Revision: Genetics and Evolution >> Molecular Basis of Inheritance Biology Science (English Medium) Class 12 CBSE

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

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.

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.

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

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

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

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

Conservative replication is a mode of DNA replication in which the original parental DNA molecule remains intact, and a completely new DNA molecule is synthesized.

Dispersive replication is a mode of DNA replication in which the parental DNA is broken into fragments, and each daughter DNA molecule contains a mixture of old and new DNA segments.

Semi-conservative replication is a mode of DNA replication in which each daughter DNA molecule consists of one parental (old) strand and one newly synthesized strand.

The movement of the ribosome from one end of the mRNA to the other end by the distance of one triplet codon during translation is known as translocation.

Translation is the process by which tRNA having anticodon to the codon on the mRNA, supplies amino acids, as per the message on mRNA.

A sequence of three adjacent nucleotides in mRNA that codes for one amino acid is known as a codon.

Central dogma is the principle that genetic information flows in one direction in a cell, from DNA to RNA to protein.

A sudden change that occurs in the nucleotide sequence of a gene, causing either a minor or considerable change in the characters of an individual is known as mutation.

Reverse transcription is the process by which DNA is synthesized from an RNA template.

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.

1. DNA structure was first studied by Rosalind Franklin (1953); later explained by Watson and Crick, who proposed the double helix model (Nobel Prize, 1962).

2. DNA is a macromolecule made of two complementary strands twisted into a double helix.

3. Each strand is made up of nucleotides, which include phosphate, sugar (pentose), and a nitrogenous base.

4. There are four nitrogenous bases:

• Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
• Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)

5. The two strands form a ladder-like structure, with bases as rungs and sugar-phosphate as the backbone.

• Miescher (1869) isolated a substance from white blood cell nuclei (pus from bandages) and called it nuclein — the first discovery of nucleic acid.
• Nuclein properties — High phosphorus content + acidic nature → renamed nucleic acid.
• Two types of nucleic acids: DNA and RNA.
• Early belief — Scientists thought proteins were the genetic material (large, complex, varied). DNA was wrongly considered simple and unimportant.
• 1928–1952 — Over 25 years, three key experiments proved that DNA (not protein) is the genetic material.
• Role of DNA — It is stable, can replicate accurately, and passes traits to the next generation — making it the true genetic material.

• DNA is extremely long (about 2.2 m in humans) and needs to be highly compacted to fit inside very small cells.
• In prokaryotes, DNA is present in a nucleoid without a true nucleus and is organised into loops and supercoils with the help of HU proteins, DNA gyrase, and topoisomerase enzymes.
• In eukaryotes, DNA is associated with positively charged histone proteins rich in lysine and arginine, which help in packaging.
• DNA wraps around a histone octamer made of H2A, H2B, H3, and H4 to form nucleosomes, which are the basic repeating units of chromatin.
• Nucleosomes further coil into higher-order structures like 10 nm fibres, 30 nm solenoids, and looped domains with the help of histone H1 and non-histone chromosomal proteins, ultimately forming chromosomes.
• Chromatin exists in two forms: euchromatin, which is loosely packed and transcriptionally active, and heterochromatin, which is densely packed and transcriptionally inactive.

• Frederick Griffith (1928), a British medical officer, experimented on Streptococcus pneumoniae to find a cure for pneumonia.
• Two strains were used - S-strain (virulent, smooth, pathogenic, encapsulated) and R-strain (non-virulent, rough, non-pathogenic, non-capsulated).
• Four experiments - R-strain injected: the mouse survived. S-strain injected: mouse died. Heat-killed S-strain injected: the mouse survived. Live R-strain + Heat-killed S-strain injected: the mouse died.
• In the 4th experiment, live S-strain bacteria were recovered from the dead mouse's blood, even though only heat-killed S-strain was used alongside the R-strain.
• R-strain bacteria picked up something from the heat-killed S-strain, transformed into a virulent S-strain, and synthesised a smooth polysaccharide coat.
• Griffith called the unknown substance responsible for this change the "transforming principle", believed to be some form of genetic material.
• Griffith proved genetic material can transfer between bacteria, causing a permanent, heritable change, but did not identify what the transforming principle was (later proven to be DNA).

• In 1952, Alfred Hershey and Martha Chase proved that DNA is the genetic material using bacteriophages and E. coli bacteria.
• They used radioactive isotopes: ³²P to label DNA and ³⁵S to label proteins.
• Viruses grown in ³²P medium had radioactive DNA, while those grown in ³⁵S medium had radioactive protein.
• These labelled viruses were allowed to infect E. coli, and then blending and centrifugation were done to separate viral coats.
• Only bacteria infected with ³²P-labelled viruses became radioactive, showing that DNA entered the bacterial cells.
• Bacteria infected with ³⁵S-labelled viruses were not radioactive, proving proteins did not enter the cells; hence, DNA is the genetic material.

