The Structure of Nucleic Acids and DNA Replication

Question 1

Describe the structure of a nucleotide.

Answer 1

• Pentose sugar
• Inorganic phosphate on 5’ carbon of pentose sugar
• Nitrogeneous base on 3’ carbon of pentose sugar

Question 2

Distinguish between purines and pyrimidines.

Answer 2

• Purines are larger in size than pyrimidines as they have a two ring structure compared to the one ring structure in pyrimidines
• Adenine & Cytosine are purines, Thymine & Guanine are pyrimidines

Question 3

Describe the formation of a nucleotide.

Answer 3

Nitrogeneous base is joined to 1’ carbon via glycosidic bond with the loss of water, forming nucleoside. Condensation reaction between nucleoside and phosphoric acid results in a nucleotide due to formation of phosphoester bond.

Question 4

Describe the formation of a polynucleotide.

Answer 4

Condensation reaction between phosphate group on 5’ carbon of one nucleotide to the hydroxyl group on the 3’ carbon of the other nucleotide –> formation of phosphodiester bond –> repeated many times to form polynucleotide

Question 5

Describe the structure of DNA.

Answer 5

• Double stranded
• Each strand is chain of deoxyribose sugars linked together by phosphodiester bonds wtih bases projecting at right angles inwards
• Adenine & Thymine –> 2 H bonds
• Cytosine & Guanine –> 3 H bonds
• Two strands are held together by hydrogen bonds between nitrogeneous bases via complementary base pairing

Question 6

State the steps of DNA replication.

Answer 6

1. Origin of Replication
2. Priming
3. Elongation
4. End Product

Question 7

Describe Origin of Replication step in DNA replication.

Answer 7

• DNA replication begins at origin of replication
• Helicase breaks H bonds between complementary bases, unwinds and separates DNA
• Each parental DNA strand acts as a template for synthesis of daughter strand.

Question 8

Describe the Priming step in DNA replication.

Answer 8

• Primase catalyses synthesis of RNA primer
• Important as DNA polymerase III cannot initiate DNA synthesis bc it can only add on new DNA nucleotides to pre-existing 3’ OH- end –> RNA primer provides 3’ OH end

Question 9

Describe the Elongation step in DNA replication.

Answer 9

• DNA polymerase III adds on free DNA nucleotides to exposed bases on parental DNA strand via comp. base pairing
• DNA polymerase catalyses formation of phosphodiester bond between two adjacent DNA nucleotides
• DNA polymerase works in 5’ to 3’ direction with respect to the growing chain
• DNA polymerase is capable of proofreading –> when DNA nucleotide is added, DNA pol will proofread against the template and can remove incorrectly paired DNA nucleotide and replace it with the correct pair
• Since DNA is antiparallel, one newly synthesised DNA strand elongates continuously in 5’ to 3’ direction and DNA pol. moves towards replication fork
• Lagging strand is synthesised discontinuously through synthesis of Okazaki fragments, aided by RNA primers
• After the first Okazaki fragment is synthesised, DNA pol. detaches from the template strand and reattaches to the newly exposed DNA template
• Overall direction is towards the replication fork
• RNA primers at each Okazaki fragment is removed and replaced with corresponding DNA nucleotides by DNA polymerase I
• DNA ligase seals up gaps between newly replaced DNA and existing fragments by catalysing phosphodiester bonds

Question 9

Describe End Product of Replication.

Answer 9

• Two daughter DNA molecules are synthesised at end of DNA replication, consisting of one parental DNA strand and one newly synthesised daughter DNA strand. (half is conserved)
• This is called semi conservative replication

Question 10

Explain end replication problem.

Answer 10

• Failure to replace RNA primer at 3’ end of parental template strand with corresponding DNA nucleotides as there is no 3’ end available for DNA polymerase to add on DNA nucleotides.
• Newly formed daughter DNA is shorter than parental strand
• DNA will shorten and critical genes may be lost after many rounds of DNA replication.