cell cycle helen filmore Flashcards
WHAT ARE THE STAGES OF THE CELL CYCLE and describe
INTERPHASE - cell growth. it is where most cells spend most of their time, except cancer cells
MITOSIS - active cell division
APOPTOSAIS
what are the phases of interphase and describe them
G1 = Growth phase, longest phase. cells organelles are made here such as ribosomes and proteins, and they all duplicate
S = Synthesis - DNA synthesis - so 23 pair chromosomes are duplicated to make 46 pairs
G2= prepare for mitosis by making microtubules
M= mitosis - cell divides into 2 cells
However, some cells go into G0 = no more cell division instead of S
what is mitosis
when the cell divides and produces 2 daughter cells
what are the phases of mitosis
prophase, prometaphase, metaphase, anaphase and telophase and cytokinesis.
DESCRIBE PROPHASE
- nuclear envelope start disappear
- centrosomes migrate to opposite ends
- DNA condenses
describe pro metaphase
- nuclear membrane is now gone
- chromosomes line up in the middle
- centrosomes are at opposite ends
describe metaphase
- centromeres form microtubules (aka spindle fibres) which extend to the each other and the chromatids
describe anaphase
the microtubules pull one chromatids of one type of chromosome to each other
- therefore, both microtubes have 1 chromatid of each of the 2 chromatid
- there is now 4 “chromosomes”
describe telophase
- cell dimples
- nuclear envelope begins to reform
- microtubules breaks down
describe cytokinesis
Cytoplasm divides- actin ring pinches cytoplasm along the crease of the two new cells- cleavage furrow.
two new cell forms
why are checkpoints important
they ensure various cellular conditions are met before proceeding to the next phase of the cycle
- they ensure that there are no DNA mutation or errors present which can potentially the passed down to future daughter cells
what happens if a cell has dna errors during checkpoint
- they stop the cell from entering mitosis so cell can fix error,
- if errors can’t be fixed, they cauuse cell to die by apoptosis
what are the check points for cell cycle to regulate them
G1/S phase checkpoint
G2/Mitosis checkpoint
Spindle checkpoint
which protein carry out these checkpoint process
- cyclin dependent kinases (CDK) - always present in a cell but their default is for them to be inactive
- cyclins - activates the cyclin dependent kinases
what are the types of cyclins
DEAB (in order )
what the types of cyclins and which phase are they produced in
G1 phase = Cyclin D , in which binds to CDK- 4 and CDK6
G1 / S Phase= Cyclin E in which binds to CDk2
S Phase = Cyclin A in which binds to CDK2
G2/M phase - Cyclin B which binds to CDK1
It’s reversed. Memorise order of DEAB and know it does from 4/6 - 1
D- cdk 4/6
E - cdk -2
A - cdk -2
B - cdk 1
what are the levels of cyclins like
their levels are cyclical. so all the cyclins are always present, but some are higher than others in different phases.
what is the check point for G1/S phase
- where cell chooses to proceed with S phase and divide or go into quiescence via the G0 phase
*it is mediated by Cyclin E/CKD2 complex and 2 cell cycle regulator proteins called Retinoblastoma protein (RB) and E2F
what does retinoblastoma protein do
+ it prevents excessive cell division by inhibiting cell cycle progression
+ apt signals for cell division are present such as cyclins, RB is inactivated by phosphorylation
What do CYCLIN E - CKD2 complex do ?
- it fully phosphorylates Rb (retinoblastoma protein) and INACTIVATES IT
- Rb is normally bound to a transcription factor E2F
- once Rb is phosphorylated, it releases E2F, so E2F is then able to bind the promotors of proteins required for cell division i.e dna polymerase and transition to S phase.
- it also phosphorylates P27 Kip 1, an inhibitor of cyclin D. Phosphorylation causes degradation of this protein which promotes expression of cyclin A.
- this allows the cell to enter S phase.
why is the RB and E2F protein so important (think cancer)
- if there is a mutation in the gene which codes for the Rb protein, you may have a complete loss of the inactivating role of the Rb.
- therefore, E2F will have nothing to bind to and will constant activate transcription for proteins required for dna replication and cell division
- as a result, cells with this mutation will constantly divide = cancer,
what is the G2/M check point
it ensures that any potential DNA errors / mutations that occurred in the S phase can be fixed before mitosis
what is the Spindle check point
occurs in mitosis , specifically between metaphase and anaphase. it ensures all of the chromosomes are positioned correctly.
what do the activated CKD2-CYCLIN A complex do
helps activate DNA replication as it allows entry into S phase.
* it ensures that DNA replication only occurs once.
