L11: CSF Mutations and Cancer

What is gene expression?

A: The process of making a protein or functional product from DNA.

What are the two key stages of gene expression and what do they do?

• Transcription: DNA → RNA

• Translation: RNA → Protein
  

Why does structure determine function in proteins?

Because the protein's 3D shape (structure) determines how it interacts with other molecules—incorrect structure often means loss of function.

How does a DNA mutation potentially affect a protein's function?

It can change the RNA and amino acid sequence, altering protein folding and function.

 Which is more likely to affect the final protein: a mutation in a coding region or an intron?

A mutation in the coding region.

What are two types of mutations based on origin?

Germline mutations: In gametes, passed to offspring
Somatic mutations: Occur during cell division, local effects (e.g. tumors)

What are the two scales of DNA mutations?

Large-scale: Chromosomal rearrangements

Small-scale: Nucleotide substitutions, insertions, deletions

What is a silent mutation?

A substitution that changes the DNA/RNA sequence but not the amino acid—protein remains the same.

What is a missense mutation?

A substitution that changes one amino acid in the protein—may or may not affect function.

What factors influence whether a missense mutation impacts function?

The chemical properties of the original and new amino acids and the role of that residue (e.g., hydrophobic → hydrophilic switch).

What is a nonsense mutation?

A substitution that changes a codon to a stop codon, truncating the protein.

What is a frameshift mutation and how does it occur?

Caused by insertion or deletion of 1–2 nucleotides, shifting the reading frame and changing all amino acids downstream.

Does a frameshift have more severe effects at the 5’ or 3’ end of mRNA?

At the 5’ end—more of the protein is affected.

Does an insertion/deletion of 3 nucleotides cause a frameshift?

No. The reading frame is preserved.

Why might deleting 3 nucleotides not severely affect a protein?

It removes one amino acid; the impact depends on whether that residue is critical for function.

What causes Huntington’s disease at the DNA level?

A CAG triplet repeat expansion—38 or more repeats causes the disease.

 How does the Huntington's mutation expand?

During replication and repair, repeating sequences make it easier to insert extra CAGs.

What mutation causes Sickle Cell Anemia?

A: A point mutation in the beta-globin gene, changing GAG to GUG (glutamic acid to valine)

What is the impact of the glutamic acid → valine change in hemoglobin?

Hydrophilic → hydrophobic change causes rigid fibres in hemoglobin

How does the Sickle Cell mutation affect red blood cells?

Makes hemoglobin molecules rigid → RBCs become sickle-shaped → block small blood vessels → tissue damage.

Why is RBC shape important?

Normal RBCs are flexible and can pass through capillaries; sickled ones are rigid and cause blockages.

Why don’t all mutations have major effects in humans?

Because we are diploid—we have two copies of each gene.

What are the three main mitotic checkpoints and what do they check for?

• G1 checkpoint: DNA damage, nutrients

• G2 checkpoint: DNA quality, readiness for mitosis

• M checkpoint: Chromosome-spindle attachment
  

What is MPF and what is its role?

Maturation Promoting Factor, a cyclin/Cdk complex that triggers mitosis at the G2 checkpoint.

What activates Cdk?

Binding to a cyclin

What are the components of MPF and what do they do?

• Cyclin: Protein that builds up during the cell cycle.

• Cdk: Enzyme that needs cyclin to be active

When is MPF active and why?

When cyclin levels peak at G2 phase—MPF forms and phosphorylates proteins to trigger mitosis.

 What happens to cyclin and Cdk after mitosis?

Cyclin is degraded, Cdk remains for the next cycle.

 What types of signals do checkpoints rely on?

"Stop" and "Go" signals mediated by specific gene products.

What are STOP and GO signals in the cell cycle?

Regulatory molecules ensuring proper progression or arrest

What causes cancer at the genetic level?

A: DNA mutations in cell cycle regulatory genes

What are the two types of genes that regulate the cell cycle?

• Proto-oncogenes: Stimulate proliferation ("Go")

• Tumor suppressor genes: Inhibit proliferation ("Stop")
  

What is a proto-oncogene?

A gene that promotes cell proliferation under normal conditions.

How does a proto-oncogene become an oncogene?

Through mutations that lead to over-activation, causing excessive cell division.

What is a tumor suppressor gene?

A gene that inhibits cell division and ensures checkpoints work properly.

What happens when tumor suppressor genes are mutated?

They lose their function, allowing uncontrolled cell division.

What protein detects DNA damage at G2?

A: ATM. It is a protein that recognizes DNA double-strand breaks and activates repair mechanisms.

what is p53?

A tumor suppressor gene that regulates the cell cycle and prevents cancer formation by inducing apoptosis in damaged cells.

What does p53 do in response to DNA damage?

Activates transcription of an inhibitory protein to halt the cell cycle

How do mutations in p53 lead to cancer?

Without p53, cells can't pause the cycle for DNA repair—damaged DNA is replicated, leading to cancer.

What activates p53, and what does it do?

ATM protein detects DNA damage → activates p53 → halts cycle and initiates DNA repair.

How does Ras mutation promote cancer?

Mutated Ras stays active without growth factor → continuous transcription → unregulated cell division.

 What are two key proto-oncogenes?

• Ras (a GTPase)

• Myc (a transcription factor).

What is Ras?

A: A GTPase involved in cell signaling and cycle progression

Q: How does mutant Ras cause cancer?

A: It remains active even without a growth signal → continuous division

Q: What is Myc?

A: A transcription factor that promotes cell cycle progression

 Do multiple mutations usually contribute to cancer?

Yes, cancer is typically the result of multiple genetic alterations.

 What are the two origins of cancer-causing mutations?

• Inherited (germline)

• Acquired (environmental/chemical/viral)

Why is loss of cell cycle control dangerous?

It can lead to uncontrolled growth and tumor formation, i.e., cancer.

What tissue would you sample for DNA analysis in suspected cancer?

The tumor tissue. If inaccessible, blood may be used.