DNA Replication

Following operations occurs in the replication of deoxyribonucleic acid (DNA):
  • Origin of Replication
  • Elongation of new DNA strand
  • The problems of Anti-parallel DNA strands
  • RNA primer
  • Protein assisting the DNA replication
  • Proofreading

  1. Origin of replication

The replication of a DNA molecule begins at special sites called origins of replication. The eukaryotic deoxyribonucleic acid or DNA comprise thousand of such replication origins. A protein initiates deoxyribonucleic acid or DNA replication. It acknowledge these sequences of origins and attach to the deoxyribonucleic acid (DNA). It separates the both two strands. These strands open up to build a replication “bubble.” Multiple or various replication bubbles are develop in eukaryotes. These bubbles fuse with each other. The replication of deoxyribonucleic acid (DNA) then progress in both directions and entire ‘molecule is replicated or copied. There is replication fork at each and every end of a replication bubble. It is Y‑shaped site or region. New strands of deoxyribonucleic acid DNA elongates replication fork.

Elongating a New DNA Strand

    An enzymes DNA polymerases catalyzes elongation of new or latest (DNA) deoxyribonucleic acid at a replication fork. The nucleotides align with complementary basis on “old” template strand of deoxyribonucleic acid  (DNA). They are added by DNA polymerase one by one. The rate of elongation is about five-hundred nucleotides per second in human beings cells
    The substrates for deoxyribonucleic acid (DNA) are nucleo side triphosphate. The nucleoside triphosphates have 3 phosphate sets like Adenosine Tri-Phosphate (ATP). Each monomer loses two phosphates and binds to the growing end of a DNA (deoxyribonucleic acid) strand. Hydrolysis of the phosphate is the exergonic process. That's why, it drives polymerization of nucleotides to yield DNA (deoxyribonucleic acid).

    The Problem of Antiparallel DNA Strands

    There is a problem of DNA (deoxyribonucleic acid) formation (synthesis) at the replication fork. The two DNA (deoxyribonucleic acid) strands (filament) are anti-parallel (3-5 and 5-3). Their sugar—phosphate backbones run in contrary orientation (direction). Phosphate group of each and every nucleotide is link to the 5′ (5 prime) carbon (C) of deoxyribose. The phosphate group of one nucleotide is connected to the 3′ (3 prime) carbon of the adjacent nucleotide. Therefore, there is dissimilar process of replication in both strands:

    (a)   Leading strand: The enzyme DNA (deoxyribonucleic acid)  polymerase can just add nucleotides to the free 3′ three prime end of a DNA (deoxyribonucleic acid) strand. It can not at all add it to the 5′ five prime end. Thus, a new DNA (deoxyribonucleic acid) strand is synthesized in 5— 3′ directions. The DNA (deoxyribonucleic acid) polymerase can develop a continuous complementary strand along 5′ —3 orientation. This DNA (deoxyribonucleic acid) strand is called the leading strand.

    (b)   Lagging strand: The DNA (deoxyribonucleic acid)  polymerase locomote away from the replication fork to elongate in 3-5 strand of DNA (deoxyribonucleic acid). The DNA (deoxyribonucleic acid) synthesized in this orientation or direction is called lagging strand. The lagging strand is first formed as a series of segments. These segments or fragments or pieces are called Okazaki fragments. These segments were discovered or introduced by Japanese scientist Okazaki. These fragments are about hundred to two hundred (100 to 200) nucleotides long in eukaryotes.

    RNA Primer

    There is other problem for DNA (deoxyribonucleic acid) polymerase. It can only add a nucleotide to a polynucleotide that is previously exactly paired with the complementary strand. This means that DNA (deoxyribonucleic acid)  polymerase cannot actually initiate formation of a (deoxyribonucleic acid) DNA strand. Nucleotides must be added to the end of an previously occurring chain. This chain of nucleotides is term as a primer. The primer is a short stretch of ribonucleic acid (RNA). It is formed by another enzyme primase. It is about ten (10) nucleotides long in eukaryotes. Only one primer is needed for the leading strand of new deoxyribonucleic acid (DNA). Each and every fragment must have separate primer in the lagging strand. An enzyme then replaces the ribonucleic acid RNA  nucleotides of the primers with deoxyribonucleic acid DNA. Another enzyme Ligase connects all the DNA fragments into a strand.

    Protein Assisting the DNA deoxyribonucleic acid replication

    Following proteins help in the formation of deoxyribonucleic acid  DNA:
    1. DNA deoxyribonucleic acid  polymerase
    2. Primase
    3. Ligase
    4. Helicase: it's a protein. It causes untwisting the double helix of deoxyribonucleic acid DNA.
    5. Single strand binding protein: It is attached to the separated strands of deoxyribonucleic acid DNA and does not permit them to recoil.

    Proofreading of deoxyribonucleic acid Replication

    The errors or faults in the completed deoxyribonucleic acid DNA molecule are only one in one billion nucleotides. These errors must be corrected. Some enzyme correct these faults or errors.

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