Nucleotides, DNA, RNA & Gene Expression NG

Purine nucleotides can be synthesized de novo on a ribose-phosphate framework or recovered through salvage pathways. Synthesis requires substantial energy and is tightly regulated. Purine degradation ultimately produces uric acid in humans; excessive production or reduced excretion can contribute to hyperuricaemia.

Pyrimidine rings are synthesized first and then attached to ribose phosphate. Their degradation differs from purines and does not produce uric acid. Balanced nucleotide pools are necessary for accurate DNA synthesis.

Salvage pathways conserve metabolic resources and can be clinically important when defective. Antimetabolite drugs exploit nucleotide synthesis because rapidly proliferating cells have high demand, though normal proliferative tissues may also be affected.

DNA replication is semiconservative: each daughter DNA molecule contains one parental strand and one newly synthesized strand. Helicase-related machinery opens the duplex, single-strand binding factors stabilize exposed templates, and topological stress must be controlled.

DNA polymerases synthesize new DNA in the 5' to 3' direction using complementary base pairing. Because the two templates are antiparallel, leading and lagging strands are synthesized differently; the lagging strand forms discontinuous fragments that are processed and ligated.

Polymerase proofreading and post-replicative repair greatly improve fidelity. Replication errors that escape correction can become stable mutations after subsequent cell divisions.

DNA is continuously challenged by spontaneous base loss, deamination, reactive species, radiation and chemicals. Cells possess repair systems specialized for different lesion types, including direct reversal in selected contexts, base-excision repair, nucleotide-excision repair, mismatch repair and double-strand-break responses.

Damage and mutation are not identical. Damage is a chemical or structural lesion; a mutation is a stable sequence alteration. Successful repair can remove damage before it becomes fixed.

Failure of repair pathways increases genomic instability and can predispose to cancer or inherited disease. The phenotype depends on which repair mechanism is defective and which tissues are most affected.

RNA polymerase uses one DNA strand as template to synthesize RNA. Transcription initiation depends on promoter recognition and regulatory proteins. Eukaryotic primary transcripts undergo processing that commonly includes 5' capping, splicing and 3' polyadenylation.

Splicing removes introns and joins exons. Alternative splicing permits one gene to generate multiple RNA and protein products, increasing functional diversity without requiring a separate gene for every protein isoform.

RNA stability and processing are themselves regulatory points. Errors can alter protein abundance or structure even when the underlying coding DNA sequence is intact.

Ribosomes read mRNA codons in the 5' to 3' direction. Transfer RNAs carry amino acids and use anticodons to recognize codons. Aminoacyl-tRNA synthetases are crucial for fidelity because they attach the correct amino acid to each tRNA.

Translation proceeds through initiation, elongation and termination. Peptide bonds form as the ribosome moves along mRNA. The genetic code is degenerate—several codons can encode the same amino acid—but each codon has a defined meaning in the standard code.

New polypeptides may require folding, cleavage, phosphorylation, glycosylation, targeting or assembly into complexes before becoming functional. Gene expression therefore extends beyond transcription and translation.

Cells regulate gene expression through chromatin state, transcription factors, enhancers/silencers, RNA processing, RNA stability, translation and protein turnover. Epigenetic mechanisms can alter accessibility without changing the DNA base sequence.

PCR amplifies selected DNA regions through cycles of denaturation, primer annealing and polymerase extension. Reverse-transcription approaches permit RNA-derived sequences to be studied, while sequencing identifies nucleotide order.

A detected molecular variant requires interpretation. Pathogenicity depends on variant type, gene function, inheritance, population frequency, functional evidence and phenotype. Molecular detection is powerful but does not eliminate clinical reasoning.

Nucleotides are required for DNA and RNA and also participate in energy transfer, signalling and activated metabolic intermediates. Purines and pyrimidines can be synthesized de novo or recovered through salvage pathways. Their synthesis and degradation are regulated because imbalances can affect replication, energy use and metabolite accumulation.

DNA stores genetic information in a complementary double-stranded structure. Replication requires controlled unwinding, template-directed polymerization, proofreading and repair. DNA damage occurs continually from replication errors, spontaneous chemistry and environmental insults; repair pathways reduce the probability that damage becomes a permanent mutation.

Transcription copies selected DNA information into RNA. Messenger RNA is processed in eukaryotic cells and then translated by ribosomes, which read codons and use transfer RNAs to assemble amino acids into polypeptides. Protein function can subsequently be modified by folding, cleavage, phosphorylation, glycosylation and other post-translational events.

Gene expression is regulated at multiple levels: chromatin accessibility, transcription-factor activity, RNA processing/stability, translation and protein degradation. Different cells can therefore share essentially the same genome while expressing very different phenotypes.

Mechanism map: DNA storage → replication when cells divide; DNA → transcription → RNA processing → translation → protein folding/modification → function. For mutation questions, distinguish DNA damage from a fixed heritable sequence change.

Clinical integration includes inherited disease, cancer biology, nucleotide-metabolism disorders and molecular diagnostic techniques. Drugs can selectively target nucleotide synthesis or DNA-related processes, but effects depend on cell type and proliferation.

Common confusion: replication copies DNA; transcription makes RNA from DNA; translation makes polypeptide from an RNA template. A mutation is not synonymous with every episode of DNA damage.