Meiosis Chapter 11.4

Chromosome Review Let’s review… What is a chromosome? Condensed chromatin (DNA + proteins) only visible during cell division!! How many chromosomes do you have in each of your cells? 46 (23 pairs) But wait..do any of your cells NOT have 46? SEX CELLS!

Chromosome Number What does it mean to have 23 pairs of chromosomes? 23 chromosomes: one copy of each from mom & one from dad! Cells that have a chromosome from each parent are DIPLOID (2N)! Somatic cells (body cells) “two sets”

contain one set of genes! Chromosome Number Some cells don’t have a pair of each chromosome! Gametes (sex cells) of sexually reproducing organisms These cells are HAPLOID (N)! “one set” Single set of genes! Why do they only have half the number of chromosomes? Each gamete can only contain one set of genes! They COMBINE to Become DIPLOID!

Chromosome Structure Review Chromatid One strand of a duplicated chromosome Joined by a centromere to its sister chromatid Sister chromatids Two chromatids joined by a common centromere Each carries identical genetic information Together called a DYAD

Meiosis The genetic information that we (and other sexually reproducing organisms) inherit comes from two cells: sperm and egg Meiosis – cell division in which the chromosome number is cut in half Gametes (sperm and egg) divide this way! Germ cells in the testis and ovary undergo meiosis and produce gametes.

Meiosis: Reduces the number of chromosomes/cell by half Occurs in two steps: Meiosis I Meiosis II Meiosis I and II each have prophase, metaphase, anaphase, and telophase stages 2N = 4 N = 2

Meiosis 2 dyads DNA replication occurs during interphase before the beginning of meiosis I but not before meiosis II Chromosomes will associate with the other member of its pair during Meiosis Homologous chromosomes (also called a TETRAD) 1 tetrad

Meiosis Meiosis I is a reductional division: the number of centromeres is reduced by half after this division Meiosis II is an equational division: the number of centromeres remains equal after this division

Meiosis 1: Prophase 1 Prophase I Chromosomes become visible Homologous chromosomes pair up to form tetrads consumes 90% of the time for meiosis Crossing Over occurs Results in Genetic Variation New allele combinations!

Two major sources of genetic variation in Meiosis I Crossing Over Creates new combinations of mom and dad’s alleles Think about chromosomal mutations! Independent Assortment

Metaphase 1 As prophase I ends, tetrads attach to spindle fibers Tetrads line up at center of cell

Anaphase 1 Dyads pulled toward opposite poles by spindle fibers Disjunction: separation of chromosomes Nondisjunction leads to polyploidy! (extra chromosomes) Note the exchange of information between paternal and maternal chromosomes

Telophase 1 Separated chromosomes cluster at opposite ends of cell Nuclear membrane forms around each cluster Cytokinesis follows and forms 2 new cells

This was a reductional division: number of centromeres reduced per cell. Prophase I: 4 centromeres, therefore 4 chromosomes Prophase II: 2 centromeres, therefore 2 chromosomes/cell

Results of Meiosis 1 Two daughter cells Neither with two complete sets of chromosomes (haploid) Sets have been shuffled and independently assorted Chromosomes differ between each other and the original cell

Meiosis II Two cells enter second meiotic division Neither cell goes through DNA replication prior to this division!

Prophase II Chromosomes (dyads) become visible Do not form tetrads because they are already separated from homologous pair!

Metaphase II & Anaphase II Chromosomes (dyads) attach to spindle fibers and line up in center of cell Remember…they aren’t paired with another chromosome! Anaphase II – chromatids (monads) separate from each other at the centromere move to opposite poles!

Telophase II & Cytokinesis Four genetically different haploid cells produced (N) Each monad may be an entirely new combination of maternal and paternal genetic information

Meiosis II

Meiosis Review Meiosis II Meiosis I Prophase II: Dyads reappear, no tetrads! Metaphase II: Dyads line up in middle Anaphase II: Monads pulled apart (separated at centromere) Telophase II: four new genetically different haploid daughter cells with monads Meiosis I Prophase I: tetrads form (homologous chromosomes pair), crossing over occurs Metaphase I: tetrads line up in middle Anaphase I: Dyads pulled apart Telophase I: two new genetically different haploid daughter cells with dyads

Meiosis: Formation of Gametes Meiosis results in two kinds of haploid, sexual gametes Males produce sperm Females produce eggs (usually only one of the four egg cells is used!) Sperm fertilizes egg to produce 2N zygote! Goes through mitosis & cell specialization to form a new organism!

Mitosis vs. Meiosis: a Comparison Both preceded by DNA replication Both are methods of cell division Both include Prophase, Metaphase, Anaphase, and Telophase Both are followed by cytokinesis Mitosis vs. Meiosis

Contrasting Meiosis & Mitosis Each daughter cell receives a complete set of chromosomes Less genetic diversity Doesn’t change the chromosome number of the original cell Single cell division Two genetically identical diploid daughter cells Asexual reproduction Makes somatic cells Meiosis Two alleles for each gene segregated and end up in different cells Greater variety of possible gene combinations Reduces the chromosome number by half Two rounds of cell division Four genetically different haploid daughter cells Sexual reproduction Makes gametes

Gene Linkage Genes on different chromosomes assort independently What about genes on the same chromosome? Tend to be linked! Chromosomes assort independently, but typically genes on the same chromosome are inherited together Especially when close together! Crossing over causes some genes on the same chromosome to assort independently

Gene Maps Frequency of crossing-over between genes during meiosis is used to determine genes’ locations Farther apart, more likely that crossing over occurs between them Close together, crossovers rare Use frequency of crossing over to determine distances from each other and map genes’ locations on chromosomes!