Lecture 11 Flashcards
What is a mutation
Change in the base sequence of DNA that can impact proteins
Mutations can occur in germ line cells (passed on to future progeny) or local/somatic (during cell division, not whole body - local effects)
Mutations in coding regions/exons are more impactful than those in introns (though those in introns can have effects as may change how splicing occurs)
Describe a Large scale alterations.
Chromosomal rearrangements
Describe Small scale alterations (focused on)
one or a few nucleotides altered
Name the types of small scale mutations
Substitution (minimal or major effect) of bases
Insertions/deletions (indels) of bases - can have major effect if within coding sequence - can cause a frameshift,
What are the different types of substituions
Silent
- Base substituted but not change to amino acid
Missense
- Base substituted and change to amino acid (effect depends on residue role – e.g. if both residues hydrophilic/hydrophobic less likely to have a major effect, also if serine or threonine – typically involved in phosphorylation – are changed, a major effect may also occur)
- Example is sickle cell anaemia
Nonsense
- Base substituted and changes to a stop codon/no amino acid (protein truncated because shortened, can have a major effect - ESPECIALLY NEAR N’ Terminus of protein/5’ end of mRNA/3’ end of DNA)
What are the different types of indels
Cause a frameshift (1 or 2 nt):
- Proteins is completely altered from point of frameshift, can have a catastrophic effect (especially nead N’ terminus/ 5’ on mRNA/ 3’ on DNA) as stop codon can be removed/produced and/or many amino acids changed
Maintain frame (3 nt)
- Only lose or gain a few amino acids
Examples are Huntington’s disease (insertion of triplets - lots of extra CAG/glutamine, like roughly 15-19 extra, so protein is very long and has different function) and TVP. These generally affect the brain
Describe sickle cell anaemia
There is a missense substitution mutation (CTC goes to CAC) in the DNA that produces on of the proteins - beta globin - in haemoglobin, which causes Glu to change to Val. Therefore amino acids goes from hydrophilic to hydrophobic
This causes the red blood cell to be sickle/weirdly shaped - less surface area, as haemoglobin molecules inside RBC form fibrils and form rigid structures. Not as good to squeeze in capillaries. Can cause protection against malaria
Describe G2 checkpoint (in cell cycle)
MPF at the G2 checkpoint:
Cyclin - proteins tat fluctuates throughout the cell cycle (purple in diagram)
Cyclin dependent kinase (Cdk) - a kinase that is activated when attached to a cyclin
Maturation (or M-phase) promoting factor/MPF - a specific cyclin/Cdk complex - key for g2 checkpoint
MPF phosphorylates many other proteins which allows mitosis to commence.
Signals go to MPF to activate/deactivate it based on certain factors e.g. state of DNA.
Describe stop and go molecules
Many gene products/proteins are considered stop and go molecules, they basically :
- Stop: genes that normally keep proliferation in check
- Go: genes that normally stimulate cell proliferation
What can happen if DNA mutations affect stop and go mutations
DNA mutations can change the function of STOP and GO molecules. If stop and go molecules are not working correctly, altered protein function can result, which may lead to loss of cell cycle control - the cell cycle could proceed when it shouldn’t.
Uncontrolled cell growth can result in tumours.
What are the stop and go molecules assocatied with cancer
In cancer, the genes affected by DNA changes are often:
- Proto-oncogenes - genes that normally stimulate cell proliferation.
- Tumour suppressor genes - genes that normally keep proliferation in check.
Oncogene - cancer relating genes.
Both can result in uncontrolled cell growth/tumour (overactivation of proto-oncogenes or deactivation of tumour suppression genes)
Mutation may cause the Go protein (e.g. Ras or Myc) to be active with or without ligand therefore too much protein (protein inhibits the cell cycle)
Two examples of proteins from proto-oncogenes that can become oncogenes are:
Ras - a GTPase
Myc - a transcription factor
Comparatively, mutations may cause Stop proteins to loose function and therefore cel divides when is shouldn’t
E.g.
TP53
BRCA 1
BRCA 2
How do the cancer-causing DNA mutations arise?
Genetic predisposition: in all cells of the body
Inherited from parents - an issue of deficiency in a gene (typically one copy, e.g. p53, BRCA)
Acquired: locally, in one cell initially
e.g. UV damage, smoking, carcinogens, viruses, drugs and treatments e.g. chemotherapy
In both, altered protein function can results which may lead to loss of cell cycle control
Can one mutation cause cancer
It’s never just one mutation that causes cancer – getting one mutation may increase chance of getting another mutation. Mutations may not accumulate in the same cell but in the same cell population
How do you analyse presence of cancer
Cell/tumour level vs systemic - you wouldn’t typically use blood to analyse presence of cancer (unless blood cancer). You would need to sample tissue itself via DNA analysis (unless using super high tech)
