Cell Cycle - Cell Biology
What is the cell cycle
A reproductive cycle of a cell where the cell is able to duplicate its chromosomes and usally the other cell components before dividing to make 2 daughter cells
What takes place in the S phase of the cell cycle
In the S phase where the chromosomes are duplicated by DNA replication where chromosome number stays the same but each chromosome has 2 chromatids so double the genetic material, that can be dividied equally
What is Chromatin
How DNA is held in eukaroyotic cells in a complex of histones and non-histone proteins + nuclear DNA. It allows for DNA compaction and involvbed in DNA activity regulation
What are nuceosomes
The basic structural monomeric unit of chromatin
What are histone octamers made from
They are formed from the dimers H3 H4 H2A H2B, and there are 2 copies of each dimer so get an overal total of 8 dimers to make the histone octamer
What are the 2 functiuonal domains in the core histone
Amino terminal tail and the histone fold
The histone fold can intercat with each other allowing the formation of dimers via handshake intercation
What are nucleosomes made of
They are made of nuclear DNA + histones
What are the linker histones
They bind both the DNA and the nucleosome core, and so can change the path of DNA that exit the nucleosome.
This means it effects the linker DNA accesibility and so then this may chnage the chromatin compaction and organisation
What is the zig-zag model
The structure of the 30nm chromatin fibres that is amde from the folding of 11nm chromatin fibres. It is thought that the nuleosomes are able to connect to the nucleosome that is opposite it, to form a helix like structure.
What are SMC proteins
They are able to form cohesins and condesisn proteins
What are cohesins and condensins needed for
Cohesins are needed during interphase and condensins are needed during mitosis, and they work by forming a dimer with another SMC protein
What is the structure of the SMC protein dimers
Both dimers contain 2 SMC protein but they also need the additon of a non SMC protein otherwise the dimer is unfunctional.
They are both able to form a ring around the nucleosome
How does cohesin work during interphase
Loop domains in the DNA are created by the additon of cohesin to the interphase chromosome due to cohesin binding. Then undergo ATP hydrolysis and the bound CTFC complex will prevent any further looping taking place. This leads to loops of interphase chromosomes with cohesin biund to it
How to visualise the chromosomes
Use multi-coloured FISH to visualise both intrerphase and mitotic chromosomes
They take uo very specifc places in the nucleus called chromosomal territories and is unique for each chromosome, that is able to regularly interact with the nucleus
What is interphase chromatin
Eukaroyic chromatin DNA us wrapped around histone octamers called nucleosomes
During interphase the fibres are arranged in loops controlled by cohesin and CTFC that define loop boundaries
Looping chromatin fibres has a function in chromatin compaction and gene expression
Geneome organisation overall does depend on the organsim tyoe
What is the mitotic chromosome structure
There is a centromere 1 per chromosome with then 2 chromatids bound to it- 2 DNA molecule per chromatid
The end of the chromatid called the telomere
How do you fold uo DNA to mitotic chromosomes
There is a short region of DNA, that have chromatin made up of nucleosomes like beads on a string
There is no cohesin but there is condensin 2, that is able to fold the chromatin into loops and then load that onto the DNA.
Condensin 1 then further fold by adding loops within the loops, to help condense the DNA into metaphase chromosome
What are mitotic chromosomes
Interphase chromatin and mitotic chromosomes are organised differentially.
During mitotic chromosome assembly interphase loops disappear completely, and DNA is folded into linear array of continuous chromatin loops attached to a scaffold. This array is compressed to form a mitotic chromosome.
Different classes of proteins participate in the mitotic chromosome assembly, but condensins seems to be the most important.
Condensins establish loops, but these loops are different from the chromatin loops that are found in interphase.
