Cell Cycle Mitosis and Meiosis Cell Division and

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 • S phase: DNA synthesis • G phase: gap for growth • M phase: mitosis (nuclear division) • Cell division or cytokinesis is part of G 1 Figure 2. 15

Stages of the Cell Cycle • Interphase – Prepares for cell division – Replicates DNA and subcellular structures – Composed of G 1, S, and G 2 • 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 Prophase I (early) (diploid) Figure 3. 4 Prophase I (late) (diploid) Metaphase I (diploid) Nuclear envelope 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 b c A B • Exchange between homologs • Occurs in prophase I a B A b C C c D D E F d E F e f Figure 3. 5 Letters denote genes and case denotes alleles 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