Molecular Biology, Gene Expression and Biotechnology
What you must understand
DNA is a double helix of nucleotides (deoxyribose, phosphate, nitrogenous base) with complementary pairing A–T and G–C via hydrogen bonds; the backbone is joined by covalent phosphodiester bonds. RNA is single-stranded, contains ribose and uracil, and comes in messenger, transfer and ribosomal forms.
Replication is semi-conservative: each new molecule keeps one old strand. Transcription copies a gene into mRNA in the nucleus using one DNA strand as template. Translation occurs on ribosomes: mRNA codons are read three bases at a time, tRNA brings matching amino acids via anticodons, and the polypeptide grows. The genetic code is triplet, non-overlapping, degenerate (several codons per amino acid), and universal.
Genetic engineering uses restriction enzymes to cut DNA, ligase to join fragments, and vectors (plasmids) to insert genes into host cells, producing insulin, growth hormone, or transgenic organisms. Biotechnology also includes cell engineering and cloning.
Traps
- Uracil replaces thymine in RNA; RNA still pairs with A.
- Transcription makes mRNA from DNA; translation makes protein from mRNA - questions swap them.
- Degeneracy means one amino acid has several codons, not one codon for several amino acids.
- Restriction enzymes cut; ligase joins. Do not reverse them.
Rule of thumb
Follow the central dogma chain: DNA → (replication) DNA; DNA → (transcription) mRNA → (translation) protein.
Worked example: A DNA template triplet ATG transcribes to mRNA codon UAC, which pairs with tRNA anticodon AUG and codes for one amino acid; a change of one base is a gene mutation.