1.5 Protein control of cell division

What does the cytoskeleton do

Gives mehcanical support and shape to cells

What does the cytoskeleton consist of

different protein structures including microtubules, which are found in all eukaryotic cells

What are microtubules

hollow cylinders composed of the protein tubulin. They radiate from the microtubule organising centre (MTOC) or centrosome

What do microtubules control

The movement of membrane-bound organelles and chromosomes

What does cell division require

remodelling of the cytoskeleton

What does the formation and breakdown of microtubules involve

Polymerisation and depolymerisation of tubulin

What do microtubules form

The spindle fibres that are active during cell division

What does the cell cycle consist of

Interphase and mitotic phase

What does interphase involve

Growth and DNA synthesis including G1, S and G2

What happens in G1 during interphase

Growth

What happens in S during interphase

DNA is replicated

What happens in G2 during interphase

Further growth

What happens in mitosis

The chromosomal material is separated by spindle microtubules. This is follows by cytokinesis in which the cytoplasm is seperared into two daughter cells

What does mitosis consist of

prophase, metaphase, anaphase, telophase

What happens during prophase

DNA condenses into chromosomes each consisting of two sister chromatids. Nuclear membrane breaks down, spindle microtubules extend from, the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region

What happens during metaphase

chromosomes are aligned at the metaphase plate (equator of the spindle)

What happens during anaphase

As spindle microtubules shorten by depolymerisation, sister chromatids are separated and the chromosomes are pulled to opposite poles

What happens during telophase

The chromosomes decondense and nuclear membranes are formed around them

What is progression through the cell cycle controlled by

Checkpoints

What are checkpoints

Mechanisms within the cell that assess the condition of the cell during the cell cycle and halt progression to the next phase until certain requirements are met

What is involved in regulating the cell cycle

Cyclin proteins that accumulate during cell growth

How does progression occur (to do with cyclins)

Cyclins combine with and activate cyclin-dependant kinases. Active cyclins-CDK complexes phosphorylate proteins that regulate progression through the cycle. Sufficient phosphorylation must be reached

What happens at the G1 checkpoint

Retinoblastoma protein (Rb) acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication

What does phosphorylation by G1 cyclin-CDK inhibit

Retinoblastoma protein (Rb)

What allows transcription of the genes that code for proteins needed for DNA replication

Inhibition of the retinoblastoma protein

What happens at G2 checkpoint

The success of DNA replication and any damage to DNA is assessed

What does DNA damage trigger

the activation of several proteins including p53 that can stimulate DNA repair, arrest the cell cycle or cause cell death

What does a metaphase checkpoint control

progression from metaphase to anaphase

What happens at the metaphase checkpoint

progression is halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules

What may result in degenerative disease

An uncontrolled reduction in the rate of the cell cycle

What may result in tumour formation

An uncontrolled increase in the rate of the cell cycle

What normal gene can mutate to from a tumour promoting oncogene

Proto-oncogene, usually involved in the control of cell growth or division

What is apoptosis triggered by

cell death signals that can be external or internal

What is an example of an external death signal

the production of death signals molecules from lymphocytes

What is an example of an internal death signal

DNA damage

what do external signal molecules bind to

Surface receptor protein

What do external death signal molecules trigger

a protein cascade within the cytoplasm

What can an internal death signal resulting from DNA damage cause

An activation of p53 tumour-suppressor protein

What do both types of death signals result in

The activation of caspases (types of protease enzyme) that cause the destruction of the cell

Why is apoptosis essential during development of an organism

To remove cells no longer required as development progresses or during metamorphosis

When may cells initiate apoptosis

In the absence of growth factors

Describe the action of caspases in cell destruction

Are proteases

Digest and break down proteins

Caspases activate other caspases