Module 2 Section 4

Question 1

define karyotype

Answer 1

visualization of the number and appearance of the chromosomes within a cell

Question 2

Define aneuplidy

Answer 2

• 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)

Question 3

About chromosomal translocations

Answer 3

• 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

Question 4

About DSB (double strand break)

Answer 4

• 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

Question 5

DNA repair in D. radiodurans

Answer 5

• 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

Question 6

About recombinational DNA repair

Answer 6

• directed at repair of DSB

- requires presence of another, undamaged homologous dsDNA

Question 7

Steps of recombinational DNA repair (not final step)

Answer 7

1. after DSB, broken ends processed by helicases+nulcleases
   - 5’ ends degraded to create 3’ overhangs at break site
   - coated by SSB proteins
2. 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
3. 2nd invasion, similar to 1st
4. DNA Pol. mediated extension of invading strands after they are paired
   - invaded homologous chromosome used as template

Question 8

recombinational DNA repair final step

Answer 8

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

Question 9

Resolution of Holliday intermediates

Answer 9

• 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