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​.