Origins of Mutations and DNA Replication/Repair Flashcards
3 common DNA lesions
- Depurination – spontaneous hydrolyzation of bond between base and deoxyribose resulting in loss of purine
- Deamination – loss of amine group due most commonly to spontaneous conversion of cytosine to uracil
- Pyrimidine dimers – dimerization of adjacent thymine bases
Difference between somatic and germline mutations
Somatic – affects only cell where mutation occurred and progeny, can be problematic if resulting in unregulated proliferation/survival of cell, if mutation occurs early in embryogenesis (can cause mosaicism)
Germline – pass on mutation to offspring of affected individual
Gene mutation
Variation in nucleotide sequence (freq. = 10^-10 per bp per cell division/10^-5 or -6 per locus) – often occurs due to nucleotide exchange, insertion, or deletion
Chromosome mutation
Rearrangements, deletions, or duplications of chromosomal regions (freq. = 6 x 10^-4 per cell division) – often follows DNA double strand breaks or faulty recombination (MORE LIKELY to cause disease than gene mutation)
Genome mutation
Surplus or loss of chromosomes (freq. = 10^-2 per cell division) – often occurs due to mis-segregation of chromosomes during meiosis or mitosis (MORE LIKELY to cause disease than gene mutation)
Point mutation (4 types)
Mutations resulting in exchange of one nucleotide for another, arising from replication errors, mutagenic chemicals, radiation, or repair of damaged DNA
- Silent
- Missense
- Nonsense
- RNA processing mutant
Silent mutation
Does not change amino acid sequence of encoded protein
Missense mutation
Changes amino acid sequence of encoded protein
Nonsense mutation
Generates stop codon (UGA, UAA, UAG)
Incorrect recombination (mechanisms leading to small insertions or deletions)
If homologous recombination between DNA molecules is done incorrectly due to misalignment, unequal crossover will occur, inserting or deleting genetic material (especially common at loci with repetitive DNA)
Strand slippage during replication (mechanisms leading to small insertions or deletions)
Newly synthesized DNA strand may loop out in a single-stranded form, causing the strand to be replicated a second time, which creates DNA with extra repeated segments
Intercalating agents (mechanisms leading to small insertions or deletions)
Chemical mutagens intercalating into DNA, distorting the structure of the molecule and resulting in insertion or deletion of one or more nucleotides during DNA replication
Frameshift mutation
Insertion or deletion mutation that is not a multiple of three, altering the codon structure downstream of the mutation to create different protein products (responsible for 25% of human genetic diseases)
Describe the process of meiosis.
- -Prophase I of meiotic division sees chromosomes condense and become visible
- -Homologous chromosomes (maternal and paternal copies) align in synapsis process, held tightly together by synaptonemal complex
- -Homologous chromosomes exchange homologous fragments by crossing over
- -Synaptonemal complex disintegrates and chromosomes are held together by chiasmata (sites of crossovers)
- -In metaphase, chrosomes are positioned at equator
- -In anaphase, homologous chromosomes are separated in disjunction
- -Cell divides to give rise to two haploid cells, each with 23 chromosomes
- -Sister chromatids are divided equally between two daughter cells in second meiotic division
Crossing over
Exchange of homologous fragments on homologous chromosomes that leads to genetic recombination (VERY IMPORTANT!)
Disjunction
Proper separation of homologous chromosomes in anaphase
Nondisjunction
Incomplete separation of chromosomes in anaphase, leading to germ cells with surplus or missing chromosomes (increases with maternal age)
Which autosomes does trisomy occur in?
13 (Patau syndrome), 18 (Edward syndrome), 21 (Down syndrome) – this is a GENOME MUTATION
Mosaicism
Cells with different genotypes occurring in the same person that results from somatic mutations in embryogenesis (occurs in 2-4% of trisomy 21)
Proofreading activity of DNA polymerases
DNA polymerase δ has 3’ –> 5’ exonuclease activity which edits 3’ end of growing DNA strand; if polymerase detects wrong base pairing, 3’ –> 5’ exonuclease activity hydrolyzes new phosphodiester bond and polymerase tries again
Strand-directed mismatch repair
DNA repair enzymes recognize nicks and gaps in newly synthesized strand and distortions in the DNA double helix that result from incorrect base pairing; these enzymes remove the incorrect base pair and surrounding area which is then filled in by DNA polymerase
HHNPCC (Lynch syndrome)
Heritable cancer syndrome that causes 80% chance of developing colorectal cancer (and others); caused by defects in the mismatch repair system
Base excision repair
DNA glycosylases recognize specific types of altered base and catalyze their removal hydrolytically (ex. removal by uracil DNA glycosylase of cytosine that has been deaminated to uracil)
Steps of base excision repair
- Nucleotide is spontaneously altered to the wrong one
- Uracil DNA glycosylase deletes the wrong nucleotide
- AP endonuclease cuts the backbone
- DNA phosphodiesterase removes deoxyribose phosphate group
- Gap is filled by DNA polymerase and backbone repaired by DNA ligase
Nucleotide excision repair
Repairs damage caused by agents that result in large changes in structure of DNA (ex. pyrimidine dimers)
How is a pyrimidine dimer repaired?
- Enzyme complex scans DNA molecule and recognizes distortions in double helix
- Helicase separates strands and phosphodiester bond is cut on either side of affected area
- Oligonucleotide containing lesion (section of strand including dimer) is peeled off
- Gap in DNA helix produced by nucleotide excision is filled/fixed with DNA polymerase and ligase
Xeroderma pigmentosa
Disorder characterized by extreme sensitivity of the skin to sunlight, pigmentation changes, and skin cancers; caused by mutations abolishing/impairing proteins required for nucleotide excision repair
Why are double-stranded DNA breaks troublesome?
Makes repair difficult due to no intact strand remaining
What mechanisms can fix double-stranded DNA breaks?
- Nonhomologous end-joining (results in loss of nucleotide at break point, permanently changing nucleotide sequence)
- Homologous end-joining (recombination processes used to repair broken DNA so that info needed to repair lesion without loss of sequence can be used)
Cytaburine
Analogue of cytidine that contains arabinose instead of ribose; converts to cytarabine triphosphate inside cells which then competes with deoxyribonucleotides for binding to DNA polymerases
Cyclophosphamide
Converted to active drug phosphoramide mustard in liver, which is a bi-functional alkylating agent that forms inter- and intra-strand DNA crosslinks
Doxorubicin
Forms tripartite copmlex with DNA and topoisomerase II and causes double-strand breaks to accumulate
