CBSE · Class 12 · Biology · Chapter 5

Molecular Basis of Inheritance — Formula Sheet

Board Formulas

10 formulas

1.Chargaff's Rule★
$[A] = [T], [G] = [C]; \frac{A + G}{T + C} = 1$
Purines equal pyrimidines in any double-stranded DNA (holds across species).
2.DNA Double Helix Geometry★
$Diameter = 2 nm; Pitch = 3.4 nm; Rise per bp = 0.34 nm; 10 bp per turn$
B-form DNA, right-handed helix.
3.Length of Human DNA
$L = (6.6 \times 1 0^{9} bp) \times (0.34 nm) \approx 2.2 m per diploid cell$
About 2 metres of DNA is packed into every human diploid nucleus (only a few micrometres wide).
4.Semiconservative Replication★
$Each daughter duplex = 1 parental strand + 1 newly synthesised strand$
Proved by Meselson-Stahl (1958) via density gradient centrifugation of ¹⁵N-labelled DNA.
5.Nucleotide = Sugar + Base + Phosphate
$Nucleotide = deoxyribose + N-base + PO_{4}^{3 -}$
Nucleoside = sugar + base only. Adding phosphate makes a nucleotide.
6.Transcription Unit★
$Promoter \to Structural gene \to Terminator$
In prokaryotes RNA polymerase reads the template strand 3' → 5' and builds mRNA 5' → 3'.
7.Genetic Code Combinatorics★
$4^{3} = 64 codons; 61 sense + 3 stop (UAA, UAG, UGA)$
Triplet, degenerate, unambiguous, universal, non-overlapping, commaless.
8.Central Dogma
$DNA transcription mRNA translation Protein$
Reverse transcription (RNA → DNA) is an addition proposed by Temin and Baltimore in retroviruses.
9.Post-transcriptional Processing (Eukaryotes)★
$hnRNA capping + splicing + tailing mature mRNA$
5' cap = methyl guanosine; 3' tail = ~200 adenylates; splicing removes introns and joins exons.
10.Human Genome Facts
$Bases \approx 3.164 \times 1 0^{9}; Genes \approx 20, 000 - 25, 000; Chr 1 largest, Chr Y smallest$
Less than 2% of the genome codes for protein; the rest was earlier called 'junk DNA'.