(i) Process of DNA Replication
• DNA replication begins at a unique and fixed point called origin of replication or ‘ori’.Initiation
▪ The complementary strands of DNA double helix are separated by two enzymes, DNA gyrase and DNA helicase. This is called unwinding of double-stranded DNA.
▪ The separated strands tend to rewind, therefore these are stabilised by proteins called single strand binding proteins (ssBPs), which bind to the separated strands.
▪ Unwinding of double-stranded DNA forms a Y-shaped configuration in the DNA duplex, which is called replication fork. Elongation
▪ An enzyme called primase initiates replication of the strand oriented in the 3′ (towards origin)→5′ (towards fork) direction. This generates 10–60 nucleotides long primer RNA (replicated in 5′→3′ direction).
▪ The free 3′–OH of this RNA primer provides the initiation point for DNA polymerase for sequential addition of deoxyribonucleotides.
▪ DNA polymerase progressively adds deoxyribonucleotides to the free 3′-end of the growing polynucleotide chain so that replication of the 3′→5′ strand of the DNA molecule is continuous (growth of the new strand in 5′→3′ direction).
▪ The replication of 3′→5′ strand is continuous and it is called leading strand, while the replication of second strand (5′→3′ strand) of the DNA molecules is discontinuous and it is known as the lagging strand.
▪ The replication of lagging strand generates small polynucleotide fragments called ‘Okazaki fragments’ (after R. Okazaki, who first identified them).
▪ These Okazaki fragments are then joined together by enzyme called DNA ligase.
(ii) A translational unit in mRNA from 5' → 3' comprises of a start codon, region coding for a polypeptide, a stop codon and untranslated regions (UTRs). UTRs are present at both 5'-end and 3'-end of mRNA.
