CBSE Class 12 Biology Revision Notes Chapter 6

Class 12 Biology Chapter 6 Notes – Molecular Basis of Inheritance

Class 12 biology Chapter 6 Notes explain the molecular basis of inheritance. By referring to these Class 12 biology Chapter 6 Notes, students can understand the key concepts in the structure and function of DNA and RNA. Deoxyribonucleic acid, or DNA, is the molecule which contains the genetic code of living beings and provides information about the living cells and  the type of protein to generate. In Class 12 biology Chapter 6 Notes, students learn about replication, transcription and translation. Students can understand and learn the chapter well and revise accurately and quickly.

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Class 12 Biology Chapter 6 Notes cover all the topics and give students a detailed explanation since our qualified teachers prepare them by following the CBSE Syllabus. Class 12 biology Chapter 6 Notes give them an easy way to study and revise and reduce the burden on the students. Extramarks’ academic team has created Class 12 biology Chapter 6 Notes by referring to the NCERT textbook and other books.

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Key Topics Covered In Class 12 Biology Chapter 6 Notes

Class 12 biology Chapter 6 Notes cover all essential topics from Class 12 Biology Chapter 6.

Following are the topics included in Class 12 Biology Chapter 6 Notes:

  • The DNA
  •  The Search for Genetic Material
  •  RNA World
  •  Replication
  •  Transcription
  •  Genetic Code
  •  Translation
  •  Regulation of Gene Expression
  •  Human Genome Project
  •  DNA Fingerprinting

For every concept included in the Class 12 Biology Chapter 6 notes, below is an explanation prepared by the expert teachers at Extramarks. Students must study the Chapter 6 biology Class 12 notes to clear all concepts. Biology Chapter 6 Class 12 notes help students prepare all key concepts, enhance their understanding of the subject, and score well in their examinations.

Class 12 Biology Chapter 6 Notes: Molecular Basis of Inheritance:


DNA is a long polymer of deoxyribonucleotides that carries genetic information in all living organisms. The length of DNA is defined as the count of nucleotides or pairs of nucleotides, also called base pairs, present in it. DNA length is also one of the characteristics of an organism. For example, the bacteriophage lambda has 48502 base pairs (bp), the bacteria Escherichia coli has 4.6 × 106 bp, and human haploid DNA content is 3.3 × 109 bp.

Structure of Polynucleotide Chain:

A nucleotide consists of three components –

  • a pentose sugar (deoxyribose for DNA and ribose in the case of RNA)
  • a nitrogenous base
  • a phosphate group.

There are two nitrogenous bases –Purines (Adenine and Guanine) and Pyrimidines (Cytosine, Uracil, and Thymine). All the bases are common in DNA and RNA, except one, uracil, which is present in the place of thymine in RNA.

  • A nucleoside is formed by attaching a nitrogenous base to the OH of 1′-C of pentose sugar via an N-glycosidic linkage.
  • A corresponding nucleotide (or deoxynucleotide) is formed when a phosphate group is attached to the OH of the nucleoside’s 5′ C via phosphodiester linkage.
  • Two nucleotides are attached through 3′-5′ phosphodiester bonds to form a dinucleotide. More and more nucleotides can be joined similarly to form a polynucleotide chain.
  • Thus the polymer formed on one end has a free phosphate moiety at the 5′ -end of sugar, which is referred to as the 5′-end of the polynucleotide chain and at the other end, there is a free OH of the 3’C group, which the sugar has, which is called to as 3′ -end of the nucleotide chain.
  • A polynucleotide chain’s backbone is made up of phosphates and sugar; the nitrogenous bases attached to the sugar moiety project inside from the backbone.
  • In RNA, each nucleotide residue has an additional –OH group present at the 2′ -position in the ribose sugar. Also, in RNA, uracil is found instead of Thymine.
  • In 1953, Francis Crick and James Watson suggested a very simple Double Helix model for DNA structure based on X-ray diffraction data.
  • The base pairs of nucleotides are said to be complementary to each other.

