Nucleic Acids & Protein Synthesis
DNA and RNA structure, transcription and translation
DNA (deoxyribonucleic acid): Double helix of two antiparallel polynucleotide strands. Each nucleotide = phosphate + deoxyribose sugar + nitrogenous base (A, T, C, G). Base pairing: A=T (2 hydrogen bonds), C≡G (3 hydrogen bonds).
RNA (ribonucleic acid): Single-stranded. Ribose sugar instead of deoxyribose. Uracil (U) replaces thymine (T). Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).
DNA Replication (semi-conservative): Helicase unzips DNA → DNA polymerase adds complementary nucleotides to each strand → two identical molecules, each with one old and one new strand.
Transcription (in nucleus): RNA polymerase copies one DNA strand (template) into mRNA. DNA: TAC GCA → mRNA: AUG CGU.
Translation (at ribosomes): mRNA codons (3 bases) are read. Each codon matches a tRNA anticodon carrying a specific amino acid. Ribosome moves along mRNA, building the polypeptide chain.
Genetic code: Triplet (3 bases = 1 amino acid), degenerate (multiple codons for same amino acid), universal, non-overlapping. AUG = start codon. UAA, UAG, UGA = stop codons.
Key Points to Remember
- 1DNA: A=T (2 H-bonds), C≡G (3 H-bonds)
- 2Semi-conservative replication by DNA polymerase
- 3Transcription: DNA → mRNA (in nucleus)
- 4Translation: mRNA → protein (at ribosomes)
Pakistan Example
DNA Forensics in Pakistan — From Crime Scenes to Paternity
Pakistan's Punjab Forensic Science Agency uses DNA profiling for criminal investigations. The semi-conservative replication of DNA is the basis of PCR (Polymerase Chain Reaction), which amplifies tiny DNA samples from crime scenes. Understanding nucleic acid structure is essential for modern forensic science.