Cell Cycle Flashcards
What are the stages of mitosis
Interphase
Prophase
Metaphase
Anaphase
Telophase
Roughly how long is each stage of the cell cycle
What happens during the G phase of the cell cycle
The cell decides whether the conditions are favourable for division
What happens in the S phase of the cell cycle
DNA is replicated or else daughter cells will only have half of the required genetic material
Involved the use of DNA polymerase (which is very accurate)
It makes 2 sister chromatids which are held together by cohesion
What happens in the prophase
Chromosomes condense
Spindle fibres begin to form, coordinated by the centrosomes
How does chromosome condensation occur
How do spindle fibres form
At the beginning of nuclear division, two wheel-shaped protein structures called centrioles position themselves at opposite ends of the cell forming cell poles. Long protein fibers called microtubules extend from the centrioles in all possible directions, forming what is called a spindle
What is the prometaphase and why is it needed
Nuclear envelope breaks down
Protein in nuclear envelope diffuses out
It’s needed as it provides more space in the cell
What happens during the metaphase
Chromosomes align at the equator
Kinetochores attached to microtubules/kinetochore spindle fibres
Chromosomes are visible
What is a kinetochore
Is the protein complex assembled at the centromere that binds to microtubules of the spindle fibres
What happens in the anaphase
Spindle fibres form
Sister chromatids seperate
Daughter chromosomes migrate to opposite poles
Spindle poles move apart
Explain the process of chromosome migration
Polymerization and Depolymerization: Microtubules undergo dynamic instability, alternating between phases of growth (polymerization) and shrinkage (depolymerization), driving chromosome movement.
Kinetochore Microtubules: Attach to the kinetochores and shorten by depolymerizing at their plus ends, pulling chromatids towards the poles.
Polar and Astral Microtubules: Help push the spindle poles apart and anchor the spindle apparatus.
Motor Proteins
Dynein: Moves towards the minus end of microtubules and helps pull chromosomes towards the spindle poles.
Kinesin: Generally moves towards the plus end of microtubules and can help slide microtubules past each other to elongate the spindle.
Kinetochore Function
Attachment and Tension Sensing: Kinetochores play a crucial role in attaching chromosomes to microtubules and sensing tension to ensure proper attachment.
Checkpoint Signaling: Kinetochores are involved in the spindle assembly checkpoint, which prevents progression to anaphase until all chromosomes are properly attached.
What happens during the telophase
Daughter chromosomes arrive at poles and decondense
Nuclear envelope reformation
What happens during cytokinesis
Cytoplasmic separation
Formation and contraction of actin ring
What are the stages of the cell cycle
G1, S, G2, M
What are the 3 major regulatory transitions in the cell cycle
What is the role of the cell-cycle control system
What does the cell-cycle control system depend on
Depends on cyclin-dependent protein kinases (Cdks)
What are Cdks and what do they do
What are the 4 classes of cyclins
G1/S cyclins
S-cyclins
M-cyclins
G1-cyclins
What do G1/S cyclins do
activate Cdks in late G1 which helps to trigger progression through ‘Start’ which results in a commitment to cell-cycle entry. The level of these cyclins decrease in the S phase
What is the role of S-cyclins
they bind to Cdks ‘start’ has been completed and help stimulate chromosome duplication. The levels of s-cyclins stay high until mitosis and is involved in the control of some early events in mitosis
role in regulating the progression of the cell cycle, specifically the transition from G1 phase to S phase and the progression through S phase
What is the role of M-cyclins
-activate Cdks that stimulate entry into mitosis at G2/M transition. Levels of M-cyclins fall during mitosis
these three are needed in all cells
What do G1-cyclins do
The binding of G1-cyclins to CDKs activates the kinases, enabling them to phosphorylate substrates required for the G1/S transition
What are the 4 different types of cyclin-Cdks complexes
• there are 4 Cdks, one each interacts with each cyclin
• The different cyclin-Cdk complexes are called G1-Cdk, G1/S-Cdk, S-Cdk and M-Cdk
What do cyclins do
• Cyclin also directs Cdk to specific target proteins so each cyclin-Cdk complex phosphorylates different substrate proteins
What happens when cyclin binds to the active site of a Cdk
• When cyclin binds to the active site it causes the protein loop (which blocks the active site) to move away from the active site which partially activates the Cdk enzyme
• When a seperate kinase, the Cdk-activating kinase, phosphorylates an amino acid near the entrance of the Cdk active site - the cyclin-Cdk complex is fully activated
• This causes a conformational change which increases the activity of the Cdk and allows the kinase to phosphorylates its target protein and cause specific cell cycle events
What happens to the Cdk in the absence of cyclin
The active site of Cdk is partly blocked by a protein looop
What are CKIs and what do they do
Binding of Cdk inhibitor proteins (CKIs) inactivate cyclin-Cdk complexes
• CKI binding rearranges the structure of the Cdk active site making the Cdk inactive
• Cells mainly use CKIs to regulate the activity of G1/s and S-Cdks early in the cell cycle
What are sister chromatids
A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be ‘one-half’ of the duplicated chromosome.
