How do we get more cells:

Mitosis vs. Meiosis and What’s going on the rest of the time

DNA has two forms: • Chromatin: unwound and stringy;

information is accessible but difficult to move

• Chromosomes: folded up chromatin; information is NOT accessible, but easy to move

• DNA SPENDS MOST OF THE TIME AS CHROMATIN!!!

Chromosome structure:

Telomeres (ends)

Centromere (middle)

During replication each chromosome is duplicated

Sister chromatids – two identical copies of chromosomes attached at the center

There are different types of chromosomes

• Each type has a different set of GENES on them.

• Chromosomes that are the same type are called HOMOLOGOUS chromosomes

Homologous chromosomes

• Have the same set of genes but may have different ALLELES

• An allele is a variation of a gene.

Homologous chromosomesGene: Hair color - allele: brown hair

Gene: Eye color - allele: blue eyes

Gene: Hair color - allele: blonde hair

Gene: Eye color - allele: brown eyes

What do we need to do before the cell can split?

• Make another copy of DNA

• Make more organelles (cell parts)

• Cells Grow

Cell Cycle

Cell CycleG1(Gap 1) – Cell growth and protein

synth.

S – DNA replication (copying)

G2 (Gap 2) – growth, prepare for cell division

M – Mitosis (nuclear division) and cytokinesis (cell division)

Why do cells need to divide?

• When we grow -

• To repair damage

• Replace cells when they die

Mitosis

• Process by which body cells duplicate and split their nucleus

• 4 stages

• Followed immediately by cytokinesis

1. ProphaseDuplicated chromatin folds up into

chromosomes

1. Prophase• Centrosomes separate and move

toward opposite poles

1. Prophase• Centrosomes start to form the

Mitotic spindle–Made of microtubules (cytoskeleton)

1. Prophase• Nuclear membrane breaks down

• Nucleolus disappears

2. Metaphase• Chromosomes line up along the

middle of the cell metaphase plate

• Spindle fibers attach to the centromeres of each chromosome

3. Anaphase• Sister chromatids separate and one

chromatid of each chromosome moves toward opposite poles

4. Telophase• Chromosomes unwind to chromatin

• Nuclear Envelope reforms

• Spindle breaks down

• Nucleolus reappears

Cell Division is not yet Done!!

• What have we made?

– One Cell with Two complete Nuclei

What is left to do?

- Cytokinesis: Divide the cytoplasm and separate the cells

Cytokinesis• Animal cells:

Membrane pinches inward until it divides the cytoplasm into two equal parts–Cleavage Furrow

Cytokinesis

• Plant cell: Cell plate forms in the middle of the cytoplasm

• Made of cellulose

• Becomes cell wall

NOW Cell division is done!

• Now you have Two Identical daughter cells (also identical to parent!)

Controlling Cell Division

• Cells know when they need to divide

–Complicated series of “stop” and “go” signals make sure cells only divide when needed

Controlling Cell Division• Cells know when they don’t need

to divide

Loss of control

• What happens?!?

– cells divide uncontrollably

–Pile up on top of each other

–Form big balls of cells called?????

»TUMORS!!!

Tumors

• Tumor cells do not respond to (or do not have) the body’s control signals

–missing a “stop” signal

–Hyperactive “go” signal

Tumor vs. Cancer

• Cells that grow out of control for tumors.

• BUT tumors aren’t necessarily cancerous

Tumor vs. Cancer

• Benign tumors: grow out of control, but are isolated and don’t interfere with healthy cells.

• Malignant tumors: grow out of control AND interfere with healthy cells.

Tumor vs. Cancer

• Malignant tumors are cancerous.

Spreading Cancer

• What if 1 cancer cell breaks off and enters the blood stream?

–When it “lands” it will form a new tumor there

–This is called metastasis

Sexual vs. Asexual Reproduction

DNA from how many parents?

Offspring Example

Asexual

Sexual

Chromosome Terms

• Diploid: two of each type of chromosome (one from each parent)

• Haploid: one of each chromosome

• Most human cells are Diploid

–We have 46 chromosomes

•22 pairs of body chromosomes

•2 sex chromosomes (XX or XY)

• What would happen if we made Egg and Sperm cells through mitosis?– Fertilized egg would have 92 chromosomes!

• What do we have to do when forming these cells?

–Divide the DNA in half!

Major differences between Mitosis and Meiosis

• one cell goes through two divisions to make a total of 4 cells

• cells created at the end are different from the cell they came from because of crossing over

Meiosis

• After the first division, cells are haploid and no longer diploid

• Happens only in reproductive cells

Genetic Variation:

- diversity between individuals

- created by crossing over and meiosis and sexual reproduction!

Meiosis I: 1st division

1.Homologous chromosomes pair up to form tetrads

2.Crossing over occurs: homologous chromosomes swap info (blonde allele for brown allele)

Meiosis I: 1st division

Similar steps to mitosis but:

3. Cells end up haploid but chromosomes are still duplicated

Meiosis II: 2nd division

Just like mitosis

- each cell from Meiosis 1 divides

- Creates 4 cells total

- all are different because of crossing over - all have ½ the original chromosomes

Compare and Contrast Mitosis and Meiosis