DNA Replication
One of the important features of DNA is that it can form its exact copy. This is known as DNA Replication. This essential feature of DNA occurs during synthesis phase of cell cycle. Each DNA molecule fives rise to two DNA molecules identical to each other as well as to the parent molecule. The precise replication without error is required for cell. If error rate of 0.01% occurs, then it can cause significant gene mutation.
Max Delbruck proposed those possible ways of DNA replication. They are:
1. Semi- conservative replication
2. Conservative replication
3. Dispersive replication
Semi-conservative replication
Watson and Crick suggested very simple mechanism of DNA replication on the basis of their own model. This is most widely accepted method of DNA replication. It occurs in the nucleus of cell. It requires a supply of free nucleotides which exist in nucleoplasm. To this, extra phosphate groups are added to activate them. Taylor 1956 first demonstrated the semi-conservative mode of replication and later Meselson and Stahl 1964 experimentally proved it in E coli.
Main features
1. A progressive separation of the two strands of a DNA molecule or DNA molecule unwinds and two strands are separated by breakage of Hydrogen bonds between bases.
2. Each strand acts as a template of the synthesis of new strand.
3. Nucleotides with appropriate complimentary bases then slot into place opposite the exposed bases on each strand by base paring that is A with T and C with G.
4. Hydrogen bonds between the complimentary bases hold them in place. The sugar of one nucleotide then joined to the phosphate of the next nucleotide to form polynucleotide chain.
5. The result is that two new DNA molecules are formed from a DNA molecule. These processes are dependent on a number of enzymes including DNA polymerase.
6. Each new DNA molecule contains one old polynucleotide strand and one new strand. During replication half of the original DNA molecule is conserved and the complementary half is newly synthesized by base pairing. So, this method is called semi-conservative replication.
Mechanism of DNA replication
DNA replication is a complex multi enzymatic process involving several enzymes and factors. The replicating units of chromosomes are called replicons. Replication starts with an incision or a cut at a specific point generally referred to as nicking. An incision enzyme called endonuclease brings this about. Bacterial and viral DNA has a single origin of replication but in eukaryotes there are many origins of replication due to a large size and association with proteins.
It includes following steps:
a) Activation of nucleotide
The deixtribibyckeotide monophosphates such as deoxyriboadenosine monophosphate(dAMP), deoxyriboguanosine monophosphate(dGMP),
deoxyribocytosine monophosphate(dCMP), deoxyribothymine monophosphate(dTMP) occur in nucleoplasm. These are activated into deoxyribonucleoside triphosphate. This process is called phosphorylation.
dAMP→dADP→dATP
dGMP→dGDP→dGTP
dCMP→dCDP→dCTP
dTMP→dTDP→dTTP
b) Unwinding of DNA
Replication of DNA begins at a specific point called initiation point. The unwinding proteins bind to the initiation point on DNA helix and initiate unwinding of its strand. Thus two strands of DNA start separating by breakdown of Hydrogen bonds between paired nucleotide.
c) Base pairing
Deoxyribonucleoside triphosphates pair with appropriate nitrogenous base of trmplate DNA strand according to base pairing tht is A with T and C with G.
d) Conversion of Triphosphate to Monophosphate
The deoxyribonucleoside triphospahate molecule on pairing with nitrogenous bases of template strand set free pyrophosphate molecule and change into dexyribonucleotide monophosphate. This occurs in the presence of enzyme pyrophosphatase during which a large amount of energy is released.
e) Formation of new DNA chain
The energy so released is utilized in joining the adjacent nucleotide to form polynucleotide chain. This process is catalyzed by DNA polymerase (enzyme).
The new strand of DNA is synthesized in 5’→ 3’ direction on 3’ →5’ template is continuous and known s leading strand while the synthesis of 3’→ 5’ strand on 5’ →3’ strand is discontinuous and lagging strand. After sometime of replication of 3’ →5’ strand, primase again initiates the replication of 5’ →3’ (lagging) strand. The replication of lagging strand generates smaller polynucleotide fragments called Okazaki fragments. The Okazaki fragments are 1000 – 2000 nucleotides long in prokaryotic and only 100 – 200 nucleotides long in eukaryotes. The Okazaki fragments are joined by enzyme polynucleotide ligase.
Conservative replication
In this, completely new DNA molecule is synthesized from old DNA. In new DNA molecule, both strands are new and in old DNA, both strands are old.
Dispersive replication
In this, original DNA chain breaks and recombines in a random fashion. The newly
formed DNA molecules contain partly new and partly old strands.
