ch.13 - meiosis.docx

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Biological Sciences
BIOSC 0150
Christine Dahlin- Universityof Pittsburgh Johnstown

1 General Biology 1 Chapter 13 – Meiosis Review – mitosis (cell division) creates 2 genetically identical copies of a single cell Single cell – asexual reproduction Diploid – all chromosomes replicate Haploid – only ½ of chromosomes replicate  Asexual reproduction makes diploid cells (mitosis/fission)  Meiosis makes haploid cells  sources of genetic variation Sexual reproduction – creation of offspring by the fusion of genetic materials from 2 parents  Offspring have a copy of each chromosome from each parent  Offspring is NOT genetically identical to each other or to parents (except twins)  Heredity – transmission of genes from one generation to the next  Genetics – study of heredity and hereditary variation  Locus – specific location of a gene on a chromosome Somatic cells – most cells in the body are somatic cells  Diploid – have 2 copies of each chromosome (2n)  One copy from each parent  Homologous chromosomes – same length and centromere position o Also same genes!! o But they do not have the same alleles because parents have different allele  Diploid – 2 sets of chromosomes, each from a different parent  Homologous chromosomes – individual chromosomes from a homologous pair o They are NOT sister chromatids How do we count our chromosome?  Karyotype – display of complete sets of chromosomes when each chromosome has been duplicated, stop mitosis during metaphase  Humans somatic cells have 23 pairs of chromosomes or 46 chromosomes total  All chromosomes are autosomes  Autosomes – homologous; the first 22 pairs;  Sex chromosomes – the 23 pair – XX (for male) and XY (for female)  Since karyotes are made during metaphase, each chromosome has been duplicated Reproduction occurs through fusion of gametes  Gametes – sperm and egg  Meiosis – organisms’ process of making new gametes What would happen if two mitotic cells combined as gametes?  Parental generation: 2n chromosomes o 2 x 23 = 46 chromosomes  Offspring: 2n + 2n = 4n chromosomes o Four copies of each genes o 2 x 46 = 92 chromosomes 2 General Biology 1 Chapter 13 – Meiosis  Third generation: 4n + 4n = 8n chromosomes o Eight copies of each genes o 2 x 92 = 184 chromosomes and continue to increase….  Problem – keep doubling make too many chromosomes  Solution – creation of haploid cells  Haploid – cell with single chromosome set (1n)  Gametes are haploid cells  Haploid cells created through meiosis Making new people  Fusion of gametes (male sperm and female egg)  Result – fertilized egg (zygote)  Zygote – 1 diploid cell results from two haploid sets of chromosomes Meiosis – cells divide into daughter cells with subsets of their genetic information  Produces – 4 haploid (1n) daughter cells from single diploid (2n) cell  Genetic material need to duplicate then divide in 4 ways: o Duplicate chromosomes to make sister chromatids o Line up homologous chromosomes o Crossing over: homologs exchange genetic info o Separate homologous pairs o Split apart dyads (sister chromatids)  Produce genetically variable gametes Meiosis occurs at the same part of the cell cycle as mitosis  It starts with duplicated 2n chromosomes  It occurs in 2 phases (vs. mitosis only occur in 1 phase)  Interphase – chromosome duplicate (in S phase)  Meiosis I – homologous chromosomes separate  Meiosis II – dyads (sister chromatids) separate Details – pair of homologous chromosomes in diploid parent cell get duplicated and become pair of sister chromatids o Meiosis I – homologous chromosomes separate to haploid cells with duplicated chromosomes o Meiosis II – sister chromatids separate (haploid cells with unduplicated chromosomes)  Meiosis I: homologous chromosome separate o Chromosomes are duplicated and expanded as chromatin as the end of interphase o Prophase 1 – chromosomes condense  Chromosome pairs are tightly associated  Crossing-over (chiasma) – exchange of DNA sequence between homologs  Occur during prophase I  Chiasmata – points where crossing-over occurred o Metaphase 1  Homologous chromosome pairs align in middle of the cell 3 General Biology 1 Chapter 13 – Meiosis  Both chromatids of one homolog are attached to microtubules o Anaphase 1  Homologs separate and move toward opposite side of the cell (homologs were replicated and became dyads)  Homologs then move towa
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