The Cell Cycle The repeating sequence of growth & division of a cell.

G1 S G2

Cell Cycle: Outline I. Interphase A. G1 B. S C. G2 II. Cell Division A. Mitosis 1. P 2. M 3. A 4. T B. Cytokinesis

Why do cells divide? l 1. They need to “grow” the organism or repair parts of the organism that are dead or damaged.

Why do cells divide? l 2. As cells grow larger: they take in more food through the cell membrane they produce more waste which must exit through the cell membrane l SO: they can’t just get bigger and bigger… …they need more cell membrane!

Why do cells divide? l If cells GROW too large, there is not enough cell membrane to be efficient. The ratio of surface area to volume is too LOW: difficulty obtaining nutrients difficulty eliminating wastes diffusion across the cell membrane is impaired

Why do cells divide? l Cells divide so they can increase surface area to volume ratio stay small and efficient. l In other words… THEY divide to get MAXIMUM SURFACE AREA to VOLUME ratio!

Two types of cell division: l Produces EXACT copies of first or “parent” cells [“biological xerox”] l Occurs only in sex cells [producing egg & sperm] MitosisMeiosis Occurs in ALL regular body cells [like skin cells] Produces genetically DIFFERENT cells from the “parent” cells [variation in DNA]

Get your note cards! l Put them in chronological order based on what you see…

Interphase l Nucleus clearly seen l Growth and repair l G1, S, G2 phases l Chromosomes are not visible; l DNA is in the form of chromatin l DNA is copied or REPLICATED l Centrioles duplicate centrioles nucleus chromatin

Mitosis: Prophase l Chromatin coils to form visible chromosomes l Nuclear membrane and nucleolus disappear l Spindle fibers form between centrioles Spindle fibers Sister chromatid Centromere

Mitosis: Metaphase l Chromosomes move to the equator l spindle fibers attach to chromatids by centromeres Equator

Mitosis: Anaphase l Centromeres divide l Sister chromatids pulled apart toward opposite poles Sister chromatid

Mitosis: Telophase/ Cytokinesis l Chromosomes arrive at the opposite poles NEXT: l Cytoplasm divides l Nucleolus and nuclear membrane reappear l Chromosomes begin to uncoil Nuclear membrane

Cytokinesis Animals Cytoplasm pinches near equator called cytokinesis Two new cells form Plants Plasma membrane does not pinch Cell plate forms across equator Two new cells form Division is Complete

Results Guaranteed genetic continuity Two new cells with identical chromosomes to the parent cell

What happens when the cell cycle goes wrong? l CANCER! l Cancer is uncontrolled cell division l Caused by one of (or a combination of): Genetic predisposition Environmental exposure to carcinogens

Meiosis The “OTHER” cell division

FAQ for MEIOSIS 1. Why meiosis? For making gametes (egg or sperm), the specials cells for sexual reproduction. l Meiosis reduces the genetic number for gametes. l Without meiosis, sexual reproduction would make offspring with too much DNA!!

2. What happens? cell division two times, with no interphase in between (Meiosis I then Meiosis II) Each division goes through P-M-A- T FAQ for MEIOSIS (cont’d)

3. What’s the result? 4 new daughter cells each have half the number of chromosomes as original cells DNA that is NOT identical to original cell or to each other FAQ for MEIOSIS (cont’d)

Meiosis I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Interphase I Prophase I * Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corre- sponding homo- logous chromo- some to form a tetrad. Crossing over occurs here… Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Meiosis I

Crossing-Over

Meiosis I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromo- somes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Meiosis I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase IIMetaphase IIAnaphase IITelophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase II Metaphase IIAnaphase IITelophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase II Metaphase II Anaphase IITelophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase IIMetaphase II Anaphase II Telophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase IIMetaphase IIAnaphase II Telophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis II

Gamete formation in humans l In males, every cell that undergoes meiosis produces 4 gametes (sperm). l …called spermatogenesis l In females, meiosis occurs once each menstrual cycle and only one of the four cells made in meiosis actually matures into an egg. l … called oogenesis

Comparing Mitosis & Meiosis Mitosis 1. Occurs in all REGULAR body cells. 2. Only 1 division. 3. Produces 2 daughter cells that are identical to parent cell and are diploid (2N). 4. No genetic variability (differences) 5. For growth and repair!!!!! Meiosis 1.Occurs only in sex cells (sperm and egg) 2.2 divisions. 3.Produces 4 daughter cells that are NOT identical to the parent cell and are haploid (1N). 4.Genetic variability from one generation to the next. 5.For sexual reproduction!