The main features of the Double helical structure of DNA:

  • Double helical DNA is made of two polynucleotide chains, where the backbone is made up of sugar-phosphate, and the bases project inside.
  • The two chains have antiparallel polarity. It means that if one chain has polarity 5′ to 3′, the other has polarity 3′ to 5′.
  •  The bases in two strands are linked through hydrogen bonds (H-bonds), forming base pairs (bp).
  • Adenine makes two hydrogen bonds with the thymine on the opposite strand, and vice-versa. Similarly, cytosine is bonded to guanine with three H-bonds.
  • As a result, pyrimidine is always the inverse of purine.This generates a uniform distance between the two strands of the helix.
  • The two polynucleotide chains are coiled in a right-handed manner. The pitch of the DNA helix is 3.4 nm, and there is roughly ten bp in each turn. The distance between two base pairs in a helix is approximately 0.34 nm.
  • The plane of one base pair stacks over the other in the double helix. This, in addition to H-bonds, confers stability to the helical structure.

Francis Crick put forward the Central dogma in molecular biology, which says that the genetic

information flows from DNA——–> RNA———> Protein.

Packaging of DNA Helix:

In prokaryotes, such as E. coli, the DNA is not scattered throughout the cell. DNA is negatively charged and is held with some positively charged proteins in a region termed ‘nucleoid’. The DNA in nucleoids is seen as very large loops held by proteins.

Eukaryotes possess a set of positively charged, basic proteins called histones. Histone proteins are rich in the basic amino acid residues that have positive changes in their side chains.

Eight histone molecules are organised into a unit called the histone octamer.The negatively charged DNA polymer wrapped around the positively charged histone octamer forms a nucleosome structure.

A nucleosome contains 200 bp of DNA helix. Nucleosomes are repeating units of chromatin, which are the thread-like stained bodies seen in the nucleus. In chromatin, the nucleosomes are seen as ‘beads-on-string’ structures when viewed under an electron microscope.

  • The chromatin nucleosomes (beads-on-string structure) are packaged to form chromatin fibres. These chromatin fibres are further coiled and condensed at the metaphase stage of cell division to form chromosomes. The packaging of chromatin also requires an additional set of proteins that are collectively referredto as non-histone chromosomal (NHC) proteins.
  • Euchromatin: The regions of chromatin that are loosely packed (and therefore stain light) are referred to as euchromatin.
  • Heterochromatin: Some chromatin regions that are more densely packed and stain dark are called Heterochromatin.
  • Euchromatin is said to be transcriptionally( where RNA synthesis is taking place) active chromatin, whereas heterochromatin is inactive.


Transforming Principle:

  • In 1928, Frederick Griffith observed a transformation (change in physical form) in the bacterium  Streptococcus pneumonia during his experiment.
  • When these bacteria are grown on a culture plate, some produce rough colonies (R), while others produce smooth, shiny colonies (S). This is so because the S-strain bacteria have a mucous coat of polysaccharides, while the R-strain bacteria do not.
  • Mice infected with the R strain bacteria do not suffer from pneumonia, but mice infected with the S strain (virulent) die from pneumonia.
  • Griffith again observed that when heat-killed S-strain bacteria were injected into mice, it did not kill them. The mice died when a mixture of heat-killed  S and  live R bacteria were injected.
  • He found living S-strain bacteria in the dead mice.
  • He concluded that the heat-killed S-strain bacteria had somehow transformed the R-strain bacteria.
  • Some ‘transforming principles’ got transferred from the heat-killed S strain, which enabled the R strain to synthesise smooth colonies and become virulent.
  • This virulence must be due to the transfer of genetic material. However, his experiments did not define the biochemical nature of genetic material.

Biochemical Characterisation of Transforming Principle:

  • The biochemical nature of Griffith’s experiment’s “transforming principle” was determined by Oswald Avery, Maclyn McCarty, and Colin MacLeod.
  • They purified biochemicals (DNA, RNA, proteins, etc.) from the heat-killed S cells to see the transformation of R cells into S cells.
  •  RNA-digesting enzymes (RNases) and protein-digesting enzymes (proteases) did not affect this transformation, so the transforming substance was not an RNA or protein.
  • DNase digestion prevents transformation, implying that the transformation was caused by the DNA.
  • They found that DNA alone from S bacteria made R bacteria transform into virulentstrains, and they concluded  that DNA is the hereditary material.