How do chromosomes condense
- DNA is wound around histones (which are non specific DNA binding proteins) which creates ‘beads of strings’
- ‘Beads of string’ wrap around eachother organised by scaffold proteins which fold ontop of eachother
Why do the chromosomes need to condense during the prophase
Cause gene expression isn’t needed
Makes it easier for the cell to move the chromosomes around the cell
What are spindle fibres made from and what is the role of spindle fibres
Made of microtubules
Coordinates movement of chromosomes
Spindle fibres attached to centrosomes which are at opposite ends of the cell
What are microtubules
Polymers of tubules which can form structures
They can grow and shrink
What are the 3 types of microtubules
Astral
Kinetichore
Inter polar
How does chromatid separation occur during the anaphase
What is the APC and what does it do
It breaks down cohesin (high holds sister chromatids together)
It also breaks down inactive seperase and securin
What is inactive seperate / seperase and what is its role
Inactive seperase is held in an inhibited form when bound to securin (securin inhibits seperase)
Seperase breaks down cohesion’s so sister chromatids seperate to form chromosomes
How does APC become activated
Becomes activated when chromatids in the middle of the cell
It breaks down securin to activate seperase
How does chromosome migration occur
-Shortening of kinetochore microtubules
-Astral microtubules pull centrosomes to opposite ends of cell
-Movement of daughter chromosomes to opposite poles
-forces generated mainly at kinetochores
How does organelle distrubution occur during the telophase
Mitochondria are relatively evenly distributed throughout the cytoplasm during cell division. They can replicate independently and are present in sufficient numbers to ensure that both daughter cells receive an adequate supply
The ER and Golgi apparatus are fragmented into vesicles during mitosis. These fragments are randomly distributed throughout the cytoplasm
Lysosomes and peroxisomesare distributed throughout the cytoplasm. They are present in multiple copies and are shared between the daughter cells during cytokinesis.
The centrosomes are duplicated during S phase, and each daughter cell receives one centrosome during mitosis
What happens to spindle fibres after mitosis ends
What is cdc2
A protein kinase (Cdk)
It phosphorylates proteins to activate them
The levels of cdc2 doesn’t change during the cell cycle
Activity of it rises and falls during the cell cycle
It is activated by a cyclin
What are the two states that retinoblastoma (Rb) proteins exist in
Hypophosphorylated
Hyperphosphorylated (lots of phosphate groups attached)
During the cell cycle, when is Rb hypophosphorylated and hyperphosphorylated
What are Rb interacting proteins
Are found in G1 but no the other stages
They bind to Rb
Rb exerts its function by interacting with a variety of other proteins
Eg CDKs , E2F, p53
What is E2F and what is its role
It is an Rb interacting protein
It is a transcription factor (it regulates expression of genes important for DNA replication and mitosis cell division)
its involved in the transactivation of genes required for S phase entry, including : s phase cyclin, DNA polymerase
It binds to specific DNA sequences in the promoters of genes that code for proteins needed to enter the S phase
What does a cyclin-Cdk complex do
It phosphorylates Rb
What is quiescence
How do cyclins make the cell cycle progress in the forward direction
They cause Cdks to phosphorylate the preceding phase cyclin which causes that Cyclin to break down
How were cyclins discovered
How were Cdks discovered
How is E2F activated
(So G1-Cdk complexes activate E2F proteins
What are astral microtubules and what do they do
They radiate outward from the poles into the cytoplasm, helping to position the spindle in the cell
What are kinetochore microtubules and what do they do
The plus end of kinetochore microtubules are attached to sister chromatid pairs of kinetochores
(Kinetochores are located at the centromere)
Kinetochore microtubules connect the spindle poles with kinetochores sister chromatids
What are inter polar microtubules and what do they do
Plus end of inter polar microtubules overlap with plus ends of microtubules from the other pole
This results in a anti parallel array in the mid zone of the cell
How are microtubules arranged in the cell
Plus end of microtubules project away from the spindle pole (which is where the centrosomes are)
Minus end of the microtubule are anchored at the spindle poles
What is the difference between chromosomes and chromatids
Chromosomes are whole DNA structures with many genes, existing in pairs in diploid cells.
Chromatids are the duplicated forms of chromosomes that appear during cell division, ensuring that genetic information is evenly divided between daughter cells
