NEET 2027 · Biology · Class 12 Chapter 5 · File MBI-03

The whole chapter on one page

Formulas, numbers, scientists, exceptions and mnemonics, plus the written defence for each of the recurring error families.

Built for printing. Nothing here needs reasoning — it all needs recall.

What this file is. Everything in the chapter that has to be recalled rather than reasoned, on one printable page. Use it for the last pass before an ILTS paper and for the March–April revision block.

Every formula in the chapter

Quantity	Formula	Worked example
Length of a DNA molecule	`bp × 0.34 nm`	`6.6×10⁹ bp → 2.2 m`
Number of helical turns	`bp ÷ 10`	`23,000 bp → 2300 turns`
Length from turns	`turns × 3.4 nm`	`2300 → 7820 nm = 7.82 μm`
Chargaff, one base given	`A = (100 − 2×given) ÷ 2`	`C = 20% → A = 30%`
Chargaff from a ratio r = (A+T)/(G+C)	`G + C = 100 ÷ (r + 1)`	`r = 1.5 → G+C = 40 → G = 20%`
DNA molecules after n generations	`2ⁿ`	`n = 3 → 8 molecules`
Hybrid molecules after n generations	`always exactly 2`	—
Percentage hybrid	`(2 ÷ 2ⁿ) × 100`	`n = 2 → 50%`
Nucleosomes in a stretch of DNA	`bp ÷ 200`	`6×10⁵ bp → 3000`
Codons unaffected by a deletion at position p	`whole part of (p − 1) ÷ 3`	`p = 901 → 300 codons`
Complete codons in a stretch of n bases	`whole part of n ÷ 3`	`98 bases → 32 codons, 2 left over`

Every number worth memorising

Value	What it belongs to
`0.34 nm`	rise per base pair
`3.4 nm`	pitch — one full turn
`10`	base pairs per turn
`2 nm`	diameter of the helix
`2 and 3`	hydrogen bonds in A=T and G≡C
`5386 nucleotides`	bacteriophage φX174 — single-stranded
`48,502 bp`	bacteriophage lambda
`4.6 × 10⁶ bp`	E. coli genome
`3.3 × 10⁹ bp`	human haploid genome
`2.2 m`	total DNA length in one human cell
`2000 bp per second`	polymerisation rate at an E. coli fork
`18 minutes`	time for E. coli to finish replication
`200–300`	adenylate residues in the poly-A tail
`200 bp`	DNA wrapped in one nucleosome
`8 = 2 × 4`	histones in the octamer core
`64 / 61 / 3`	total codons / sense codons / stop codons
`2`	amino acids with a single codon — Met and Trp
`6th codon`	position of the sickle-cell change in β-globin
`70S = 50S + 30S`	bacterial ribosome
`80S = 60S + 40S`	eukaryotic ribosome
`23S rRNA`	the ribozyme that forms the peptide bond in bacteria
`3164.7 million bp`	size of the human genome (HGP figure)
`3000 bases`	average human gene
`2.4 million bases`	dystrophin, the largest known human gene
`about 30,000`	estimated human gene count
`99.9%`	of bases identical between any two people
`less than 2%`	of the genome that codes for protein
`2968 / 231`	genes on chromosome 1 / the Y chromosome
`1.4 million`	known SNP sites
`3300`	books of 1000 pages needed to print the genome
`1990–2003`	the Human Genome Project
`0.1 to 20 kb`	size range of a VNTR

Every scientist, with the attribution trap attached

Scientist	Year	What they are credited with	The attribution trap
Friedrich Miescher	1869	isolated DNA from pus cells, called it nuclein	not the structure — that was 84 years later
Frederick Griffith	1928	transformation in Streptococcus pneumoniae	showed that something transfers, not what
Avery, MacLeod, McCarty	1933–44	DNA is the transforming principle	the enzyme-subtraction logic is theirs, not Griffith's
Erwin Chargaff	—	the equal base ratios	measured them; did not build the model
Wilkins and Franklin	—	the X-ray diffraction data	Franklin died in 1958, so is not a Nobel laureate
Watson and Crick	1953	the double-helix model; predicted semiconservative replication	the prediction is theirs, the proof is not
Hershey and Chase	1952	bacteriophage T2, ³²P and ³⁵S — DNA is the genetic material	the blender step is essential, not decorative
Meselson and Stahl	1958	⁵N experiment proving semiconservative replication	in E. coli
Taylor and colleagues	1958	the same result in Vicia faba using radioactive thymidine	the plant version, often confused with Meselson–Stahl
George Gamow	—	argued the code must be a triplet	a physicist, not a biologist
Nirenberg and Matthaei	—	the cell-free system that cracked the code	—
Har Gobind Khorana	—	synthetic RNA of defined repeating sequence	—
Severo Ochoa	—	polynucleotide phosphorylase, for making RNA of known sequence	the enzyme, not the code itself
Robert Holley	—	the structure of tRNA	shared the 1968 Nobel with Nirenberg and Khorana
Francis Crick	—	the central dogma; predicted the adapter molecule	predicted tRNA before it was found
Jacob and Monod	1961	the operon model of gene regulation	Nobel 1965, shared with André Lwoff
Reiji Okazaki	—	the short lagging-strand fragments	—
Frederick Sanger	—	the sequencing method the HGP automated	—
Alec Jeffreys	—	DNA fingerprinting using VNTR probes	—

