Cell Division & Specialization

The Eukaryotic Cell Cycle The eukaryotic cell cycle consists of four phases.

Gap 1 (G1) Phase - cell growth

S Phase -DNA replication occurs

Gap 2 (G2) Phase -preparation for division -G1, S, and G2 all are part of interphase.

M Phase -all of the cell’s energy is focused on division into two daughter cells (mitosis and cytokinesis )

Most of the time…. Most of the cell cycle is spent in Interphase, preparing for nuclear <mitosis> and cytoplasmic <cytokinesis> division so that the cell is ready to undergo the next stage as soon as new cells are needed.

Cyclins -group of proteins that regulate timing of the cell cycle -help to start & stop the cycle

Mitosis &Cell Division

M Phase: Cell Division Mitosis is the division of the nucleus. In eukaryotes, cell division occurs following interphase: it is the M phase and includes mitosis and cytokinesis. Mitosis is the division of the nucleus. Cytokinesis is the division of the cytoplasm.

Mitosis Process in which a cell’s replicated nucleus is divided in preparation for division of the cell

Mitosis occurs in 4 continuous phases Prophase Metaphase Anaphase Telophase

1. Prophase -chromatin condenses and duplicated chromosomes become visible -centrioles move to opposite sides of nucleus and radiate the spindle -the nucleolus disappears and nuclear envelope breaks down

Metaphase -spindle fibers attach to centromeres and chromosomes line up at the equatorial plate

Anaphase -the centromeres split and the paired chromatids separate -chromosomes move along the shortening spindle to opposite poles of the cell

Telophase -a new nuclear membrane forms around each new group of chromosomes -spindle fibers break down - chromosomes unwind to chromatin -a nucleolus becomes visible in each daughter nucleus

Cytokinesis -final phase of the cell cycle Results in two cells that are genetically identical (daughter cells)

Cytokinesis in animal cells The cell membrane pinches in, creating a cleavage furrow, until the mother cell is pinched in half.

Cytokinesis in Plants Cellulose & other materials are transported to the midline of the cell and a new cell wall is constructed.

Cytokinesis in Plants

End Results The process of DNA replication, the division of the nucleus, and cytokinesis results in two new cells that are genetically identical.

Meiosis

Chromosomes Chromosomes—the strands of DNA and protein inside the cell nucleus—are the carriers of genes.

Diploid Cells Half of the chromosomes come from your male parent, and half come from your female parent, giving you two alleles for each gene.   The two sets of chromosomes are homologous, meaning that each of the chromosomes from the male parent has a corresponding chromosome from the female parent. These chromosomes have the same genes, but may have different alleles.

Diploid Cells A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.” The diploid number of chromosomes is sometimes represented by the symbol 2N. For a human the diploid number is 46 or 2N=46.

Haploid Cells Some cells contain only a single set of chromosomes, and therefore a single set of genes. Such cells are haploid, meaning “one set.” They have one possible allele for each gene. The gametes of sexually reproducing organisms are haploid. The haploid number in humans is 23 or N=23.

What is Meiosis? Meiosis is the production of sperm and egg cells in organisms that reproduce sexually. Each cell has to go through the division process twice in order for the cell to end up with half the number of chromosomes.

How many sets of genes do multicellular organisms inherit? A human egg is haploid (has 23 chromosomes) and a sperm is haploid (has 23 chromosomes). Fertilization of the egg by the sperm restores the diploid number of 46 chromosomes.

Meiosis contains 2 separate divisions Meiosis I Meiosis II

Phases of Meiosis

Meiosis I Interphase Just prior to meiosis I, the cell undergoes a round of DNA replication during interphase.

Prophase I -Chromatin condenses, the nuclear membrane breaks down, & a spindle forms.

Prophase I The duplicated chromosomes pair up, forming a structure called a tetrad; they undergo a process called crossing-over. First, the chromatids of the homologous chromosomes cross over one another.

Prophase I Then, the crossed sections of the chromatids are exchanged. Crossing-over is important because it produces new combinations of alleles in the cell, increasing genetic variation.

Metaphase I -paired homologous chromosomes attach to the spindle and line up

Anaphase I -spindle fibers pull members of each homologous chromosome pair toward opposite ends of the cell

Telophase I and Cytokinesis -a nuclear membrane forms around each cluster of chromosomes and cytokinesis follows, forming two new cells.

Meiosis I The two cells produced have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Chromosome number has been halved.

Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

Prophase II -chromosomes—each still consisting of two chromatids—become visible and the spindle forms

Metaphase II -chromosomes line up in the center of each cell

Anaphase II -chromatids to separate and move to opposite poles of the cell

Telophase II -Nuclei reform around each group of chromosomes,the spindles break down, and cytokinesis occurs.

The two nuclear divisions in meiosis result in four daughter cells forming from an original parent cell, each with half the chromosomes of the parent cell.