Gene: Its Nature, Expression and Regulation

So DNA is basically the instruction manual for your entire body. Every cell in your body — skin, liver, brain, bone — contains the same DNA, the same 3 billion base pairs, the same ~20,000 genes. Yet a skin cell is nothing like a liver cell. How does a molecule — literally just a chemical — encode the information needed to build and run a human being? And how do different cells read different parts of the manual? That's what this chapter is about: the nature of the gene, how it expresses itself, and how that expression is regulated. The story of DNA begins with its structure. In 1953, James Watson and Francis Crick, using X-ray crystallography data from Rosalind Franklin and Maurice Wilkins, proposed the double helix model. It's two strands of nucleotides running in opposite directions (antiparallel), held together by hydrogen bonds between complementary base pairs: adenine with thymine (two hydrogen bonds) and guanine with cytosine (three hydrogen bonds). The sugar-phosphate backbones form the rails, and the base pairs form the steps. This structure immediately suggested how DNA could replicate: just unzip the two strands and use each as a template. The central dogma of molecular biology, proposed by Francis Crick, states that genetic information flows from DNA to RNA to Protein. DNA is transcribed into mRNA, which is then translated into a polypeptide chain. This is the fundamental information pathway in all living cells. There are exceptions (retroviruses reverse the flow, using RNA to make DNA), but for most life, this dogma holds. DNA replication happens before cell division. The double helix unwinds with the help of enzymes like helicase, and DNA polymerase reads the template strand and adds complementary nucleotides. Because the two strands are antiparallel, replication is continuous on the leading strand but discontinuous on the lagging strand (Okazaki fragments). The result is two identical DNA molecules, each with one old strand and one new strand — semi-conservative replication. Meselson and Stahl proved this elegantly using nitrogen isotopes. Transcription is the process of making an RNA copy of a gene. RNA polymerase binds to the promoter region and moves along the DNA template, synthesizing a complementary RNA strand. In eukaryotes, the initial transcript (pre-mRNA) undergoes processing: 5' capping, 3' polyadenylation, and splicing (removing introns and joining exons). This mature mRNA then leaves the nucleus for the cytoplasm. Translation happens on ribosomes. The mRNA is read in triplets called codons, each coding for a specific amino acid. tRNA molecules act as adapters — they have an anticodon that pairs with the mRNA codon and carry the corresponding amino acid. The ribosome facilitates codon-anticodon pairing and catalyzes peptide bond formation. The genetic code is degenerate (multiple codons for the same amino acid), nearly universal, and read continuously without overlaps. Gene regulation determines which genes are expressed when. In bacteria, the lac operon is a classic model: when lactose is absent, a repressor protein binds to the operator and blocks transcription. When lactose is present, it binds the repressor and inactivates it, allowing transcription. In eukaryotes, regulation is more complex, involving transcription factors, enhancers, silencers, chromatin remodeling, and epigenetic modifications. Not all DNA codes for protein. Only about 1.5% of the human genome actually codes for proteins. The rest includes introns, regulatory sequences, repetitive DNA, satellite DNA, and mysterious 'junk DNA' that may have regulatory or structural functions. Introns are transcribed but spliced out before translation. Their discovery was surprising — why would cells waste energy making RNA they're going to chop up? We now know introns allow alternative splicing (one gene producing multiple proteins) and may have evolutionary significance. Mutations are changes in the DNA sequence. Point mutations affect a single base pair — silent (same amino acid), missense (different amino acid), or nonsense (premature stop codon). Frameshift mutations (insertions or deletions not in multiples of three) shift the reading frame and usually ruin the protein. Mutations can be spontaneous or induced by mutagens (radiation, chemicals). Cells have DNA repair mechanisms, but unrepaired mutations can cause cancer or genetic disease.