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Cell Cycle, Mitosis, and Meiosis Cell Division and Death • Are required for normal growth and development. • Mitosis produces new cells – Mitosis occurs in somatic cells (all cells but egg and sperm) • Apoptosis is cell death that is part of normal development • Necrosis is cell death in response to injury The Cell Cycle The sequence of events associated with cell division Figure 2.15 • S phase: DNA synthesis • G phase: gap for growth • M phase: mitosis (nuclear division) • Cell division or cytokinesis is part of G1 Stages of the Cell Cycle • Interphase – Prepares for cell division – Replicates DNA and subcellular structures – Composed of G1, S, and G2 • Mitosis division of the nucleus • Cytokinesis division of the cytoplasm Replication of Chromosomes • Process of duplicating chromosomes during S phase • Occurs prior to division • Produces sister chromatids held together at centromere Mitosis •Produces two identical daughter cells •Chromosomes align •Sister chromatids separate and move to opposite poles. •Nuclear membranes form around each new nucleus •Division of cytoplasm or cytokinesis occurs. Overview of Mitosis • Continuous process divided into – Prophase – Metaphase – Anaphase – Telophase Mitosis in a Human Cell Figure 2.17 Prophase • Replicated chromosomes condense • Microtubules organize into a spindle • Nuclear membrane breaks down Figure 2.17 Metaphase • Chromosomes line up on the metaphase plate • Spindle microtubules are attached to centromeres of chromosomes Figure 2.17 Anaphase • Centromeres separate • Chromosomes move to opposite ends of the cell Figure 2.17 Telophase • Chromosomes uncoil • Nuclear membranes form • Spindle disappears Figure 2.17 Cytokinesis • Cytoplasmic division • occurs after nuclear division is complete. • Two cells are formed. Gametes • Form from cell division of germ line cells • Meiosis is cell division to produce gametes • Meiosis has two divisions of the nucleus (Meiosis I and Meiosis II) and produces cells with half the number of chromosomes (haploid) Homologous Chromosomes • Carry the same genes • Pair during Meiosis I • Separate in the formation of gametes • One copy of each pair is from the mother and one is from the father. Figure 1.3 Meiosis I : homologous chromosomes separate Spindle fibers Nucleus Nuclear envelope Prophase I (early) (diploid) Figure 3.4 Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Meiosis II : sister chromatids separate Prophase II (haploid) Figure 3.4 Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four nonidentical haploid daughter cells Results of Meiosis Gametes •Four haploid cells •Contain one copy of each chromosome and one allele of each gene •Each cell is unique Figure 3.4 Recombination (crossing over) • Occurs in prophase of meiosis I A A B B C • Homologous chromosomes exchange genes F • Generates diversity b C D D E E F Figure 3.5 a a e f c b c d d e f Recombination (crossing over) A a B • Exchange between homologs • Occurs in prophase I C C c D D E F d E F e f Figure 3.5 Letters denote genes and case denotes alleles b c d e f Recombination (crossing over) a A B b C •Creates chromosomes with new combinations of alleles for genes A to F D E F A a B c b c d d C D E F Figure 3.5 e f e f