Meiosis Week 14 Slides 2024 PDF

LIVING SYSTEMS (BIOL14406) MEIOSIS [email protected] IBRC 116  What’s this lecture about? This lecture should help you discuss the following: The diploid or haploid-dominant or alternating life-cycles in humans and fungi and plants The stages of meiosis The diff...

LIVING SYSTEMS (BIOL14406) MEIOSIS [email protected] IBRC 116  What’s this lecture about? This lecture should help you discuss the following: The diploid or haploid-dominant or alternating life-cycles in humans and fungi and plants The stages of meiosis The differences between mitosis and meiosis Sources of genetic variation in a diploid organism  What’s this lecture about? https://www.youtube.com/watch? https://www.youtube.com/watch? v=84jlwjvrJwY&t=2s v=eFgVQHkUEq0 DEFINITIONS MEIOSIS: Involves DNA replication followed by two rounds of cell division, producing four haploid cells from the parent diploid cell (each haploid cell has = 1 chromosome 2 chromatids…. half the number of chromosomes of the parent diploid cell). HAPLOID CELLS: also known as gametes, and contain one set of chromosomes. Two haploid cells can fuse together (e.g. fertilisation) to produce one diploid cell. PLOIDY the no. of chromosome sets in a cell Haploid cells (n) have 1 set of chromosomes (n = 23 in human gametes) or the diploid number is Fertilisation >> a zygote with 2 sets of chromosomes = a diploid& 56 2 cell: 2n (2n = 46 in our somatic 250 46 for humans Amoeba Jumpingproteus jack for ring-tailed ant lemurs cells) for Mitosis produces 2n cells Meiosis produces n gametes PLOIDY in the animal sexual life cycle GERM CELLS Diploid dominant HOMOLOGOUS CHROMOSOMES  Usually PLOIDY in the fungal sexual life Haploid cycle Dominant 2N N Zygosporangium Thread-like Hyphae (multicellular, haploid structures) come together from two different fungi and fuse to form Zygosporangium. This has haploid nuclei from both parents within a single cell Haploid nuclei then fuse to form diploid nuclei in the Zygospore The diploid neuclei in the Zygospore undergo Meiosis to form the haploid Spores The Spores germinate and divide to make new, multicellular and haploid Fungi.  Both Haploid P L O I D inYthe Algae sexual life And cycle Diploid 2N N Multicellular haploid Gametophytes make haploid sperm and egg cells The sperm and egg cells fuse to form the diploid Zygote The Zygotes undergo mitosis to form diploid Sporophytes The Sporophytes undergo meiosis to form haploid Spores The Spores develop into multicellular haploid Gametophytes  What would be the lifecycle of the following ? Green algae Mushrooms Fruitflies Sea-weed Yeasts Mice  Answers to the Quiz Fungi Insects Algae Haploid Dominant Diploid dominant Both haploid and diploid Algae Fungi Mouse Both haploid and diploid Both haploid and diploid Diploid dominant  Part 2- What’s this lecture about? This lecture should help you discuss the following: Meiosis I : Overview M E I O S I S : an O V E R V I E W 2 successive divisions with no DNA replication in between: 1st cell division = MEIOSIS I – Prophase I – Metaphase I – Anaphase I – Telophase I 2nd cell division = MEIOSIS II – Prophase II DNA entering PROPHASE 1 Paternal Maternal chromosome Chromosome S Phase G2 Phase  PROPHASE 1 has 5 STAGES Synaptonemal Complex Leptotene : Homologues condense and pair and recombination begins Zygotene : The synaptonemal complex forms at the sites of recombination Pachytene : Assembly is complete and the homologues are synapsed Diplotene : Desynapsis and dissasembly begins and the cross-over events are seen at the chiasmata CROSSING OVER Condensed non-sisters exchange Enzymes DNA in recombination nodules break/rejoin Homologous Chromosomes (HC) (e.g. DNA mismatch repair protein) Each chromatid has few (1-3) crossovers (DNA exchanges) with a non-sister Synaptonemal complex breaks down after