The DNA: Deoxyribonucleic acid (DNA) is a long polymer of deoxyribonucleotides. The length is usually defined by the number of nucleotides (or base pairs). The Double Helix Model was proposed by James Watson and Francis Crick in 1953, based on X-ray diffraction data by Wilkins and Franklin. One of the hallmarks of their proposition was base pairing between the two strands: Adenine pairs with Thymine (2 H-bonds) and Guanine with Cytosine (3 H-bonds). The two chains have anti-parallel polarity (5′ → 3′ and 3′ →  5′). The backbone is constituted by sugar-phosphate, and the bases project inside. Erwin Chargaff observed that for double-stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equal one.

Packaging of DNA Helix: In prokaryotes (like E. coli), DNA is held with some positively charged proteins in a region called nucleoid. In eukaryotes, the organization is more complex. Positively charged, basic proteins called histones (rich in lysine and arginine) organize to form a unit of eight molecules called a histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called a Nucleosome. Nucleosomes constitute the repeating unit of a structure in the nucleus called chromatin (seen as “beads-on-string” under electron microscope).

The Search for Genetic Material:

• Transforming Principle: Frederick Griffith (1928) conducted experiments with Streptococcus pneumoniae (S-strain virulent, R-strain non-virulent). He concluded that R-strain bacteria had been transformed by the heat-killed S-strain bacteria due to the transfer of some “transforming principle.” 

• Biochemical Characterisation: Avery, MacLeod, and McCarty (1933-44) discovered that the transforming biochemical nature was DNA (proteases and RNases did not affect transformation; DNases did).

• The Unequivocal Proof: Hershey and Chase (1952) worked with bacteriophages. They used radioactive Phosphorus (³²P) to label DNA and radioactive Sulfur (³⁵S) to label protein. They found that bacteria infected with viruses that had radioactive DNA were radioactive, proving DNA is the genetic material. 

Replication: The scheme suggested that the two strands would separate and act as a template for the synthesis of new complementary strands (Semiconservative DNA Replication). This was experimentally proved by Meselson and Stahl (1958) using heavy nitrogen (¹⁵N) in E. coli. The main enzyme is DNA-dependent DNA polymerase. Replication begins at the origin of replication. On the template with polarity 3’→ 5′, replication is continuous (leading strand), while on the 5’→ 3′ template, it is discontinuous (lagging strand with Okazaki fragments), joined by DNA ligase.

Transcription: The process of copying genetic information from one strand of DNA into RNA. The transcription unit has a Promoter (binding site for RNA polymerase), Structural gene, and Terminator. The strand with polarity 3’→ 5′ acts as the template strand; the other (5’→ 3′) is the coding strand. In eukaryotes, the primary transcript (hnRNA) contains both exons (coding sequences) and introns (non-coding). It undergoes Splicing (introns removed), Capping (methyl guanosine triphosphate at 5′ end), and Tailing (polyadenylate residues at 3′ end) to become mRNA.

Genetic Code: The code is a triplet (61 codons code for amino acids, 3 are stop codons). It is degenerate (some amino acids are coded by more than one codon), unambiguous, and universal. AUG functions as a start codon and codes for Methionine.

Translation: The process of polymerization of amino acids to form a polypeptide. tRNA (adapter molecule) has an anticodon loop and an amino acid acceptor end. The Ribosome is the cellular factory for protein synthesis.

Regulation of Gene Expression: In prokaryotes, predominant control is at the transcriptional initiation level.

The Lac Operon: Elucidated by Jacob and Monod. It consists of one regulatory gene (i gene) and three structural genes (z, y, a). The i gene codes for the repressor of the lac operon. The repressor binds to the operator region and prevents RNA polymerase from transcribing. In the presence of an inducer (Lactose or Allolactose), the repressor is inactivated, allowing transcription. This is negative regulation.

Human Genome Project (HGP): A mega project to sequence the human genome (3 × 10⁹ bp). Methodologies included Expressed Sequence Tags (ESTs) and Sequence Annotation. Key findings: The human genome contains 3164.7 million nucleotide bases; the average gene consists of 3000 bases; less than 2% of the genome codes for proteins; chromosome 1 has the most genes (2968) and the Y has the fewest (231).

DNA Fingerprinting: Developed by Alec Jeffreys. It involves identifying differences in some specific regions in DNA called repetitive DNA (Satellite DNA). It uses Variable Number of Tandem Repeats (VNTR) as a probe. The steps involve isolation, digestion, separation (electrophoresis), blotting (Southern blot), hybridization with radioactive VNTR probe, and autoradiography.