UNIT 1 - KA5 Flashcards
What is the cytoskeleton
The cytoskeleton is a network of proteins throughout the cytoplasm
What type of support does the cytoskeleton give to cells
The cytoskeleton gives mechanical support
and shape to cells
What does the cytoskeleton consist of
It consists of different protein structures
including microtubules, which are found in all eukaryotic cells
What is a microtubule composed of
Microtubules are hollow cylinders composed of the protein tubulin.
Where do microtubules radiate from
They radiate from the microtubule organising centre (MTOC) or centrosome
What do microtubules control the movement of
Microtubules control the movement of
membrane-bound organelles and
chromosomes
What does cell division require
Cell division requires remodelling of the
cytoskeleton. Microtubules also form the spindle fibres that are active during cell division.
What does formation and break down if microtubules involve
Formation and breakdown of microtubules
involves polymerisation and depolymerisation of tubulin
What are the three functions of the cytoskeleton
- the cytoskeleton gives mechanical support and shapes to cells
- microtubules control movement of membrane bound organelles and chromosome
- microtubules play an important role in cell decision as this requires remodelling of cell cytoskeleton and also forms the spindle fibres which are needed during cell division
What Two parts can the cell cycle be divided into
- Interphase
- Mitotic (M) phase
Interphase
Interphase lasts much longer than the mitotic phase. Interphase is an active period of growth. Interphase is divided into three sub phases.
What are the three sub phases of interphase
- G1 phase
- Sphase
- G2 phase
G1 phase
The G1 phase is an initial growth phase where proteins and organelles are synthesised
Sphase
During the Sphase the cell continues to grow and DNA is replicated in preparation for mitosis
G2 phase
A further growth phase. Proteins and organelles synthesis continues in preparation for mitosis
Mitotic phase
At the end of G2 cells enter the Mitotic phase (M)
Mitosis
The chromosome material is separated
Cytokinesis
The separation of the cytoplasm into daughter cells
Draw a diagram of cell cycle
G1 —> Sphase —> G2 —> mitosis —> cytokinesis
The process of mitosis is broken up into four stages
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
- DNA condenses into discrete chromosomes and appear as two identical sister chromatids joined at the centromere
- nuclear membrane breaks down
- spindle microtubules extend to form the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centrosome region
Metaphase
The chromosomes line up at the metaphase plate (equator of the cell)
Anaphase
- as spindle fibres shorten by depolymerisation, sister chromatids are separated.
- once apart, each chromatid can now Be called a chromosome
- the chromosomes are pulled to opposite poles of the cell
- by the end of the anaphase the two poles of the cell each Have An individual and complete set of chromosomes.
Telophase
During telophase, the chromosomes decondense and nuclear membrane are formed around them
Cytokinesis
Cytokinesis follows telophase and the cytoplasm of the cells splits to give two daughter cells
Three roles of microtubules in the cell cycle
- aligning the chromosomes on the metaphase plate
- separating sister chromatids
- formation of daughter nuclei
Why must the cell cycle be controlled
The cell cycle must be controlled to ensure that events proceed in the correct order and are completed before the next starts
What are cell cycle checkpoints
Checkpoints are 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
Name and describe the three checkpoints in the cell cycle
- G1 checkpoint - occurs towards the end of G1; sufficient cell growth must have occurred before the cell can enter the Sphase
- G2 checkpoint - occurs at the end of G2; success of DNA replication is assessed
- M checkpoint - occurs during metaphase and controls entry to anaphase. Checks that the chromosomes are aligned correctly on the metaphase plate; therefore ensures that each daughter cell receives correct number of chromosomes
G1 =
G1 checkpoint = most important
What happens if a cell receives the ‘go ahead’ signal at the G1 checkpoint
if a cell receives the ‘go ahead’ signal at the G1 checkpoint it will usually complete the cycle. If not it will exit the cycle and switch to a non - dividing state called G0 phase.
What are cyclin proteins that accumulate during cell growth involved in
Cyclin proteins that accumulate during cell growth are involved in regulating the cell cycle
What do cyclins combine with and what happens if sufficient phosphorylation is reached
Cyclins combine with and activate cyclin- dependent kinases (CDKs). Active cyclin-CDK complexes phosphorylate proteins that regulate progression through the cycle. If sufficient phosphorylation is reached, progression occurs.
At the G1 checkpoint what does the retinoblastoma protein act as
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
Describe how action of G1 -CDK allows progression of the cell cycle into the S phase
Phosphorylation by G1 cyclin-CDK inhibits the retinoblastoma protein (Rb)
This allows transcription of the genes that code for proteins needed for DNA replication.
Cells progress from G1 to S phase.
What happens to DNA in relation to G2 checkpoint
At the G2 checkpoint, the success of DNA replication and any damage to DNA is
assessed
Describe some functions of p53
- stimulate DNA repair
- arrest the cell cycle - this can allow time to recognise and fix damage so the cell cycle can restart
- initiate apoptosis if damage is too severe
Metaphase checkpoint
At the metaphase checkpoint, progression is halted until the chromosomes are aligned
correctly on the metaphase plate and attached to the spindle microtubule
An uncontrolled reduction in the rate of the cell cycle may result in…
An uncontrolled reduction in the rate of the cell cycle may result in degenerative disease
An uncontrolled increase in the rate of the cell cycle may…
An uncontrolled increase in the rate of the cell cycle may result in tumour formation
What is a proto-oncogene
A proto-oncogene is a normal gene usually involved in the control of cell growth or division which can mutate to form a tumour promoting oncogene.
Name and describe the process used to destroy an organisms own cells
The destruction of cells must be carefully controlled in a multicellular organism. It is brought about by a process known as apoptosis - programmed cell death.
Why must apoptosis be carefully controlled in a multicellular organism
Apoptosis must be carefully controlled as it is important in organ/tissue formation. Excessive apoptosis may result in degenerative conditions, while inhibition of apoptosis may result in tumour formation or cancer.
What is apoptosis triggered by
Apoptosis is triggered by cell death signals that can be external or internal
What is an example of production of death signal molecules from lymphocytes.
The production of death signal molecules from lymphocytes is an example of an
external death signal.
What is an example of an internal death signal
DNA damage is an example of an internal death signal.
What is the mechanism of an extrinsic signal
External death signal molecules bind to a surface receptor protein and trigger a protein
cascade within the cytoplasm
What is the mechanism of an intrinsic signal
An internal death signal resulting from DNA damage causes activation of p53 tumour-suppressor protein
What do death signals result in the activation of
Both types of death signal result in the activation of caspases (types of protease
enzyme) that cause the destruction of the cell
When is apoptosis essential
Apoptosis is essential during development of an organism to remove cells no longer
required as development progresses or during metamorphosis
When May cells initiate apoptosis
Cells may initiate apoptosis in the absence of growth factors.
