Since cells can’t continue to grow larger, they must split, or divide at

some point.

Cell Cycle2 parts:• Interphase – period of growth & DNA synthesis

(copying)

• Mitotic phase – period of cell division (1 cell divides into 2 cells)I love cells,

baby!

Interphase – time of cell growth & copying of DNA

3 phases of interphase:

• G1 – growing & ribosomes making proteins

• S – synthesis (or copying) of DNA (sister chromatids formed)

• G2 – growing & getting ready to divide

Cells spend most of their time in interphase.

DNA is in chromatin (stringy) form.

Mitotic Phase – time of cell division

Consists of 2 main phases:1. Mitosis – division of nucleus2. Cytokinesis – division of cytoplasm

Sister Chromatids

• Identical copies of chromosomes attached by a centromere.

Mitosis

4 phases:• Prophase • Metaphase• Anaphase• Telophase

http://www.cellsalive.com/mitosis.htm

Prophase

• DNA condenses into chromosomes

• Nuclear membrane disappears

• Centrioles move toward poles

• Spindle fibers form & attach to chromosomes

Metaphase

• Sister chromatids lined up on metaphase plate with spindle fibers attached to them.

Anaphase• Sister chromatids get pulled apart to poles

as spindle fibers shorten.

Telophase• New nuclear membrane forms around

chromosomes at each pole• Chromosomes “unpack” into chromatin• Cell membrane begins to pinch in (animals cells)

or cell plate develops (plant cells)

Cytokinesis – division of the cytoplasm

Cleavage furrowCell plate

Control of the Cell Cycle

• Enzymes control cell cycle so cells divide at the right times

• Cancer – uncontrolled cell division caused by a change in the DNA that codes for the making of the control enzymes

Cell Cycle Control• http://wps.prenhall.com/esm_freeman_biosci_

1/7/1948/498717.cw/index.html

Internal Vs external signals

• Internal – the checkpoints

• External – growth factors, density dependent inhibition, anchorage dependence

Cell cycle checkpoints

• length varies depending on cell type

cell cycle checkpoints control the cell’s progression.

Check points determine if a cell is ready to progress to the next stage. G1 – mostly controlled by growth factors

G2 - determines if cell will enter the M phase & requires proper completion of DNA synthesis. M - b/t metaphase & anaphase; requires the proper attachment of all chromosomes to spindle apparatus.

MPF = mitotic cyclin dependent kinase complex

How MPFs work to control the cell cycle

Once bound to cyclin, the mitotic cyclin depdendent kinase complex (or MPF) phosphorylates proteins involved in the early stages of mitosis. The active MPF stimulates the following:

• breakdown of the nuclear envelope• chromosome condensation,• mitotic spindle formation • degradation of key proteins. • activating the anaphase promoting

complex

To sum up…

• CDKs are only active when bound to cyclin.• When working, they cause the cell to go

through the mechanics of mitosis.• The cell doesn’t continually go through mitosis

b/c the cyclin gets destroyed when the MPF complex reaches a certain concentration.

cancer

• Tumor• Begign• Malignant• Metastasis• Radiation & chemo

Cancer

• Forms tumors & can spread through body - metastasize

Causes of Cancer

• Environmental factors

• UV radiation from sun

• Tobacco• Viral

infections • Genetic

How to prevent cancer:

• Low fat, high fiber diet

• Vitamins• Exercise• Don’t smoke• Use sunscreen• Regular doctor

appointments

Removal of colon polyps

• http://www.gihealth.com/flashvideo/flash.html

Review of the cell

cycle

• http://www.cellsalive.com/mitosis.htm

2 Basic Modes of Reproduction:

1. Asexual – one parent– Produces offspring ID to parent– Ex: sponges, some worms, some plants,

bacteria, some fungi– Variation occurs through mutation

2. Sexual – two parents– Genetic variation occurs

Meiosis

• The making of gametes

• Reduces chromosome # by ½ so the 2 halves can come together & make a whole.

• Creates genetic variation.

Terms you need to know:

• Somatic cell – all cells other than sperm & egg. Humans = 46 chromosomes;diploid ( 2n)

• Gametes – sperm & egg. Humans = 23 haploid (n)

• Gene – seg of DNA that codes for the making of a polypeptide

Karyotype• An ordered picture of an individuals chromosomes

used to detect abnormal chromosome #

Trisomy 21 karyotype

More terms you need to know:

• Homologous chromosomes – paired chromosomes that contain genes that code for the same traits.

Homologous Chromosomes

Meiosis has 2 parts:

• Meiosis produces 4 non – identical haploid gametes.

• Meiosis I – separates homologous pairs of chromosomes.

• Meiosis II – separates sister chromatids – just like mitosis.

•http://www.cellsalive.com/meiosis.htm

Meiosis I & II

2 Notable events in meiosis I that cause genetic variation in gametes:

1. Random assortment of homologous chromosomes - there are 2 possible arrangement of maternal & paternal chromosomes on the metaphase plate. 2n –

humans = 8 million possible combinations

2 Notable events in meiosis I that cause genetic variation in gametes …

2. Crossing over – occurs when nonsister chromatids of homologous chromosomes exchange genes. Can happen at up to 3 different locations along the pair.

http://www.csuchico.edu/~jbell/Biol207/animations/recombination.html

A third notable event that causes variation in offspring, not gametes, &

occurs after meiosis if sperm & egg meet:

• Random fertilizaiton – chance determines which sperm (all of which are different) fertilizes which egg (all of which are different).

Meiosis & Mitosis compared:

• http://www.pbs.org/wgbh/nova/baby/divi_flash.html

Mistakes in Meiosis• Nondisjunction – when chromosomes don’t

separate during meiosis.• Results in:• Trisomy – 3 of a chromosome– Ex: Down Syndrome – Ex: Klinefeltersyndrome xxy

• Monosomy – 1 of a particular chromosome– Ex: Turner syn

http://www.tokyo-med.ac.jp/genet/anm/domov.gif

Trisomy 21

Some terms

• Autosomes – chromosomes other than the sex chromosomes

• Tetrad• synapsis• chiasmata

Evolutionary adaptation depends on a

population’s genetic variation.

• Those individuals best adapted to their env will live longer & therefore reproduce more, passing on their well adapted genes.

• More variation means a greater chance of individuals being better adapted, allowing for the survival of the species.