Revision: Class 12 >> Molecular Basis of Inheritance NEET (UG) Molecular Basis of Inheritance

Transfection is the process of inserting a vector into eukaryotic cells.

The thread-like complex of DNA and proteins present in the nucleus of eukaryotic cells is called chromatin.

The process by which a very long DNA molecule is compactly organised inside the cell nucleus so that it fits within the limited nuclear space and remains functional is called DNA packaging.

Positively charged basic proteins rich in lysine and arginine that associate with DNA to help in its packing in eukaryotic cells are called histones.

A structural unit composed of eight histone protein molecules around which DNA is wrapped is called histone octamer.

The basic repeating unit of chromatin formed by DNA wrapped around a histone octamer is called nucleosome.

Proteins other than histones that are associated with chromatin and help in higher-order DNA packaging and regulation are called non-histone chromosomal (NHC) proteins.

Nucleoid is the region in prokaryotic cells where DNA is organized and associated with proteins, despite the absence of a true nucleus.

When DNA directs the synthesis of DNA itself, then such function of DNA is called autocatalytic function. Eg. Replication.

The process by which DNA duplicates itself is called replication. 

When DNA directs the synthesis of chemical molecules other than itself, then such functions of DNA are called heterocatalytic functions. Eg, Synthesis of RNA (transcription), synthesis of protein (Translation), etc.

Sudden inheritable change in the genetic material is called mutation.

A sequence of three nucleotides on mRNA that codes for a specific amino acid is called a triplet codon.

The technique of identifying an individual by analyzing the unique DNA sequence present in each person, similar to fingerprints, is called DNA fingerprinting.

• Nucleic acids are polymers of nucleotides and serve as the building blocks of genetic material.
• Two types of nucleic acids found in living systems are DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid).
• DNA acts as the genetic material in most organisms; it was identified as the genetic material over a century after Mendel's era.
• RNA acts as genetic material in some viruses; in other organisms, it primarily functions as a messenger (mRNA) and also acts as an adapter, structural and catalytic molecule.
• Human genome sequencing has opened a new era of genomics, building on the understanding of DNA as genetic material.

• A nucleotide has three parts: nitrogenous base, pentose sugar and phosphate group.
• Sugars differ → RNA has ribose, DNA has deoxyribose.
• Two types of bases:
  • Purines → Adenine (A), Guanine (G)
  • Pyrimidines → Cytosine (C), Thymine (T), Uracil (U)
• Cytosine is common to both DNA and RNA; Thymine is in DNA, and uracil is in RNA.
• Nucleoside = base + sugar; nucleotide = nucleoside + phosphate group.
• Nucleotides join by 3′–5′ phosphodiester bonds to form a polynucleotide chain.
• The chain has a 5′ end (free phosphate), a 3′ end (free OH), and its backbone is made of sugar and phosphate.

• DNA has two polynucleotide chains forming a double helix structure.
• The two strands run in an anti-parallel direction (one 5′→3′, the other 3′→5′).
• Base pairing occurs through hydrogen bonds:
  • A pairs with T (2 H-bonds)
  • G pairs with C (3 H-bonds)
• Purine always pairs with pyrimidine, maintaining a uniform distance between strands.
• The helix is right-handed, with 10 base pairs per turn and a pitch of 3.4 nm.
• Base pairs stack over each other, which increases the stability of the DNA structure.

• Base pairing makes DNA strands complementary, so the sequence of one strand helps predict the other.
• During DNA replication, each strand acts as a template to form identical daughter DNA molecules.
• The double helix model explains how genetic information is stored and copied.
• Central dogma: genetic information flows as DNA → RNA → Protein.
• In some viruses, reverse transcription occurs (RNA → DNA).

Prokaryote vs Eukaryote Packaging

• The discovery of nuclein and the proposal of inheritance principles occurred simultaneously, yet confirming DNA as the genetic material required considerable time.
• By 1926, the scientific investigation into the mechanisms of genetic inheritance had advanced to the molecular level.
• Cumulative research by scientists such as Mendel, Sutton, and Morgan successfully narrowed the source of genetic inheritance to chromosomes within the cellular nucleus.
• Key historical milestones include Hofmeister's 1848 observation of chromosomes during mitosis and Miescher's 1869 isolation of nuclein, which Altman later renamed nucleic acid.
• By 1920, the establishment that chromosomes are composed of both proteins and DNA initiated further experimental studies to identify the exact molecular carrier of genetic information.