Every exception

Every exception in the chapter, in one list

Chargaff applies only to double-stranded DNA. Bacteriophage φX174 is single-stranded and its base amounts are unequal.
Some viruses carry RNA, not DNA — tobacco mosaic virus, Qβ bacteriophage, HIV and other retroviruses.
Reverse transcriptase is an RNA-dependent DNA polymerase and runs the central dogma backwards.
DNA polymerase cannot start a chain from nothing; it needs a free 3′-OH, so a primer is required.
The whole molecule never unwinds — only a small replication fork travels along.
Prokaryotes have one origin of replication; eukaryotes have many per chromosome.
Tailing is template-independent — there is no run of Ts in the DNA to copy.
Capping, tailing and splicing happen in eukaryotes only. Bacterial mRNA is used as made.
5S rRNA is made by RNA polymerase III, not polymerase I, even though it is a ribosomal RNA.
Eukaryotic mRNA is monocistronic; prokaryotic mRNA is polycistronic.
There is no tRNA for any stop codon. Termination is done by a release factor, a protein.
The peptide bond in bacteria is made by RNA (23S rRNA), not by a protein enzyme.
Only methionine and tryptophan have a single codon each.
AUG does two jobs — start signal and methionine.
The code is nearly, not fully, universal — mitochondrial codons and some protozoan codons differ.
The code is degenerate but unambiguous. Many codons per amino acid, never many amino acids per codon.
Deleting or inserting three bases does not cause a frameshift — it adds or removes one amino acid.
H1 is not part of the histone octamer — it sits outside, on the linker DNA.
Heterochromatin stains dark and is inactive; euchromatin stains light and is active.
The letter i in the lac operon means inhibitor, not inducer. The inducer is lactose.
The repressor binds the operator, not the promoter.
Identical twins give the same DNA fingerprint — the one case where the technique cannot separate two people.
VNTRs are minisatellites, not microsatellites.

Mnemonics and anchors

Structure and code

Pure As Gold — purines are Adenine and Guanine
CUT the pie — pyrimidines are Cytosine, Uracil, Thymine
G≡C has three bonds, so GC-rich DNA is harder to pull apart
Degeneracy arrow points codons → amino acid. The other direction would be ambiguity, and the code is not ambiguous.

Transcription and translation

Polymerases I, II, III → r, m, t. The strays, 5S rRNA and snRNA, hang off III.
σ for start, ρ for release — s with s, r with r
z, y, a → beta, permease, acetylase; i makes no enzyme at all
H1 is the linker, so it links beads and cannot be inside one

The five personal traps, and the written defence for each

These are the error families that have recurred across Aamirah's papers. Each one has a specific written action, not a general warning — the action is what makes the difference under time pressure.

Error family	Where it appears in this chapter	The written defence
Ratio reversal	Chargaff ratio questions, (A+T)/(G+C)	Before any arithmetic, write the sentence “the group on top is the bigger group”. The flipped answer is always one of the options.
Right content, wrong arrangement	Coding versus template strand; sickle-cell amino acid direction; match-the-column items; the histone list	Write your own answer out fully, with labels, before reading the options. Match label to label, not shape to shape.
Assertion–reason instability	Q6 and Q14 in the question file; roughly one in five NEET questions from this chapter	Mark T or F beside each of the two lines on the paper before looking at the four options. Never read the pair as one sentence.
Attribution errors	Griffith versus Avery versus Hershey–Chase; Meselson–Stahl versus Taylor; Gamow versus Nirenberg	For ‘which follows from X’ questions, check who did it, not only whether the statement is true. One option is always a true fact from the wrong experiment.
Blank-rate discipline	The three numerical types, which most candidates skip	These are mechanical and unambiguous. They are the cheapest marks in the chapter and should never be left blank.

Beyond the NCERT text, but asked

Nothing has been removed from this chapter by rationalisation, so there is no syllabus gap to cover. There is, however, content that NEET has asked which the NCERT text does not carry. Learn these as labels; do not build study time around them.

DNA polymerase I versus III — III does the bulk synthesis, I replaces the RNA primer with DNA
Primase, topoisomerase, single-strand binding proteins — NCERT names only polymerase, helicase and ligase
Melting temperature — GC-rich DNA melts higher; also A-DNA and Z-DNA forms
TATA box, enhancers, spliceosome composition, alternative splicing
A, P and E sites of the ribosome; formyl-methionine as the bacterial initiator
Amber, ochre and opal as names for UAG, UAA and UGA; the wobble hypothesis; missense, nonsense and silent mutations
Catabolite repression — positive regulation of the lac operon by glucose through cAMP and CAP; and the contrast with the repressible trp operon
Lalji Singh and DNA fingerprinting in India; NCERT credits only Alec Jeffreys