PinkMonkey Online Study Guide-Biology
The various steps involved in this process are summarized as follows:
The mechanism of replication starts at a specific
point of the DNA molecule. This is called the origin.
At the origin, the DNA strand breaks because
of an incision (nick). This is made by an enzyme called
incision enzyme (endonuclease).
The hydrogen bonds joining the two strands
are broken by the enzyme.
The two strands start unwinding. This
takes place with the help of a DNA unwinding protein.
The two polynucleotide strands are thus separated.
The point where the two strands separate appears
like a fork or a y-shape. This is described as a replicating
A new strand is constructed on each old strand.
This takes place with the help of a small RNA primer
molecule which is complimentary to the DNA at that point. Each
old DNA strand acts as a template (site) for the construction
of new strand. The RNA primer attaches itself to the old strand
and attracts the enzymes which add new nucleotides. The deoxyribose
nucleotides are present in the surrounding nucleoplasm. Appropriate
nucleotides are selected from the nucleoplasm, and are attached
by H-bonds to their respective complementary bases on the old
strand. A new DNA strand is thus constructed opposite to each
The formation of new complementary strand always
begins at the 3' end of the template strand (original strand)
and progresses towards the 5' end (i.e in 3' ®
5' direction). Since the new strand is antiparallel to the template
strand, it is obvious that the new strand itself is always developed
in the, 5'®3' direction. For
this reason when the two original strands separate (then with
respect to the origin of separation), one acts as 3'®5'
template while the other acts as 3'®5'
template. Of the two, the replication of 3'®5'
template begins first. Hence the new strand formed on it is
called the leading strand. The other template (5'®3')
must begin replication at the fork and progress back toward
the previously transcribed fragment. The new strand formed on
it is called the lagging strand.
Replication of the lagging strand takes place
in small fragments called Okazaki fragments.
These are then connected together by the enzyme polynucleotide
Replication may take place in only one direction
on the DNA helix (unidirectional) or in two directions
At the end of the process, two double stranded
DNA molecules are formed from the original DNA molecule.
- In each newly formed DNA, one strand is old while the other
is new. Hence, it is described as semi-conservative replication.
The semi-conservative nature of DNA replication was confirmed
by Meselson and Stahl (1958) with the help of an experiment. They marked
the DNA in Esherichia coli with heavy isotope of nitrogen
(15N) and then traced it in the following generations of the E-coli progeny.
It was demonstrated that every daughter DNA is an exact copy of the parent
DNA and that each daughter DNA has one strand of the parent (old strand)
and the other strand in newly formed.
Table of Contents8.0 Introduction
8.1 Packaging of Hereditary Material
The Structure of DNA
Replication Of DNA In Eukaryotes
Replicatin of Pokaryotic Chromosome
RNA: Structure and Types
The Genetic Code
The Central Theme of Protein Synthesis
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