• Frederick Griffith (1928) performed experiments on Streptococcus pneumoniae.
• There are two strains:
  • S strain (smooth, virulent) → causes disease
  • R strain (rough, non-virulent) → does not cause disease
• S strain kills mice, while R strain does not.
• Heat-killed S strain alone does not kill mice.
• A mix of heat-killed S + live R strain kills mice, and live S bacteria are found.
• Griffith concluded that a “transforming principle” from S strain converted R strain into virulent form.

• Avery, MacLeod & McCarty identified the transforming principle.
• They tested DNA, RNA and proteins from bacteria.
• Only DNA could transform R cells into S cells.
• Protease and RNase did not stop transformation (not protein/RNA).
• DNase stopped the transformation, proving that DNA is the genetic material.

• Hershey and Chase (1952) proved that DNA is the genetic material using bacteriophages.
• They used radioactive phosphorus (³²P) to label DNA and radioactive sulphur (³⁵S) to label proteins.
• Phages infected E. coli bacteria, injecting their genetic material.
• After blending and centrifugation, DNA (³²P) was found inside bacteria.
• Protein (³⁵S) remained outside in the supernatant.
• This showed that DNA, not protein, is the genetic material.

• Discovery of Ribozymes - Sidney Altman and Thomas Cech independently discovered that RNAs can act as biocatalysts.
• RNA World hypothesis - The RNA World hypothesis suggests that early life was based exclusively on nucleic acids, most probably RNA, and was first proposed by Carl Woese, Francis Crick, and Leslie Orgel in 1960.
• Evidence for RNA World - RNA is found abundantly in all living cells, structurally related to DNA, and can evolve, replicate, and catalyse reactions.
• Formation of primitive cells - RNA molecules underwent replication, mutation, and developed their own machinery to form primitive cells.
• Formation of DNA - Double-stranded DNA formed eventually, resulting in rich biodiversity.

• DNA replication is the process by which a DNA molecule makes exact copies of itself, each parental strand acting as a template for a new complementary strand, giving two identical daughter molecules with one old and one new strand each (semi-conservative).
• It occurs during the S-phase (Synthesis phase) of interphase in the cell cycle.
• Proposed by Watson and Crick (1953) and experimentally confirmed as semi-conservative by Meselson and Stahl (1958) in E. coli.
• Of the three proposed models, semi-conservative was proved correct, while conservative and dispersive were disproved.
• Replication is an autocatalytic function (DNA making DNA), unlike heterocatalytic functions, where DNA directs the synthesis of other molecules, like RNA (transcription) or protein (translation).

• In 1958, Meselson and Stahl proved that DNA replicates semi-conservatively in E. coli.
• E. coli was first grown in heavy nitrogen (¹⁵N) medium, making all DNA heavy (¹⁵N-¹⁵N); cells were then transferred to normal nitrogen (¹⁴N) medium.
• DNA samples were separated using CsCl (caesium chloride) density gradient centrifugation to distinguish heavy, light and hybrid DNA.
• After one generation in ¹⁴N medium, DNA showed hybrid/intermediate density (¹⁵N-¹⁴N); after two generations, DNA was half hybrid + half light (¹⁴N-¹⁴N).
• In 1958, Taylor and colleagues also proved semi-conservative replication in Vicia faba (faba beans) using radioactive thymidine.

• Main enzyme = DNA-dependent DNA polymerase; polymerises at ~2000 bp/sec; mistakes cause mutations.
• Deoxyribonucleoside triphosphates act as substrates and provide energy for polymerisation.
• Replication occurs at the replication fork; only in the 5' to 3' direction; one strand = continuous, the other = discontinuous; fragments are joined by DNA ligase.
• Replication starts at a fixed point called the origin of replication; DNA polymerase cannot initiate randomly.
• In eukaryotes, replication occurs in the S-phase; failure of cell division after replication causes polyploidy.

