LEADING STRAND

1. RNA primase synthesises RNA primer by attaching NTPs (nucloeside triphosphates) to the single stranded DNA.

2. Begins at the origin of replication site (RNA primer).

3. DNA polymerase III synthesises the new strand by attaching dNTPs (deoxynucloeside triphosphates) to the 3 end of the RNA primer where the formation of the new strand is continued. This allows the replication process to occur in a 5 to 3 direction following the replication fork.

4. DNA polymerase I removes RNA primer.

5. The two attached extra phosphate groups of dNTPs pair up with the exposed DNA by complementary base pairing.

6. DNA polymerase III links together the sugar and innermost phosphate groups of adjacent nucleotides.

7. The two extra phosphate groups are broken off and then released.

8. This way, continuous DNA strand is built up on the leading strand.

PROCESS ON LEADING & LAGGING STRAND

LAGGING STRAND

1. RNA primase synthesises RNA primer by attaching NTPs (nucloeside triphosphates) to the single stranded DNA.

2. DNA polymerase III synthesises the new strand by attaching dNTPs (deoxynucloeside triphosphates) to the 3 end of the RNA primer in a direction away from the replication fork.

3. The replication occurs discontinuously whereby short lengths of new DNA called Okazaki fragments are formed from each primer.

4. The fragments grows away from the replication fork until it reaches the next fragment.

5. DNA polymerase I removes RNA primer by attaching dNTPs to replace the RNA with DNA.

6. DNA ligase seals up each break between the fragments by making sugar-phosphate bond to allow a continuous strand of new DNA to be formed.

CHEMICAL NAME

FUNCTION (S)

Nucloeside triphosphates (NTPs)

Building unit of RNA

A ribose nucleotide with 2 additional phosphates which are chopped off during RNA synthesis process.

Deoxynucloeside triphosphates (dNTPs)

Building unit of DNA

Helicase

Unwinds the DNA strands and exposes them as single strands (template strands).

Separates the DNA double helix by breaking the hydrogen bonds.

Single-stranded proteins (SSBs)

Stabilise the structure and protect the single stranded DNA structure by binding the regions on the DNA.

DNA gyrase

Relieves the tension put on the DNA molecule as it is being unwound.

RNA primase

Synthesises RNA primer by binding NTPs to the single stranded DNA using complimentary base pairing.

DNA polymerase III

Synthesises the new DNA strand in a 5 to 3 direction by firstly binding dNTPs to the 3 prime end of RNA primer.

DNA polymerase I

Removes RNA nucleotides of RNA primer on the lagging strand and replacing them with DNA by attaching dNTPs.

DNA ligase

Joins Okazaki fragments together by forming sugar-phosphate bond between deoxyribose and the phosphate of adjacent nucleotides.

TRANSCRIPTION

1) RNA polymerase unwinds the strands of DNA double helix leaving them separate whereby one of them will become the template strand.

2) RNA polymerase synthesises mRNA by attaching free NTPs to the DNA strand using complementary base pairing.

3) As the NTPs are linked to the DNA strand, one strand of mRNA is formed. The mRNA strand is much shorter than the DNA strand as only one section of the DNA is transcribed.

4) mRNA separates from the DNA and the DNA double helix is zipped back by RNA polymerase.

TRANSLATION