DNA Replication

Question 1

Describe the process of semi-conservative DNA replication.

Answer 1

• DNA helicase causes the two strands of DNA to separate.
• DNA helicase completes the separation of the strand. Meanwhile, free nucleotides that have been activated are attracted to their complementary bases on the two DNA strands.
• Once the activated nucleotides are lined up, DNA polymerase begins to catalyse the formation of phosphodiester bonds between adjacent free activated DNA nucleotides.
• Finally, all the nucleotides are joined to form a complete polynucleotide chain using DNA polymerase.
• In this way, two identical molecules of DNA are formed.
• Each new molecule of DNA is composed of one original strand and one newly formed molecule. It is therefore semi-conservative.

Question 2

Which direction does DNA polymerase always move in?

Answer 2

• Along the template strand always in the same direction (3’ to 5’). As DNA only unwinds and unzips in one direction, DNA polymerase has to replicate each of the template strands in opposite directions.

Question 3

What is the leading strand?

Answer 3

• The strand that is unzipped from the 3’ end can be continuously replicated as the strands unzip. This strand is called the leading strand and is said to undergo continuous replication.

Question 4

What is the lagging strand?

Answer 4

• The other strand is unzipped from the 5’ end, so DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.
• This results in DNA being produced in sections (Okazaki fragments), which then have to be joined.
• This strand is called the lagging strand and is said to undergo discontinuous replication.