Why cells divide

• Cells divide because:– Demands the cell places on its DNA– Moving enough nutrients and wastes across

the cell membrane

• Cell division – process forming two “daughter” cell

Cell Division

• 1st stage – mitosis (division of the nucleus)

• 2nd stage – cytokinesis – division of cytoplasm

• Chromosomes – contain DNA and proteins

• Before cell division, each chromosome replicates and consists of two “sister” chromatids

Cont.

• Each pair of chromatids is attached at the centromere, which will separate during cell division and each new cell will have identical copies of the old cell

• Cell cycle: – Cell grows– Prepares for division (replicates DNA)– Divides into two daughter cells– Cycle begins again

Mitosis

• 4 phases:• Prophase – chromatin condense into

chromosomes, centriole separates and a spindle begins to form, the nuclear envelope breaks down

• Metaphase – chromosomes line up across center of cell, connected to spindle fiber (helps separate chromosomes) at centromere

Cont.

• Anaphase – mitosis: centromeres that join the sister chromatids split, chromatids separate and become individual chromosomes (two groups)

• Telophase – chromosomes disperse into tangle of dense material, nuclear envelope re-forms around chromosome clusters, spindle breaks apart, nucleolus is visible in daughter nucleus.

Cont.

• Cytokinesis – division of cytoplasm in the M phase, cell membrane folds into cell pinching cell into two equal parts containing nucleus and cytoplasmic organelles

Cell regulating

• Cyclin – (protein) regulates the timing of the cell cycle in eukaryotic cells

• Regulation – internal (replication) and external (growth) are controlled by proteins

• Cancer – uncontrolled cell growth (disease of the cell cycle)

Meiosis

• Homologous – two sets of chromosomes (set from female, set from male), two complete sets of chromosomes and two complete sets of genes

• Diploid – “two sets” cell that contains homologous chromosomes (2N=8)

• Haploid – “one set” (N=4)

Phases of Meiosis

• Meiosis – process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

• Divided into two distinct phases

• Meiosis I and II

Meiosis I

• Prophase I – each chromosome pairs with its corresponding homologous chromosome to form a tetrad (4 chromatids)

• Metaphase I – spindle fibers attach to the chromosomes

• Anaphase I – fibers pull the homologous chromosomes toward opposite ends of the cells

Cont.

• Telophase I and cytokinesis – nuclear membrane form and the cell separates into two haploid (N) daughter cells, each with half the number of chromosomes as the original cells

• Crossing-over – exchange of chromatids which results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles

Meiosis II

• Prophase II - two haploid (N) daughter cells, each with half the number of chromosomes as the original cells

• Metaphase II – chromosomes line up in a similar way to the metaphase stage of mitosis

• Anaphase – sister chromatids separate and move toward opposite ends of the cell

Cont.

• Telophase and cytokinesis – Meiosis II results in 4 haploid (N) daughter cells (2 chromosomes each)

Gamete Formation

• Males – haploid gamete is the sperm

• Females – only one haploid gamete, egg

• Comparing mitosis and meiosis:– Mitosis produces 2 genetically identical diploid

(2N) daughter cells: allows the body to grow and replace cells

– Meiosis produces four genetically different haploid cells: used in sexual reproduction

Mitosis Meiosis

Asexual Reproduction Sexual Reproduction

Produces two identical cells Produces four different cells

New cell contains a full set (diploid) of chromosomes

New cells contain half the number of chromosomes (haploid)

DNA replication once during interphase

DNA replication once during interphase before Meiosis 1

One nuclear division Two nuclear divisions

No crossing over – no genetic variation

Crossing over during prophase 1 to ensure genetic variation

Sister chromatids pulled to opposite poles during anaphase resulting in 2 identical cells. The centromere splits ensuring that identical chromatids are pulled to the opposite poles

Homologous chromosomes move to opposite poles during anaphase 1. Centromeres hold sister chromatids together resulting in non-identical cells

Occurs in all types of body cells

Occurs only in sex cells (egg & sperm)

Purpose: cell growth and repair

Purpose: produce sex cells