COHESIN plays a very important role in the organisation of interphase genome, CONDENSIN plays equally important role during mitosis
What is Interphase
The period between the end of 1 M phase and beginning of the next one
What is G1
Cell cycle gap between M phase and S phase
What is G2
Cell cycle gap between S phase and the M Phase
What is the S phase
Cell cycle stage during which the DNA replication takes place and chromosone duplication takes place
What is M phase
Where mitosis and cytokinesis takes place
What is mitosis
nuclear division, the process in early M phase by which the duplicated chromosomes are segregated by the mitotic spindle and packaged into daughter nuclei
What is cytokinesis
cell division, the process in late M phase by which the duplicated nuclei and cytoplasmic components are distributed into daughter cells by division of the mother cell
What is chromatin
Complex of DNA, histones, and non-histone proteins found in the nucleus of a eukaryotic cell
What is a histone
a group of small abundant proteins, rich in basic amino acids, that combine to form the nucleosome cores around which DNA is wrapped in eukaryotic chromosomes
What is a nucleosome
beadlike structure in eukaryotic chromatin, composed of a short length of DNA wrapped around an octameric core of histone proteins; the fundamental structural unit of chromatin
What are the stages of the cell cycle
M phase is where the mitosis and cytokinesis and interphase all take place, so DNA replication happen here as well
G1 and 2 stages are when there is a loss of signalling cells and cell decides if the cells is going to enter the cell cycle or not, so uave many molecular pathways to decide
What is cell cycle control
Is a control system of molecular pathways that regulate the timing and coordination of cell cycle events that govern the progression through the cycle
What are cell cycle checkpoints
Stop the cycle at 3 different stages if somehting goes wrong and work like a binary switch so once complete are irreversable
What is the start checkpoint
In the mid to late G1 phase and can't be progressed path if the cell growth is insufficiant, DNa damage ir incomplete preperations
Enter a long non-dividing state if condtions are not met
What is the G2/M checkpoint
Transition into the M phase is regulated and can be stopped due to DNA damage or incompletion of DNA replication
What is the metapahase to anaphase checkpoint
Transition checkpoint in M phase where the initiation of a sister chromatid can be blocked, this is due to chromosomes not being properly attached to microtubules in the mitotic spindles
What is the cell cycle control system dependent on
It is relinant on cyclically activated cyclin dependen protein kinases
When cyclin form complex with Cyclin dependent Kinase the protein kinase is activated to trigger cell cycle events amd without the cyclin the CDK is inactive and so no cell cycle event
How does CDK and cyclin regulate cell cycle progression
Due to a constant concentration of CDK the level of cyclin fluctuates and always more CDK than cyclin so all the CDK is active at once
What are the 4 types of cyclin
G1 cyclins
G1/S cyclins
S cyclins
M cyclins
How does phosphorylation of proteins drive transition in the cell cycle
In G1 have the phosphorylation of G1/CDK substrate
To enter the S phase have phosphorylation of S-CDK complex
To enter M phase have phosphorylation of M-CDK complex
And to enter anaphase have to destrcution of APC/C substrate
How do cyclins trigger different cell cycle events
Different cyclins bind to different CDK
The cyc;oin protein wont only activate the CDK but also the direct target specific proteins so each Cyclin-CDK complex will phosphorylate a different protein set
The same complex may also induce different effect at different times in cell cycle
What are CDK-activating Kinases
They regulate the whole cyclin CDK complex
Good to increase or decrease rate of reaction for the complex dependent on the cell environment in cell cycle
What is the Wee-1CDC25 regulatory pathway
Wee1/Myt1 be used to inhibit the cyclin CDK complex by further phosphorylation at 1 or 2 sites of the active site to make the complex inactive, but5 the CDC25 can dephosphorylate the sites to then reactivate the cyclin CDK complex
What does G1 cyclin do
Stimulate entry into a new cell cycle, and concentration depend on cell growth rate or on the growth promoting sequences being signalled
What do G1/S cyclins do
Stimulate the progression ofr the start so the commitment to the cell cycle entry, peak conc at end of G1
What do S cyclins do
They are necassary for DNA synthesis and contribute to control of early mitotic events
M cyclins what do they do
Necassary for full entry into mitosis and peak conc in metaphase of mitosis
What are CDK inhibitor proteins
CKI binding cause a large conformationla change in the CDK active site so make it inactive, helps to govern the G1/S and S CDK complexes
What is regulated proteolysis
The ubiquitin pathway is one of several ways to degrade proteins in a controlled manner, due to ubiqutin is attached to a lysine of a protein targetred for degradation. This produce a polyubiquitin chain
What is the proteasome in proteolysis
Large protein complex with proteolytic activity for degrading proteins marked by polyubiquitn chains added to them on the lysine
How does a proteasome work
Proteasome cap with a polyubiqitin receptor, recgonise the ployubiqutin chain and move the target protein to the proteasome core where they are digested after the ubiqutin removed by ubiquitin hydrolase and ATPases unfold the protein as it enters
How does proteolysis help to regulate cell cycle
Due to regulate the M stage by activating the APC/C that then degrade the M cyclin so then no entry to mitosis and especially the metaphase to telophase checkpoint
