Chapter 16 - Molecular Basis of Inheritance

Question 1

Proved Theory of DNA Replication

Answer 1

Semiconservative Model: Proposed that when a double helix replicates, each of the daughter molecules will have an original strand plus a new strand

Question 2

Alternate Theories of DNA Replication proven to be false

Answer 2

Conservative Model: after 1 round of replication, parental strands are joined together and two new strands come together, therefore 1 daughter cell has original parental DNA and 1 daughter cell has new DNA

Dispersive model: parental and new strands break in different parts and enzymes fill in the gaps with the alternating DNA strand so after 1 round of replication, each daughter cell has mixed and matched pairs of new and parental DNA strands

Question 3

Origin of Replication

Answer 3

a short stretch of DNA that has a specific sequence of nucleotides

- Proteins recognize this sequence, bind to it and open up the circular DNA sequence to separate the two strands
- DNA replication then proceeds in both directions until all is copied, in eukaryotes there are a few hundred replication bubbles speeding up the process

Question 4

Replication Fork

Answer 4

a Y shaped region at the end of each replication bubble where the parental strands are being unwound by several enzymes
- This allows single strand binding proteins to bind to the unpaired strands to prevent them from re-paringT

Question 5

Topoisomerase:

Answer 5

an enzyme that helps relieve strain caused by the untwisting of the double helix at the replication form by breaking, swiveling and rejoining DNA strands.

Question 6

How a New DNA Strand is Synthesized

Answer 6

• a DNA primer is synthesized by primase and binds to the unwound strands to start a complementary chain
• DNA polymerase catalyzes the synthesis of the rest of the DNA copy by adding nucleotides
• strands are formed in antiparallel directions as DNA is only added to the 3’ of a primer
• They begin in the middle at the replication origin with forks on either end, with 2 DNA polymerase working concurrently, 1 starting in the middle and at the 3’ end of the template strand moving both from 5’ to 3’ end of the new strand.

Question 7

Semi Discontinuous Model of DNA synthesis: Leading Strand

Answer 7

the new complementary DNA strand being synthesized along the template strand towards the replication fork(the strand elongating in the direction as to where the template strands were opened up)
- Only 1 primer is required to synthesize the whole leading strand
- Synthesized continuously compared to lagging
- Requires a primer, DNA polymerase III, helicase enzyme and an SSB protein

Question 8

Semi discontinuous model of DNA synthesis: Lagging Strand

Answer 8

a discontinuously synthesized DNA strand that elongates by means of Okazaki Fragments in a 5’-3’ direction away from the replication fork.
- DNA polymerase 3 works along the template strand away from origin of replication and towards replication fork until it hits a primer where it falls of, before binding further back and repeating process(move in direction of arrows above but starting at middle arrow before working backwards)
- Each okazaki fragment requires separate primers
- DNA ligase joins adjacent Okazaki fragments together by

- Requires primase and RNA primers, DNA polymerase III and I, DNA ligase

Question 9

Proteins used within DNA replication

Answer 9

• Helicase: unwinds parental double helix at replication forks
  
  • Singe-strand binding protein: binds and stabilizes single stranded DNA
  • Topoisomerase: relieves overwinding strain ahead of replication forks by breaking, swiveling and rejoining DNA
  • Primase: synthesizes an RNA primer at the 5’ end of each strand(start)
  • DNA Polymerase 3: synthesizes new DNA strand off of template strand in 5’-3’ direction
  • DNA ligase: joins Okazaki fragments and
    
    • DNA Polymerase 1: removes primers in lagging strand and replaces with nucleotide sequences

Question 10

Trombone Model to describe DNA replication Complex

Answer 10

two DNA polymerase molecules, one on each template strand, ‘reel in’ the parental DNA and extrude newly made DNA molecules with the lagging strand being looped back through the complex

Question 11

Proofreading and repairing DNA

Answer 11

• DNA polymerase proofread each nucleotide against its template as soon as it is covalently bonded
  
  • If it finds an incorrect one, nucleases removes the nucleotide and DNA polymerase resumes synthesis

Question 12

Mismatch repair

Answer 12

other specific enzymes which remove and replace incorrectly paired nucleotides
- if not repaired, it leads to a mutation

Question 13

Nucleotide Excision Repair

Answer 13

A segment of the strand containing the damage is cut out by a DNA nuclease, and the resulting gap is filled in by DNA polymerase, ligase and nucleotides using the undamaged strand as a template

Question 14

Issue with Replicating ends of DNA Molecules

Answer 14

• In eukaryotic organisms with Linear DNA strands, usual replication machinery cannot complete the 5’ ends of daughter DNA strands because there is no 3’ end of a preexisting
  
  • As a result, repeated rounds of replication produce shorter and shorter DNA molecules with uneven ends

Question 15

Telomeres

Answer 15

the multiple repetitions of one short DNA sequence at the end of a chromosome molecule which protects the organisms genes from being eroded during successive rounds of DNA replication

Question 16

Telomere function

Answer 16

1. Specific proteins associated with telomeric DNA prevent the staggered ends of daughter molecules from activating the cells system for monitoring DNA damage(cell death)
   2. Acts as a buffer zone which provides protection against the organisms genes shortening and eroding over successive replications(over time as more replications correlates with older ages)
      Doesn’t prevent by merely postpone

Question 17

Process of Telemorase in Cells

Answer 17

• Telomerase contains its own RNA molecule which acts like a template, binding to a complementary sequence on the normal length DNA template strand, artificially extending that strand
  
  • Creates a primer upstream of the shortened, newly made DNA strand and gives DNA polymerase a template to join complementary nucleotide pairs to lengthen the shortened strand
    
    • Not active in most human somatic cells

Question 18

Telomere functioning in cancer cells

Answer 18

• Normal shortening of telomeres may protect against cancer as it limits the number of divisions the cell can undergo
  • Cells from large tumors have unusually short telomeres, which is normal since they replicate uncontrollable, over time leading to self-destruction
    • Telomerase activity is abnormally high in cancer cells, suggesting it stabilizes telomere length and allows cancer cells to persist