DNA repl

In conservative DNA replication, each new DNA molecule contains one newly synthesized daughter strand and one paternal template strand of DNA.

• False

• True

Answer explanation

Correct statement: In semi-conservative DNA replication, each new DNA molecule contains one newly synthesized daughter strand and one paternal template strand of DNA.  

Explanation:  In the conservative model, the 2 DNA molecules resulted where one DNA molecule contains of both parental strands and the other DNA molecule contains of both newly synthesised daughter strands.

In DNA replication, only 1 strand is used as the template.

• False

• True

Answer explanation

Both strands are used as templates in DNA replication process.

DNA replication starts at special sites called origins of replication (Ori).

• False

• True

Answer explanation

Note:

Do not confuse Ori with OriT, which is the origin of transfer in bacterial conjugation which you will learn later under the topic “Genetics of bacteria”.

DNA primase binds to DNA by complementary base pairing.

• False

• True

Answer explanation

Correct ans: Interaction between active site of the DNA Primase (enzyme) with DNA (substrate) is by complementary 3D conformation.

RNA primers are required as DNA Polymerase requires a free 5’ OH group from pre-existing chain, to synthesis new DNA.

• False

• True

Answer explanation

Correct ans: DNA polymerase requires a free 3’ OH group of a pre-existing polynucleotide strand which can be either RNA primers (at initial stage of DNA replication) or Okazaki fragment (at later stage of DNA replication after RNA primers are removed). Both DNA Polymerase III & I are unable to synthesise DNA from scratch (de novo), both enzymes will require the 3’OH ends of a pre-existing polynucleotide strand.

DNA polymerase and DNA ligase help to catalyse the formation of phosphodiester bonds.

• False

• True

Both leading and lagging strands are synthesised in the 5’ to 3’ direction.

• False

• True

Answer explanation

Explanation: Leading strand and lagging strand are both daughter strands. DNA polymerase reads DNA template strand from 3’ to 5’ direction and makes the new DNA strand in 5’ to 3’ direction. Thus, new DNA molecule will still have an anti-parallel structure.  Any new polynucleotide chain (DNA or RNA) is always synthesised in 5’ to 3’ direction as new nucleotide is added to the 3’OH end of the chain.

There is a need for multiple primers for the synthesis of lagging strand as the DNA molecule opens up gradually to expose new 3’ ends on the template (for lagging strand) in the replication fork.

• False

• True

Answer explanation

There is a need for multiple primers for the synthesis of lagging strand as the DNA molecule opens up gradually to expose new 3’ ends on the template (for lagging strand) in the replication fork.

DNA ligase helps to remove RNA primers from the daughter strand.

• False

• True

Answer explanation

Correct ans: RNA primers are hydrolysed by DNA Polymerase I and the gap is filled with free/activated DNA nucleotides. DNA ligase join Okazaki fragments together with phosphodiester bonds.

The failure of DNA polymerase I to replace the last primer removed with deoxyribonucleotides on the lagging strand results in a 3’ overhang, in eukaryotes.

• False

• True

Answer explanation

Explanation:

DNA polymerase I can only add deoxyribonucleotides to the 3’ OH end of a pre-existing polynucleotide. So the 3’ end of the template DNA is not copied to the 5’end of the daughter DNA strand (lagging strand) as there is no free OH group at the 5’end, resulting in staggered (uneven) ends, also known as 3’ overhang.