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York University
Natural Science
NATS 1610
Barbra Czebhan

05/02/2013 SC NATS 1610 STUDY NOTES The Cell Cycle and Mitosis Why Cells Divide  Replace dead and damaged cells  Cell size is limited, cannot grow indefinitely  Single cell life form- asexual reproduction Somatic and Germ Cells  Typical body cell  Diploid cell  Two sets of each chromosome  Human have 46 chromosomes  All cells in the body are somatic cells besides gametes o Sperm o Egg  Each has 23 chromosome in humans Stages of Mitosis  Interphase (preparation stage) o Make proteins o Copies organelles and chromosomes  Division Stage (Mitosis and Cytokinesis) o Nuclear division o Division of the cytoplasm  Interphase (before mitosis) o Before replication the chromosome consists of one chromatid o After replication chromosomes consist of 2 identical chromatids attached at the centromere  One chromosome made out of 2 DNA molecule  G1-Phase- Primary growth phase  S-Phase- Cell synthesize replica of genome and producing identical sister chromatids  G2-Phase- Second growth phase, prep for mitosis  M-Phase- sister chromatids pull apart by microtubules  C-Phase- cytoplasm divides  Prophase o Chromosomes become coiled o Mitotic spindle forms, cell pole formation  Prometaphase o Nuclear membrane breaks down o Microtubules capture chromosomes and begin to move them towards the cells equator 05/02/2013 SC NATS 1610 STUDY NOTES  Metaphase o Chromosomes lined up at equator  Anaphase o Sister chromatids separate o Microtubules pull chromosomes to poles  Telophase o Nuclear envelopes form, chromosomes stretch out o Spindle breaks down Types of Chromosomes  Homologs o Two chromosomes composing a pair o Chromosomes of the sae type with the same genes in same location o Autosomes  Sex chromosomes o X and Y o Determine the sex of an individual XX being female and XY being male Meiosis is Necessary in Sexual reproduction  Meiosis divides the nuclei of germs cells in a way that halves the number of chromosomes in daughter cells o It is the first step toward the gametes required for sexual reproduction  Takes place in two sets of division, meiosis l and meiosis ll  Meiosis l o Reduces the number of chromosomes from diploid to haploid  Meiosis ll o Produces four haploid daughter cells  Spermatogenesis o Diploid germ cell increases in size o The primary spermatocyte- four haploid spermatid- change in form to become sperm Stages of Meiosis (l,ll)  Prophase l o Chromosomes condense o Homologous chromosomes pair o Crossovers occur o Spindle forms and attach chromosomes to pole o Nuclear envelope breaks 
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