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Lecture 14 - Cell Division 05-30-11

Lecture 14 - Cell Division 05-30-11 - Lecture 14 Chapter 2...

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Unformatted text preview: Lecture 14 05-31-11 Chapter 2 Chromosome and Cell Division How genetic material pass from cell to cell, from generation to generation? 1. Somatic cell division 2. Gametic cell division mitosis meiosis Chromosomes Structure of organized DNA in theeukaryotic . __________ cell In the eukaryotic nucleus, one of the threadlike structures consisting of a single DNA molecule packaged in a protein complex. Morphological parts: Centromere, 2 arms and 2 telemeres Chromosome Morphology Centromere: constricted region on chromosome where the two chromatids are connected p and q arms: p top, q bottom Metacentric: p=q Submetacentric p<q Acrocentric p<<q Telocentric p=0 Telomere: tip of the chromosome Chromosome Structure Replicated for cell division Essential for chromosome segregation Somatic Cell Cycle 4 phases G1 = Gap 1: prepare for DNA replication, it is the longest phase of the cell cycle S = Synthesis: DNA replication G2 = Gap 2: prepare for cell division G1 + S + G2 = Interphase M = Mitosis: cell division Chromosomes must be exactly replicated and accurately divided into 2 daughter cells Daughter cells are identical to each other and to the parent cell Note the DNA amount changes: G1:normal S:double G2: double M:dynamic Mitosis Prophase Metaphase Anaphase Telophase http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations%20%20Podcasts/ch02/0201_cell_cycle.html Mitosis - Prophase >1/2 of mitosis is spent in prophase Condensation of chromosomes - Threadlike chromosomes become visible - Double structure with 2 chromatids - Chromatids are held together by the centromere Formation of mitotic spindles (microtubules) - Pull the chromatids apart to the opposite poles. - Poles - axis for chromosomal segregation Mitosis - Prometaphase Nuclear membrane disappears Microtubules move and make contact with chromosomes Further condensation of DNA Chromosomes move toward the equatorial plane of the cell (middle of the cell Mitosis - Metaphase Chromosomes are fully condensed Centromeres align on the metaphase plate Sister chromatids remain intact, but are ready for migration to the opposite poles Occurs via the spindle fibers attached to the kinetochore Mitosis - Anaphase Shortest stage of mitosis Sister chromatids disjoin and migrate to the opposite ends of the cell. Spindle fibers pull the chromatids apart at the centromere. Each chromatid is now At the end of anaphase each pole has an identical set of chromosomes. Mitosis - Telophase Final stage of mitosis A complete set of chromosomes arrive to each pole of cell. Chromosomes relax and uncoil into chromatin Nuclear membrane reforms. Cytokinesis occurs Division of the cytoplasm into two cells. Two daughter cells contain identical copies of the genetic material. Gamete cell division Mitosis results in daughter cells with the same amount of genetic material in the somatic cells How does the genetic material pass to gamete cells: egg and sperm (then to next generation)? - Egg and sperm combine their genetic material as for baby - Why does a child have 46 instead of 92 chromosomes after the genetic material combination? Meiosis results in daughter cells with half the genetic material as the paternal cell during gamete cell division. Homologous Chromosomes In most eukaryotic cells, there are two identical sets of chromosomes Identical with respect to their gene content, structure, and centromere placement. One set is inherited from male and the other from female parents. Total Chromosome Number (2n) Number of chromosomes in an organism. Human 2n=46 Haploid Number (n) ½ the total chromosome number. Number of chromosomes in a gamete. One set of chromosome. Human n=23 Meiosis Meiosis: from the Greek meioum Two successive nuclear divisions produce gametes, in which chromosome number is reduced by half. Two divisions produce four cells with each containing only one member of each pair of homologous chromosomes. Meiosis Phases Interphase occurs prior to meiosis just as it did before mitosis DNA or chromosomes replicates Stages of Meiosis: Meiosis I (First meiotic division or reduction division) Prophase I, Metaphase I, Anaphase I, and Telophase I Meiosis II (Second meiotic division or equational division) Prophase II, Metaphase II, Anaphase II, and Telophase II http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations %20-%20Podcasts/ch02/0202_meiosis.html Meiosis - Prophase I Several days 5 sub-stages: Chromosomes condense and become more and more visible. Homologous Chromosomes pair (synapsis). Paired homologs Crossing over occurs between non-sister chromotids Chiasma result of breakage and rejoining between nonsister chromatids. Crossing over Crossing over: Physical exchange of genetic material between chromatids of homologous chromosomes. Chromosomes become a mosaic of the maternal and paternal homologs. I n c r e a ses ge n e t i c v a r i a t i o n . P a i r e d homologs C r ossi ng ov e r C h i asm a - Poi n t of c r oss i ng ov e r R esu l t i ng c h r om a t i ds G e n e t i c r e com b i n a t ion Chromosome Crossing Over Causes Genetic Variation Locus: gene site on homologous chromosomes (pl. loci). Each diploid organism contains two copies of each gene Alleles: alternative forms of a gene at certain locus Each locus may have different or identical alleles. The alleles specifically control expression of the related trait. Meiosis I Metaphase I : Tetrads move and align on the mataphase plate. Anaphase I: Homologs disjoin - Homologous chromosomes are drawn to opposite poles; centromeres do not divide Telophase I: haploid set chromosomes are at each poles; cytoplasm divides Interkinesis period between meiosis I & II Nuclear membrane forms; chromosomes relax and each one still consists of two sister chromatids. Meiosis II Meiosis II resembles normal mitosis- separation of sister chromatids The chromosome number remains the same in each cell before and after the second division (equational division). No DNA replication occurs before meiosis II. Resulting only one member of each chromosome pair in the nucleus of cell and each cell contains one single (haploid) set of chromosomes and becoming gametes. Meiosis II Prophase II: Chromosomes condense; Nuclear envelopes break down Metaphase II: Spindle fibers form attaching to the centromeres (kinetochores); the centromeres line up on the metaphase plate. Anaphase II: Centromeres separate and each sister chromatids move to opposite poles = chromatids disjoin Telophase II: Chromosomes reach the poles; nuclear membranes form around each set of chromosomes; cytokinesis occurs (cytoplasm divides) resulting in 4 daughter cells Mitosis vs Meiosis Postmeiotic Events Gamete Formation and Fertilization Meiosis produces four haploid cells. These cells must mature to become functional gametes: eggs and sperm. Fertilization: occurs with the union of egg and sperm. The sperm nuclei unites with the egg producing a zygote (embryo) Meiosis Creates Genetic Variation Two processes in meiosis creates genetic variation Chromosome crossing over at prophase I (genetic recombination) Refer to exchange of genes between non-sister chromatids Basis for intra-chromosomal recombination Random distribution of chromosomes in anaphase I Refer to independent assortment and random separation of homologous chromosomes http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations%20%20Podcasts/ch02/0203_genetic_var_meiosis.html Homologous Chromosome Random Separation Causes Genetic Variation Determined by the number of chromosomes in the haploid genome of each organism and is equal to 2n, where n is the haploid number. Example: n = 4, then 24 = 16 or 16 different gamete combinations. Example: Human with 23 chromosome pairs (n = 23). 223 = 8,388,608 different gametes Does not include the additional genetic diversity created through recombination or crossing over. Reason we are each distinct in appearance and individuality except identical twins (monozygotic) Homologous Chromosome Random Separation N=3, 23 = 8 ...
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