Question
Write a note on Mechanism of $ \text{(DNA)}$ replication
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Answer
$\rightarrow$ In living cells, such as $E.$coli, the process of replication requires a set of catalysts $($enzymes$).$
$\rightarrow$ The main enzyme is referred to as $ \text{(DNA)}-$ dependent $ \text{(DNA)}$ polymerase, since it uses a $ \text{(DNA)}$ template to catalyse the polymerisation of deoxynucleotides.
$\rightarrow$ These enzymes are highly efficient as they have to catalyse polymerisation of a large number of nucleotides in a very short time.
$\rightarrow$ $E.$ coli that has only $4.6 \times 10^6 \ bp \ ($compare it $6$ with human whose diploid content is $6.6 \times 10^9 \ bp),$ completes the process of replication within $18$ minutes; that means the average rate of polymerisation has to be approximately $2000 \ bp$ per second.
Not only do these polymerases have to be fast, but they also have to catalyse the reaction with high degree of accuracy.
Any mistake during replication would result into mutations. Furthermore, energetically replication is a very expensive process.
$\rightarrow$ Deoxyribonucleoside triphosphates serve dual purposes.
$(i)$ act as a substrates.
$(ii)$ provide energy for polymerisation reaction $($the two terminal phosphates in a deoxynucleoside triphosphates are high$-$ energy phosphates, same as in case of $ \text{ATP).}$
$\rightarrow$ $ \text{(DNA)}$-dependent $ \text{(DNA)}$ polymerases:
$\rightarrow$ In addition to $ \text{(DNA)}$$-$dependent $ \text{(DNA)}$ polymerases, many additional enzymes are required to complete the process of replication with high degree of accuracy.
$\rightarrow$ Replication fork:
$\rightarrow$ For long $ \text{(DNA)}$ molecules, since the two strands of $ \text{(DNA)}$ cannot be separated in its entire length (due to very high energy requirement), the replication occurs within a small opening of the $ \text{(DNA)}$ helix, referred to as replication fork.
$\rightarrow$ Continuous, discontinuous. Strands:
$\rightarrow$ The $ \text{(DNA)}-$dependent $ \text{(DNA)}$ polymerases catalyse polymerisation only in one direction, that is $5\ '$ $\rightarrow$ $3\ '.$
$\rightarrow$ This creates some additional complications at the replicating fork.
$\rightarrow$ Consequently, on one strand (the template with polarity $3\ '$ $\rightarrow$ $5\ '),$ the replication is continuous, while on the other (the template with polarity $5\ '$ $\rightarrow$ $3\ '),$ it is discontinuous.
$\rightarrow$ The discontinuously synthesised fragments are later joined by the enzyme $ \text{(DNA)}$ ligase.
$\rightarrow$ Origin of replication:
$\rightarrow$ The $ \text{(DNA)}$ polymerases on their own cannot initiate the process of replication.
$\rightarrow$ Also the replication does not initiate randomly at any place in $ \text{(DNA)}$.
$\rightarrow$ There is a definite region in $E.$ coli $ \text{(DNA)}$ where the replication originates. Such regions are termed as origin of replication.
$\rightarrow$ It is because of the requirement of the origin of replication that a piece of $ \text{(DNA)}$ if needed to be propagated during recombinant $ \text{(DNA)}$ procedures, requires a vector.
$\rightarrow$ The vectors provide the origin of replication.
$\rightarrow$ Cell division $ \text{(DNA)}$ replication:
$\rightarrow$ In eukaryotes, the replication of $ \text{(DNA)}$ takes place at $S-$phase of the cell$-$cycle.
$\rightarrow$ The replication of $ \text{(DNA)}$ and cell division cycle should be highly coordinated.
$\rightarrow$ A failure in cell division after $ \text{(DNA)}$ replication results into polyploidy $($a chromosomal anomaly$).$
$\rightarrow$ The main enzyme is referred to as $ \text{(DNA)}-$ dependent $ \text{(DNA)}$ polymerase, since it uses a $ \text{(DNA)}$ template to catalyse the polymerisation of deoxynucleotides.
$\rightarrow$ These enzymes are highly efficient as they have to catalyse polymerisation of a large number of nucleotides in a very short time.
$\rightarrow$ $E.$ coli that has only $4.6 \times 10^6 \ bp \ ($compare it $6$ with human whose diploid content is $6.6 \times 10^9 \ bp),$ completes the process of replication within $18$ minutes; that means the average rate of polymerisation has to be approximately $2000 \ bp$ per second.
Not only do these polymerases have to be fast, but they also have to catalyse the reaction with high degree of accuracy.
Any mistake during replication would result into mutations. Furthermore, energetically replication is a very expensive process.
$\rightarrow$ Deoxyribonucleoside triphosphates serve dual purposes.
$(i)$ act as a substrates.
$(ii)$ provide energy for polymerisation reaction $($the two terminal phosphates in a deoxynucleoside triphosphates are high$-$ energy phosphates, same as in case of $ \text{ATP).}$
$\rightarrow$ $ \text{(DNA)}$-dependent $ \text{(DNA)}$ polymerases:
$\rightarrow$ In addition to $ \text{(DNA)}$$-$dependent $ \text{(DNA)}$ polymerases, many additional enzymes are required to complete the process of replication with high degree of accuracy.
$\rightarrow$ Replication fork:
$\rightarrow$ For long $ \text{(DNA)}$ molecules, since the two strands of $ \text{(DNA)}$ cannot be separated in its entire length (due to very high energy requirement), the replication occurs within a small opening of the $ \text{(DNA)}$ helix, referred to as replication fork.
$\rightarrow$ Continuous, discontinuous. Strands:
$\rightarrow$ The $ \text{(DNA)}-$dependent $ \text{(DNA)}$ polymerases catalyse polymerisation only in one direction, that is $5\ '$ $\rightarrow$ $3\ '.$
$\rightarrow$ This creates some additional complications at the replicating fork.
$\rightarrow$ Consequently, on one strand (the template with polarity $3\ '$ $\rightarrow$ $5\ '),$ the replication is continuous, while on the other (the template with polarity $5\ '$ $\rightarrow$ $3\ '),$ it is discontinuous.
$\rightarrow$ The discontinuously synthesised fragments are later joined by the enzyme $ \text{(DNA)}$ ligase.
$\rightarrow$ Origin of replication:
$\rightarrow$ The $ \text{(DNA)}$ polymerases on their own cannot initiate the process of replication.
$\rightarrow$ Also the replication does not initiate randomly at any place in $ \text{(DNA)}$.
$\rightarrow$ There is a definite region in $E.$ coli $ \text{(DNA)}$ where the replication originates. Such regions are termed as origin of replication.
$\rightarrow$ It is because of the requirement of the origin of replication that a piece of $ \text{(DNA)}$ if needed to be propagated during recombinant $ \text{(DNA)}$ procedures, requires a vector.
$\rightarrow$ The vectors provide the origin of replication.
$\rightarrow$ Cell division $ \text{(DNA)}$ replication:
$\rightarrow$ In eukaryotes, the replication of $ \text{(DNA)}$ takes place at $S-$phase of the cell$-$cycle.
$\rightarrow$ The replication of $ \text{(DNA)}$ and cell division cycle should be highly coordinated.
$\rightarrow$ A failure in cell division after $ \text{(DNA)}$ replication results into polyploidy $($a chromosomal anomaly$).$
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A
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B
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C
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D
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(a)
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anabolic
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(b)
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anabolic
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inducer
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off
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active
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(c)
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catabolic
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inducer
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off
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active
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(d)
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catabolic
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corepressor
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on
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inactive
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