Week 8 - The cell cycle, regulation mechanics. Flashcards
Cell cycle
(simple expl.)
◼ The life cycle of a cell, the period between successive divisions of a cell.
Cell cycle: prokaryotes edition™
Binary fission
◼ Binary fission is the normal life cycle of a bacterial cell
which involves:
➢Replication phase (R-phase = C-period);
➢Division phase (D-phase = D-period);
➢Interval phase (I-phase = B-period).
2.50
pt.1
R-phase, D-phase and I-phase
◼Replication phase - the duration to replicate the bacterial genome
(e.g. E.coli 40 min).
Results in the formation of new chromosome which has anindependent point of attachment to the membrane.
◼Division phase - the segregation of daughter chromosome and other cellular components into daughter cells.
The D-phase is initiated by the FtsZ proteins, which assemble in form of a ring (Z-ring) at the midpoint of the cell and leads to formation of septum.
◼Interval phase (I-phase = B-period) the period between division and the initiation of chromosome replication.
3 main groups of the cell when it comes to cell division
◼ Highly specialized cells
lack ability to divide
e.g. RBC, neurones.
◼ Cells which can induce cell
division by specific
stimulus
e.g. liver cells, lymphocytes.
◼ Cells with relatively high
level of mitotic activity – e.g.
hemapoetic stem cells, epithelia cells.
Divide the cell cycle in 2 main parts?
OBS fråga om man ska kunna hur lång tid phasen lasts ex
➢For a typical rapidly proliferating human cell with a total cycle
time of 24 hours, the G1 phase might last about 11 hours, S
phase about 8 hours, G2 about 4 hours, and M about 1 hour.
Dividing part (mitosis) and
Interphase -
G1, S, and G2 phases.
OBS! G0 is not a phase that belong to interphase!!
G1 phase
Gap phase after cell division.
1st phase in the interphase.
Growth and biosynthesis
activity phase.
Duration of G1: highly
variable, also among different
cells of the same species.
Cell conducts many checks before entering the S phase.
S phase
DNA synthesis phase.
Starts with replication of DNA and finishes then → amount of DNA in the cell is doubled.
Duplication of centrioles takes place.
(Important to get ready for mitotic spindle)
G2 and M phase
➢G2 - Gap phase after synthesis of DNA and before celldivision.
Cell conducts series of checks before entering M phase.
➢M phase - the cell actually divides.
G0 phase
G0 – the «resting phase». In multicellular organisms, most
differentiated cells «exit» the cell cycle and survive for
days, weeks, or in some cases (e.g., nerve cells and cells of
the eye lens) even the lifetime of the organism without
dividing again. Some G0 cells can return to the cell cycle
and resume replicating.
◼These cells may be quiescent (dormant) or senescent
(aging or deteriorating).
◼ Quiescent cells may re-enter the cell cycle, senescent cells
do not re-enter the cell cycle.
◼ Most somatic cells of an organism are differentiated and
quiescent - they reside in the G0 phase of the cell cycle.
Explain restriction point
At the restriction point, the cell checks for certain signals and resources:
Is there enough energy and nutrients?
Are there growth signals from the body?
Is the DNA in good shape?
Two Choices:
Pass: If everything looks good, the cell will “pass” this checkpoint, committing to continue through the cycle and eventually divide.
Pause or Exit: If conditions aren’t right, the cell will pause in G1 or even exit to a resting state called G0 (a non-dividing phase) until conditions improve.
The restriction point helps make sure cells only divide when they’re in good condition and there’s a real need for new cells, preventing unnecessary or risky divisions.
CDK
regulation
◼ The genes encoding cyclins and CDKs are conserved among all eukaryotes.
Meaning not changes much during evolution + similarities across different species.
◼ Each CDK can associate with different cyclins, and the
associated cyclin determines which proteins are phosphorylated by a particular cyclin-CDK complex.
◼ Three main cyclin-CDK complexes exist:
➢G1 cyclin-CDK;
➢S-phase cyclin-CDK;
➢Mitotic cyclin-CDK (also known as maturation promoting factor)
What is this picture?
Obs slide is not for learning by heart but to show complexity in level of reg.
Schematic rep. of how each step of CDK activity (change or decrease) progress the cell cycle.
Fill in the table
Draw image of overview of cell cycle checkpoints
The cell cycle checkpoints
◼ Cell cycle checkpoints are surveillance mechanisms that
monitor the order, integrity, and fidelity of the major events of the cell cycle:
➢growth to the appropriate cell size
➢the replication
➢integrity of the chromosomes
➢accurate segregation at mitosis.