What happens during prophase
The replicated chromosomes will then condense in the nucleus and outside the nucleus the mitotic spindles will start to develop between the 2 centrosomes that replicated and moved apart
What happnes in prometaphase
This starts as soon as the nuclear envelope breakdown
The chromosomes attach to the spindle microtubules via the kinetochores in the chromosome and they undergo active movemenent in the cytoplasm
What happens during metaphase
Chromosomes align on the equator of the spindle fibres on the metaphase plate
The kinetochores spindles attach to the sister chromatdis to the opposte pole of the spindles
The kinetochroes loctaed in the chromosome centromere
What are Kintochores and how ard they assembled
Asembled during early mitoisis on centromeres and they are used for the attachment of microtubules
What is the centromere
The part of the chromosome where the primary construction is made of DNA which allows chromatid segragation during mitosis
How do kinetochores attach to microtubules
Via 20-30 microtubule attached to the kinetochore to connect the mitotic spindle and the chromosome
What is cohesion of sister chromatids
Established immediatly after DNA synthesis by a protein complex called cohesin and DNA catenation
Cohesin can do this due to hold the sister chromatids together until anaphase at the centromeres
It is removed in anaphase due to activity of the Spindle assembly checkpoint mechanism
How does the spindle assembly checkpoint work
It will detect incorrect attachemnt at the kinetochores
The cell is then arrested in metaphase
This then give more time to correct the improper
attachements
When there is then proper attachement the APC/C is acti9vated and the cell can enter anaphase
How to silence SAC and APC activation
APC remain inactive until all the Kintochores are attached properly to the microtubules due to SAC
After the attachment the ubiquitinates securin after the securin destructes and releases and so activates to seperate the cohesin from the centromere
What takes place during Anapahse
Sister chromatids seperate tor form 2 daughter chromosomes and each is pulled away by spindles to opposite poles of the cell
Pulled by the contraction of the kintechore microtubules
This leads to chromosome segragationn
What takes place during telophase
Chromosomes arrived at the poles of the cell and decondense
New nuclear envelope envelopes around both the poles of the cell
Central spindles is formed and the divison of the cytoplams started by the contrcatile ring
What takes place during cytokinesis
Cytoplasm is dividied into 2 by the contractile ring of actin and myosin and leads to the formation of 2 identicla daughter cells
What is mitotic spindle assmebly
Microtubules have polarity, due to the - end depolarises and + end will rapidly polymerise
What is a cancer cell
It is an abnormal cell type that is able to rapidly divide and uncontrollably in an autonomous process
It can lead to the formation of tumour cells that happens after repeated round of mutation and division that then lead to clone of cell that form a tumour that can be malignant or benign
How is cell divison, growth and cell survival is regulated
By intraceullular and extraceullular signal molecules are able to regulate this
What are mitogens
These are extracellular factors that stimulate cell division that is triggered by G1/s - CDK activity
What are growth factors
Are able to stimulate cell growth by promoting synthesis of new proteins and macromolecules
Survival factors
These promote the cell survivla by surpressing the form of programmed cell death called apoptosis
How do mutations of these lead to cancer
Mutations in the receptor on the cell membrane
Mutations cause the active area of the receptor so the receptor then always signalling so no longer responsive to outside areas
May be overexpression due to too many receptors so then gbetb too large a signal so excessive activity
What is an oncogene
A gene that the protein synthesized will promote cancer due to mutation in the gene and are a dominant gene
What is a protoncogene
Resust in an over sythesized protein that generally regualte the cell cyle and are too active so too much cell division
What is a tumour surpressor gene
Code for a protein that restrain cell proliferation such that the loss of the gene lead to greater chance of cancer formation
What is genetic and chromosome instability
An abnormal increase of defect in the chromosomes number of structure along with increase in the rate of defects in the genes
What is Meiosis
Nuclear divison that leasd to the fortmation of 4 haploid cells that are all gametes and gentically different to each other
What is a bivalent
Structure formed from 2 closely aligned duplicated homologous chromosomes thar are then joined together by a synaptonemal complex
How do synapotomial complex work
They hold bivlant together by rod shaped transverse filaments that hold them together and the axial core each homolog intercat with the cohesin in complexes thatbhold sister chromatids together
What is a Chiasmata
When there is a crossover between 2 sections of non sister chromatids and lead to genetic variation
What happens in meiosis 1
The homologous chromosomes are segragated and so sister chromatids are held together and this lead to 2 diploid cells with 2 chromatids each
There is DNA replication before hand
What takes place in meiosis 2
The sister chromatids are then segragated and lead to 4 genetically different haploid cells
There is no DNA replocation
What is the differnce of meiosis vs Mitosis
In meiosis 1 homologous seperate rather than sister chromatids
Both sister kinetochores in a homolgue must attach stably to the same spindle pole and the chiasamta hold homologous together allowing bi-orientation at the equator