Cell Cycle Some basic concepts DNA is a polymer of nucleotides Genome the complete set of DNA in an organism Role of DNA the sequence of nucleotides in DNA represents the sequence of amino acids in a protein Gene A sequence of DNA that codes for the production of a single protein A gene is a recipe for a protein Genes are located along the chromosome like beads on a string Humans have about 22 000 genes in their genome each cell has a complete copy of this genome Any given cell only uses part of that genome However when a cell duplicates the entire genome needs to duplicate too DNA in chromosomes does not occur naked in the nucleus it is complexed with a protein and is collectively known as chromatin Chromatin DNA is complexed with wrapped up with a protein in cell nucleus Chromosomes are like pairs of books you get one volume of 23 from mom another volume of 23 from dad Each has the same set of hereditary instructions for hair color eye color etc These above are referred to as homologous chromosomes The 23 chromosome determines the gender where it differs Sex Chromosomes Females have set of chromosomes that are XX which are homologous so a total of 23 pairs of homologous chromosomes Males have a set of chromosomes that are XY so only 22 pairs of homologous chromosomes and one non homologous pair Mitosis Somatic Cells Daughter cells identical to parent cells No Genetic Recombination Meiosis Reproductive Cells Daughter cells have half of the chromosomes of the parent Genetic recombination The Cell cycle The cell cycle depicts a repeating pattern of Cell growth DNA replication cell division Two main phases Interphase and Mitotic phase S Phase synthesis DNA replication occur each chromosome duplicates to form two identical chromatids G2 Phase Newly replicated DNA begins to condense Mitotic Phase Two phases Mitosis the division of chromosomes Cytokinesis the division of the cytoplasm 1 Prophase Chromosomes condense and become visible Nuclear membrane disappears Spindle fibers form near poles 2 Metaphase Chromosomes align at center of cell along the metaphase plate Each chromatid faces an opposite pole Nuclear membrane has disappeared Spindle fibers attach to the centromere 3 Anaphase Spindle fibers pull chromatids to each pole Each chromatid is now a single unmatched chromosome 4 Telophase Chromosomes begin to unwind and lose their distinct shape Nuclear membrane re forms A cleavage furrow begins to develop this is the beginning of cytokinesis Cytokinesis a contractile ring made of protein begins to tighten forms a cleavage furrow one cell becomes two membrane fuse G1 Phase Growth and metabolism cell accumulates material and doubles in size Bacterial cell division is called binary fission CANCER Control of cell growth Density dependent growth growth stops when contact made with adjacent cell Anchorage dependent growth solid surface needed for growth Two main causes Genes that induce cell division become hyperactive Oncogenes are mutated genes that result in excessive cell division Genes that suppress cell division are referred to as tumor suppressor genes When these genes mutate cell division is not suppressed MEIOSIS The point of meiosis is genetic re combination Somatic cells body cells any cell that is not an egg or sperm cell Somatic cells undergo mitosis Gamete reproductive cells egg or sperm Germ cells undergo meiosis to form gametes Are formed as a result of meiosis Diploid a cell having the full compliment of paired chromosomes in humans 46 chromosomes abbreviated as 2n Somatic cells are diploid Haploid cells in the reduced chromosome state in humans 23 chromosomes abbreviated as 1n Gametes are haploid Meiosis halving of chromosome number from diploid state to haploid state Meiosis produces the gametes for sexual reproduction A gamete precursor called a germ cell which is diploid undergoes meiosis Meiosis creates gametes or sex cells Male starting cell is called a spermatagonium in the testes It is the cell that enters Meiosis I Female starting cell is called an oogonium in the ovaries At fertilization the diploid state is restored Summary Meiosis I homologous chromos separate Meiosis II sister chromatids separate Prophase I as in mitosis the chromosomes condense out Homologous chromosomes 1 from mother 1 from father pair up is referred to as a tetrad Two sources of genetic recombination 1 Crossing over 2 Independent assortment Metaphase I Homologous chromosomes move to metaphase plate Has exchanged genetic info from crossing over Second source of genetic recombination 2 Independent assortment Anaphase I Paired chromosomes pull away from each other sister chromatids still together Telophase I and Cytokinesis Diploid cell divide to find haploid cells Prophase II just a pause no DNA replication Metaphase II The 23 matched chromosomes line up at the metaphase plate each sister chromatid faces a separate pole Anaphase and Telophase II sister chromatids separate resulting haploid cell 23 chromatids
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