The Genetic Material is DNA (experimental proof):

  • In 1952, Alfred Hershey and Martha Chase worked with bacteriophages (viruses that infect bacteria).
  • The bacteriophage infects the bacteria, and its genetic material enters the bacterial cell. Hershey and Chase worked to find whether it was the protein or the DNA from the viruses that had entered the bacteria.
  • They grew some viruses on a medium containing radioactive phosphorus and others on a medium containing radioactive sulphur.
  •  Viruses grown on radioactive sulphur carry radioactive protein but not radioactive DNA because DNA does not contain sulphur. Similarly,
  • Viruses supplied with radioactive phosphorus contain radioactive DNA but not radioactive protein because DNA contains phosphorus, but protein does not.
  • E. coli bacteria were infected with radioactive phages. Then, after some time, as the infection progressed, the viral coats and the bacteria were separated by agitating them in a blender.
  • The bacteria were separated from virus particles by centrifugation.
  • Bacteria infected with viruses with radioactive DNA were radioactive, indicating that DNA was the material passed from the virus to the bacteria. Bacteria that were infected with radioactive proteins that contained viruses were not radioactive. This showed that viral proteins did not enter the bacteria. DNA is the genetic material that is passed to bacteria.

Properties of Genetic Material (DNA versus RNA):

The following criteria must be fulfilled by a molecule that can act as genetic material:

(1) It should be able to give rise to its replica (Replication).

(2) It should be stable chemically and structurally.

(3) It should provide the chance for slow changes (mutation)

required for evolution.

(4) It should be able to express itself in the form of ‘Mendelian


  • Both nucleic acids (DNA and RNA) can direct their duplications. Proteins fail to fulfil the first criteria themselves.
  • The two complementary strands, if separated by heating, come together when appropriate conditions are provided. The 2′-OH group in every nucleotide in RNA is reactive, making RNA labile and easily degradable. RNA has catalytic properties, hence its reactive nature. The presence of thymine instead of uracil also gives DNA additional stability. Therefore, DNA is structurally more stable and chemically less reactive when compared to RNA.
  • Both DNA and RNA can mutate. Because it is unstable, RNA mutates at a faster rate.
  • RNA can directly translate the proteins and hence  easily express the characters. DNA, however, is dependent on RNA for protein synthesis. The protein-synthesising complex has evolved around RNA.

Hence, both RNA and DNA can function as genetic material, but DNA, being more stable, is preferred for storing genetic information. RNA is better for the transmission of genetic information.


RNA was the first genetic material. Necessary life processes (like metabolism, translation,

splicing, etc.) developed around RNA. DNA has evolved from RNA but has become more stable with chemical modifications.


Watson and Crick( 1953) proposed a semiconservative scheme for DNA replication. The replication scheme suggested that the two strands would separate and act as a template for synthesising new complementary strands. After the replication process, each DNA molecule possesses one parent and one newly synthesised strand.

It is now proved that DNA replicates semi-conservatively. Matthew Meselson and Franklin Stahl did the following experiment in 1958:

  • They grew E. coli bacteria in a medium containing ¹⁵NH₄Clas, the only nitrogen source for many generations (¹⁵N is the heavy isotope of nitrogen); as a result, ¹⁵N was incorporated into newly synthesised DNA (as well as other nitrogen-containing compounds). A density gradient could easily distinguish this heavy DNA molecule from normal DNA.
  •  Then they shifted the cells into a medium with normal ¹⁴NH₄Cl. Then the DNA that was extracted from the culture after one generation, that is, after 20 minutes, [ E. coli divides in 20 minutes] had an intermediate or hybrid density that is in between the ¹⁵N and ¹⁴N-DNA because the new DNA molecule has one ¹⁵N old strand and ¹⁴N new complementary strand (semiconservative DNA replication). After 40 minutes, DNA extracted from the culture after the 2nd generation was composed of equal amounts of this hybrid and ‘light’ DNA.


