CBSE Class 12 Biology Chapter 6 Revision Notes Part 1

Chapter 6: Molecular Basis of Inheritance Revision Notes Part 1

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  • DNA is the genetic material for most organisms, except for viruses which use RNA instead
  • DNA helps in the synthesis of RNA, which in turn helps in protein synthesis, and these proteins control traits of individuals.
  • DNA, RNA, and the genetic code are responsible for the transmission of genes from parents to progeny.

Molecular Basis of Life

  • At the molecular level, all living organisms are made up of cells
  • Some organisms are made up of a single cell, while others contain millions of cells that carry out particular or specialized functions in the organism as a whole

Nucleotides: DNA & RNA

  • The nucleolus of the cell contains chromatin which makes chromosomes. Chromosomes, in turn, consist of genes that are made up of twisted DNA strands.
  • Living organisms have a fixed number of chromosomes. For example, human beings have 23 chromosome pairs, amounting to 46 total.
  • DNA is responsible for the transfer of traits from one generation to another

dna packaging


  • DNA functions:
    • genetic material passed from parent to offspring
    • provides all of the required information to direct and regulate the construction of the proteins necessary for a cell to perform all of its functions
  • RNA is essential for various biological roles like coding, decoding, regulation, and expression of genes
  • RNA is the genetic material in viruses
  • RNA functions:
    • convert the information stored in DNA into proteins

DNA and RNA Structure

  • DNA and RNA molecules are polymers formed by single units called nucleotides (called ribonucleotides for RNA)
  • DNA is double stranded, whereas RNA is single stranded
  • Nucleotide components:
    • Pentose Sugar
    • Nitrogenous Bases
    • Phosphate Group



  • Multiple nucleotides joined together form polynucleotides
  • The structure of polynucleotide chain is as follows:
    • adenine (A), guanine (G), cytosine (C), or thymine (T) and a phosphate group together make the polynucleotide chain.
    • The phosphate groups are connected by phosphodiester bonds to the subsequent nucleotide’s sugar molecule, thus forming the polynucleotide chain.

DNA: Double Helix Structure

  • made up of two polynucleotide chains
  • backbone = sugar + phosphate
  • interior parts made of nitrogenous bases
  • complementary base pairing – only certain bases will pair with the another
  • Maintains a uniform distance between these pairs – pairing occurs between one purine and one pyrimidine

DNA Structure and Packaging

  • Cells make histone proteins that bind the DNA to counterbalance negative charge of the double helix



  • DNA Packaging is the process of folding the DNA molecule into chromatin to fit into the nucleus of a cell
    • required because DNA is around 3 meters in length, and it needs to be packed into the small nucleus
  • DNA strands are helically coiled (right-handed coil)
  • Pitch of each helix is 3.32 nm (10 nucleotides per turn)
  • A typical strand of DNA is 2.2 meters long



DNA Replication

  • Replication results in multiple copies of a DNA strand, that takes place in three stages: initiation, elongation, and termination
  • Primary enzyme involved in the process: DNA Polymerase
  • DNA replication occurs after the unwinding of the double strand by helicase

replication DNA

Check out our DNA Replication Simulation in 3D!

Multiple Choice Questions

  1. DNA Polymerase synthesises the new DNA strand in which direction?

  2. Packaging of the DNA is important because ____________________

  3. TRUE or FALSE: Chromosomes are made up of wound up RNA and DNA.

  4. The different stages of DNA replication are ______, _________, and ________.

  5. The negative charge of the DNA double helix is neutralized by ______ proteins.

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