Write four salient features of genetic code.

The genetic code is a set of instructions that direct the translation of DNA into 20 amino acids, the basic units of proteins in living cells. The genetic code is made up of codons. The salient features of genetic code are as follows: 1. Codon is a triplet. Out of 64 codons, 61 code for 20 amino acids and 3 codons (UAA, UGA and UAG) do not code for any amino acids. Thus, they function as terminating or stop codons. 2. Genetic code is unambiguous and specific.Thus, one codon codes for only one amino acid. 3. Some amino acids are coded by more than one codon. Hence, the code is degenerate. 4. The codon is read in mRNA in a contiguous fashion,i.e. without punctuation. Thus, the code is commaless.

What is genetic code?

The genetic code is a set of rules to translate information encoded within the genetic material. The sequence of the nucleotides on the mRNA chain that is translated into the amino acids depends on the triplet sequence of the nucleotides that decides which amino acid will be formed. One triplet codon codes only for one amino acid. The genetic code is unambiguous and specific.

Describe the structure and function of Lac operon.

The gene regulation in bacteria is explained by the operon model. Proteins that are needed for a selected function, or that are involved within the same biochemical pathway, are encoded together in blocks called operons. Lac operon is an example of this that consists of structural genes, promoter, operator, regulator and inducer. The three types of structural genes are z, y and a genes. Promoter is the region where RNA polymerase bind and initiate the transcription of structural genes. Operator is the DNA sequence next to the promoter in which the specific repressor protein binds. Hence, it acts as the switch for transcription. Regulator codes for the repressor protein. Inducer is the substrate that prevent the binding of repressor protein into operator. In Lac Operon, lactose acts as the inducer. If the lactose is absent, the operon is switched off because the repressor protein is produced by the regulatory gene that binds to the operator gene. The lactose inactivated the repressor protein, hence, in the presence of an inducer (lactose), enzymes are produced.

Write polarity of template strand and coding strand. Write three regions of transcription unit of DNA.

DNA is made up of two complementary strands twisted into each other in the form of a double helix. Two strands of DNA have opposite polarity. The strand that has the polarity 3' to 5' is called template strand that acts as template for transcription. The other strand which has the polarity 5' to 3' that has the sequence same as the mRNA except in the place of thymine is called coding strand. This strand does not code for anything. Transcription is a process where the DNA sequence of a gene is transcribed to form an mRNA molecule. It is the first step of genetic expression. A transcription unit in DNA has primarily three regions: A Promoter The Structural gene A Terminator