define karyotype
visualization of the number and appearance of the chromosomes within a cell
Define aneuplidy
- the presence of an abnormal number of chromosomes in a cell
- result of chromosomal non-disjunction (failure of homologous chromosome to separate normally during/after mitosis)
About chromosomal translocations
- abnormality caused by rearrangement of parts between non-homologous chromosomes
- can be direct switching of material or large scale deletions/insertions
- generally caused by error in homologous recombination/ double strand break repair
About DSB (double strand break)
- break in phosphodiester backbone on both strands of DNA at the same site
- usually arise during replication when fork reaches a single strand break, becomes propagated into a DSB in template + datughter strands
- can arise from UV/gamma radiation
- if not repaired, DSB typically leads to call death
DNA repair in D. radiodurans
- regenerates its DNA every 12-24 hr
- contains many copies of its cellular DNA so it has backup templates for repair
- high levels of Mn, important for repair
- after gamma irradiation, cell growth stops, DNA repair begins
About recombinational DNA repair
- directed at repair of DSB
- requires presence of another, undamaged homologous dsDNA
Steps of recombinational DNA repair (not final step)
- after DSB, broken ends processed by helicases+nulcleases
- 5’ ends degraded to create 3’ overhangs at break site
- coated by SSB proteins - 3’ ss extensions invade homologous chromosome, catalyzed by recombinases
- invading strand displaces one strand of intact homologous chromosome, base pairs with other
- structure called D-loop - 2nd invasion, similar to 1st
- DNA Pol. mediated extension of invading strands after they are paired
- invaded homologous chromosome used as template
recombinational DNA repair final step
A) synthesis dependent strand annealing (SDSA)
-lengthened invading strands displaced by helicases, anneal to each other, remaining gaps filled by DNA Pol., DNA ligases finish repair
B) double strand break repair (DSBR)
-strands ligated while still linked, forms 4-branch crossover junction called Holliday intermediate
-Holliday junction resolvases cleave the intermediates in 1 or 2 ways
Resolution of Holliday intermediates
- if Holliday intermediates cleaved using same method, then there is a non-crossover
- meaning genetic material is exchanged but chromosomes are not
- If X sites and Y sites cleaved, results in crossover
- genetic material outside of site of repair is from two separate chromosomes