  • The main enzyme for DNA replication is DNA-dependent DNA polymerase because it uses a DNA template to catalyse the polymerisation of deoxynucleotides.
  • DNA helix unwinds by the action of the helicase enzyme.
  • The replication occurs within a small opening in the DNA helix, called the replication fork.
  • The DNA-dependent DNA polymerases catalyse polymerisation only in the 5′–>3′ direction.
  •  On one template strand (with polarity 3′–>5′), the replication is continuous, while on the other (the template with polarity 5′–>3′), it is discontinuous.
  • The enzyme DNA ligase joins the discontinuously synthesised fragments (Okazaki fragments).

There is a definite region in the DNA where the replication originates. Such regions have been termed the origin of replication. In eukaryotes, DNA replication occurs at the S-phase of the cell cycle.


Transcription involves copying genetic information from a DNA strand into RNA. In this process, one segment of only one strand of DNA is copied into RNA.

Transcription Unit

A transcription unit in the DNA is defined by the three regions primarily in the DNA:

(i) A Promoter

(ii) The structural gene

(iii) A Terminator

  • The DNA-dependent RNA polymerase also catalyses the polymerisation in only one direction, that is, 5’→3′, the strand with the polarity 3’→5′ acts as a template strand. The other strand, with the polarity 5’→3′ and the same sequence  as RNA (except Thymine base at the place of uracil), is displaced during the process of transcription. This displaced strand (which does not code for anything) is called the coding strand.
  • The promoter and terminator regions flank the structural gene in a transcription unit. The promoter region is situated toward the 5′ -end (upstream) of the structural gene on the template strand. It is a DNA sequence that forms a binding site for the RNA polymerase, and the presence of a promoter region in a transcription unit  also defines the template and coding strands.
  • The terminator is located towards the 3′ -end (downstream) of the coding strand and usually defines the end of the transcription process.

Cistron is the segment of DNA coding for a polypeptide (structural gene in a transcription unit). It could be monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes).

Split Genes: In eukaryotes, the monocistronic structural genes have interrupted coding sequences – and the genes in eukaryotes are split. The coding sequences are called exons. Exons are the sequences that appear in mature RNA. The exons are interrupted by introns. These introns or  intervening sequences do not appear in the mature or processed RNA.

Types of RNA and the Transcription process:

In bacteria, there are three major types of RNAs found:

  • mRNA (messenger RNA)- which provides the template
  • tRNA (transfer RNA), and- transports amino acids while also reading the genetic code.
  • rRNA (ribosomal RNA)-  play a structural and catalytic role during translation.

A single DNA-dependent RNA polymerase catalyses the transcription of all types of RNA in bacteria.

Initiation- RNA polymerase associates transiently with initiation factor (σ), binds to the promoter and initiates Transcription.

Elongation- It uses nucleoside triphosphates as substrate and polymerises in a template-dependent fashion following the rule of complementarity. It also helps the opening of the DNA helix, and elongation continues. Only a small portion of RNA remains bound to the enzyme.

Termination-Once the polymerase reaches the terminator region, the nascent RNA falls off, and so does the RNA polymerase with the help of the termination- factor (ρ). This results in the termination of transcription.

  • In eukaryotes, three different RNA polymerase enzymes, I, II, and III, are found, catalysing the synthesis of all types of RNA. RNA polymerase I transcribes rRNAs, whereas RNA polymerase II transcribes the precursor of mRNA, the heterogeneous nuclear RNA (hnRNA). RNA polymerase III transcribes tRNA, 5srRNA, and snRNAs (small nuclear RNAs).
  • In eukaryotes, the primary transcripts (hnRNA) contain both the exons and the introns and are non-functional; hence, the introns are removed, and the exons are joined in a defined order (gene splicing). hnRNA undergoes further processing called capping and tailing. In the capping process, an unusual nucleotide called  methyl guanosine triphosphate is added to the 5′-end of hnRNA. In tailing, adenylate residues are added at the 3′-end in a template-independent manner. Fully processed hnRNA, now called mRNA, is transported out of the nucleus for translation.


The encoded information in genetic material is translated into proteins with the help of the genetic code.

George Gamow said there are only four bases; if they have to code for 20 amino acids, the code should be different combinations of four bases. Gamow recommended that the code consist of three nucleotides for all 20 amino acids.

