MitosisCh. 10

Why Divide?• Growth of organism• Repairs• Reproduction• Genetic Variation2 Types:Mitosis:

– Parent cell produces genetically identical diploid (2n) daughter cells

Meiosis:– Parent cell produces

genetically varied haploid (n) daughter cells

Follow the Chromosomes • Chromosome compact DNA;

easier for transportingHow many in humans?

– 23• Homologous chromosomes

pairs of chromosomes in 2n cells; one copy from each parent

How many in humans?– 46 (2n= 2x23)

Word for when too many chromosomes are present; 47XXY?

– Ploidy; deadly in animals but helpful in plants

• Sister chromatids copies of chromosomes during mitosis

The Cell Cycle and Mitosis• What is the growth and

development phase of a cell called?– Interphase

• 3 Phases in interphase:1) G1 Phase normal cell growth

(Growth 1)– Various in length between cell

types

2) S Phase DNA is replicated and proteins for making chromosomes (Synthesis)– 10-12 hours long

3) G2 Phase cell growth and preparation for division (Growth 2)– 4-6 hours long

No Growth Phase• G0 Phase cell continues

to function but no longer grows to do division– Cells can be permanently

be in G0 or pushed back into G1 by signals

• What cells never leave G0?– Most human nerve cells

• What signals start up growth again?– Growth factors, hormones,

and other external signal molecules

M Phase• M Phase mitosis;

completed in 5 steps1) Prophase2) Prometaphase3) Metaphase4) Anaphase5) Telophase

What is the last step to division?– Cytokinesis; splitting of

the cytoplasm

Prophase1) Chromosomes begins to

form as chromatin is condensed– 2m of DNA changes to 23

pairs of chromosomes

2) Nucleolus shrinks and disappears

3) Spindle fibers formed between two centromeres

4) Centromeres move to opposite sides of the cell (spindle poles)

Late Prophase1) Nuclear envelope

disappears2) Spindle fibers extend

and attach sister chromosomes at the centromeres

3) Spindle fibers meet up with other spindle fibers across the cell

Metaphase1) Spindle fibers

lengthen and shorten to move chromosomes to the middle of the cell (metaphase plate)

Anaphase1) Spindle fibers shorten

and pull sister chromatids to opposite spindle poles

2) Sister chromatids are now called “daughter chromatids”

Telophase1) Opposite of prophase2) Spindle fibers

disassemble 3) Chromosomes

decondense4) Nuclear envelope and

nucleolus reappear

CytokinesisIn animals, protists, and many fungi:1) Furrow forms and cuts

cells in half2) Organelles that were

copied are moved to each cell

In plants:3) Cell plate forms as

guide for new cell wall4) Cell wall forms slowly

and divides cell in two

The Powerful Mitotic Spindle Fibers• All movement of

chromosomes and cytokinesis depend on spindle fibers

• Spindle fiber:– complex made of

microtubules and motor proteins

– Grow in all directions from centrioles creating the centrosome or Microtubule Organizing Center (MTOC)

Two Spindle Types1) Kinetochore

microtubules:– Bind chromosomes to

spindle poles– Chromosomes “walk” along

tube– Tube is broken down as

chromosome moves

2) Nonkinetochore microtubules:– Overlap cross the cell and

push against each other– Lengthening tubes pushes

cells apart

Cell Cycle Regulation• 3 Checkpoints• Each ensures the major step is

completed and the cell is ready to commit to the next big step

1) G1/S checkpoint: cell must either commit to whole division cycle or not– May require an external factor

(growth hormone)

2) G2/M checkpoint: commits cell to mitosis or not– Wont continue if DNA is to

damaged

3) Metaphase checkpoint: cell wont finish division unless chromosomes are lined up properly

Cdks Levels in the Cycle• Checkpoint regulated

internally by cyclin proteins and cyclin-dependent kinase enzymes (Cdks)

• Cdks need cyclins to be active and they start a phosphorylation cascade leading to a target protein

• What does this mean for cyclin levels in the cycle?– They fluctuate through

divison but Cdks levels are constant

Physical Inhibition• Growth factors typically cause

the levels of cyclin/Cdk activity to increase while inhibitors lower it

• What might inhibit cell growth after a wound has healed?– Physical contact of over cells;

receptors recognize cells of the same type and stop growing

• Contact Inhibition keeps cells in check inside organs and systems inside complex organism– Also limits bacterial growth in

cultures– Pushes cells in G0 phase until

contact is broken

The Big “C”• What happens if contact inhibition

is inhibited?– Cells continue to grow and form large

masses (tumors)• Most cancers are caused by a

shutdown of contact inhibition or an over stimulation of cyclin/Cdk complexes

• Tumor mass of cells– Benign local or no growth; may not

need to be removed– Metastasis growing and spreading

through the body; must be treated • Why does cancer hurt the body?

– Pressure from tumors, stealing blood and nutrition, over production of signals, etc…

What Causes Cancer?• Most cancers are the

result of damaged DNA caused by age, chemicals

(carcinogens), or naturally occurring mutations in the

DNA (heredity) • Oncogenes mutated

genes causing the cancer• What type of oncogenes

would you see in most cancers?– Cyclin/Cdk regulation– Growth hormone receptors

Cell Cycle in Prokaryotes• Prokaryotes go through a

similar G1, S, and G2 cycle, however they divide by binary fission

• Their single circular chromosome is replicated and then each is pulled to opposite sides of the cell

• The cell, once large enough, divides in half by cytokinesis

• Process is very short and simple, which is why bacteria can divide so quickly; most of the time is spent coping the DNA