eukaryotic cell growth and death

describe the cell cycle

process that starts when a new cell forms, and ends when the cell divides

G1 → S → G2 → M

G0 phase - cells that do not divide stay here

why do cells divide?

• to compensate for growth and division - smalls have small surface area to volume ratio

• repair and renewal ; replace damaged/dead cells ensuring constant cell number

• reproduction; gamete cells producing offspring

cell cycle in diff cell types

• some cells divide continuously replacing lost cells eg stem cells

• others do not divide eg mature muscle and nerve cells

• some cells only divide when stimulated eg hepatocytes

cell cycle divisions

can be divided into

1. interphase

2. mitotic phase

interphase

• consists of g1, s phase, g2

• growth phase

• increase in mass, synthesis of cell contents

• cells stay here majority of the time

mitotic phase ( m phase)

cellular division via mitosis ( nuclear divison ) and cytokineses ( cytoplasmic divison)

interphase : G1

g= gap

• synthesis of RNA, proteins & organelles

  • but NOOOO!!! chromosomes

  • cell increases in mass

• depenidng on cell type, duration varies

  • typically 8-10 hours, but a few minutes to hours in faster cells

  • decide if cell should through cellular divisions

    • to G0 or stay in G1

centromere

denotes the number of chromosome, holds sister chromatids together

interphase : G2

• cell double checks for errors

• DNA repair

• prepares cell for mitosis ( cell growth + protein synthesis )

• typically 4-6 hours

interphase S-phase

s= synthesis

• replication of DNA ( in prep for cellular division)

• relatively short ( like 5 hrs.)

mitotic (m) phase

m phase : mitosis ( nuclear) + cytokinesis ( cytoplasmic)

• results in 2 genetically identical cells

  • typically 30 - 45 minutes

  • VERYYYY SHORT

cell cycle regulation

• checkpoints ensure proper progression, error free division

• if conditions are met cell progresses further

  • using cyclins & cyclin-dependent kinases (Cdks)

• if conditions are not met, cell will not continue

how do check points regulate regulation

1. ensure appropriate timing of cell cycle phases

2. ensures phase completion before transition to next phase

3. able to respond to external conditions eg growth factors + nurtients

cyclins & cyclin dependent kinases

remember kinases are enzymes that transition phosphate groups through hydrolysis of atp/gtp

cyclins - proteins that bind to cylin-dependant kinases ( Cdks)

cyclin dependant kinases - enzymes that bind to proteins that activate proteins required for the cycle

• name given due to fluctuating concentrations during diff stages

• Cdk-cylin complex allows cell to pass checkpoint to the next stage

cyclins

protiens that bind to Cdks

form cyclin-Cdk complexes which allow cells to pass the checkpoint to the next phase

cyclin dependant kinases

bind to cyclin formng Cdk-cylclin, passingg checkpoint, letting cells transition to next stage

cylin dependant kinases activate proties required to progess the cycle

active mitotic cdk-cyclin stimulates

• nuclear envelope breakdown

• chromosome condensation

• mitotic spindle formation

• targeted protein degradation

main checkpoints

3 main checkpoints

• metaphase- anaphase transition control

• G1-S transition control

• G2- M transition control

G1-S check point

• occurs towards the end of G1

• checks whether cell is ready for DNA replication @ S-phase

  • cell is big enough

  • has the necessary proteins

  • is the dna damaged

• if all the conditions r not met, cell goes through resting period (g0) until ready to divide

G0 phase

cell exits from cell cylce if cell is not progressing to S-phase

• G0 cells are dormant

• non-dividing cells undergo terminal differentiation ( eg nerve cells)

  • can be triggered to re-enter g1 if theres a signal ( eg hepatocytes)

G2-M transition control

occurs @ the end of g2 ( boundary of g2 - m)

checks whether the cell is ready for m phase ( cellular division)

• cell is big enough

• DNA replication complete

• have necessary proteins

  • if conditions unmet here, arrest of cel can occur here if division is not necessary

metaphase-anaphase checkpoint control

occurs in the mitotic phase ( specifically metaphase and anaphase)

• checks whetehr mitosis process is going well

  • ensures chromatids are properly attached to spindle fibers pro

  • ensures daughter cells receive complete set of chromosomes

apoptotic cell death process

1. chromosomes condense, cytoplasm shrinks

2. nucleus becomes fragmented, bleb formations

3. apoptotic bodies are ingested by phagocytic cells

what if a cell makes an irreparable mistake

p53 proteins triggers apoptosis ( programmed cell death)

necrosis

premature death where cells swell and rupture, releasing cellular contents

• result of external factors ( disease, injury, hypoxia)

apoptosis

programmed cell death

part of development and growth ( healthy process)

triggered by DNA damage by UV, xrays and chemotherapeuatic agents

apoptic vs necrosis