The salient features of the genetic code:

  • The codon is a triplet (made up of three nucleotides). Sixty-one codons code for amino acids, and three codons function as stop or terminator codons (UAA, U
  • GA, UAG); hence, they do not code for amino acids.
  • There is more than one codon for some amino acids; hence, the code degenerates.
  •  The codon is read into mRNA continuously. There are no punctuations.
  •  The code is nearly universal for every organism: for example, from bacteria to humans, UUU codes for phenylalanine (phe). Some exceptions have been found in some protozoans and mitochondrial codons.
  •  AUG has dual functions. It codes for methionine (met) and acts as an initiator codon.

Mutations and Genetic Code:

In the gene, a change of a single base pair (point mutation) for the beta-globin chain results in the change of the amino acid residue to valine instead of glutamate. It results in a diseased condition called sickle cell anaemia.

Frameshift Mutation- Deletion or Insertion of one or two bases changes the whole reading frame from the point of deletion or insertion. However, such mutations are called frameshift deletion or insertion mutations. Deletion or insertion of three or its multiple bases insert or delete in one or multiple codons hence

one or multiple amino acids, and the reading frame remains unaltered from that point onward.

tRNA– the Adapter Molecule:

  • tRNA has an amino acid acceptor end to which it binds  amino acids and an anticodon loop with bases complementary to the code for protein synthesis.
  • tRNAs are specific for each amino acid.
  • For initiation, another specific tRNA is referred to as the initiator tRNA. There are no tRNAs for stop codons.
  • The secondary structure of tRNA is similar to that of a  clover leaf. In its real structure, the tRNA is a compact molecule which appears like an inverted L.


Translation refers to the polymerisation of the amino acids to form a polypeptide chain.

The amino acids are linked by a bond  known as a peptide bond.

  • Charging of tRNA- Amino acids are thus activated in the presence of  ATP and linked to their cognate tRNA-  through aminoacylation of tRNA. Suppose two such charged tRNAs are brought closer. In that case,  peptide bond formation isfavourably influenced by them—the presence of a catalyst ( ( peptidyl transferase enzyme) increases the rate of peptide bond formation.
  • A translational mRNA sequence is flanked both by the start codon (AUG) and by the stop codon and codes for a polypeptide. The sequence of amino acids is according to the sequence of bases in the mRNA.
  • An mRNA has a few additional sequences that are not translated, called untranslated regions (UTR), present at both the 3′ -end (after the stop codon) and the 5′ -end (before the start codon).
  • In its inactive state, the ribosome( cellular factory for protein synthesis) exists as two subunits; a small subunit and a large subunit. When the small subunit finds an mRNA, the process of translation of the mRNA into protein starts. The ribosome’s larger subunit has two sites for upcoming amino acids to bind to. Thus, amino acids remain close enough to each other to form a peptide bond.
  • The ribosome also acts as a catalyst ( ribozyme enzyme -23S rRNA in bacteria) for the peptide bond formation.
  •  The ribosome binds to the mRNA at the start codon (AUG) for initiation, recognised only by the initiator tRNA. Then aminoacylated tRNA binds to the appropriate codon in mRNA sequentially
  • by forming the complementary base pairs with the tRNA anticodon. The ribosome slides from codon to codon along the mRNA. Amino acids are added by tRNA and translated into polypeptide sequences (elongation).
  •  In the end, a release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome.


  • Gene expression to form polypeptides can be regulated at many levels in eukaryotes:

-Transcriptional level at the time of formation of a primary transcript, i.e. Transcription

-At the time of processing or splicing

-During the transport of mRNA from the nucleus to the cytosol-Translational level

  • Environmental, metabolic, or physiological conditions regulate gene expression.
  • An embryo’s development and differentiation result from the coordinated regulation of several sets of genes.
  • Control of gene expression occurs primarily during transcription initiation., in the case of prokaryotes, .
  • Regulatory proteins regulate RNA polymerase activity at the start site and can be activators or repressors.
  • The availability of the promoter region is controlled by an operator sequence that binds to a specific protein, most commonly the repressor protein.There is a specific operator sequence and repressor protein for each operon.

