Lec 46: Nucleic Acid Structure/DNA Replication and Repair Flashcards Preview

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Flashcards in Lec 46: Nucleic Acid Structure/DNA Replication and Repair Deck (17):

Describe the structure of DNA

Double helical structure. Two polynucleotide chains (strands) which run in opposite directions around the common axis.


Describe the monomeric units of DNA

Deoxynucleoside monophosphates (dNMPs)

1) five carbon deoxyribose sugar (-OH at 3' carbon) 2) phosphate group (attached to sugar at 5')

3) Nitrogenous base (attached to sugar at 1')


Describe the key structural features of B-form DNA


  1. B-DNA is the most common form in vivo
  2. Right handed double helix



Explain the concept of base pairing

Specific hydrogen pairing between nitrogenous bases.

Thymine binds to Adenine with two hydrogen bonds

Cytosine binds to guanine with three hydrogen bonds


What is antiparallelism in DNA?

Each DNA strand is polar. Each strand has 5' and 3' ends. When two strands bind together, they run parallel but opposite of each other. 


Describe the relevance of polarity in DNA.

DNA polarity is relevant in DNA replication and transcription. DNA is only synthesized in the 5’ to 3’ direction.


Describe the structural differences between DNA and RNA.

  1. DNA has deoxyribose and RNA has ribose
  2. DNA has Thymine and RNA has Uracil

  3. other features are the same.


Explain the concepts of ‘semiconservative’ replication.

In DNA replication, the helix is split apart and both old strands are used as templets for strands. The copies will contain one original strand and one new strand. 




Explain the concepts of ‘bidirectional’ replication.

At the origin of DNA replication, there are two replication forks. Each replication fork consists of one leading strand and one lagging strand. 

The two replication strands move away from each other. 


Describe the various steps involved in eukaryotic DNA replication.


  1. Helicase splits the helix. Forming two replication forks
  2. Single-stranded binding proteins bind to exposed bases to prevent reannealing. Topoisomerase binds and prevents helix strain.
  3. DNA Pol α/ RNA Primase binds to 5' strand and lays down RNA primers
  4. DNA Pol ε elongates in 5' to 3' direction on leading strand
  5. DNA Pol δ elongates on lagging strand. Also replaces RNA primer with DNA
  6. DNA Ligas seals Okazaji fragments



List the various enzymatic activities needed for eukaryotic DNA replication in vivo

DNA polymerase- catalyses the nucleophilic attack by the 3'-hydroxyl of the previous dNTP by the first/alpha phosphate of the next dNTP

DNA ligase - joins DNA strands by catalyzing the formation of phosphodiester bonds.


Explain the function of topoisomerase in replication and why it is important.

enzymes that regulate the overwinding or underwinding of DNA.


Explain what a telomerase is and why it is important for DNA replication.

Adds DNA sequence repeats ("TTAGGG" in all vertebrates) to the 3' end of DNA strands in the telomere regions, which are found at the ends of eukaryoticchromosomes.

This region of repeated nucleotide called telomeres contains noncoding DNA and hinders the loss of important DNA from chromosome ends. As a result, every time thechromosome is copied, only 100–200 nucleotides are lost, which causes no damage to the organism's DNA.



Explain the key features of the DNA polymerase reaction.


  • Adds nucleotides only to the 3’-end of the primer. DNA synthesis is always in the 5’ to 3’ direction
  • Can only extend pre-existing DNA.
  • Cannot synthesize DNA from just dNTPs
  • Has 3’ to 5’ exonuclease proof reading activity in addition to the polymerase activity


Explain why the synthesis of one strand is continuous and the other is discontinuous during replication and how such syntheses are accomplished and coordinated.

DNA Pol only elongates in 5' to 3' direction.

Continous on leading strand.

Discontinuous on lagging strand because DNA pol must wait for helicse to unwind more DNA.

Accomplished by Primase (adds RNA primers)

DNA Pol ε/δ (elongation) (replaces RNA primers with DNA)

DNA ligase ( joins osaki segments on lagging strand)


Explain the various mechanisms of DNA repair.

  1. Nucleotide excision repair – acts on lesions causing large distortions, pyrimidine diners.
  2. Base excision repair – acts on small lesions involving damage to a single base
  3. Mismatch repair pathway- corrects base mismatch based ny replication errors that escaped DNA Pol proof reading. Older DNA strands are methylated. So error non-methylated strand is cut and replaced. 



What are the diseases that result from the impairment of DNA repair processes?

Impariment of mismatch repair pathway causes Lynch syndrome/ Hereditary Nonpolyposis Colorectal Cancer (higher risk of cancer, especailly colon)

Impariment of nucleotide excision repair pathway causes Xeroderma pigmentosum (hypersensitive to sunglight/ cigarette smoke)


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