Maharashtra State Board · Class 12 · Biology · Chapter 4

Molecular Basis of Inheritance — Formula Sheet

Board Formulas

11 formulas

1.Chargaff's Rule★
$% A = % T, % G = % C, A + G = T + C$
Empirical base-pair rule preceding Watson–Crick model. Purines = pyrimidines.
2.Watson–Crick Structure Parameters★
$Diameter = 2 nm; Rise/bp = 0.34 nm; Pitch = 3.4 nm; 10 bp/turn$
Right-handed double helix. Antiparallel strands with major and minor grooves.
3.Base Pairing★
$A = T (2 H-bonds), G \equiv C (3 H-bonds)$
Purine (A, G) pairs with pyrimidine (T, C). GC-rich regions are more stable.
4.DNA Length Calculation
$Length = No. of bp \times 0.34 nm$
Human genome ≈ 6.6 × 10⁹ bp ≈ 2.2 m long DNA.
5.Semi-conservative Replication★
$Each daughter DNA = 1 parental strand + 1 new strand$
Verified by Meselson–Stahl experiment (1958) using ¹⁵N/¹⁴N.
6.Central Dogma★
$DNA replication DNA transcription RNA translation Protein$
Proposed by Crick. Reverse transcription (RNA → DNA) exists in retroviruses.
7.Codon and Anticodon
$Codon: mRNA triplet; Anticodon: tRNA triplet complementary to codon$
64 codons; 61 code for 20 amino acids; 3 stop codons (UAA, UAG, UGA).
8.Genetic Code Properties★
$Triplet, degenerate, universal, non-overlapping, comma-less$
Balbharati lists all five properties — memorise as a bullet list.
9.Lac Operon Components★
$i \to repressor, P, O \to control, z, y, a \to structural genes$
Regulated by lactose (inducer). Model of prokaryotic gene regulation.
10.Start and Stop Codons
$Start: AUG (Met); Stop: UAA, UAG, UGA$
AUG codes for Methionine (Met) and initiates translation. Stop codons do not code for any amino acid.
11.Enzymes in Replication
$Helicase, Primase, DNA polymerase, Ligase, Topoisomerase$
DNA polymerase reads 3' to 5'; synthesises new strand 5' to 3'.