Notes On DNA Replication : Machinery and Enzymes - CBSE Class 12 Biology
DNA replication is the basis of biological inheritance where the DNA is copied with the help of various enzymes, such as helicases, topoisomerases, DNA polymerase and DNA ligase. In a bacterial cell, DNA replication begins at a specific point called the ‘origin of replication’. This is the reason why a piece of DNA is cloned into a vector during recombinant DNA procedures, as it is the vector that provides the origin of replication.
 
In eukaryotes with large DNA molecules, there may be many origins of replication that finally merge with one another. During replication, helicases unwind and uncoil the DNA double helix into single strands of DNA by the breakdown of hydrogen bonds. DNA polymerase uses the DNA template to catalyse the polymerisation of deoxyribonucleotides. 

The deoxyribonucleoside triphosphates act as substrates and also the terminal phosphates in it break down to provide energy as ATP. Due to the high energy requirement, two DNA strands do not always separate completely. But, to facilitate the separation of a DNA helix, the enzyme topoisomerase cuts and rejoins one strand of the DNA, which then unzips the double-stranded DNA. As a result, replication bubbles are formed that extend as a Y-shaped replication fork. This replication fork is actually a small opening in the DNA helix where replication occurs.

The DNA-dependent DNA polymerase enzyme can catalyse the replication reaction in only one direction, five prime to three prime. This causes continuous replication on the leading strand that has three prime to five prime polarity, whereas discontinuous replication on the lagging strand with five prime to three prime polarity. These discontinuously replicated fragments, also known as Okazaki fragments are then joined by the enzyme DNA ligase. In case wrong bases are entered into the DNA helix by mutation, the enzyme nuclease can identify and cut off the defective segment of the DNA and introduce the correct segment, which is then joined by the enzyme DNA ligase. The replication of DNA in eukaryotes takes place in the synthesis phase of the cell cycle. If the replication is not followed by cell cycle, it leads to polyploidy.

Summary

DNA replication is the basis of biological inheritance where the DNA is copied with the help of various enzymes, such as helicases, topoisomerases, DNA polymerase and DNA ligase. In a bacterial cell, DNA replication begins at a specific point called the ‘origin of replication’. This is the reason why a piece of DNA is cloned into a vector during recombinant DNA procedures, as it is the vector that provides the origin of replication.
 
In eukaryotes with large DNA molecules, there may be many origins of replication that finally merge with one another. During replication, helicases unwind and uncoil the DNA double helix into single strands of DNA by the breakdown of hydrogen bonds. DNA polymerase uses the DNA template to catalyse the polymerisation of deoxyribonucleotides. 

The deoxyribonucleoside triphosphates act as substrates and also the terminal phosphates in it break down to provide energy as ATP. Due to the high energy requirement, two DNA strands do not always separate completely. But, to facilitate the separation of a DNA helix, the enzyme topoisomerase cuts and rejoins one strand of the DNA, which then unzips the double-stranded DNA. As a result, replication bubbles are formed that extend as a Y-shaped replication fork. This replication fork is actually a small opening in the DNA helix where replication occurs.

The DNA-dependent DNA polymerase enzyme can catalyse the replication reaction in only one direction, five prime to three prime. This causes continuous replication on the leading strand that has three prime to five prime polarity, whereas discontinuous replication on the lagging strand with five prime to three prime polarity. These discontinuously replicated fragments, also known as Okazaki fragments are then joined by the enzyme DNA ligase. In case wrong bases are entered into the DNA helix by mutation, the enzyme nuclease can identify and cut off the defective segment of the DNA and introduce the correct segment, which is then joined by the enzyme DNA ligase. The replication of DNA in eukaryotes takes place in the synthesis phase of the cell cycle. If the replication is not followed by cell cycle, it leads to polyploidy.

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