◼ If the presence of a defect is detected, the arrest of cell
cycle progression takes place.
G1/restriction checkpoint
◼ Check for – cell size, nutrients, growth factors, DNA damage (environmental factors). Primary decision point.
◼ The cell should progress through restriction point – after growth‐factor‐independent cell cycle progression starts.
Transition to the S phase.
◼ If damage is found – G1 arrest and/or cell enters G0 phase
(non-dividing state).
S-phase checkpoint
◼ The S-phase checkpoint is a surveillance mechanism, that
responds to DNA damage (spontaneous mutations).
◼ The stabilization of DNA replication forks, which is critical for cell survival and genome stability.
◼ A checkpoint is a cascade of signalling events that puts
replication on hold until a problem is resolved.
G2-phase checkpoint
◼ DNA damage checkpoint serves to prevent the cell from entering cell division (M-phase) with genomic DNA damage.
◼ A number of highly conserved proteins that sense DNA
damage and signal the cell-cycle machinery, are involved.
◼ In response to extensive DNA damage, p53 activates gene
expression for specific proteins to induce apoptosis.
M phase checkpoint
◼ Check for: mitotic spindle assembly.
◼ Prevents separation of the duplicated chromosomes until
each chromosome is properly attached to spindle apparatus.
◼ Cell ability to enter anaphase will be blocked in case of mistake until problem solved.
P53 protein
- What is it?
- What is it able to do?
- Activated by what?
- How is activation achieved?
◼ Nuclear DNA-binding phosphoprotein,
may serve as a transcription factor.
◼ Is able to bind specific DNA sequences.
◼ Activated by
increasing the protein’s half-life and the rate of translational initiation of its mRNA.
◼Activation achieved by either:
- Posttranslational modification of the protein
- Alternative splicing
- Binding of regulatory proteins
Picture shows what?
Fråga om man ska kunna denna
pt. 2
19:16
How p53 works when activated
OBS fråga om man ska kunna detta
om ja - lägg i ps och markera med rött
lect 8
pt 2
21:35
Cell aging is called what?
Name the causes and consequenses
pt 2
25:37
process of gradual deterioration of cells, tissues, and organisms as they age
What does cell proliferation mean?
Multiplying or increasing in number. In biology, cell proliferation occurs by a process known as cell division.
What does cellular senescence mean?
It is considered what?
What pathways are there?
◼ Cellular senescence - irreversible arrest of cell proliferation.
◼ The senescence arrest is considered irreversible.
◼ Two main pathways are involved - p53/p21 and p16/pRB.
What does this picture show
Schematic rep. of both pathways of scenescence arrest
Main problem, DDR could be persistent = different responses
Apoptosis
- What is it
- Where and when does it take place
◼ Form of cell death, also known as programmed cell death
is a normal phenomenon, occurring frequently in a multicellular organism.
◼ Apoptosis takes place:
➢embryonic development
➢adult human body – cells with genomic damage, senescent cells, cells which are not required anymore (activated T cells after responce to infectious agent).
➢human diseases – overactive initiation of apoptosis (type 1 diabetes).
- What are Caspases
- Their synthesization
- It is considered what because it cannot turn back after a critical point?
A family of proteases that are responsible for triggering apoptotic changes in the cell.
◼Caspases are synthesized in the cell as inactive precursors - procaspases, and activated by cleavage at aspartic acids by other caspases.
◼The protease cascade is irreversible - once a cell reaches a critical point along the path to destruction, it cannot turn
back.
2 pathways of Apoptotis
- Extrinsic pathway –extracellular messenger protein called tumor necrosis factor (TNF).
- Intrinsic pathway – activation is regulated by Bcl-2 proteins.
Q, should we know the 2 processes
33:19
pt.2
After the cell death pathway is activated….
Endogenous and exogenous agents
Does DNA repair processes exist in both prokaryotes and eukaryotes
+ What is involved in it?
Link between DNA reperation and regulation of cell cycle
What does checkpoint mechanisms during cell cycle do? What does failure in these checkpoints lead to?
Endogenous agents: Stuff inside the organism → cause damage, e.g natural mistakes during DNA replication
Exogenous agents: Stuff outside the organism that can cause damage, like radiation or chemicals.
Yes and there are many proteins involved.
DNA reperation and regulation of cell cycle are closely linked
◼ During the cell cycle, checkpoint mechanisms ensure that a
cell’s DNA is intact before DNA replication and after
replication before cell division starts. Failures in these
checkpoints can lead to an accumulation of damage.
