The Cell Cycle

When do cells divide?

• Reproduction• Replacement of damaged cells• Growth of new cells• In replacement and growth cell divisions

how should daughter cells compare to parent cell?

• The daughter cells should be identical copies of the parent cell.

How can identical daughter cells form?

• The genome must be copied and then divided such that each daughter cell gets one of the copies.

• Genome = all the genes in an organism

Bacterial Reproduction

• How do bacterial cells reproduce?

Important terms in eukaryotic cell division

• Chromosome = threadlike structures that are composed of DNA + protein

• replication = process whereby DNA is identically copied (before cell division)

• mitosis = division of the nucleus• cytokinesis = division of the cytoplasm• chromatin = DNA + protein complex that

is thin and fibrous; it will condense into distinct chromosomes during cell division

• Chromatid = after replication the chromosome consists of 2 sister chromatids joined at the centromere.

• Centromere = specialized region of the chromosome, where chromatids are joined. Each chromosome has one centromere.

The Cell Cycle

Interphase

• Around_____of cell cycle is spent in this phase

• G1 = first growth phase• S = synthesis phase, DNA synthesis

(replication) occurs here• G2 = second growth phase

G2 phase of Interphase in animal cells:

• Nuclear envelope is visible• One or more nucleoli are present• Centrioles are replicated and the 2 pairs

are near nucleus• aster forms around each pair of centrioles• chromosomes are loosely packed into

chromatin fiber, not distinguishable

General Overview of Mitosis

Late Interphase:

Prophase:

• In the Nucleus:• Nucleoli disappear• chromosome fibers condense into

discrete chromosomes• each chromosome consists of 2 sister

chromatids joined at the centromere

In the Cytoplasm:• mitotic spindle begins to form• spindle consists of microtubules

arranged between the centrosomes• centrosomes move apart due to

lengthening of microtubules

Prometaphase:• Nuclear envelope breaks apart• each chromatid has specialized structure

called kinetochore located at the centromere region

• kinetochore microtubules (km) interact with chromosomes at the kinetochore region

• The km’s cause the chromosomes to move• nonkinetochore microtubules radiate from

each pole

Metaphase• Chromosomes move to the metaphase plate

and line up there• the centromeres of the chromosomes are all

aligned on the metaphase plate• each sister chromatid of one chromosome, has

a kinetochore microtubule attached to it from opposite poles

• kinetochore microtubules + nonkinetochore microtubules = spindle fiber

Anaphase• Kinetochore microtubules shorten and non-

kinetochore microtubules lengthen• Centromeres divide and each chromosome

has no sister chromatid component• the shape of the cell elongates into an

elipse• chromosomes are pulled to the opposite

poles

Telophase• Nonkinetochore microtubules continue to

elongate the cell• new daughter nuclei form at the two poles• new nuclear envelopes are formed

around the chromosomes• nucleoli reappear• chromosomes uncoil into chromatin fiber• last phase of mitosis

Cytokinesis

• Begins before telophase has completed• evidenced by cleavage furrow in animal

cells and cell plate in plant cells

Mitochondrial Division

Evolution of mitosis:

Bacterial cells

Dinoflagelates; chromosomes attach to nuclear envelope.

Diatoms; nuclear envelope stays, microtubules inside nucleus

Most other eukaryotes; spindle forms outside of nucleus, and nuclear envelope breaks apart

Examples

Dinoflagellates Diatoms

Checkpoints in the cell cycle: If it

passes the G1 checkpoint

cell divides if not enters G0

phase and does not

divide

Cyclin protein levels fluctuate according to cell cycle stage. When cyclin is high the Cdk attaches and phosphorylation leads to breakdown of nuclear envelope. Later MPF initiates cyclin breakdown

Cancer cells• How does abnormal cell division of cancer

cells differ from normal cell division?• Cancer cells are not under density dependent

inhibition• Continue to grow until all nutrients are used up• Cancer cells are immortal, do not shorten

telomeres.• Cancer cells often have a mutated p53 gene.

p53 Gene• Known as the tumor suppressor gene• Found on the 17th chromosome• Codes for a p53 protein (393 amino acids long)• The protein does three things-

– Arrests growth by stopping the cell cycle– Activates DNA repair enzymes if mutations are

detected– Causes apoptosis (cell death) if cell is irreparable – http://www.youtube.com/watch?v=witLM--V2v8