Errors in Replication and DNA Repair (Lecture 5)

Telomerase

-A special enzyme that catalyzes the lengthening of telomeres in eukaryotic germ cells -Not active in most cells -Proteins with RNA template of the six repeating nucleotides of the telomere (Humans: TTAGGG) -There to keep re-building the Telomeres (Protein has RNA in it: complementary RNA contemplate to continue the cell, and keep it going) -Telomerase activity: Increasing DNA molecule so it increases the clock until you get to something important

Proofreading and repair

-By the very end of DNA replication, the error rate is only around 1 in 10 billion nucleotides -DNA pol III has an error rate of 1 in 100,000 nucleotides (that is 100,000 times more common!!) -DNA polymerases proofread each new nucleotide against the template strand. When an incorrect nucleotide is found, the polymerase removes the nucleotide and resumes synthesis (Basically it deletes the incorrect nucleotide)

Telomeres

-Eukaryotes have linear genomes, prokaryotes have circular -Due to lagging strand synthesis, the ends of DNA molecules are not fully replicated -After many generations, the DNA strands get shorter and shorter and shorter -Each replication the DNA molecule gets shorter -Staggered ends = double strand breaks -Special proteins associate with telomeres to prevent cell cycle arrest or cell death -It is thought that shortening telomeres is related to the overall aging process -Shortening process connected to overall aging, after so many replications your DNA gets too short and dies -Eukaryotes have special nucleotide sequences called telomeres at the ends of each DNA molecule -Telomeres do not contain genes (Humans: TTAGGG) -Telomeres are repeated 100-1000x -Telomeres act as a buffer zone that protects the organisms genes -Telomeres "buffer" to protect the organism, takes longer because of this to effect the organism (important DNA that codes for things don't get hurt as fast) -As one would expect, older cells have shorter DNA, so it is thought that telomeres are connected to the aging process -If DNA continues to shorten, eventually essential genes will be missing!

Xeroderma Pigmentosum

-Genetic disease where individuals have a defect in a nucleotide excision repair enzyme -These individuals are extremely sensitive to UV radiation (sunlight) because the DNA is not repaired properly resulting in skin cancer -UV radiation causes thymine dimers, causing DNA to buckle, which interferes with DNA replication

Cancerous Cells

-Have incredibly short telomeres! -Some tumors have telomerase activity in somatic cells allowing for cancerous cells to divide indefinitely

Double Strand Breaks

-If not repaired quickly, results in cell death -Could be two single strand breaks within 10 nucleotides (Results in DNA with "staggered" ends) -Apoptosis: If cells detect enough damage to die

DNA Damage

-Incorrect nucleotides can arise outside of replication as well (DNA maintenance is an ongoing project) -DNA is constantly subjected to carcinogens and mutagens (Cigarette smoke, X-rays, etc.) -DNA bases can also undergo spontaneous changes under normal cellular conditions -These types of changes are typically repaired before they are passed on to subsequent generations

Preventing Cancer

-Normal shortening of the DNA molecule may protect the cell from cancer -Overtime, DNA does get damaged and mutations accumulate -With each division, telomeres shorten -Eventually, the cell gets "old enough" that it finally dies

Nucleotide Excision Repair

-Nuclease - cuts out a section of DNA that contains the incorrect bases -DNA polymerase fills in the gap -DNA ligase reconnects the DNA strands -Can't just cut out one nucleotide at a time -Thymine dimer- Two T's connected to each other, can't be properly read

Evolution Connection

-Occasionally mistakes do slip through! -Once that mistake is replicated into a new DNA molecule, then it is permanent -This is how evolution occurs! Without a slight error rate, there would be no changes to DNA and there would be no changes to anything -Would be no variation if the DNA was replicated perfectly

Mismatch Repair

-Sometimes, DNA polymerase does not catch every mistake -Other enzymes will then remove and replace an incorrectly paired nucleotides -In a type of colon cancer, one of these mismatch repair enzymes are dysfunctional causing mutations to increase at a higher rate -Around 100 different enzymes in E. coli -130 identified in humans so far -Sometimes changes parental strand by accident and that's permanent change