E.G., Lac operon- It is the prototype operon in bacteria, which codes for genes responsible for lactose metabolism. The operon is controlled by the amount of lactose (which acts as a repressor protein) in the medium where the bacteria are grown. Therefore, regulation by the Lac operon could also be seen as the regulation of enzyme synthesis by its substrate.


A  project to sequence the human genome was launched in 1990 to find the complete DNA sequence of the human genome using genetic engineering techniques and Bioinformatics.

The human genome is estimated to contain nearly 3 x 109 bp.The enormous amount of data necessitates  using high-speed computational devices for data storage, retrieval, and analysis.

Goals of HGP

  • identification of all the approximately 20,000-25,000 genes present in human DNA;
  • Find out the sequences of these 3 billion chemical base pairs that form the human DNA;
  •  Stories this information in their databases;
  •  Improve tools for data analysis;
  •  Transfer related information and technologies to other sectors, such as industries and research organisations.
  •  Address the project’s emerging ethical, legal, and social issues (ELSI).

HGP was a 13-year project coordinated by the U.S. Department of Energy and the National Institute of Health. The project was completed in 2003.

  • The methods involved two major approaches;
  • First was focused on identifying all the genes is expressed as RNA (referred to as Expressed Sequence Tags (ESTs).
  • The other approach involved sequencing the whole genome set containing all the non-coding and coding sequences and later assigning different regions within the sequence with functions which is a term referred to as Sequence Annotation.
  • The DNA fragments were sequenced using automated DNA sequencers, which worked on the principle of a method developed by Frederick Sanger.

Salient Features of the Human Genome

(i) The human genome consists of 3164.7 million bp.

(ii) An average gene consists of 3000 bases, but dystrophin’s largest known human gene consists of 2.4 million bases.

(iii) The total number of genes is evaluated at 30,000. In humans, nearly all nucleotide bases (99.9%) are identical.(iv) The functions of over 50 percent of the discovered genes are unknown.

(v) Less than 2 percent of the total genome codes for proteins.

(vi) A large portion of the human genome comprises repeated sequences.

(vii) Repetitive sequences are extended DNA sequences repeated many times, sometimes hundreds to thousands of times. Repetitive DNA sequences have no direct coding functions, but they definitely shed light

on chromosome structure, dynamics and evolution.

(viii) Chromosome 1 has the maximum genes (2968), and the Y has the minimum (231).

(ix) Scientists have discovered about 1.4 million locations in humans where single-base DNA differences (SNPs—single nucleotide polymorphisms) occur.


DNA fingerprinting is a very quick method of comparing the DNA sequences of any two individuals and finding variations in individuals within a population at the DNA level.

DNA Fingerprinting works on the principle of polymorphism in DNA sequences. It has many applications in forensic Science, genetic biodiversity and evolutionary biology.

In DNA fingerprinting, there is the identification of differences in some specific regions of a DNA sequence called repetitive DNA because, in these sequences, a small extent of DNA is repeated multiple times. These repetitive DNA sequences get separated from bulk genomic DNA by gradient centrifugation, referred to as satellite DNA. These sequences show a great deal of polymorphism and form the basis of DNA fingerprinting.

Since DNA from every tissue of an individual shows the same degree of polymorphism, it has become a very useful identification tool in forensic applications. Further, as polymorphisms

are inherited from the parents to their children, thus DNA fingerprinting is key to resolving paternity testing  disputes.

Alec Jeffreys developed the technique of DNA Fingerprinting. He used satellite DNA as a probe that showed a very high degree of polymorphism. It is known as the Variable Number of Tandem Repeats (VNTR). The technique involves Southern blot hybridisation using radio labelled VNTR as a probe. It included

  • isolation of DNA,
  • digestion of DNA by restriction endonucleases enzyme,
  • DNA fragments are separated by gel electrophoresis,
  • transferring (blotting) of separated DNA fragments to synthetic membranes, such as nitrocellulose filter membrane or nylon,
  • hybridisation using a radiolabeled VNTR probe, and
  • detection of hybridised DNA fragments by autoradiography.

After hybridisation with the VNTR probe, the autoradiogram gives many bands of differing sizes. These bands provide a characteristic pattern for an individual’s  DNA.

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