DNA replication
Watson and Crick suggested a very simple mechanism of DNA replication or DNA transcription on the basis of its double helical structure. During replication the weak hydrogen bonds between the nitrogeneous bases of the nucleotides separate so that the two polynucleotide chains of DNA also separate and uncoil. The chains thus separated are complementary to one another. Because of the specificity of base pairing, each nucleotide of separated chains attracts it complementary nucleotide from the cell cytoplasm. Once the nucleotides are attached by their hydrogen bonds, their sugar radicals unite through their phosphate components, completing the formation of a new polynucleotide chain.
The method of DNA replication is semi-discontinuous and described as semi-conservative method, because each daughter DNA molecule is a hybrid conserving one parental polynucleotide chain and the other one newly synthesized strand. DNA replication occur in S-phage in cell cycle.
(i) Mechanism of DNA replication
The entire process of DNA replication involves following steps in E.coli.
(a) Recognition of the initiation point : First, DNA helix unwind by the enzyme “Helicase” which use the energy of ATP and replication of DNA begin at a specific point, called initiation point or origin where replication fork begins.
(b) Unwinding of DNA : The unwinding proteins bind to the nicked strand of the duplex and separat the two strands at DNA duplex. Topoisomerase (Gyrase is a type of topoisomerase in E.coli) helps in unwinding of DNA.
(c) Single stranded binding protein (SSB) : Which remain DNA in single stranded position and also known as helix destabilising protein (HDP).
(d) RNA Priming : The DNA directed RNA polymerase now synthesizes the primer strands of RNA (RNA primer). The priming RNA strands are complementary to the two strands of DNA and are formed of 50 to 100 nucleotides.
(e) Formation of DNA on RNA primers : The new strands of DNA are formed in the 5'-->3' direction from the 3'-->5' template DNA by the addition of deoxyribonucleotides to the 3' end of primer RNA.
Addition of nucleotide is done by DNA polymerase III. The leading strand of DNA is synthesized continuously in 5'-->3' direction as one piece. The lagging strand of DNA is synthesized discontinuously in its opposite direction in short segments. These segments are called Okazaki fragments.
The method of DNA replication is semi-discontinuous and described as semi-conservative method, because each daughter DNA molecule is a hybrid conserving one parental polynucleotide chain and the other one newly synthesized strand. DNA replication occur in S-phage in cell cycle.
(i) Mechanism of DNA replication
The entire process of DNA replication involves following steps in E.coli.
(a) Recognition of the initiation point : First, DNA helix unwind by the enzyme “Helicase” which use the energy of ATP and replication of DNA begin at a specific point, called initiation point or origin where replication fork begins.
(b) Unwinding of DNA : The unwinding proteins bind to the nicked strand of the duplex and separat the two strands at DNA duplex. Topoisomerase (Gyrase is a type of topoisomerase in E.coli) helps in unwinding of DNA.
(c) Single stranded binding protein (SSB) : Which remain DNA in single stranded position and also known as helix destabilising protein (HDP).
(d) RNA Priming : The DNA directed RNA polymerase now synthesizes the primer strands of RNA (RNA primer). The priming RNA strands are complementary to the two strands of DNA and are formed of 50 to 100 nucleotides.
(e) Formation of DNA on RNA primers : The new strands of DNA are formed in the 5'-->3' direction from the 3'-->5' template DNA by the addition of deoxyribonucleotides to the 3' end of primer RNA.
Addition of nucleotide is done by DNA polymerase III. The leading strand of DNA is synthesized continuously in 5'-->3' direction as one piece. The lagging strand of DNA is synthesized discontinuously in its opposite direction in short segments. These segments are called Okazaki fragments.
