NCERT Solutions for Class 12 Biology Chapter 5 Molecular Basis of Inheritance

Short Note:

• Experiments by Griffith (transformation), Avery, MacLeod and McCarty, and Hershey-Chase confirmed that DNA, not protein, is the hereditary genetic material.

• DNA is a double helix (Watson-Crick model) made of two anti-parallel polynucleotide strands held together by hydrogen bonds between complementary nitrogenous bases: adenine pairs with thymine (2 hydrogen bonds), and guanine pairs with cytosine (3 hydrogen bonds).

Short Note:

• DNA replication is semi-conservative, meaning each daughter DNA molecule retains one parental strand and synthesises one new strand, as demonstrated by the Meselson and Stahl experiment.

• The enzyme DNA polymerase catalyses the synthesis of new DNA strands using the parental strands as templates, and replication occurs during the S-phase of the cell cycle.

Short Note:

• Transcription is the process of copying genetic information from one strand of DNA into messenger RNA (mRNA), carried out by the enzyme RNA polymerase, and in eukaryotes, the primary transcript undergoes processing (splicing, capping, tailing) to form mature mRNA.

• The genetic code is a triplet code (a codon of three nucleotides specifies one amino acid), is nearly universal across organisms, is degenerate (most amino acids are coded by more than one codon), and includes start (AUG) and stop codons (UAA, UAG, UGA).

Short Note:

• Translation is the process of polypeptide (protein) synthesis using the sequence of codons on mRNA as a template, occurring on ribosomes with the help of tRNA molecules that bring specific amino acids.

• The lac operon in E. coli is a classic example of gene regulation, where the structural genes for lactose metabolism are switched on in the presence of lactose (the inducer) and switched off in its absence, illustrating how gene expression can be regulated at the transcriptional level.