of replication In prokaryotes, replication process requires three types of DNA polymerases (DNA polymerase I, II, and III). DNA polymerase III is the main enzyme involved in DNA replication. DNA polymerase I (also known as Kornberg enzyme ) and DNA polymerase II are involved in DNA repair mechanism. Eukaryotes have five types of DNA polymerases that catalyses the polymerization of nucleotides at the ' OH of the new strand within a short period of time.
E. coli that has only . × bp, completes its replication process within minutes; that means the average rate of polymerisation has to be approximately bp/sec. Replication takes place faster at the same time accurately.
Any error will lead to mutation. However replication errors are corrected by repair enzymes such as nucleases. Deoxy nucleoside triphosphate acts as substrate and also provides energy for polymerization reaction. Replication begins at the initiation site called the site of ‘ origin of replication’ (ori) .
In prokaryotes, there is only one origin of replication, whereas in eukaryotes with giant DNA molecules, there can be several origins of replication (replicons). Since the two strands of DNA cannot be separated throughout at a time (due to large requirement of energy) the replication occurs within a small opening of the DNA helix called as replication fork. Unwinding of the DNA strand is carried out by DNA helicase. Thus, in one strand (template strand with polarity ' ') the replication is continuous and is known as the leading strand while in the other strand (coding strand with polarity ' ') replication is discontinuous, known as the lagging strand (Fig.
. ) . The discontinuously synthesized fragments of the lagging strand (called the Okazaki fragments ) are joined by the enzyme DNA ligase. Fig .
Mechanism of replication showing a replication fork XII Std Zoology Chapter XII Std Zoology Chapter Molecular Genetics of information is reversed. RNA synthesizes DNA by reverse transcription, then transcribed into mRNA by transcription and then into proteins by translation. For a cell to operate, its genes must be expressed. This means that the gene products, whether proteins or RNA molecules must be made.
The RNA that carries genetic information encoding a protein from genes into the cell is known as messenger RNA (mRNA). For a gene to be transcribed, the DNA which is a double helix must be pulled apart temporarily, and RNA is synthesized by RNA polymerase. This enzyme binds to DNA at the start of a gene and opens the double helix. Finally, RNA molecule is synthesized.
The nucleotide sequence in the RNA is complementary to the DNA template strand from which it is synthesized. Both the strands of DNA are not copied during transcription for two reasons. . If both the strands act as a template, they would code for RNA with different sequences.
This in turn would code for proteins with different amino acid sequences. This would result in one segment of DNA coding for two different proteins, hence complicate the genetic information transfer machinery. . If two RNA molecules were produced simultaneously, double stranded RNA complementary to each other would be formed.
This would prevent RNA from being translated into proteins.