• DNA is the primary genetic material in most organisms, while RNA acts as genetic material in some viruses.
• A genetic material must be capable of replication, which both DNA and RNA can achieve through base pairing.
• DNA is chemically and structurally more stable than RNA because it lacks the reactive 2′-OH group and contains thymine instead of uracil.
• Both DNA and RNA can undergo mutations, but RNA mutates faster due to its unstable nature, leading to rapid evolution in RNA viruses.
• DNA stores genetic information efficiently, whereas RNA helps in expression and transmission of genetic information through protein synthesis.

• 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 of synthesis of DNA from parental DNA, occurring in the S-phase of interphase and follows a semi-conservative mode.
• It is semi-conservative, meaning each daughter DNA has one parental strand and one newly synthesized strand, as proved by Meselson and Stahl experiment (E. coli).
• Replication begins at a specific site called the origin (Ori), where nucleotides are activated, and DNA unwinds using the enzyme helicase, forming a replication fork.
• Single-strand binding proteins (SSB) stabilise separated strands, and RNA primers initiate synthesis of new strands.
• DNA polymerase synthesizes new strands in the 5’ → 3’ direction, continuously on the leading strand and discontinuously on the lagging strand.
• On the lagging strand, short DNA segments called Okazaki fragments are formed and later joined by DNA ligase.
• RNA primers are removed and replaced by DNA, and finally, two identical daughter DNA molecules are formed.

• Semiconservative replication means each new DNA molecule has one old (parental) strand and one newly synthesised strand.
• This was proved by Meselson and Stahl in 1958 using E. coli and density gradient centrifugation.
• E. coli were first grown in ¹⁵N (heavy nitrogen), so the DNA became heavy.
• When transferred to ¹⁴N (light nitrogen), after first generation a hybrid (¹⁴N–¹⁵N) DNA band was formed.
• After the second generation, two bands appeared (hybrid and light DNA), confirming semiconservative replication.

• Initiation at Origin: DNA replication begins at a specific site called the origin of replication; prokaryotes have a single origin, while eukaryotes have multiple origins (replicons).
• Unwinding of DNA: The DNA double helix is unwound and strands are separated by helicase, while topoisomerase (DNA gyrase) relieves supercoiling, forming replication forks.
• Primer Formation: A short RNA primer is synthesized by the enzyme primase to provide a free 3′-OH end for DNA synthesis.
• Elongation of New Strands: DNA polymerase adds nucleotides in the 5′ → 3′ direction using parental strands as templates; synthesis is continuous on the leading strand and discontinuous on the lagging strand forming Okazaki fragments.
• Completion and Ligation: RNA primers are removed, gaps are filled with DNA, and Okazaki fragments are joined by DNA ligase to form complete daughter DNA molecules.

• Protein synthesis is the process by which cells produce proteins, which act as structural components, enzymes, and hormones.
• It involves two main steps: transcription (DNA → RNA) and translation (RNA → protein).
• In transcription, genetic information from DNA is copied into mRNA, where uracil (U) replaces thymine (T).
• Central dogma, proposed by Francis Crick (1958), states that information flows from DNA → RNA → protein.
• In retroviruses, reverse transcription occurs (RNA → DNA), explained by Temin and Baltimore (1970) using RNA-dependent DNA polymerase.

• 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.

• The genetic code is the relationship between the sequence of nucleotides in DNA/mRNA and the sequence of amino acids in a protein, thus controlling protein synthesis.
• It is a triplet code in which three consecutive nucleotides (codon) specify one amino acid, giving a total of 64 codons (4³ combinations).
• Out of 64 codons, 61 code for amino acids while 3 (UAA, UAG, UGA) act as stop codons, and AUG serves as the start codon coding for methionine.
• The genetic code is degenerate, meaning more than one codon can code for the same amino acid, often due to variation at the third base (wobble position).
• The triplet nature of the genetic code was confirmed by experiments such as frame-shift mutations by Francis Crick and the poly-U experiment by Marshall Nirenberg, which showed that UUU codes for phenylalanine.
• Further decoding of codons was achieved using synthetic RNA techniques developed by Har Gobind Khorana, helping establish the full genetic code dictionary.
• Changes (mutations) in the nucleotide sequence can alter amino acid sequences in proteins, showing that the genetic code directly determines protein structure and function.

