The Cell Cycle:Cell Division and

MitosisHow and Why Cells Divide

Cell Division

• Why do Cells Divide?• Surface area to volume ratio decreases as cell

grows. SA can no longer accommodate nutrients and wastes from increased volume that must cross the CM.

• Cells divide to maintain high SA to volume ratio. • DNA can no longer accommodate cell functions

Controls on Cell Division

• Growth and Development• Tissue Repair• Contact with other cells • Internal & External Regulators cyclins

(proteins) respond to events inside & outside of cell

What do you call uncontrolled cell growth?

CANCER (does not respond to cyclins)

*drugs cut off blood supply to cancer cellsdie

The Details:– Chromosomes contain an organism’s genetic

information.– Each new cell gets the same number of

chromosomes and the exact same genes as the original cell.

How Are Chromosomes Replicated ?

• Each Chromosome is a gigantic molecule of DNA

• Starting at one end, DNA molecule UNZIPS• Each Nitrogen base attracts its Complementary

Nucleotide (A with T and G with C)• Chromosomes are duplicated so the parent cell

contains two full identical sets of chromosomes.• Cell then divides resulting in each daughter cell

ending up with one full set of chromosomes.

Chromosomes

• Chromosomes: DNA coiled around proteins called histones.

• DNA & histones form beadlike structure nucleosomes.

• After DNA replication, chromosome structure: 2 identical sister chromatids attached by centromere.

The Cell Cycle

Interphase

G1 cell growth & development

S DNA synthesis

G2 Organelle synthesis

Cell Division

Mitosis & Cytokinesis

Interphase

Interphase is all the time BETWEEN cell divisions• Cells grow in size • Cells synthesize proteins by joining Amino Acids

together in specific sequences• DNA (of each chromosome) unzips and Replicates• This doubles the total number of chromosomes.• Condensing & coiling of chromatin (DNA & proteins)

mesh. • Centrioles replicate

Mitosis Overview

The Steps (phases) of Mitosis

• Prophase

• Metaphase

• Anaphase

• Telophase– Cytokinesis

Prophase (longest phase of Mitosis)

• Chromosomes condense & coil, becoming visible under microscope

• Nuclear membrane disintegrates

• Centrioles begin to migrate to opposite poles

• Microtubules form a structure called a Spindle

Metaphase

• Paired chromatids

lineup in the

middle of the cell,

sometimes called

the equator• Attach to spindle

via centromere

Anaphase

• Centromeres split• Sister Chromatids

separate, moving away from the equator towards opposite Poles

• Microtubules of the Spindle pull them to opposite ends of the cell

Telophase

• Chromosomes uncoil, becoming invisible chromatin mesh

• Spindle disappears• Nuclear membrane

reappears• Cytokinesis (division of

cytoplasm usually follows)– Cytoplasm is divided

evenly into two cells

The Importance of Mitosis

Mitosis is Nature’s 99.9999 percent guarantee

that when ANY Eukaryotic cell divides,

it produces two Genetically Identical Cells.

What kind of cells divide by Mitosis?

Somatic (body) cells

Somatic Cells/Body Cells

• Heart, Muscle, and Nerve cells rarely divide if at all.

• Skin, digestive, and connective tissue cells divide rapidly, replacing worn out, broken down, dead cells.

• Following injury, rapid cell division of these cells slows as healing progresses, due to cyclins.

Different Organisms and Their Chromosome NumbersCat 38

Cow 60

Fruit fly 8

Goldfish 94

Human 46

Onion 16

What do all these chromosome numbers have in common?

How Chromosomes Appear

• Chromosomes are found in homologous pairs. (one from each parent)– Each chromosome has a homolog that

carries traits for the same genes.

• Diploid number (2n)– The total number of chromosomes that

exist in a cell.

Mitosis

in Humans

46

92

ProMAT

DNA Replication

4646

46

92

ProMAT

DNA Replication

4646

2n

4n

ProMAT

DNA Replication

2n2n

MitosisIn Humans In ANY Species