Molecular Basis of Inheritance Class 12 Notes Biology Chapter 6 - CBSE

Chapter: 6

What are Molecular Basis of Inheritance?

Dna: Structure Of Polynucleotide Chain

  • DNA is the polymer of deoxyribonucleotides and are composed of nitrogenous base, Pentose sugar, Phosphate.
  • Nucleoside = Nitrogenous base + Pentose sugar (linked through glycosidic bond) Nucleotide = Nucleoside + Phosphate group (linked through phosphodiester bond)

Features Of Double Helix Structure Of Dna

  • It is a double-helical structure.
  • Complementary bases gets paired up through hydrogen bond. Purines always pair with their corresponding pyrimidines. For e.g. A = T and G ≡ C.
  • The helix is right-handed, Pitch − 3.4 nm, 10 bp in each turn.

Packaging Of Dna Helix

  • Distance between two consecutive base pairs in a DNA = 0.34 nm = 0.34 × 10−9 m
  • Total number of base pairs in a human DNA = 6.6 × 109bp
  • Total length of human DNA = 0.34 × 10−9 × 6.6 × 109= 2.2 meters



  • ➥ Loosely packed regions of chromatin which stains light.
  • ➥ Transcriptionally active chromatin.


  • ➥ Densely packed regions of chromatin which stains dark
  • ➥ Transcriptionally inactive.

Discovery Of Dna As A Genetic Material

  • Griffith performed experiments with the bacteria Streptococcus pneumoniae. This bacterium has two strains − S strain and R strain.
  • Live R strain in the presence of heat-killed S strain results in pneumonia because somehow R strain bacteria is transformed by heat-killed S strain bacteria. Hence, it was concluded that there must be transfer of genetic material.

$$\bull\space\text{Heat-killed S strain} + \text{Live R strain}\xrightarrow[\text{Digest RNA and proteins}]{\text{RNase . Proteases}}\text{Transformation occurs and mouse dies}$$

Hershey And Chase Experiment To Confirm Dna As The Genetic Material

  • When a bacteriophage infects and transfer its genetic material to the bacteria, Bacterial cell treats it as its own genetic material and replicates it leading to production of many copies of viral particles
  • They labelled some phages with radioactive Sulphur and the others with radioactive phosphorus.
  • These phages were used to infect E. coli.
  • E.coli was then blended and centrifuged to remove viral particles.
  • It was observed that bacteria with radioactive DNA were radioactive while those with radioactive proteins lost their radioactivity.

It was found that DNA enters the bacteria from viruses and not proteins. Hence, it was concluded that DNA is the genetic material which was responsible for radioactivity.

Dna Replication With Experimental Proof

Experiment to Prove That DNA Replicates Semi-Conservatively

  • Performed by − Messelson and Stahl
  • E.coli was grown in a medium containing heavy isotope N15 as the nitrogen source.
  • N15 was incorporated into newly synthesized DNA as well and the DNA became heavy DNA.
  • Heavy DNA molecule can be differentiated from normal DNA by density gradient centrifugation using cesium chloride as the gradient.
  • This implies that the newly synthesized DNA obtained one of its strands from the parent. Thus, replication is semi-conservative.


It is the formation of RNA molecules from the DNA. A transcriptional unit majorly have three regions:

  • Promoter
  • Structural gene
  • Terminator

Template Strand

  • The strand with polarity 3' → 5' acts as a template.

Coding Strand

  • The strand with polarity 5' → 3' acts as a coding.


  • The DNA sequence which codes for tRNA or rRNA molecule defines a gene.
  • Cistron − Sequence that code for a polypeptide
  • Regulatory genes − Sequences that do not code for anything, but perform regulatory functions

Transcription Process


  • RNA polymerase binds with the promoter to initiate the process of transcription.
  • Association with initiation factor (σ) alters the specificity of RNA polymerase to initiate the transcription.


  • RNA polymerase uses nucleotide triphosphate as substrate, and polymerize the process.


  • Termination occurs when termination factor (rho) alters the specificity of RNA polymerase to terminate the transcription.

Genetic Code

  • Codon is triplet. 43 = 64 (61 codons code for amino acids while 3 are stop codons)
  • One codon codes for a single specific amino acid. Codons are unambiguous.
  • Codons are degenerate since some amino acids are coded by more than one codon.
  • Genetic code is universal. 1 codon codes for same amino acid in all species.



  • Amino acids are activated in the presence of ATP and are linked to their corresponding tRNA. The tRNA now called as charged tRNA.


  • tRNA moves from codon to codon on the mRNA and amino acids are added one by one.


  • Release factor binds to stop codon to terminate the translation.

Regulation Of Gene Expression

  • Expression of genes coding for enzymes is required only when substrate for that enzyme is available.
  • For example:

$$\text{Lactose} \xrightarrow{\text{beta-g alactosidase}}\text{Glucose + Galactose}$$

Regulation Of Lac Operon


  • An arrangement where a polycistronic gene is regulated by a common promoter and regulatory genes.
  • Lac Operon was given by Jacob and biochemist Monod.


  • Lactose acts as an inducer for lac operon since it regulates the switching on and off of the operon.

Regulation In Absence Of Inducer

  • Inducer binds with the protein product of gene i (repressor) and inactivates it.
  • This inactivated repressor is unable to bind to operator sequence and thus RNA polymerase remains activated and z, y, and a genes synthesize their respective mRNA, which in turn gets translated to form β-galactosidase, permease, and transacetylase enzymes.
  • In presence of all these enzymes, the metabolism of lactose proceeds in a normal manner.

Human Genome Project(Hgp)

  • Started in 1990, completed in 2003.
  • Human genome contains 3 × 109 (3164.7 million) nucleotide bases.


Single nucleotide polymorphism occur at about 1.4 million locations in human DNA. They are believed to have significance in explaining diseases and evolutionary history of human beings.

DNA Fingerprinting

  • DNA fingerprinting is a method for comparing the DNA sequences of any two individuals.
  • VNTR (variable number of tandem repeats) are satellite DNAs and are used as probes in DNA fingerprinting.
  • DNA fingerprinting forms the basis of paternity testing, forensics and for studying genetic diversity in a population and evolution.