DNA REPLICATION, TRANSCRIPTION AND TRANSLATION
BDU, TRICHY, INDIA
STUDY MATERIAL..
MOLECULAR BIOLOGY
REPLICATION
INTRODUCTION :
DNA Replication is the process of copying the double stranded DNA
molecule into two identical DNAs which have two different strands, one is
the original strand , another one is daughter strand.
Replication process occurs in the S phase of the cell cycle.
How DNA replication works:
● DNA is composed of two complementary strands.
● The two strands separate and each strand of the original DNA
● molecule serves as template for the production of a new
complementary strand.
● Finally,the fixing enzymes help to link the aligned nucleotides by
phosphodiester bonds to form a continuous strand.
Three models for DNA replication :
1. Conservative,
2. Semiconservative and
3. Dispersive.
,Conservative Model :
After DNA replication, the parental DNA remains together, and the
newly formed daughter strands are together.
Semiconservative Model :
The semi-conservative method suggests that each of the two parental
DNA strands act as a template for new DNA to be synthesised; after
replication, each double-stranded DNA includes one parental or “old”
strand and one “new” strand.
Dispersive Model:
Both new copies of DNA have double-stranded segments of parental
DNA and newly synthesised DNA interspersed.
Steps involved in DNA replication:
1. Recognition of initiation point
DNA replication starts at a specific point called initiation point
or origin where the replication fork begins. This is a nucleotide sequence
,of 100 to 200 pairs of bases. Specific initiation proteins recognize the
initiation site on DNA. Such proteins along with DNA directed RNA
polymerase Initiate the synthesis of RNA primer for the formation of DNA
chain. Prokaryotic chromosomes usually possess one initiation. point or
replication fork., whereas the eukaryotic chromosomes may possess
several replication forks.
Nicks are produced by the endonuclease enzyme.
2. Unwinding of DNA -
When the DNA duplex molecule is cut open (nicked) to form a
bubble or fork the unwinding proteins get attached at the point of nick
which helps in the separation of the strands of the DNA duplex.
3. Template DNA
The single stranded DNA on which the new DNA is synthesised
is called template DNA..
4. RNA Primer
Pre-formed polynucleotide chain is necessary to start the
synthesis of DNA, RNA polymerase synthesises RNA primer on template
DNA. In the absence of RNA primer the DNA replication is irregular.
5. Chain Elongation
, New DNA strand is formed due to DNA polymerase III
enzyme. This enzyme adds nucleotides in the 5' to 3' direction. This
activity of DNA polymerase is called polymerizing.
6. Replication forks
A replication fork develops after the DNA strand splits.
Okazaki fragments: This polymerase III enzyme is able to polymerize in
the 5' to 3' orientation. As a result, one of the two DNA strands with 3 to 5'
polarity experiences continuous DNA synthesis; this strand is known as
the continuous strand. The strand with 5' to 3 polarities on the other hand
undergoes synthesis of DNA in tiny pieces known as Okazaki fragments,
named for the researcher who made the first identification of them.
The strand is referred to as a discontinuous strand as it is produced in
fragments. On this strand, DNA synthesis is occurring in opposition to the
BDU, TRICHY, INDIA
STUDY MATERIAL..
MOLECULAR BIOLOGY
REPLICATION
INTRODUCTION :
DNA Replication is the process of copying the double stranded DNA
molecule into two identical DNAs which have two different strands, one is
the original strand , another one is daughter strand.
Replication process occurs in the S phase of the cell cycle.
How DNA replication works:
● DNA is composed of two complementary strands.
● The two strands separate and each strand of the original DNA
● molecule serves as template for the production of a new
complementary strand.
● Finally,the fixing enzymes help to link the aligned nucleotides by
phosphodiester bonds to form a continuous strand.
Three models for DNA replication :
1. Conservative,
2. Semiconservative and
3. Dispersive.
,Conservative Model :
After DNA replication, the parental DNA remains together, and the
newly formed daughter strands are together.
Semiconservative Model :
The semi-conservative method suggests that each of the two parental
DNA strands act as a template for new DNA to be synthesised; after
replication, each double-stranded DNA includes one parental or “old”
strand and one “new” strand.
Dispersive Model:
Both new copies of DNA have double-stranded segments of parental
DNA and newly synthesised DNA interspersed.
Steps involved in DNA replication:
1. Recognition of initiation point
DNA replication starts at a specific point called initiation point
or origin where the replication fork begins. This is a nucleotide sequence
,of 100 to 200 pairs of bases. Specific initiation proteins recognize the
initiation site on DNA. Such proteins along with DNA directed RNA
polymerase Initiate the synthesis of RNA primer for the formation of DNA
chain. Prokaryotic chromosomes usually possess one initiation. point or
replication fork., whereas the eukaryotic chromosomes may possess
several replication forks.
Nicks are produced by the endonuclease enzyme.
2. Unwinding of DNA -
When the DNA duplex molecule is cut open (nicked) to form a
bubble or fork the unwinding proteins get attached at the point of nick
which helps in the separation of the strands of the DNA duplex.
3. Template DNA
The single stranded DNA on which the new DNA is synthesised
is called template DNA..
4. RNA Primer
Pre-formed polynucleotide chain is necessary to start the
synthesis of DNA, RNA polymerase synthesises RNA primer on template
DNA. In the absence of RNA primer the DNA replication is irregular.
5. Chain Elongation
, New DNA strand is formed due to DNA polymerase III
enzyme. This enzyme adds nucleotides in the 5' to 3' direction. This
activity of DNA polymerase is called polymerizing.
6. Replication forks
A replication fork develops after the DNA strand splits.
Okazaki fragments: This polymerase III enzyme is able to polymerize in
the 5' to 3' orientation. As a result, one of the two DNA strands with 3 to 5'
polarity experiences continuous DNA synthesis; this strand is known as
the continuous strand. The strand with 5' to 3 polarities on the other hand
undergoes synthesis of DNA in tiny pieces known as Okazaki fragments,
named for the researcher who made the first identification of them.
The strand is referred to as a discontinuous strand as it is produced in
fragments. On this strand, DNA synthesis is occurring in opposition to the
