Chapter 12 The Cell Cycle

Cell division creates duplicate offspring in unicellular organisms (Asexual Reproduction) Provides for growth, development, and repair in multicellular organisms (Sexual Reproduction) The cell cycle extends from the creation of a new cell by the division of its parent cell to its own division into two cells

Cell division results in genetically identical daughter cells The process of recreating a cell necessitates the EXACT duplication and equal division of the DNA

Each diploid eukaryote has a characteristic number of chromosomes in each somatic cell Reproductive cells, or gametes, have half that number of chromosomes. Diploid: Haploid: Mosquito 6 Frog 26 Corn 20

Chromosomes Each c’some is a very long DNA molecule with associated proteins. This DNA-protein complex is called chromatin.

Distribution of Chromosomes During Eukaryotic Cell Division Before cell division, a cell copies its DNA and each c’some densely coils and shortens. Duplicated c’somes consist of 2 identical sister chromatids. A duplicated c’some has a centromere where the chromatids are most closely attached.

The sister chromatids separated during mitosis (division of the nucleus) Then the cytoplasm divides during cytokinesis, producing 2 separate, genetically equivalent daughter cells.

Phases of the cell cycle Cell cycle (or life of the cell) lasts for about 20 hours There are 2 main stages in the cycle of the eukaryotic organism The cell cycle consists of the mitotic (M) phase, which includes mitosis and cytokinesis, and interphase, during with the cell grows and duplicates its c’somes

interphase Usually lasting 90% of the cell cycle Includes the G1 phase, the S phase, and the G2 phase Gap 1 (G1) phase- growth period of the cell S (synthesis) phase- period when DNA replication occurs Gap 2 (G2) phase- cell prepares for division

mitosis Mitosis- equal division of genetic information (chromosomes) between the two new cells (division of the nucleus) 4 phases of M: prophase, metaphase, anaphase, and telophase

prophase 1. Prophase- nuclear envelope disintegrates; nucleolus disappears; and, the chromatin fibers coil and fold into visible

In cells that have centrioles, such as animals’ cells, the centrioles replicate Filaments called asters develop & radiate from the centrioles The asters lengthen and become connected to the centromeres The asters are now called spindle fibers

Metaphase Chromatids become aligned at the center of the cell or at its equator Centrioles have migrated to opposite sides of the cell, called the poles

anaphase The centromeres separate and the daughter chromosomes begin to migrate to opposite poles This migration occurs as a result of a shortening of the spindle fibers.

telophase Begins when all chromosomes reach the 2 poles Spindle fibers begin to disappear and the nuclear membranes begin to reappear Chromosomes begin to disappear as the chromatin material disperses through the new nuclei

cytokinesis Cytokinesis begins during telophase In animal cells the cytoplasm begins to pinch inward In plant cells, a partition called a cell plate begins to form and divides the cytoplasm

Meiosis One special type of cell reproduction is for organisms that produce offspring by sexual reproduction Special reproductive cells must be produced by each parent. These are the cells that unite to form the new cell.

Sex cells Gamete- special type of reproductive cell that contains only one set of chromosomes (sperm & egg) A special reproductive process that cells use to produce gametes is called meiosis (reduction division)

First cell division: meiosis I Prophase I- nuclear membrane disappears. The homologous chromosomes (made up of 2 chromatids) pair up next to each other. At this time, genetic material can be exchanged by a process called crossing over.

Metaphase I Homologous chromosomes move to the equator of the cell

Anaphase I Homologs of each pair go to opposite poles (chromatids remain attached at the centromeres)

Telophase I 2 daughter cells results but the chromosome number has not been halved which results in a second division…Meiosis II