Protein Control Of Cell Division Flashcards
(30 cards)
Describe the structure of the cytoskeleton.
The cytoskeleton is a network of protein fibres that extend throughout the cytoplasm in all eukaryotic cells. It is attached to membrane proteins and gives mechanical support and shape to the cell, acting as scaffolding to maintain its shape.
What is the purpose of a cytoskeleton other than providing support?
Organelles are attached to the cytoskeleton and it is involved in the movement of cellular component such as vesicles and chloroplasts. The cytoskeleton is also responsible for the movement of whole cells — flagella and cilia both rely on cytoskeletal activity.
What are microtubules?
Microtubules are hollow cylinders. They are polymers of a dimer made from soluble globular proteins α-tublin and β-tubulin.
The dimers are arranged to form microtubules and these have a diameter of 25mm. The length of the microtubules is determined by the cell through polymerisation or depolymerisation of tubulin at the ends.
What is the function of microtubules?
Microtubules govern the location and movement of membrane bound organelles and other cell components.
They are found in all eukaryotic cells and radiate from the microtubule-organising centre (MTC) or centrosome. In animals, centrioles form part of the centrosome and are involved in the organisation of the spindle fibres during cell division.
What are the requirements of cell division involving microtubules?
Cell division requires the remodelling of the cells microtubules.
The cytoplasmic microtubules are disassembled to form spindle fibres to control movement of chromosomes during mitosis and meiosis.
What are the 3 roles of microtubules during cell division?
- The star shaped tuft of microtubules at each centrosome ensures that the cell division apparatus is correctly located.
- Some of the spindle microtubules consist of microtubules attached to kinetochore proteins at the centromeres of each chromatid.
- Other microtubules in the spindle do not attach to chromosomes but instead attach to microtubules from the opposite centrosome.
What are the benefits of the roles of microtubules?
These roles allow the separation of chromatids by the disassembly of the microtubules at the kinetochores. This has a pulling effect on chromatids to poles.
The rate of assembly and disassembly of microtubules is far higher during cell division.
What is the purpose of cell cycle?
This cell cycle regulates the growth and replacement of genetically identical cells throughout the life of the organism.
Summarise the process of cell cycle.
The cell cycle is composed of interphase and mitosis. Interphase builds up cell contents which has three phases called G1, S and G2. Interphase is followed by the mitotic or M phase, which is consisted of mitosis and cytokinesis.
The whole cell cycle is therefore G1 -> S -> G2 -> M
Describe in detail, interphase.
Interphase involves growth and DNA synthesis.
This is a very active period of growth and metabolism. Interphase consists of the initial growth phase G1 which is followed by an S phase where the cell continues to grow and copy its chromosomes. There is then a further growth phase called G2.
In short:
- G1 is the first growth stage where the cell makes new proteins and copies of its organelles.
- S is the DNA replication stage.
- G2 is the second growth stage, the cell makes more proteins and copies the organelles in preparation for mitosis.
What does the mitotic phase involve?
The mitotic phase consists of mitosis and cytokinesis.
FIRST… mitosis is the dynamic continuum of sequential changes described as prophase, metaphase, anaphase and telophase (PMAT). These phases flow from one to the next to divide the nucleus. Spindle fibres move the chromosomes during mitosis.
Describe in detail, the different phases involved in mitosis.
PROPHASE - the first visible sign of cell division. DNA condenses into chromosomes in preparation. Each chromosome is consisted of two sister chromatids as replication has already occurred in interphase. The microtubules start to disassemble and start to assemble spindle fibres and asters from the MTOC by polymerisation. Some spindle fibres attach to the kinetochore proteins at the centromeres of chromosomes.
Tension sensitive proteins at the kinetochore ensure that one of each chromatid pair is attached to each centrosome. The nuclear membrane then breaks down to facilitate the formation of the new nuclei.
METAPHASE - involves the movement of chromosomes lining up at the equator of the spindle on the metaphase plate. The movement is achieved by polymerisation and depolymerisation of tubulin dimers into the microtubules forming the spindle fibres.
ANAPHASE - a rapid phase that requires the depolarisation of the kinetochore end of the spindle fibre. This separates the sister chromatids. Once they are separated, the chromatids are called chromosomes and are pulled toward opposite poles.
TELOPHASE - this is the phase where a cell briefly has two nuclei. The separated chromosomes are pulled by spindle fibres to opposite poles to form daughter nuclei. The chromosomes decondense and a nuclear membrane is made again.