• Transcription is the process in which genetic information from one strand of DNA (template strand) is copied into RNA with the help of RNA polymerase.
• It occurs in the nucleoid in prokaryotes and in the nucleus in eukaryotes; mRNA then moves to the cytoplasm for translation.
• A transcription unit has three parts: promoter (start site), structural gene, and terminator (stop site).
• The process occurs in three stages: initiation (RNA polymerase binds promoter), elongation (RNA chain is formed), and termination (RNA polymerase detaches).
• Only one DNA strand acts as a template (3′→5′), while the other is the coding strand (5′→3′).
• In eukaryotes, primary RNA (hnRNA) is processed by capping, tailing, and splicing to form mature mRNA.

• A transcription unit has 3 regions - Promoter, Structural gene and Terminator.
• Template strand = 3' to 5' polarity; Coding strand = 5' to 3' polarity; RNA matches coding strand (U instead of T).
• Promoter is upstream (5' end); binds RNA polymerase; defines template and coding strands.
• Terminator is downstream (3' end); signals end of transcription.
• RNA polymerase works only in 5' to 3' direction.
• Switching promoter and terminator positions reverses the template and coding strands.

• A single RNA polymerase synthesises all types of RNA (mRNA, tRNA, rRNA).
• Initiation starts when RNA polymerase binds to the promoter region.
• Elongation occurs using nucleotides based on complementary base pairing.
• Termination happens at a terminator sequence, releasing RNA.
• No mRNA processing; transcription and translation occur simultaneously.

• Transcription copies DNA → RNA using RNA polymerase, following complementarity rules (A pairs with U).​
• In eukaryotes, 3 RNA polymerases: Pol I (rRNA), Pol II (hnRNA → mRNA), and Pol III (tRNA, 5S rRNA, snRNA).​
• The template strand has 3′→5′ polarity; RNA is synthesised 5′→3′.​
• Transcription unit = Promoter + Structural Gene + Terminator.​
• In eukaryotes, the primary transcript = hnRNA, which contains both exons and introns.​
• 3 processing steps: Splicing (remove introns) → 5′ Capping (m⁷G) → 3′ Poly-A tailing (200–300 A residues).​
• Mature mRNA is transported to the cytoplasm for translation.​

• The genetic code is the coded information in the base sequence of DNA/mRNA that determines the amino acid sequence in a protein.
• It is a triplet code - three consecutive bases form one codon, proposed by George Gamow (1954).
• There are 64 codons in total: 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA).
• AUG is the start codon and codes for methionine.
• It was deciphered mainly by Nirenberg, Khorana, and Ochoa (poly-U mRNA showed UUU = phenylalanine).
• It is degenerate (one amino acid can have several codons, usually differing in the third base - the wobble effect) and nearly universal.
• A change in the base sequence alters the amino acid sequence, so the code directly controls protein synthesis.

• A mutation is a sudden heritable change in the genetic material (DNA).
• A point mutation involves a change in a single base pair; for example, sickle cell anaemia.
• A frame-shift mutation occurs due to the insertion or deletion of bases.
• Frame-shift mutations change the reading frame of the genetic code, affecting the entire protein sequence.

• tRNA is the adapter molecule (Crick) that picks up amino acids and matches them to the correct mRNA codon.
• Robert Holley proposed the cloverleaf model (2D); in 3D, tRNA looks like an inverted L.
• It has three arms - DHU (binds aminoacyl-tRNA synthetase), anticodon (pairs with codon), TΨC (binds ribosome) - plus a 3′ CCA end for amino acid attachment.
• The amino acid is attached by aminoacyl-tRNA synthetase in a process called aminoacylation (charging), which needs energy.
• Each amino acid has a specific tRNA, with a special initiator tRNA for translation start; no tRNAs exist for stop codons.

• Translation is the process by which the codon sequence on mRNA is decoded with the help of tRNA at the ribosome to form a specific sequence of amino acids in a protein.
• It needs mRNA (template), tRNA (adapter), ribosome (with A, P, and E sites), amino acids, aminoacyl-tRNA synthetase, ATP/GTP for energy, and Mg²⁺ ions.
• Before translation, amino acids are activated and linked to their specific tRNAs (charging) by aminoacyl-tRNA synthetase.
• Initiation: the small ribosomal subunit binds mRNA at the start codon (AUG), the initiator tRNA carrying methionine attaches, and the large subunit joins to form the initiation complex.
• Elongation: amino acids are added one by one through codon–anticodon pairing, peptide bonds form between them, and the ribosome moves forward by one codon at a time (translocation).
• Termination occurs at a stop codon (UAA, UAG, UGA), where release factors free the polypeptide and the ribosomal subunits separate.