• Mutation studies help explain the relationship between genes and DNA; large mutations cause loss or gain of genes, while point mutations affect single base pairs.
• A point mutation in the β-globin gene (glutamate replaced by valine) causes the genetic disorder sickle-cell anaemia.
• Insertion or deletion of one or two bases shifts the reading frame of codons, producing frameshift mutations.
• Insertion or deletion of three or multiples of three bases removes or adds whole codons, so the reading frame remains unchanged.

• tRNA (transfer RNA) is called the adapter molecule because it links mRNA codons to their corresponding amino acids during the process of translation.
• The structure of tRNA resembles a cloverleaf in 2D (with loops and arms) and a hairpin/L-shape in 3D. It has two important sites — the anticodon loop (for codon recognition on mRNA) and the amino acid attachment site at the 3' end (acceptor end).
• tRNA has the following main structural parts: Acceptor arm (3' end, carries amino acid), TΨC arm/loop, DHU arm/loop, Anticodon arm with anticodon loop (recognises mRNA codon), and a Variable loop.
• The anticodon present on tRNA is complementary to the codon on mRNA. This ensures the correct amino acid is added during protein synthesis.
• The 3' end of tRNA always ends with the sequence CCA, where the amino acid attaches. The 5' end starts with G. Hydrogen bonds hold the structure together.

• Translation is the process in which mRNA codons are read to form a sequence of amino acids, producing a polypeptide (protein) on ribosomes.
• It requires amino acids, mRNA, tRNA, ribosomes, ATP, Mg²⁺ ions, enzymes, and release factors.
• Ribosomes are the site of protein synthesis and have three tRNA binding sites: A site, P site, and E site.
• Initiation begins with the start codon AUG, where the initiator tRNA binds at the P site, and ribosomal subunits join.
• During elongation, amino acids are added one by one, peptide bonds are formed, and tRNA shifts from A site to P site.
• Termination occurs when a stop codon (UAA, UAG, UGA) is reached, the release factor binds, and the polypeptide chain is released.

• 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 a gene regulatory system in E. coli that controls lactose metabolism by regulating transcription of structural genes. It includes the regulator gene (lacI), operator, promoter, and structural genes (z, y, a).
• The lacI gene produces a repressor protein that binds to the operator in the absence of lactose, preventing RNA polymerase from initiating transcription.
• In the absence of lactose, the operon remains “off” because the repressor blocks RNA polymerase, so structural genes are not expressed.
• In the presence of lactose, a small amount enters the cell via permease and is converted into allolactose by β-galactosidase.
• Allolactose acts as an inducer by binding to the repressor and changing its shape, inactivating it so it cannot bind the operator.
• Once the repressor is inactivated, RNA polymerase binds to the promoter and transcribes the structural genes, producing enzymes for lactose metabolism.
• This leads to the synthesis of β-galactosidase, permease, and transacetylase, enabling efficient utilisation of lactose.

• The Human Genome Project (HGP) was an international mega project launched in 1990 and completed in 2003 to sequence the entire human genome.
• It was coordinated mainly by the US Department of Energy and the National Institutes of Health (NIH), with participation from about 18 countries.
• The main aim was to identify all human genes, determine their locations, and sequence the complete human DNA (about 3.2 billion base pairs).
• The human genome contains approximately 20,000–25,000 genes, and less than 2% of DNA codes for proteins, while most consists of non-coding and repetitive sequences.
• The project used advanced techniques like automated DNA sequencing, cloning using BAC and YAC vectors, and genome mapping approaches.
• HGP revealed that about 99.9% of human DNA is identical, with variations such as SNPs and CNVs responsible for individual differences.
• The project has applications in disease diagnosis, gene therapy, genetic counselling, and understanding human evolution, along with ethical concerns related to genetic data use.

• DNA fingerprinting is a technique used to identify individuals based on unique patterns in their DNA, mainly using VNTRs (Variable Number Tandem Repeats), also called minisatellites.
• VNTRs are short repetitive DNA sequences that show high variation among individuals, making each DNA profile unique (except identical twins).
• The technique was developed by Alec Jeffreys and even a very small amount of DNA can be used for analysis.
• The process involves DNA extraction from samples like blood, hair, semen, or tissue, followed by PCR amplification if needed and restriction enzyme digestion.
• DNA fragments are separated using gel electrophoresis, transferred to a membrane by Southern blotting, and hybridised with specific VNTR probes.
• The hybridised DNA is visualized using autoradiography, producing a unique banding pattern for each individual.
• DNA fingerprinting is widely used in forensic science, paternity testing, criminal investigations, identification, genetic diversity studies, and disease diagnosis.