Life and death of cells

Apoptosis

Programmed cell death/ cell suicide, essential part of normal health and development

Post mitotic cells

Cells that never divide

Senescent cells

During aging cells lose the ability to divide, permanently stuck in G0 phase (can’t enter cell cycle)

M phase

Mitosis (nuclear division), cytokinesis (cytoplasmic division)

G1 phase

Cell undergoes protein synthesis (gene transcription & rna synthesis occurs), duplicate organelles. Cells highly metabolically active during this stage

S phase

Cell duplicates dna, extra copy of each chromosome is made, sister chromatids joined at the centromere

G2 phase

Rapid period of cell growth, cell checks dna to make sure it has been copied correctly (phase is not always necessary)

M phase

Fastest stage, nuclear envelope breaks down, mitotic spindle forms & chromosomes are separated

Cytokinesis

Cell splits in two

Stages within M phase

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

In metaphase…

Chromosomes align at the centre of the cell along the metaphase plate

Kinetochore

Attaches chromosomes to microtubules, leading to segregation of chromosomes

Hayflick limit

Most cells from multicellular organisms have a limited lifespan even when all nutrients are provided (lifespan of around 40-60 divisions)

G0 phase

Cells that are not actively dividing, when more cells are needed cells can leave G0 and enter G1 phase

What limits the number of times a cell divides

Telomere shortening following cell division

Why are tumour cells immortal

Can rebuild their telomeres using enzyme telomerase

HeLa cells

Isolated from Henrietta lacks 1951 from cervical tumour, cells could be cultured indefinitely, used to test first polio vaccine

Commitment points

Ensure correct functioning of cell cycle

Necrosis

Uncontrolled cell death associated with disease

Importance of apoptosis

Embryo development

Immune system

Homeostasis

Cancer

Pathways for triggering apoptosis

Receptor mediated - extrinsic

Mitochondria mediated - intrinsic

Intrinsic pathway

Activated via mitochondria by variety of cell stresses (free radical damage, dna damage, viral infection or loss of survival signals)

Caspases

Executioners of apoptosis

Initiator caspases

Activate other caspases

Effector caspases

Break down cellular components such as cytoskeleton & dna

Role of cytochrome c

Located in the inner mitochondrial membrane

How is apoptosis triggered

Pores form in outer mitochondrial membrane allowing release of cytochrome c into cytosol, cytochrome c binds to other cytosolic proteins forming multi protein complex - apoptosome

The formation of the apoptosome requires…

Cytochrome c, apaf 1, procaspase 9 and atp

Pro-apoptosis members of bcl2

Insert themselves into mitochondrial surface, promote formation of large pores in outer membrane (leads to release of cytochrome c)

Anti apoptotic members

exist in the mitochondrial outer membrane, block action of pro apoptotic members

Caspases

Responsible for destruction of the cell

Bcl2 proteins function

Act on mitochondria to regulate formation of pores in the outer mitochondrial membrane