• Ribosomes are the site of protein synthesis and are made of rRNA and proteins.
• They have two subunits: large and small.
• Translation starts when the small subunit binds to mRNA at the start codon (AUG).
• The large subunit helps in peptide bond formation and has sites for amino acid attachment.
• rRNA acts as a catalyst (ribozyme) for peptide bond formation.
• During elongation, the ribosome moves along the mRNA and adds amino acids one by one.
• Termination occurs at a stop codon, releasing the completed protein.

• Gene regulation = switching genes ON or OFF based on the cell's requirement and stage of development.
• In eukaryotes, regulation occurs at 4 levels: Transcriptional (primary transcript), Processing (splicing), Transport (mRNA from nucleus to cytoplasm), and Translational.
• In prokaryotes, control of transcriptional initiation rate is the main site for gene expression control.
• E. coli produces β-galactosidase to break lactose → galactose + glucose. If lactose is absent, the enzyme is not produced, proving the environment regulates gene expression.
• Enzymes synthesized based on substrate availability are called inducible enzymes. The process = induction; the triggering molecule = inducer. This is a positive control.
• Feedback repression = when the end product (e.g., amino acid) is already available, genes for its production are switched OFF. This is a negative control.

• The lac operon is an inducible operon in E. coli, proposed by Jacob and Monod (1961), that controls the metabolism of lactose.
• It consists of a regulator gene (i), promoter (P), operator (O), and three structural genes - lac z, lac y, lac a - coding for β-galactosidase, permease, and transacetylase respectively.
• When lactose is absent: the regulator gene produces an active repressor that binds the operator and blocks RNA polymerase, so the operon remains switched OFF.
• When lactose is present: lactose is converted into allolactose (the inducer), which binds the repressor and inactivates it, leaving the operator free.
• RNA polymerase then transcribes the structural genes into a single polycistronic mRNA, producing the enzymes that break down lactose - the operon is now switched ON.

• The Human Genome Project (HGP) was an international mega-project launched in 1990 and completed in 2003, coordinated mainly by the U.S. DOE and NIH.
• Its aim was to identify all human genes and sequence the entire human genome of about 3 billion base pairs.
• The main goals were to identify genes, sequence the genome, store the data, develop analysis tools, transfer technologies, and address ethical issues.
• Methodology: DNA was isolated, fragmented, cloned into vectors like BACs and YACs, sequenced by automated methods, and assembled using computers.
• Salient features: the genome has ~3 billion base pairs and 20,000–25,000 genes; less than 2% codes for proteins, and humans are 99.9% identical.
• Most genetic variation between individuals is due to SNPs (single-nucleotide polymorphisms).
• Applications: disease gene mapping, early diagnosis, personalised medicine, evolutionary studies, and advances in biotechnology.
• ELSI (Ethical, Legal, Social Issues): genome data must be kept confidential to prevent misuse and discrimination.

• Human genome sequencing helps understand biological systems in detail.
• It requires the collaboration of many scientists worldwide from different fields.
• Research has shifted from studying single genes to whole genomes.
• Scientists can now study all genes, transcripts and protein interactions together.
• This approach helps in understanding complex life processes and networks.

• DNA fingerprinting is a technique used to identify an individual by analysing the unique DNA pattern present in every person (except identical twins).
• It is based on satellite DNA, especially VNTRs (Variable Number Tandem Repeats) - short sequences repeated in tandem, whose number varies among individuals and creates DNA polymorphism.
• Principle: the differences in VNTR repeat number produce DNA fragments of different lengths, which appear as a unique banding pattern.
• Steps: DNA isolation → PCR amplification → restriction digestion → gel electrophoresis → Southern blotting → probe hybridisation → autoradiography → comparison of band patterns.
• Applications: forensic identification, paternity/maternity testing, pedigree studies, medical research, conservation biology, and evolutionary/anthropological studies.

• VNTRs (Variable Number of Tandem Repeats) are short DNA sequences repeated many times in a row.
• They show high variation (polymorphism) due to different numbers of repeats in individuals.
• VNTR patterns are unique for each person (except identical twins).
• Used in DNA fingerprinting for identification, paternity testing and forensic analysis.
• PCR has improved DNA fingerprinting, allowing analysis even from very small DNA samples.