1.5 - Protein control of cell division

Cytoskeleton

Cytoskeleton

Network of different types of proteins that extends throughout the cytoplasm. The cytoskeleton gives mechanical support and shape to cells.​

Microtubules

Make up the cytoskeleton. They are composed of hollow straight rods made of globular proteins called tubulins.

Role of microtubules

Govern the location and movement of membrane-bound organelles and other cell components​.

Centrosome (microtubule organising centre/MTOC).​

Where microtubules radiate from

Role of microtubules in cell division

Microtubules form spindle fibres which are active during cell division. Cell division requires the remodelling of a cell's cytoskeleton

Polymerisation and depolymerisation

Formation and breakdown of microtubules involves the build up and break down of tubulin

Two main phases of the cell cycle

interphase and mitotic phase (M phase)

The phase in which a cell spends the majority of its time

Interphase

Three parts of interphase

G1, S, G2

The two processes that occur during the M phase

Mitosis and Cytokinesis

The stages of mitosis

Prophase, metaphase, anaphase and telophase

Cytokinesis

The division of the cytoplasm

Prophase

Replicated chromosomes condense (become shorter and fatter) and appear as pairs of identical sister chromatids. ​ Centrosomes move apart and microtubules radiate out from them to form spindle fibres.​ Nuclear membrane is broken down.

Metaphase

Multiple spindle fibres become attached to each sister chromatid. ​ Replicated chromosomes are aligned at the equator of the spindle fibres that are attached to them. This position is called the metaphase plate.

Anaphase

Proteins holding the sister chromatids together are degraded.​ Sister chromatids separate.​ Spindle fibres attached to each of the daughter chromosomes shorten to pull them to opposite poles of the cell.

Telophase

Chromosomes arrive at each pole of the cell and decondense (unravel). ​ A new nuclear membrane forms around each group of chromosomes, forming daughter nuclei.

Checkpoints

Critical control points where stop and go ahead signals regulate the cycle.​

Cyclin

Proteins that accumulate as the cell size increases during G1.​

Cyclin-dependent kinases (Cdks)

Regulatory proteins that are activated by cyclin. Active Cdks cause the phosphorylation of proteins that stimulate the cell cycle.

G1 Checkpoint

Progression occurs at this checkpoint if a sufficient threshold of phosphorylation is reached.​ If an insufficient threshold is reached then the cell is held at the checkpoint.

Retinoblastoma

A transcription factor phosphorylated by the G1 Cdks. This allows DNA replication in the S phase.

G2 checkpoint

At this checkpoint, the success of DNA replication and any damage to DNA is assessed​.

DNA damage

This triggers the activation of several proteins, include p53​ This can

• Stimulate DNA repair​

• Arrest the cell cycle​

• Cause cell death

M phase Checkpoint

A checkpoint that controls progression from metaphase to anaphase. Spindle fibre assembly is checked for.

The role of the cell cycle

Regulates the growth and replacement of genetically identical cells throughout the life of the organism. ​

Uncontrolled reduction in the rate of the cell cycle

This leads to degenerative diseases such as Alzheimer's or Parkinson's.

Uncontrolled increase in the rate of the cell cycle

This leads to tumour formation

Proto-oncogene

A normal gene, usually involved in the control of cell growth or division, which can mutate to form a tumour-promoting oncogene

Apoptosis

This is triggered by cell death signals that activate inactive forms of DNAase and caspases (type of proteinase).​​ These enzymes then destroy the cell.

Cell death signals outwith the cell

Cell death signal binds to a surface receptor protein to activate a protein cascade, resulting in active caspases.​

Cell death signals within the cell

DNA damage results in the presence of p53 which can activate a caspase cascade​

Importance of apoptosis

​ It is essential during development of an organism to remove cells no longer required as development progresses or during metamorphosis​.