization. Which sperm fertilizes which egg. Independent Assortment The Law of Independent Assortment states that different genes and their alleles (version of genes) are inherited independently within sexually reproducing organisms. During meiosis, chromosomes are separated into multiple gametes. Genes on a chromosome can rearrange themselves through the process of crossing-over. Therefore, each gene is inherited independently. Meiosis Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis. Sperm: Meiosis produces 4 equal sperm. Egg: Meiosis produces 3 polar bodies and 1 egg (unequal). Summary Meiosis Produce haploid cells (gametes) (1n) Genetic variation - crossing over - Prophase I Reduction Division - Meiosis I Equational Division - Meiosis II Males Occurs at start of puberty Females Meiosis I - before birth Meiosis II - at puberty/fertilization Zygotes Zygote (2n) Fertilization Sperm (1n) + oocyte (1n) Human Chromosomes Karyotype 23 Pairs One from each parent A karyotype is an individual’s complete set of chromosomes. The term also refers to a laboratory- produced image of a person’s chromosomes isolated from an individual cell and arranged in numerical order.  Learning Objectives Explain the process and steps of meiosis and the purpose. Compare sister chromatids and homologous chromosomes and where they are found in the meiosis stages. Differentiate between a zygote and a gamete and how each are formed. Compare Mitosis and Meiosis in relation to DNA and ploidy. Describe why and where crossing over occurs and how it contributes to genetic variation. Explain what a germ-line cell is and its role in gametogenesis. Explain how chromosomes are duplicated and separated during Mitosis and Meiosis.