DNA Replication and Protein Synthesis

Replication: The DNA moleculecan make an exact copy of itself. The two strands separate and new complementary strands are formed. The double helix unwinds and each of the DNA strands acts as a template for a new complementary strand. Nucleotides move into the replication fork and align themselves against the complementary bases on the template. The addition of

nucleotides is catalyzed by an enzyme called DNA polymerase.

Transcription: Transcription is the process of transfer of information from DNA to RNA. The complementary single-stranded RNA molecule is called messenger RNA (mRNA). mRNA carries the information from the DNA to the ribosomes where it controls protein synthesis. Transcription is catalyzed by an enzyme called RNA polymerase.

Enzyme regulation (repression or induction) occurs at the level of transcription. Sometimes, the product formed through the action of an enzyme represses the synthesis of that enzyme. The enzyme product acts as a co-repressor, which, along with a repressor, combines with the operator gene to block transcription and, therefore, enzyme synthesis.

The synthesis of other enzymes, called inducible enzymes, occurs only when the substrate is present in the medium. Enzyme synthesis is induced because the substrate, the inducer, combines with the repressor to form a complex that has no affinity for the operator gene.

Translation: mRNA controls protein synthesis in the cytoplasm. This process is called translation. Another type of RNA is the transfer RNA (tRNA), which has attachment sites for both mRNA and amino acids and brings specific amino acids to the ribosome.

Each combination of three nucleotides on the mRNA is called a codon or triplet. Each of these triplets codes for a specific amino acid. The sequence of codons determines that of amino acids in a protein. Some triplets code for the initiation and termination of amino acid sequences. There are 64 possible codons.