Lecture 8 Cell divison I

What is cell division crucial for?

Growth, development and tissue repair

Define prokaryotic cell division

A single cell division cycle to ensure survival

Define eukaryotic cell division

Cell cycle divided into 2 stages: interphase and M phase

Two types of cell division

Meiosis and Mitosis

Prokaryotic cell divison

Chromosome remains condensed as a nucleoid during replication 

What are the 3 periods of prokaryotic cell division

B, C, and D

Define the B period

time between cell birth and initiation of DNA replication

Define C period

DNA replication begins and proceeds bidirectionally

Define D period

“binary fission”

time between end of replication and cell divison

Four phases of eukaryotic cell cycle

G1, S, G2, M

What does eukaryotic cell cycle end in?

Two identical daughter cells

What does the activation of each phase depend on?

The completion of the previous phase

What is the G0 phase?

A resting phase where the cell has left the cycle and stopped dividing

What cells can enter during the resting phase?

Fully differentiated and non-proliferating cells → RBCs

What cells don’t enter during the resting phase?

Epithelial cells → continue dividing throughout life

Define G1 phase

The first gap within interphase

Timeline of G1 phase

From end of previous M phase until DNA synthesis begins

When does “growth phase” resume? and what does the cell gain?

after M phase

cell increases proteins and organelles

Where is the restriction point?

S phase

Define S phase

DNA synthesis occurs

What does the DNA synthesis result in?

Replication of all chromosomes

When does most histone synthesis occur?

during S phase to support new chromatin formation

Define G2 phase

the 2nd gap within interphase

What occurs during G2 phase

DNA replication, protein synthesis and rapid cell growth

What is the G2 phase prepping for?

Mitosis

Microtubules during G2 phase

form into centrosomes and form “mitotic spindle”

What does the “mitotic spindle” wait for?

Chromosome seperation

What does the G2 checkpoint do?

ensure no DNA damage before mitosis

Quiescence definition

Reversible, cells metabolically active

Senescence defintion

Irreversible, cells exit due to DNA damage or stress

Key regulators in the eukaryotic cell cycle

Cyclins, Cyclin-dependent kinases, inhibitors

What are CDKs? (Cyclin-dependent kinases)

Enzymes that modify protein substrates involved in cell cycle progression

How do CDKs phosphorylate their substrates?

By transferring phosphate groups from ATP to specific amno acids on the substrates

Are CDKs active or inactive?

Inactive

Which CDKs are crucial for cycle regulation?

CDK1, CDK2, CDK4, CDK6

Key cyclins

A, B, D, E

Which CDK activates during G1 phase? and G1/S transition

PHASE: Cyclin D activates CDK4,CDK6

TRANSITION G1/S: Cyclin E activates CDK2

Which CDK activates during S phase? and G2/M transition

PHASE: Cyclin A activates CDK2 for DNA replication

TRANSITION: Cyclin A activates CDK1

Which CDK in M phase?

Cyclin B activates CDK1 to drive mitosis completion

What are the three cell cycle checkpoints?

G1, G2, and metaphse checkpoints

what occurs in the G1 checkpoint?

cell size, nutrient avalibaility, and DNA integrity assessed before S phase

what occurs in the G2 checkpoint?

ensures DNA replication is complete and checks for DNA damage

What occurs in the metaphse checkpoint?

verifies that chromosomes are attached to spindle appartus before anaphase begins

What is the cell cycle inhibitor?

p21

What is p21 triggered by?

DNA damage

What does p21 do?

stops cell cycle in g1 by inactivating cyclin-CDK

What does this stop allow?

time for repair or triggers cell death if damage too bad

what is p21 activated by?

tumor suppressor protein p53

6 phases of mitosis

M phase, prophase, pro-metaphse, metaphase, anaphase, telophase

What occurs in M phase?

Eukaryotic cell divides chromosomes into two identical sets, each in seperate nucleus

What occurs in prophase?

• chromosomes condense

• mitotic spindle forms from centrosomes

• centrosomes move to opposite poles of cell

• nuclear envelope starts to breakdown → allows spindle fibres to attach to chromosomes

What occurs in pro metaphase?

• chromosomes move toward cell equatorial plane

• the ER and Golgi detach

What occurs in metaphase?

• chromosomes align at metaphase plate

• sister chromatids alinged for seperation

• metaphse checkpoint

What occurs in Anaphase?

• sister chromatids seperated as centromeres split

• cell elongates

Two parts of anaphase

A: chromosome pulled to opposite poles

B: spindle poles move further apart

What occurs in Telophase?

• chromosome de-condense

• nuclear envelopes reform around nucelus 

• nuceloli reappear → mitotic spindle disassembles 

• cyclin levels drop → inhibit CDKs

• cell preps for cytokinesis 

What is cytokinesis?

divides the cytoplasm, organelles and cell membrane into two daughter cells

What two things together complete the cell cycle?

Mitosis and cytokinesis