05Class 12 Biology
Molecular Basis of Inheritance short notes
Short notes on DNA structure, replication, transcription, translation and gene regulation. It's one of the most tested chapters in NEET Biology, so learn the sequence of each process.
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DNA and RNA
- DNA is a double helix with antiparallel strands, from Watson and Crick, using Franklin's and Wilkins's X-ray data.
- Base pairing: A with T (two hydrogen bonds), and G with C (three).
- One turn of the helix is 3.4 nm and has about 10 base pairs.
- Griffith, then Avery, MacLeod and McCarty, then Hershey and Chase showed DNA is the genetic material.
- RNA has ribose and uracil, and is usually single-stranded.
Replication
- It's semi-conservative, as Meselson and Stahl showed.
- DNA polymerase builds in the 5' to 3' direction only.
- The leading strand is continuous, and the lagging strand is made in Okazaki fragments, joined by ligase.
Transcription and translation
- RNA polymerase copies the template strand into RNA, in the 5' to 3' direction.
- In eukaryotes, hnRNA is processed by capping, tailing and splicing, which removes introns.
- The genetic code is a triplet, degenerate and nearly universal, with 64 codons.
- AUG starts translation, while UAA, UAG and UGA stop it.
- Ribosomes read mRNA and tRNA brings the amino acids, joined by peptide bonds.
Regulation and tools
- The lac operon in E. coli has a regulator gene (i), and structural genes z, y and a. Lactose acts as the inducer.
- DNA fingerprinting uses repeating DNA sequences such as VNTRs.
- The Human Genome Project found about 3 billion base pairs. NCERT gives about 30,000 genes, though newer estimates are lower.
Where students slip up
- Mixing up the template and coding strands. RNA matches the coding strand, with U in place of T.
- Saying polymerase builds in the 3' to 5' direction. It builds 5' to 3'.
- Forgetting introns are removed and exons are kept.
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