Question

1. What does it mean that DNA is an anti-parallel dimer?

2. Describe the mechanism of how a nucleotide is incorporated at the 3' end of newly synthesized DNA strand.

3. What are the roles of primase, DNA pol I and DNA pol III in the replication fork?

Answer 1

• Ends of polynucleotide chain of DNA are named on carbon number of deoxyribose sugar.
• 3' end represents 3rd carbon and 5' end represents 5th carbon of deoxyribose sugar.

Question 1.
• DNA is formed by two polynucleotide chains.
• DNA is called antiparallel dimer because of antiparallel arrangement of two polynucleotide chains or antiparallel arrangement of sugar molecules of DNA.
• If one strand of DNA is in 5' to 3' direction then other one will be 3' to 5' direction.

Question 2.
• DNA is polymer of nucleotides.
• Nucleotides are formed by deoxyribose sugar, phosphate group and nitrogenous base.
• Sugar molecules forms backbone of DNA strand.
• New nucleotides on 3' end of polynucleotide joins by phosphodiester bond. This bond is formed between hydroxyl group (-OH) of 3rd carbon of sugar and phosphate group of 5th carbon of sugar.
• Newly added nucleotide provides free hydroxyl group on 3' end for joining of upcoming nucleotides.

Question 3.
Primase :- This enzyme forms a short nucleotide sequence called RNA primer. Primer is used in initiation of DNA replication process. It recognises starting point of replication and provides attachment site for DNA polymerase enzyme.

DNA polymerase 1 :- This enzyme removes RNA primers from okazaki fragments on lagging strands and fill the space between fragments by adding nucleotides. This is also involved in DNA repair mechanisms.

DNA polymerase 3 :- This is main enzyme for adding nucleotides on template DNA strands and formation of new DNA strands. This is also involved in proof reading process.