Cellular Growth Regulation Flashcards
What are some considerations for cell growth when looking at the growth of a population of cells?
You have to distinguish between an increase in cell numbers (hyperplasia) and an increase in cell size (hypertrophy).
It depends on integration of intra- and extracellular signals (checks on cellular physiology, growth and inhibitory factors, cell adhesion etc.)
What are some considerations for cell growth when looking at the growth at a cellular level?
Cell growth = increase in size (sometimes growth refers to this only) and cell division.
There are 4 cell cycle phases (G1, S, G2, and M).
Their progression is controlled at three key checkpoints (restriction points).
What are some considerations for cell growth when looking at the loss of cells by programmed cell death (apoptosis)?
It is a coordinated program of cell dismantling ending in phagocytosis; it is distinct from necrosis.
It occurs during normal development (e.g. separation of the digits, involution, immune and nervous system development), and in response to DNA damage and viral infection.
It could happen throughout the cell cycle as well.
Briefly describe growth factors, cytokines and interleukins.
They are proteins that:
- stimulate proliferation (called mitogens) and maintain survival (they’re usually named after originally identified target e.g. EGF, FGF, Interleukins (IL2 & IL4), NGF;
but, see also PDGF (platelet-derived GF) and IGF1 (Insulin-like GF – the main effector of pituitary growth hormone)
- stimulate
differentiation and inhibit proliferation e.g. TGF - induce apoptosis e.g. TNFα and other members of the TNF family
What are the three broad classes of growth factors, cytokines and interleukins?
PARACRINE: produced locally to stimulate proliferation of a different cell type that has the appropriate cell surface receptor
AUTOCRINE: produced by a cell that also expresses the appropriate cell surface receptor
ENDOCRINE: like conventional hormones, released systemically for distant effects
Describe the phases of the cell cycle.
Cells that aren’t in the cell cycle are considered to be in phase G0 (growth 0), as they are not growing. They are quiescent, and can be shuttled into the cell cycle to divide and multiply.
When they enter the cell cycle, they enter phase G1, which doubles all their organelles, etc.
They then go on to S1 (synthesis 1). This is where DNA replication occurs (with the incorporation of thymidine).
They then go on to G2, where they grow a bit more in size. Then, finally, they enter M1 (mitosis 1), where the cell itself splits into two.
From there, the cell goes back into G0, where it can either go back into the cell cycle again, or go on to become a terminally differentiated cell (post-mitotic), where it will go on to be shed or apoptose.
What is one technique where you can find out the DNA content of a cell?
You can use a fluorescence-activated cell sorter to analyse the cell DNA content.
As a recap, go through DNA replication.
1) DNA is replicated semiconservatively (daughter cells inherit one parental and one new strand).
2) New DNA is synthesized in the 5’ to 3’ direction from deoxynucleotide triphosphate precursors at a replication fork by a multienzyme complex (a replication machine).
3) Fidelity is determined by base pairing (A=T, G≡C) and presence of a proof reading enzyme in DNA polymerase.
4) Synthesis of the new DNA strand uses an RNA primer and occurs continuously on the leading strand and discontinuously on the trailing strand (giving rise to Okazaki fragments, which are ligated together after removal of the RNA primer).
Mitosis has 4 main stages.
Describe them.
1) PROPHASE:
- Nucleus becomes less definite
- Microtubular spindle apparatus assembles
- Centrioles (yellow) migrate to poles
PROMETAPHASE:
- Nuclear membrane breaks down
- Kinetochores attach to spindle in nuclear region
2) METAPHASE:
- Chromosomes (blue) align in equatorial plane
3) ANAPHASE:
- Chromatids separate and migrate to opposite poles
4) TELOPHASE:
- Daughter nuclei form
Another phase that is considered is cytokinesis:
- Division of cytoplasm
- Chromosomes decondense
What are the two types of drugs that act on the cell cycle?
- S-Phase active
- M-Phase active
Describe S-Phase active drugs.
- 5-Fluorouracil (an analogue of thymidine blocks thymidylate synthesis)
- Bromodeoxyuridine (another analogue that may be incorporated into DNA and detected by antibodies to identify cells that have passed through the S-phase)
Describe M-Phase active drugs.
- Colchicine (stabilizes free tubulin, preventing microtubule polymerization and arresting cells in mitosis – used in karyotype analysis)
- Vinca alkaloids (similar action to colchicine)
- Paclitaxel (Taxol, stabilizes microtubules, preventing de-polymerization)
Define and list the cell cycle checkpoints.
They are points at which controls (involving specific protein kinases and phosphatases) ensure the strict alternation of mitosis and DNA replication.
The checkpoints are:
- G1: a restriction point to check that the DNA is not damaged, cell size and metabolite and nutrient stores are also checked.
- Just before M: check that the DNA is completely replicated, and that no DNA is damaged.
- During M: when the chromosomes are aligned on the spindle.
What enzyme activity controls cells cycle progression?
Cyclin-dependent kinase activity controls cell cycle progression.
CDKs are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins. The attached phosphate group acts like a switch, making the target protein more or less active. When a cyclin attaches to a CDK, it has two important effects: it activates the CDK as a kinase, but it also directs the CDK to a specific set of target proteins, ones appropriate to the cell cycle period controlled by the cyclin. For instance, G1_11/S cyclins send CDKs to S phase targets (e.g., promoting DNA replication), while M cyclins send CDKs to M phase targets (e.g., making the nuclear membrane break down).
How does the retinoblastoma protein play a role in cyclin-dependent kinase activity?
The retinoblastoma (RB) protein is a key substrate of the G1 and G1/S cycling-dependent kinases.
Unphosphorylated RB binds E2F, preventing its stimulation of S-phase protein expression.
When phosphorylated by cyclins, RB releases E2F. The released E2F stimulates expression of more Cyclin E and
S-phase proteins e.g. DNA polymerase, thymidine kinase, PCNA etc.
DNA replication starts.
