Genetics: DNA replication

Question 1

Why is DNA replication important?

Answer 1

Each newly formed cell requires a complete set of genetic instructions (Carried via DNA) to function properly. Ergo they all receive exact copies of the DNA molecule.

Question 2

What is the copying of a DNA molecule (code) and what is it?

Answer 2

DNA replication. It is semi-conservative (Each new molecule has one original and one new strand)

Question 3

What are the major steps in DNA replication?

Answer 3

• Initiation
• Elongation
• Termination

Question 4

What occurs during initiation?

Answer 4

Enzyme helicase breaks complementary DNA strands’ base pairs’ hydrogen bonds. Molecule unzips and unwinds. (May occur in many places simultaneously)

Question 5

What are replication forks?

Answer 5

Sites where replication and separation occur.

Question 6

What are the key enzymes in DNA replication?

Answer 6

• Helicase: Cleaves and unwinds short DNA sections ahead of the replication fork
• Primase: Synthesizes an RNA primer to begin the elongation process
• DNA polymerase: Adds new nucleotides to the 3’ OH group of existing nucleotide strands / Dismantles RNA primer / Proofreads base pairing
• DNA ligase: Joins together Okazaki fragments in the lagging strand

Question 7

What is a primer?

Answer 7

A short section of RNA that serves as replication’s starting point. Formed with the help of primase.

Question 8

What happens in elongation?

Answer 8

Strand separation exposes DNA bases to free unbonded DNA nucleotides in the nucleus which join the exposed ones assisted by DNA polymerase. It only attaches nucleotides to the original strand’s 3’ end, meaning the new strand’s replication direction is 5’ to 3’, resulting in one strand continuously elongating (Leading strand). The other new strand forms in a series of Okazaki fragments opposite to the unzipping direction (Lagging strand). Each fragment needs a primer.

Question 9

What happens in termination?

Answer 9

After new strands form, they rewind into the double helix. New daughter strands are slightly shorter due to the first/last primer on the leading/lagging strand. End primers can’t be replaced by free nucleotides when removed. Several DNA replication rounds reduce the molecules size. (End of replication problem)

Question 10

How is the “End of replication problem” solved?

Answer 10

Putting DNA caps called telomeres on the ends of DNA. They code for no protein, therefore losing that information has very little consequences. They also protect fragile ends from damage.