Lecture 15 - Maintenance of Genomic Integrity Flashcards
How does tissue organization protect stem cells?
How are stem cells protected in gastrointestinal crypts?
What is hematopoietic differentiation?
Cells being differentiated, undergoing mitosis, or becoming specialized move further and further away. Therefore, mutations (which typically occur later on in cell life) happen in cells that are far away from the stem cell compartment.
In gastrointestinal crypts, the stem cells are sequestered away from the “adult” cells, which are present on the lumen of the small intestine. The stem cells are stored far down deep in the tissue, away from harm. Also, cells are jammed together tightly with junctions to prevent harm.
Hematopoietic differentiation refers to when stem cells form blood cells or related vesseld. This occurs through a highly compartmentalized process where full development of a cell occurs far away from the source (stem cell compartment).
What are the two main ways of minimizing mutations in stem cells?
1 - Apoptosis: detection of DAN damage favours apoptosis. Repair machinery often skipped/glossed over.
2 - Dreg Pumps: plasma membrane protein that pump out compounds (out of cell) which can lead to multi-drug resistance –> drug resistant cancer.
What is the immortal strand theory?
In stem cell replication, there are two strands of DNA, one conserved, the other nonconserved. The conserved strand is passed on to 1 out of the 4 daughter cells while the three others are just “transit-amplifying cells”.
How do DNA replication errors threaten stem cell genomes? When does this happen? How? What are the two mutagenic agent types that can attack?
Errors during DNA replication (S phase) can threaten genomic stability via spontaneous base changes (occur naturally).
The DNA can also be attacked by mutagenic agents:
1 - Endogenous - products of metabolism
2 - Exogenous - chemical and physical mutagens
What can correct these genetic errors? What is the molecule?
What are mismatch repair enzymes? How do they help?
DNA polymerases have the ability to proofread and correct the majority of genetic errors that occur naturally, but they aren’t perfect. They get old and are replaced and can occasionally miss something.
Loss of polymerase function can lead to increased genomic instability and eventual tumorigenesis.
Mismatch repair enzymes monitor freshly made strands for replication errors missed by DNA polymerases. They are critical for repeat regions of DNA and useful in preventing micro-satellite mutations.
Mismatch repair enzymes can become defective in certain cancers.
How can DNA become damaged during replication?
Single stranded DNA often breaks at the replication forks which is the equivalent of a double stranded break.
Usually, they are fixed, but occasionally errors can occur.
What are the 5 kinds of Endogenous mutations?
1 - Depurination (A/G): loss of purine bases
[H or OH ions]
2 - Depyrimidination (C/T): loss of pyrimidine bases
[less common]
3 - Spontaneous events
4 - Deanimation: loss of an amine group
[C to T point mutations]
5 - Oxidation: ROS generated by mitochondria
[causes transversions, DS breaks]
What are the 3 kinds of Exogenous mutations?
1 - Ultraviolet (UV) radiation: cross-links between pyrimidine bases.
[Very Stable]
2- Alkylating Agents: attach alkyl groups covalently to DNA bases.
[depurination, depyrimidation]
3 - DNA Adducts: acetylaldehyde, aflatoxin, heterocyclic amines.
What are some of the DNA protection mechanisms to Physical and Chemical carcinogens?
Give some examples. Describe de-alkylating repair enzymes, base excision repair, nucleotide excision repair, error prone repair.
Protection against physical carcinogens:
- shielding
- melanin protecting keratinocytes
Protection against chemical carcinogens:
- protective enzymes
- antioxidants
–> de-alkylating repair: removes alkyl group from DNA, causes genome to tighten and prevents transcription
–> base excision repair: recognizes chemically alterd bases and removes them, gap is filled and sealed.
–> nucleotide excision repair: detects helix distortion and chemically altered bases. Trancription coupled repair and global genomic repair follow to remove large sections & fix them.
–> error prone repair: “last ditch attempt” polymerase is stalled and needs to make the best guess to repair and error in the template strand.
Defects in DNA Protection mechanisms: describe the health issues related to Xeroderma pigmentosum (XPP).
Defects in nucleotide excision repair. Individuals especially sensitive to sunlight as UV rays can cause 100,000 thymine dimers in just one hour.
XPA, XPG, & XPV
Not one patient lives past age 20. At ten years of age at least 50% have been diagnosed with skin cancer. Average lifespan is 15-17 years old.
Defects in DNA Protection mechanisms: describe the healthy issues related to Hereditary non-polypopsis colon cancer (HNPCC).
represents 2-3% of all colon cancers. Accelerated progression of tumorigenesis.
DNA mismatch repair defects and microsatellite instability accumulates.
BRCA1 and BRCA2: talk
BRCA1 is associates to sites of DNA damage (DS breaks). It is associated with DNA repair proteins.
BRCA2 is also associated with DNA repair proteins, and is considered to be a caretaker gene since it is involved with homologous recombination.
What is the difference between homology directed repair (HDR) and nonhomologus end joining?
In homology directed repair, two unwound sister chromatids from each portion of the double helix overlap with the unwound DNA of a sister chromatid. Then strands then extend, disengage and pair once more.
In non homologous end joining, the two strands of each part of the split helix are joined by limited base pairing and fill in the missing sections by ligation. [Absence of sister chromatid]
Is the exact mechanism of BRCA1 and BRCA2 and DNA repair known?
Exact function is still unknown, but it is involved in many many many processes.
Discuss karyotype changes in cancer cells. What are translocations, intrachromosomal and interchromosomal swaps, and chromosomal rearrangements?
What is CIN Vs. MIN, which is more common?
Cancer cells have abnormalities in both chromosome structure as well as number. Dysfunction of the mitotic spindle can cause changes of chromosome number.
Translocations are when one gene swaps for another. Intrachromosomal translocation is when two genes on the same chromosome swap places, while interchromosomal translocation is when two different chromosmes swap genes.
Chromosomal rearrangements are localized firestorms of alterations and abberations which is a cluster-f#@$%. Basically, only one in 1,000 cells becomes “the one” cell that can survive while all others die off.
Chromosomal instability (CIN) means allelic losses and is far less common than chromosomal microsatellite instability (MIN) which is far, far, far more common.
