Molecular Basis of Inheritance
Class 12 · Biology
4 ideas in Class 12 Biology, explained in plain words, with games and quizzes to practise.
Structure and Packaging
Part of: DNA Molecule
DNA links sugar-phosphate backbones with bases pairing A to T by two bonds and G to C by three. The helix turns every 3.4 nm with 10 pairs per turn, so 6.6 billion pairs stretch about 2.2 metres. Histone octamers spool 200 pairs each into nucleosomes, with loose active euchromatin and dense silent heterochromatin.
Why it matters. Base arithmetic and packing numbers anchor calculation questions.
Formulas
- A plus T plus G plus C equals 100 percent
- Rise per pair is 0.34 nm
- One nucleosome holds 200 base pairs
Key words
- Nucleotide
- : A base plus sugar plus phosphate unit.
- Chargaff rule
- : A equals T and G equals C.
- Histone octamer
- : Eight proteins spooling DNA.
- Nucleosome
- : 200 pairs wrapped on proteins.
- Euchromatin
- : Loose, active chromatin.
- Heterochromatin
- : Dense, silent chromatin.
Try it in two minutes
- Pair A-T and G-C with bond counts.
- Divide 3.4 nm into 0.34 nm steps.
- Spool one nucleosome mentally.
Try at home. Show why 20 per cent C forces 30 per cent A.
Genetic Material Proof and Replication
Part of: DNA Molecule
Heat-inactivated virulent bacteria transformed live harmless ones, DNase ended the change, and Hershey-Chase traced phosphorus-labelled DNA into cells. DNA copies semiconservatively: each old strand templates one new strand, shown by density shifts across generations, with polymerases, ligase and fixed origins driving forks.
Why it matters. Experiment logic plus generation arithmetic is heavily tested.
Formulas
- Each generation halves hybrid DNA share
- Two forks share about 2000 bp per second each
- Phosphorus marks DNA, sulphur marks protein
Key words
- Transformation
- : Uptake of outside DNA changing traits.
- DNase
- : The enzyme destroying DNA.
- Semiconservative
- : One old plus one new strand.
- Replication fork
- : The Y opening copying DNA.
- DNA ligase
- : The enzyme joining fragments.
- Origin of replication
- : The fixed start site.
Try it in two minutes
- Order Griffith, Avery and Hershey-Chase conclusions.
- Halve density bands per generation.
- Place enzymes at the fork.
Try at home. Explain why sulphur labelling marked protein, not DNA.
Transcription, Code and Translation
Part of: Reading the Code
RNA polymerase copies the 3-to-5 template into RNA, guided by promoters and terminators with sigma and rho helpers. Eukaryotic transcripts splice introns, gain caps and 200 to 300 tail bases. Triplet codons number 64 with 61 sense and 3 stop; AUG starts, tRNA adapts, ribosomes join amino acids.
Why it matters. Strand choice and codon reading are the top scoring skills here.
Formulas
- 64 codons equal 61 sense plus 3 stop
- AUG starts and codes methionine
- UAA, UAG and UGA stop translation
Key words
- Template strand
- : The 3-to-5 strand copied.
- Coding strand
- : The strand matching RNA sequence.
- Exon
- : A kept coding stretch.
- Intron
- : A removed intervening stretch.
- Degenerate code
- : Many codons per amino acid.
- tRNA
- : The adapter reading codons.
- UTR
- : Untranslated ends aiding translation.
Try it in two minutes
- Pick template versus coding strand.
- Process hnRNA to mRNA.
- Read codons to amino acids.
Try at home. Explain why both strands are never copied together.
Operon, Genome and Fingerprinting
Part of: Control and Identity
The lac repressor blocks the operator until lactose inactivates it, letting z, y and a genes digest milk sugar. The genome project (1990 to 2003) read 3164.7 million bases with 30,000 genes under 2 per cent coding. Fingerprinting reads VNTR repeats differing in everyone but identical twins.
Why it matters. Switch logic plus big-number reasoning finishes the chapter strongly.
Formulas
- Genome spans 3164.7 million bases
- Less than 2 percent codes proteins
- 0.1 percent difference equals 3 million bases
Key words
- Operator
- : The DNA switch bound by repressor.
- Inducer
- : Lactose switching the operon on.
- Repressor
- : The protein blocking transcription.
- EST
- : A tag marking expressed genes.
- VNTR
- : Variable repeats naming persons.
- Polymorphism
- : Variants above one per cent frequency.
Try it in two minutes
- Flip the operon with lactose.
- Quote four genome numbers.
- Match VNTR bands to persons.
Try at home. Explain why chromosome 1 leads and Y trails in gene counts.
What your child will practise
- Genetic Material Proof and Replication: Candy Puzzle (Clue Puzzle)
- Genetic Material Proof and Replication: Candy Quick Fire (Quick Fire)
- Operon, Genome and Fingerprinting: Desert Puzzle (Clue Puzzle)
- Structure and Packaging: Candy Quest (Flip and Match)
- Transcription, Code and Translation: Ocean Quick Fire (Quick Fire)
- Quiz: Genetic Material Proof and Replication (8 questions)
- Quiz: Operon, Genome and Fingerprinting (8 questions)
- Quiz: Structure and Packaging (8 questions)
- Quiz: Transcription, Code and Translation (8 questions)