Ch 5Molecular Basis of Inheritance — Class 12 Biology, concept-first.

The previous chapter covered inheritance patterns, but at Mendel's time nobody knew what the underlying 'factors' actually were.

DNA — deoxyribonucleic acid — is the molecule that carries hereditary information in most living organisms.

A polynucleotide chain is the fundamental chemical unit of both DNA and RNA. To understand how this chain stores genetic information, you must first know what a single building block — a nucleotide —…

The DNA in a typical mammalian cell is astonishingly long. If you take the distance between two consecutive base pairs as 0.34 nm (that is 0.34 × 10⁻⁹ m) and multiply it by the total number of base pa…

The central question that early geneticists faced was simple: what molecule inside the cell actually carries hereditary information? By the early twentieth century, scientists knew that chromosomes —…

The question of what actually carries hereditary information — the genetic material — was settled decisively by Alfred Hershey and Martha Chase in 1952.

The debate over whether proteins or DNA carried genetic information was settled by the Hershey-Chase experiment. DNA became established as the genetic material for most organisms.

The story of the origin of life on Earth has a central puzzle: which came first — the information-carrying molecule (nucleic acid) or the functional molecule (protein)? For a long time, this was a chi…

The moment Watson and Crick proposed the double helix, they also saw how it could copy itself. In their own words from 1953: “It has not escaped our notice that the specific pairing we have postulated…

The central question in molecular biology after Watson and Crick proposed their model was: how does DNA copy itself? The model predicted a semiconservative mechanism — each new DNA molecule would keep…

The entire process of DNA replication is not carried out by a single enzyme. It requires a team of molecular machines, each performing a specific task with remarkable speed and accuracy.

Transcription is the first step in gene expression — the process by which a gene’s DNA sequence is copied into RNA. In molecular terms, it is the synthesis of RNA from a DNA template.

A transcription unit is not just any stretch of DNA that gets copied into RNA. It is a precisely defined segment, and its boundaries are set by three specific DNA regions that work together to ensure…

A gene is the functional unit of inheritance. While we know that genes are physically located on DNA, arriving at a precise definition of a gene in terms of a DNA sequence is surprisingly difficult.

In a bacterial cell, three distinct kinds of RNA work together to build a protein. Messenger RNA (mRNA) provides the actual template — the sequence of codons that will be read.

The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA) is translated into proteins.

The relationship between genes and DNA is best understood by studying mutations. You have already encountered mutations and their effects in Chapter 4.

The story of how the genetic code is actually read is a beautiful piece of biological logic. When Francis Crick first thought about the problem, he realised something crucial: amino acids themselves h…

Translation is the process by which the genetic information carried by mRNA is decoded to produce a specific sequence of amino acids, forming a polypeptide chain.

A cell does not express all its genes at once. That would be wasteful and chaotic. Instead, gene expression is tightly controlled so that a bacterium, for example, makes the enzymes to digest lactose…

The lac operon was the first transcriptionally regulated system to be understood, and its discovery came from the close collaboration of geneticist Francois Jacob and biochemist Jacque Monod.

The Human Genome Project (HGP) was a landmark international scientific research project with one primary goal: to determine the complete sequence of DNA base pairs that make up the human genome, and t…

The human genome is the complete set of genetic material present in a human cell. Before the Human Genome Project, scientists had wildly different guesses about how many genes we had — estimates range…

The Human Genome Project gave us the complete DNA sequence of Homo sapiens, but a sequence alone is just a string of letters.

As stated earlier, 99.9 per cent of the base sequence among humans is the same. If the human genome is roughly 3 × 10⁹ base pairs, then differences exist in about 0.1 per cent of that — that is, in ro…

- DNA as genetic material: Griffith's transformation experiment (1928) showed a "transforming principle" in heat-killed S. pneumoniae. Avery, MacLeod, and McCarty (1944) proved it was DNA.

Higher-order thinking problems from the NCERT Exemplar.

Questions from official CBSE sample papers.