One of the important features of DNA is that it can form its exact copy. This is known as DNA Replication. This essential feature of DNA occurs during synthesis phase of cell cycle. Each DNA molecule fives rise to two DNA molecules identical to each other as well as to the parent molecule. The precise replication without error is required for cell. If error rate of 0.01% occurs, then it can cause significant gene mutation.
Max Delbruck proposed those possible ways of DNA replication. They are:
1. Semi- conservative replication
2. Conservative replication
3. Dispersive replication
Semi-conservative replication
Watson and Crick suggested very simple mechanism of DNA replication on the basis of their own model. This is most widely accepted method of DNA replication. It occurs in the nucleus of cell. It requires a supply of free nucleotides which exist in nucleoplasm. To this, extra phosphate groups are added to activate them. Taylor 1956 first demonstrated the semi-conservative mode of replication and later Meselson and Stahl 1964 experimentally proved it in E coli.
Main features
1. A progressive separation of the two strands of a DNA molecule or DNA molecule unwinds and two strands are separated by breakage of Hydrogen bonds between bases.
2. Each strand acts as a template of the synthesis of new strand.
3. Nucleotides with appropriate complimentary bases then slot into place opposite the exposed bases on each strand by base paring that is A with T and C with G.
4. Hydrogen bonds between the complimentary bases hold them in place. The sugar of one nucleotide then joined to the phosphate of the next nucleotide to form polynucleotide chain.
5. The result is that two new DNA molecules are formed from a DNA molecule. These processes are dependent on a number of enzymes including DNA polymerase.
6. Each new DNA molecule contains one old polynucleotide strand and one new strand. During replication half of the original DNA molecule is conserved and the complementary half is newly synthesized by base pairing. So, this method is called semi-conservative replication.
Mechanism of DNA replication
DNA replication is a complex multi enzymatic process involving several enzymes and factors. The replicating units of chromosomes are called replicons. Replication starts with an incision or a cut at a specific point generally referred to as nicking. An incision enzyme called endonuclease brings this about. Bacterial and viral DNA has a single origin of replication but in eukaryotes there are many origins of replication due to a large size and association with proteins.
It includes following steps:
a) Activation of nucleotide
The deixtribibyckeotide monophosphates such as deoxyriboadenosine monophosphate(dAMP), deoxyriboguanosine monophosphate(dGMP),
deoxyribocytosine monophosphate(dCMP), deoxyribothymine monophosphate(dTMP) occur in nucleoplasm. These are activated into deoxyribonucleoside triphosphate. This process is called phosphorylation.
dAMP→dADP→dATP
dGMP→dGDP→dGTP
dCMP→dCDP→dCTP
dTMP→dTDP→dTTP
b) Unwinding of DNA
Replication of DNA begins at a specific point called initiation point. The unwinding proteins bind to the initiation point on DNA helix and initiate unwinding of its strand. Thus two strands of DNA start separating by breakdown of Hydrogen bonds between paired nucleotide.
c) Base pairing
Deoxyribonucleoside triphosphates pair with appropriate nitrogenous base of trmplate DNA strand according to base pairing tht is A with T and C with G.
d) Conversion of Triphosphate to Monophosphate
The deoxyribonucleoside triphospahate molecule on pairing with nitrogenous bases of template strand set free pyrophosphate molecule and change into dexyribonucleotide monophosphate. This occurs in the presence of enzyme pyrophosphatase during which a large amount of energy is released.
e) Formation of new DNA chain
The energy so released is utilized in joining the adjacent nucleotide to form polynucleotide chain. This process is catalyzed by DNA polymerase (enzyme).
The new strand of DNA is synthesized in 5’→ 3’ direction on 3’ →5’ template is continuous and known s leading strand while the synthesis of 3’→ 5’ strand on 5’ →3’ strand is discontinuous and lagging strand. After sometime of replication of 3’ →5’ strand, primase again initiates the replication of 5’ →3’ (lagging) strand. The replication of lagging strand generates smaller polynucleotide fragments called Okazaki fragments. The Okazaki fragments are 1000 – 2000 nucleotides long in prokaryotic and only 100 – 200 nucleotides long in eukaryotes. The Okazaki fragments are joined by enzyme polynucleotide ligase.
Conservative replication
In this, completely new DNA molecule is synthesized from old DNA. In new DNA molecule, both strands are new and in old DNA, both strands are old.
Dispersive replication
In this, original DNA chain breaks and recombines in a random fashion. The newly
formed DNA molecules contain partly new and partly old strands.
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