Spectrum biology

Pro cess of DNA Rep li ca tion

The process of DNA replication occur in nucleus of the cell. It begins at the

origin of replication with a nick or incision made by endonuclease on one

strand of DNA. Further opening/unzipping of more nucleotides takes place

by helicase. It leads to the formation of replication fork. DNA replication

occurs in 5 ′→ 3 ′ direction. It is continuous on one strand called leading

strand and in form of small fragments, by forming loop (trombone loop) at

another strand (lagging strand).

Sense and Antisense Strands

Both the strands of DNA do not take part in controlling heredity and metabolism. Only

one of them does so. The DNA strand which function as template for RNA synthesis is

known as template strand or minus (−) strand or antisense strand or non-coding

strand. Its complementary strand is named non-template strand or plus ( )+ strand or

sense strand or coding strand.

Following are the different enzymes with its

function in DNA replication:

Enzymes for DNA replication and their

functions

Enzymes Functions

DNA helicase The enzyme that is responsible for the

unwinding of the double helix by disrupting

the hydrogen bonds between the

antiparallel strands.

DNA gyrase An enzyme that relieves any form of

tension that’s produced after the DNA

double helix is unwound.

SSB (Single

Stranded

Binding)

proteins

The proteins that bind to the outside of the

two single template strands and prevent

the separated strands of DNA from

re-anneling.

Primase An enzyme that builds RNA primers and

assembles them at the origin of replication

site.

DNA

polymerase-III

An enzyme that creates complementary

strands of DNA during replication.

Attaches to RNA primer, can only add

nucleotides to a 3 ′-end of a DNA strand.

DNA

polymerase-I

The enzyme that removes RNA primers

and replaces them with the appropriate

DNA nucleotides.

DNA ligase The enzyme that joins DNA fragments

together by phosphodiester bonds.

Note RNA primer is composed of multiple bases that

get attached (through hydrogen bonds) to the

template strand to initiate the DNA replication.

RNA

The other nucleic acid present in cell is RNA,

i.e. ribonucleic acid. It is present predominantly

in cytoplasm and mostly in the form of single

strand. The pyrimidine, i.e. thymine of DNA is

replaced by uracil in RNA. All normal RNA

chains begin with adenine or guanine. The RNA

can be of following types

RNA Types Func tions

mRNA (messenger

RNA)

Carries information specifying

amino acid sequences of proteins

from DNA to ribosomes.

tRNA (transfer RNA) Transfer amino acids to the protein

synthesis machinery to produce

proteins.

rRNA (ribosomal

RNA)

It is a part of protein synthesis

machinery. Plays structural and

catalytic roles in ribosomes.

siRNA (small

interfering RNA)

Used for gene regulation.

snRNA (small

nuclear RNA)

Used to edit mRNA, regulate gene

expression and maintain

chromosome tips (telomeres).

Leading strandOrigin of replication

Lagging strand

5 ′^3 ′

3 ′^5 ′

Template

DNA HelicasesThe enzyme which unwind the

double helical DNA for its replication.

Primase

The enzyme which synthesise

the RNA primer.

It is removed by DNA

polymerase-I, before DNA-

ligase joins these fragments.

Lagging/Discontinuous Strand

(Trombone Loop)

As this strand is already 5′– 3′,

the parallel synthesis cannot

take place (DNA

replication always takes place in

5 ′–3′ direction)

5 ′

Priming site

RNA Primer

These are small (10-60 bp) RNA

fragments, synthesised by

primase, acts as receptor for

primary nucleotides.

Single Stranded

Binding Protein

The protein complex

which maintain the

single stranded DNA,

It prevent the

recoling of DNA.

DNA Polymerase
The enzyme DNA-
polymerase was
discovered by
Kornberg in 1957.
The DNA
polymerase-III
catalyze DNA
replication in
prokaryotes, in
eukaryotes it is
done by DNA
polymerase δ and ε
DNA polymerase α.

Leading/Continuous Strand

The strand with the direction 3′– 5′,

on which the continuous synthesis

of new strand takes place in 5′– 3′ direction.

Okazaki Fragments

Small 200 bp segments

synthesised by

DNA polymerase-III at

lagging strand.

3 ′^5 ′

3 ′

Machinery of DNA replication showing the trombone loop