Draw a small scheme of DNA damage response
What factors impacts what the cell does in response to DNA damage
Short about every aspect
Response depends on level of DNA damage
E.g. Repair: DNA polymerase during DNA replication has exonuclease activity (aka proofreading ability) - recognise wrong inserted base during synthesis and can repair it.
Tolerance: meaning cell cycle is not stopped → all processes continues
Apoptosis: If high DNA damage level and global genomic instability is high → can be triggered
Moderate DNA damage and checkpoint can detect it → cell cycle arrest led by dna reperation
DNA reperation: important role in stability of genome and ability to continue cell cycle without any DNA damage
DNA damage repair mechanisms
3.39 pt. 3
Understand image, below is result of above?
Mismatch repair mechanism
(MMR)
Repairs 2 types of DNA replication errors:
Removes base mismatch and small insertion/deletion loops
MMR is strand specific = able to distinguish between newly synthesized strand and template strand.
Understand this picture
Ensured by group of proteins that varies between single base mismatch and repeat mismatch (look at black box)
(OBS these are main proteins, exists more)
These proteins → involved in recognition of mismatch
When recognised → complex of DNA polymerase and DNA ligase with exonuclease activity (strand is cleaved) → wrong mismatch is removed
Human pathology and MMR
(OBS remove image)
Deeper into this pict.
allelic variant in MMR genes →predisposition to Lynch syndrome
Nucleotide excision repair (NER)
Repairs DNA damage caused by enviromental factors:
radiation, mutagens (includes chemical)
Important in excision of UV light induced DNA damage
Two repair sub-pathways exists, (one for) :
1. transcriptional active DNA
(TC-NER)
- global genomic (GG-NER)
OBS ASK should we be able to explain this schematic?
11:10 pt.3
Human pathology linked to NER
Example 1: XP
Xeroderma pigmentosum
Symptoms: leading to an inability to properly repair UV-induced DNA damage.
- This results in extreme sensitivity to sunlight, skin abnormalities, a high risk of skin cancer, visible small blood vessels on the skin, eye changes, and intellectual disability.
XP proteins have functions in NER and in transcription (as chromatin remodelling complex). Allelic variants in
germ cells leads to the disease.
Human pathology linked to NER
Example 2: CS
◼ Cockayne syndrome (CS)
◼ Symptoms - mental and developmental retardation,
photosensitivity, progressive sensorineural hearing loss,
short stature, a typical bird like face, deep-set eyes, loss of
subcutaneous fat and, premature ageing and progressive
neurodegeneration.
◼ Defect in NER, connected to mitochondrial base excision
repair (BER), defective XP and CS proteins → result in
defective TCR, also play a role in transcription.
OBS ASK we should know CS in detail??
◼CS-proteins are involved in signalling of the TCR-part of NER, which is the reason for the photosensitivity.
◼ CS-proteins localize to mitochondria after oxidative stress, interact with mitochondrial proteins to protect the cell from oxidative stress-induced damage. In case of defect, this may
contribute to the premature ageing.
◼ NER defects can not fully explain the clinical features of CS.
The mitochondrial changes may be suggested as the underlying
cause of disease.
OBS ASK if we should know case reports
Base excision repair (BER)
◼ Removes damaged bases in DNA sequence. Responsible for removing small, non-helix-distorting errors. (dvs small enough to only affect one strand not the entire helix)
◼ BER can repair:
➢oxidized bases,
➢alkylated bases,
➢deaminated bases,
➢inappropriately incorporated uracil,
➢single strand DNA breaks.
◼ BER is initiated by a DNA glycosylase that recognizes and removes the damaged base.
◼ Two pathways exists
- Short (removes one base lesions)
- Long (removes 2-10 base lesions).
◼ No human disease is currently known to be associated with
a defect in BER, which may be due to embryonic lethality.
Schematic repr. of BER
The two major DNA double-strand break (DSB) repair pathways in eukaryotic cells?
- Homologous recombination repair HR
- Non-homologous end joining NHEJ
◼ Both can repair (DSB) as mentioned
◼HR uses a homologous DNA template and is highly accurate.
◼NHEJ re-joins the broken ends without using a template and is often accompanied by loss of some nucleotides.
◼Choice of mechanism depends on cell cycle stage.
◼NHEJ being more active in G1
HR dominating during S and G2 phases.
Human pathology involving HR and NHEJ
◼ Defective repair of DSBs → chromosomal instability; rearrangements and loss of chromosomes.
◼ A number of human syndromes, are associated with defects
in DSB repair (e.g. hereditary breast/ovarian cancer).