crossover HC stay as a tetrad linked at chiasmata (exchange Recombination 2 crossovers Synaptonemal …and The tetrad is points) nodules between complex non- recombination held together appear sisters breaks nodules by in down… chiasmata synaptonemal disappear complex P R O P H A S E I- summary Chromosomes keep condensing HC align – synapsis (pairing) Exchange of DNA – crossing over Tetrad (bivalent) then maintained: Centromeres hold sisters together Chiasmata hold non-sisters together Nuclear envelope breaks up Centrosomes move to poles Spindle formation Diagrammatic representation of the main events that occur at each stage of prophase I  PROPHASE 1 TO METAPHASE 1 Steps 1 The centrioles are at opposite poles of the Cell 2. Pairs of homologous chromosomes become arranged on the Metaphase plate 3. Spindle fibers from one pole of the cell attach to one pair of chromosomes with the sister chromatids. Spindle fibres from the opposite pole of the cell attach to another pair of sister chromatids 4. These Chromosomes form a Tetrad Metaphase plate These Tetrads are randomly oriented Tetrad Shape  METAPHASE 1 to ANAPHASE 1 Steps 1. Two homologous chromosomes of the Tetrad separate and move towards opposite Poles 2. Sister Chromatids remain Attached at the centromeres and Move together toward the poles 3. Orientation of Chromosomes is completely random Note: A key difference between Mitosis and Meiosis is that sister chromatids remain joined after metaphase in Meiosis I whereas in Mitosis they separate  ANAPHASE1 TO TELOPHASE1 1 Homologous chromosomes migrate to the opposite poles 2 Haploid set of chromosomes at each pole Sister chromatids still remain attached Spindle disappears Nuclear envelope forms around the Cytoplasmic chromosomes Cleavage 3 Cytokinesis follows Each of the daughter cells has a nucleus with a haploid set of chromosomes  Questions Somatic cells reproduce by , while sex cells reproduce by a) meiosis; mitosis b) mitosis; mitosis c) mitosis; meiosis d) meiosis, meiosis Which of the following statements is true in humans ? a) Mitosis produces cells that have a haploid number of chromosomes b) Meiosis produces cells that have a diploid number of chromosomes c) Meiosis produces cells that have a haploid number of chromosomes  QUIZ Q1. Which of the following are true of Meiosis I ? (Choose 1 answer) (a) Sister chromatids separate to opposite ends of the cell. (b) Most homologue pairs do not have any crossovers during prophase I. (c) Chromosome number is reduced from diploid to haploid. (d) Sister chromatids in a cell undergoing meiosis are always identical. Q2. Which of the following is true of crossing over? (Choose 1 answer) (a) Crossing over occurs more often in male gametes than female gametes. (b) Segments of DNA are traded between unrelated chromosomes. (c) Most homologue pairs do not have any crossover events (d) A chiasma indicates that a crossover has occurred.  Part 3: What’s this lecture about? Meiosis II The differences between mitosis and meiosis MEIOSIS II PROPHASE II Chromosomes condense Nuclear envelopes breakdown Centrosomes move apart Spindles start forming METAPHASE II Chromosomes line up along the Metaphase plate ANAPHASE II Sister chromatids separate and they are pulled towards opposite poles Nuclear membranes form around TELOPHASE II each set of chromosomes Cytokinesis splits the chromosome sets to new cells Four haploid cells are formed  MEIOSIS AND MITOSIS (SIMILARITIES) Preceded by G1, S and G2 stages Both stages have DNA replication The stages in each case are called Prophase Metaphase Anaphase Telophase MEIOSIS and MITOSIS (DIFFERENCES) SUMMARY VIDEO  QUESTIONS Q1. What are the differences between Meiosis and Mitosis ? Q2. What are the similarities between Meiosis and Mitosis ? Q3. What is the cell-cycle sequence in the figure below ? References