Revision: Molecular Genetics Zoology HSC Science Class 12 Tamil Nadu Board of Secondary Education

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

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.

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

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.

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

• 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.

• In eukaryotes, chromosomes are gene carriers, and each chromosome consists of a single long DNA molecule associated with histone proteins (unineme model).
• DNA is packaged into repeating units called nucleosomes, where DNA is wrapped around a histone octamer made of H₂A, H₂B, H₃, and H₄ proteins.
• Nucleosomes form a 10 nm beaded fibre, which coils further into a 30 nm solenoid structure with the help of histone H₁.
• Higher-level folding condenses chromatin into looped domains attached to a protein scaffold, finally forming metaphase chromosomes with the help of non-histone chromosomal proteins.
• Chromatin exists as euchromatin (loosely packed, transcriptionally active) and heterochromatin (densely packed, transcriptionally inactive).

• 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.

• The genetic code is the specific sequence of nitrogenous bases in DNA or mRNA that determines the order of amino acids in a protein.
• It is a triplet code, where a sequence of three nucleotides called a codon codes for one amino acid.
• There are 64 codons (4³), of which 61 code for amino acids and 3 act as stop codons (UAA, UAG, UGA).
• AUG functions as the start codon and codes for the amino acid methionine.
• The genetic code is degenerate, meaning more than one codon can code for the same amino acid, showing the wobble effect at the third base.

• 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.

• 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.