1 Jan 9, 2012 Biology 321 NOTE: Since you will have already had an introduction to both mitosis and meiosis in Biol 204 & 205, this lecture is structured as a review. See the text for a more thorough discussion of these processes. Your obligation for Biol 321 is to understand chromosome movements during these processes and all material in this lecture2 Jargon Review Genomes, Chromosomes & Genes • The genome is the total genetic content of a single set of chromosomes • Genomes are organized into DNA polymers (often of different lengths) also known as chromosomes • Genes are stretches of DNA located at specific positions along the length of the DNA polymer (chromosome)3 The next page examines a shorthand way of describing chromosome content and genome copy numberOrganisms vary with respect to the • Number of genome copies • Size of genome •Number of DNA polymers/chromosomes that the genome is divided into4 scanning electron micrograph of human chromosomes 2n=46 in humans = shorthand way of describing chromosome content) n = ? The number preceding the n = ? The number following the n = ? metaphase chromosome spread Note variation in size and centromere position5 2n=46 in humans (shorthand way of describing chromosome content) n = number of chromosomes in one genome copy or one set of chromosomes The number preceding the n indicates the number of genome copies (or number of copies of each chromosome or each chromosome set) 1n = haploid 2n = diploid 3n= triploid 4n = tetraploid 6n= hexaploid The number following the n indicates the total number of chromosomes found in a somatic cell metaphase chromosome spread Note variation in size and centromere position6 Handing down DNA/chromosomes/genomes throughout the millennia Transmission or propagation of any genetic program requires two basic steps: 1. Copying step (replication of the DNA molecule) 2. Distribution step (getting the duplicated information properly distributed to the progeny) How old are these copying and distribution processes?7 In the nucleus of eukaryotic cells, the distribution process occurs in the form of 2 different types of cell division Mitosis is a conservative propagation of genetic information: the daughter cells have the same genetic composition as the mother cell Meiosis is a non-conservative propagation of genetic information: the products of meiosis have half the DNA content (half the number of chromosomes) and a different genetic composition than the mother cell8 Review cell cycle and stages of mitosis9 At the end of prophase of mitosis each duplicated chromosome is maximally condensed • Each chromosome consists of 2 identical halves known as sister chromatids • Each chromatid contains one continuous double-stranded DNA molecule coiled into a compact form • Sister chromatids are held together at a constriction called the centromere Connecting the dots: semi-conservative DNA replication, sister chromatids and mitosis10 Meiosis I (reductional division) • Homologous chromosomes are separated into different cell • Each daughter cell contains one duplicated copy of each type of chromosome: the number of chromosomes is cut in half) What events occur during prophase of the first meiotic division that do not occur during prophase of mitotis?11 synapsis: close pairing of homologs during prophase of Meiosis I What is a homolog?12 Homologous chromosomes • chromosomes that pair with each other during meiosis • chromosomes that possess the same genes at corresponding positions • homologs may carry different alleles of a given gene) • [non-homologous chromosomes carry different genes]13 KNOW THIS WELL & FOREVER Connecting the dots: DNA polymers, alleles, sister chromatids, homologous chromosomes14 Summary of chromosome movements in mitosis and meiosis Animation illustrating the differences between mitosis & meiosis: http://www.pbs.org/wgbh/nova/miracle/divide.html G1, S, G2Diploid celln = 2, 2n = 4homologscentromere(= kinetochore + DNA) sisterchromatidsMitosisMeiosis IIMeiosis IOR15 How many genome copies? How many chromosomes per genome (per set) Shorthand for genetic content of this organism’s cells? __ n = __16 Meiosis II (equational division) • Sister chromatids are separated into different cells • Meiosis II is sort of like a mitotic division, BUT the sister chromatids in mitosis are genetically identical (assuming no errors during DNA replication or other mishaps to the DNA • In contrast, after prophase of Meiosis I, sister chromatids are not typically genetically identical -- WHY NOT?17 crossing over: • a precise breakage and reunion event that occurs between two non-sister chromatids • pieces of the DNA strands in the two chromatids are exchanged • paired homologs (also known as bivalents are held together at the chiasmata) Note: we will look at the genetic implications of crossing-over in detail later in the quarter18 Walter Sutton was a Kansas farm boy and in 1902 was the first person to point out that: Chromosomes “obey” Mendel’s rules of segregation and independent assortment Interested in the History of genetics? check out this web site: http://www.genomenewsnetwork.org/resources/timeline/index.php19 Chromosomes “obey” Mendel’s rules of segregation and independent assortment: Mendel’s principle of segregation: Paired hereditary factors (R and r alleles in this diagram) segregate into different gametes Segregation of alleles into different gametes is ensured • by the pairing of homologs during prophase of the first meiotic division • and the separation of homologs into different daughter cells in anaphase of the first meiotic division20 What does Mendel’s principle of independent assortment say about the pool of gametes produced by an AaBb heterozyote? Genotypes and proportions? What events in meiosis explain Mendel’s principle of independent assortment?21 An AaBb heterozyote will produce 4 gamete genotypes in equal proportions NOTE: Genotype of F1 can be written as either AaBb or A/a; B/b / separates alleles on homologous chromosomes ; separates alleles on non-homologous chromosomes Cells/organisms of genotype AaBb will produce gametes of genotype A B, a b, A band a B.A Aa ab B Bbab ABA Aa abB B baBAb50% of the meiotic divisions:50% of the meiotic divisions:metaphase/anaphase of first division22 State a