CYTOKINESIS is the division of the cytoplasm to form two daughter cells. In animal cells, the membrane is pinched by actin and myosin fibres (cell cleavage). In plant cells, they form a middle lamella and a new cell wall between daughter cells before the membrane is made.
Why should the cell cycle be controlled? What happens when there is a lack of control?
The cell cycle is complex and has to work perfectly to produce new daughter cells. Those new daughter cells have to be produced at the correct rate and in the correct locations to allow regulated growth and repair.
An uncontrolled reduction in the rate of th cell cycle may result in degenerative disease such as Alzheimer’s or Parkinson’s. Which is the insufficient replacement of cells for normal tissue function.
An uncontrolled increase in the rate of the cell cycle may result in tumour formation. Tumours can be benign or malignant cancer.
What is a proto-oncogene?
A proto-oncogene is a normal gene involved in control of cell g Roth or division which may mutate to form a tumour promoting oncogene.
What are cell checkpoints?
Progression of the cell cycle involves checkpoints at G1, G2 and metaphase. Checkpoints are mechanisms which assess the condition of the cell during the cycle and halts progression to the next stage until requirements are met.
What does DNA damage trigger?
DNA damage triggers the activation of several proteins including p53.
Describe in detail, the G1 checkpoint.
The G1 checkpoint sits near the end of the stage. The cell size is monitored here and sufficient cell mass is required for the formation of two daughter cells. This checkpoint controls entry into the S phase.
If approval is not gained at the G1 checkpoint, the cell may switch to a non-dividing state called G0 phase. This phase means that cyclin proteins are not produced, so the cell has gone into a resting state. The cell will still be active and maybe a fully functioning differentiated cell, but is no longer concerned with the business of cell division ! G0 can return to the cell cycle if conditions change.
Describe in detail, the G2 checkpoint.
The G2 checkpoint is also near the end of the stage. This assesses the success of DNA replication and any damage to DNA to ensure a complete copy of the DNA is received by the daughter cell. This checkpoint controls entry into mitosis.
Describe in detail, the metaphase checkpoint.
The metaphase checkpoint is during metaphase (shocker). This monitors the chromosome alignment to ensure each daughter cell receives one chromatid from each chromosome. Progression is halted until chromosomes are aligned correctly.
Describe in detail cyclins and their function
Cyclin proteins accumulate during cell growth. They are involved in regulating the cell cycle.
As size increases during G1, cyclin proteins accumulate and combine with active regulatory proteins called CDKs. The binding of the cyclin to CDKs form a mitosis promoting factor and activates the CDK.
Active CDKs cause phosphorylation of target proteins that regulate progression through the cell cycle. If a sufficient threshold of phosphorylation is reached, the checkpoint is passed and the cell cycle continues. Insufficient phosphorylation results in the hold of the cell cycle.
Without any active CDKs the cell enters the G0 resting state.
Describe in detail, the retinoblastoma protein.
The retinoblastoma protein or Rb acts as a tumour suppressor. It does this by inhibiting the transcription genes that code for proteins needed for DNA replication.
Rb is a transcription factor inhibitor. It inhibits transcription factors that are involved in the production of proteins required from DNA replication in S phase.
Low levels of CDK activity allows Rb to bind to transcription factor E2F. This binding of Rb to the transcription factor inhibits the transcription of genes required to enter the S phase. It is the molecular stop signal of the checkpoint.
If the activity of G1 cyclin-CDK reaches a sufficient threshold where each Rb has been repeatedly phosphorylate 14x, this phosphorylate Rb protein can no longer bind to transcription factors therefore releasing them to promote the transcription of genes required for DNA replication.
The pathway = CDK — Rb — E2F
Lack of regulation of this pathway may lead to malignant cancer cells in humans.
Describe in detail the p53 protein.
The p53 protein is a transcription factor that can stimulate DNA repair, arrest the cell cycle or trigger cell death.
If DNA damage occurs, p53 expresses genes that stimulate DNA repair and arrests the cell cycle. If DNA repair is successful, the cell cycle may continue. However if the damage cannot be repaired - the p53 protein instructs the cell to kill itself through apoptosis.
Define ‘apoptosis’.
Apoptosis can be described as a programmed cell death. It is the careful control of the deliberate destruction of cells.
