Cell Division, Differentiation, Development & Inheritence Flashcards
What happens in the S phase of the cell cycle?
DNA replication.
What happens in the G1 phase of the cell cycle?
Cells that do not divide are arrested here and sent to G0.
What happens in the M phase of the cell cycle?
Mitosis and cytokinesis.
What are the 2 main parts of the cell cycle?
Interphase (G1, S and G2) and M phase (mitosis and cytokinesis).
What are cyclin-dependant kinases CDK’s and what are they regulated by?
Protein kinases that are the ‘engines’ of the cell cycle. The activity of these is regulated by cyclins. There are 4 types of mammalian CDK. Different cyclins bind to different CDK’s. The cyclin must be present for the CDK to bind to it and work. Different cyclins accumulate and are destroyed at different points of the cycle controlling when the CDK’s are active in the cell cycle.
At what points in the cycle does the cell undergo checks and what do they do?
- End of G1 - checks for cell size and favourable environmental conditions. Checks for DNA damage. Restriction point here.
- During G2 - Check for damaged or unduplicated DNA. Check for unduplicated chromosomes.
- End of M - Check for chromosome attachment to mitotic spindle.
Why might a cell divide uncontrollably and cause a tumour?
If a cell becomes defective in the signalling machinery it might enter the cell cycle permanently and give rise to identically defective daughter cells.
What are the 4 main ways cancer drugs can target the cell cycle?
- Drugs such as taxol work in the M phase to block the mitotic spindle.
- Drugs such as etoposide work in the G1 phase to inhibit growth factor stimulation at the restriction point.
- Drugs such as 5-flurouracil work in the S phase to block DNA damage.
- Radiation can damage DNA and cause apoptosis at the S and G2 checkpoints.
What are nucleosomes?
DNA tightly packed and wrapped around histones.
What happens to the nucleosome prior to mitosis?
They become supercoiled (even tighter wrapped) to make it inaccessible to proteins involved in replication and transcription.
What is the role of the mitotic spindle in mitosis?
Mediates the segregation of chromosomes. The condensed chromosomes attach to the microtubules which radiate from the 2 centrosomes.
What happens in prophase?
Chromatin supercoils to condense into visible chromosomes consisting of identical, paired sister chromatids. The centrosomes move to opposite poles.
What happens in prometaphase?
The nuclear envelope breaks down. Kinetochore microtubules appear and connect the kinetochores to the poles.
What happens during metaphase?
The centrosomes become aligned in a plane at the cells equator.
What happens during anaphase?
The paired chromatids separate and the new daughter chromosomes move towards the poles.
What happens during telephase?
The daughter chromosomes reach the poles. The nuclear envelopes and nuclei reform, the chromatin decondenses. Cytokinesis follows and the daughter cells then enter interphase again.
What is cytokinesis driven by in animal cells?
The contraction of an actin-myosin ring.
What is cytokinesis driven by in plant cells?
Construction of a new cell wall inside the cell.
What is cell necrosis?
Cell death caused by cell-damaging agents or oxygen/nutrient starvation. Does not require energy. Symptoms include cell swelling and cell lysis. Generates an inflammatory response.
What are the characteristic morphological features of apoptosis?
Cell condensation, membrane blobbing, nuclear shrinkage, chromatin condensation and fragmentation, and formation of apoptotic bodies. Cell contents are not released, DNA and proteins are broken down. Does not cause inflammatory response.
Why is apoptosis as essential as cell division?
Because it regulates cell numbers by balancing cell division. Important for developmental processes and removal of pathogenic cells (e.g. virus infected cells, immune cells, cells with DNA damage). Also needed in the developing CNS because there is insufficient neurotrophic to support all the neurons produced though cell division.
What are the 2 cell death genes that cause apoptosis and the gene that prevents apoptosis?
Causes apoptosis - Ced-3 = caspase, and Ced-4 = Apaf 1.
Prevents apoptosis - Ced-9 = Bcl-2.
What does the ced-3 gene do?
Ced-3 produces cysteine proteases (caspases) that are present in the cytoplasm and mitochondria. They are inhibited by IAP proteins and need to be cleaved to become active. They cleave after an aspartic acid residue.
What are the 2 distinct classes of caspases?
- Initiator caspases (8, 9, 10) - activated by pro-apoptotic stimuli. Cleave and activate executer caspases.
- Executer caspases (3, 6, 7) - Activated by initiator caspases. Cleave a variety of target proteins resulting in execution of apoptosis.
What are the 2 things caspases can be activated by?
Death receptor pathway and intrinsic pathway.
What is the role of mitochondria and Bcl-2?
Opening of pores in the mitochondrial outer membrane triggers cytochrome C release and caspase activation. Bcl-2 prevents the formation of pores in the outer membrane so the cell must overcome Bcl-2 to activate programmed cell death pathway.
What are stem cells?
Undifferentiated quiescent (slow-cycling) cells that undergo cell division when activated (e.g. after injury).They have the capacity for self-renewal through asymmetric cell division and the capacity to differentiate into mature cells (potency).
What is the meaning of totipotency, pluripotent and multipotency?
Totipotency - Can give rise to all embryonic and extra-embryonic cell types.
Pluripotency - Gives rise to all cell types found in adult organism.
Multipotency - Gives rise to multiple but not all cell types in the adult organism.
How does differentiation work?
Cells switch on certain genes and switch off others. So the type of cell formed is determined by the pattern of gene expression in the cell.
What is the role of transcription factors in differentiation?
Specific transcription factors called master regulators are expressed at an early stage in the differentiation process and they induce (or suppress) transcription of various target genes that collectively lead to cell differentiation.
What is MyoD in muscle for?
To mediate the shutting down of cell division for differentiation to occur through up regulation of the cell cycle inhibitor p21.
What is determination?
Before differentiation occurs, the fate of the cell is determined. Determination does not show in an outward phenotype.
What 2 main mechanisms determine cell fate in differentiation?
- Cytoplasmic segregation (asymmetric cell division) - cytoplasmic determinants that are unequally distributed between daughter cells influence cell fate.
- Induction - Factors that are secreted by neighbouring cells influence cell fate.
What are cytoplasmic determinants (cell differentiation)?
Agents differentially inherited at each cell division that make cells different from each other.
What are inducers (cell differentiation)?
Extracellular molecules to which the cell is exposed. A high concentration may activate a transcription factor that triggers the cascade of cellular differentiation.
Is determination (differentiation) reversible?
In the early embryo yes, by changes in the environment. Not possible in later stage embryo’s however.
Is cell differentiation reversible?
Mature plant cells can be induced to dedifferentiate to a totipotent state in a process called cloning.
What are the 2 types of technology available to reprogram mammalian somatic cells into pluripotent/totipotent stem cells?
Somatic cell nuclear transfer (cloning) and induced pluripotent stem cells (iPSCs).
What does genomic equivalence meaning (cloning)?
No information is lost from the nuclei of cells as they differentiate into mature somatic cells during cloning.
what are choanoflagellates?
Close relatives to animals. Unicellular eukaryotes that form colonies during part of their life cycle. Information from genome projects indicates they are the closest known unicellular living relatives of animals.
