DNA Replication Cr 5

Question 1

DNA Replication

Answer 1

In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule.

Question 2

First Step of DNA replication

Answer 2

Initiation

Question 3

Initiation

Answer 3

In the initiation phase of DNA replication, helicase unwinds the double-stranded DNA at the origin of replication by breaking the hydrogen bonds between complementary base pairs, creating two single-stranded DNA templates. Single-stranded binding proteins (SSBs) bind to the exposed single strands, preventing them from re-annealing or forming secondary structures. This stabilized single-stranded DNA serves as a template for the synthesis of new complementary strands.

Question 4

Helicase

Answer 4

The function of the helicase is to unpack an organism’s genetic material. Helicases are motor proteins that move directly along two hydrolysed nucleic acid strands, separating them.

Question 5

Single-Stranded Binding Proteins

Answer 5

SSB’s are proteins that bind to the separated single strands of DNA at the replication fork, preventing them from re-annealing (coming back together) and maintaining them as separate templates for new DNA synthesis.

Question 6

Second Step

Answer 6

Priming

Question 7

Priming

Answer 7

In the priming phase of DNA replication, DNA primase synthesizes short RNA primers complementary to the single-stranded DNA template. These primers provide the free 3’-hydroxyl (OH) group required for DNA polymerase to initiate DNA synthesis. Primase lays down primers on both the leading and lagging strands, ensuring that DNA synthesis can proceed in the 5’ to 3’ direction.

Question 8

DNA Primase

Answer 8

DNA Primases are enzymes whose continual activity is required at the DNA replication fork. They catalyse the synthesis of short RNA molecules used as primers for DNA polymerase. It provides a free 3’ hydroxyl (-OH) group for DNA polymerase to start adding nucleotides.
Simplified: Makes a small piece of RNA called a ‘primer’ which marks the starting point for the construction of the new strand of DNA.

Question 9

Third Step

Answer 9

Elongation

Question 10

Elongation

Answer 10

During elongation in DNA replication, DNA polymerase synthesizes new DNA strands in the 5’ to 3’ direction. On the leading strand, replication is continuous, as it proceeds in the same direction as the replication fork. On the lagging strand, replication is discontinuous, producing short fragments called Okazaki fragments due to its opposite orientation relative to the fork. DNA primase lays down RNA primers for each Okazaki fragment, and DNA polymerase extends them.

Question 11

DNA polymerase

Answer 11

Polymerase adds nucleotides to the 3’ end of the RNA primer, synthesising the DNA in the 5’ to 3’ direction. In essence, polymerase adds separate nucleotides onto the strand of DNA (starting at the primer) from the 5’ to 3’ direction.

Question 12

Okazaki fragment

Answer 12

An Okazaki fragment is a short segment of DNA synthesized on the lagging strand during DNA replication.

Since DNA polymerase can only synthesize DNA in the 5’ to 3’ direction, the lagging strand, which runs in the opposite direction of the replication fork’s movement, is replicated discontinuously.

Each fragment begins with an RNA primer laid down by primase, which DNA polymerase extends to form a short piece of DNA. These fragments are later joined together by DNA ligase to form a continuous strand.

Question 13

Leading Strand

Answer 13

The leading strand is the continuously synthesized strand of DNA that is replicated in the same direction as the replication fork’s movement.

Question 14

Lagging Strand

Answer 14

The lagging strand is the discontinuously synthesized strand of DNA that is replicated in the opposite direction of the replication fork’s movement, forming Okazaki fragments.

Question 15

Step 4

Answer 15

No name yet (the one with exonuclease)

Question 16

Exonuclease

Answer 16

An enzyme that breaks down nucleotides one at a time from the end of a polynucleotide chain.
Simplified: Removes RNA primers, allowing polymerase to fill the gaps with nucleotides.

Question 17

Step 5

Answer 17

No name yet (final step that involves ligase)

Question 18

DNA Ligase

Answer 18

DNA ligase is an essential enzyme that facilitates the joining of DNA strands by catalysing the formation of a phosphodiester bond between the 3’ - hydroxyl end of one DNA strand and the 5’ phosphate end of another.

Simplified: DNA ligase seals up gaps between the 3’ end of one and the 5’ end of another strand of DNA with a phosphodiester bond. This is because after the exonuclease removes RNA and polymerase fills the large gap with nucleotides, there is still a small gap left over (which ligase must fill).