NEET 2027 · Biology · Class 12 Chapter 5 · File MBI-02
The genetic code, the historical experiments and DNA packaging, then the three short recall topics that finish the chapter.
What is in this file — the six remaining topics. The first three are worth about a fifth of the chapter's marks and deserve full attention; the last three are small but the cheapest marks in the chapter, because they are almost pure recall.
There are four bases and twenty amino acids, so bases have to be read in groups. Read one at a time and you get four possibilities — far too few. Two at a time gives sixteen — still short. Three at a time gives sixty-four, comfortably more than enough. So the code is read three letters at a time, and those triplets are called codons.
Sixty-four codons for twenty amino acids means there is spare capacity. Most amino acids have more than one codon; three codons have no amino acid at all and mean “stop”.
| Who | What they contributed |
|---|---|
| George Gamow, a physicist | argued on pure arithmetic that the code had to be a triplet |
| Marshall Nirenberg and Heinrich Matthaei | the cell-free system that let synthetic RNA be translated in a test tube |
| Har Gobind Khorana | chemically synthesised RNA with defined, repeating base combinations |
| Severo Ochoa | polynucleotide phosphorylase, the enzyme that let RNA of known sequence be made |
| Nobel Prize 1968 | Nirenberg, Khorana and Robert Holley (Holley for working out the structure of tRNA) |
| Property | Meaning | The catch |
|---|---|---|
| Triplet | three bases per codon | — |
| Degenerate | one amino acid may have several codons | not the same as ambiguous |
| Unambiguous and specific | one codon codes for one amino acid only | the reverse of degeneracy, and both are true at once |
| Nearly universal | UUU means phenylalanine in a bacterium and in a human | nearly, not fully — see the exceptions |
| Non-overlapping | a base belongs to one codon only | — |
| Comma-less / contiguous | no punctuation between codons; read straight through | this is why a single deletion wrecks everything downstream |
Degenerate versus ambiguous — the distinction that decides marks. Degenerate means one amino acid, many codons. Ambiguous would mean one codon, many amino acids. The code is degenerate; it is not ambiguous. A statement saying “one codon can specify more than one amino acid” is always false.
Exceptions to hold on to
A point mutation changes a single base pair. The standard example in the syllabus is sickle-cell anaemia: in the sixth codon of the β-globin gene, GAG becomes GUG, so glutamic acid is replaced by valine. One base, one amino acid, a whole disease. This is also the direct bridge to Principles of Inheritance and Variation, where the same condition appears as an autosomal recessive trait.
A frameshift mutation is an insertion or deletion. Because the code is read straight through with no punctuation, losing or gaining a base shifts the reading frame from that point onwards, and everything after it is read wrongly.
The reasoning NCERT builds from this. Inserting or deleting one or two bases shifts the frame and garbles the rest. Inserting or deleting three bases adds or removes exactly one amino acid and the frame recovers. That result is itself the proof that the code is read in threes.
| Step 1 | Codons that finish before the changed base are untouched: (position − 1) ÷ 3, taking the whole-number part |
| Step 2 | Count the bases remaining after the deleted or inserted base |
| Step 3 | Divide that count by 3 to get the number of complete, misread codons; any remainder is an incomplete codon |
| Sanity check | unchanged codons + altered codons should account for the whole message |
| Total codons | 64 |
| Codons that specify an amino acid | 61 |
| Stop codons | 3 — UAA, UAG, UGA |
| Start codon | AUG |
| Amino acids with only one codon | 2 — methionine and tryptophan |
| Sickle-cell change | GAG → GUG, 6th codon of β-globin |
| Amino acid swap in sickle cell | glutamic acid → valine |
For a long time nobody knew whether heredity was carried by protein or by DNA, and most people bet on protein — proteins have twenty different building blocks and DNA only four, so protein looked like the richer alphabet. Three experiments settled it, and each one narrowed the answer further.
Griffith worked with Streptococcus pneumoniae, the bacterium that causes pneumonia. It came in two forms:
| Strain | Colony | Coat | Effect on mice |
|---|---|---|---|
| S | smooth | has a mucous polysaccharide coat | virulent — mice die |
| R | rough | no coat | non-virulent — mice live |
He injected mice four ways:
| Injected | Outcome |
|---|---|
| Live S | mice die |
| Live R | mice live |
| Heat-killed S | mice live |
| Heat-killed S + live R | mice die, and living S bacteria are recovered from them |
Something from the dead S bacteria had passed into the living R bacteria and permanently changed them. Griffith called it the transforming principle — but he had no idea what it was chemically.
They purified biochemicals from heat-killed S bacteria and asked which one could still transform R into S. The test was subtraction: destroy one class of molecule and see whether transformation survives.
| Enzyme added | What it destroys | Did transformation still happen? |
|---|---|---|
| Proteases | proteins | yes — so protein is not it |
| RNases | RNA | yes — so RNA is not it |
| DNases | DNA | no — transformation stopped |
Conclusion: DNA is the transforming principle. Even so, many biologists remained unconvinced.
They used bacteriophage T2, a virus that infects E. coli, and exploited one chemical asymmetry: DNA contains phosphorus but no sulphur, and protein contains sulphur but no phosphorus.
Phages grown with radioactive phosphorus had labelled DNA; phages grown with radioactive sulphur had labelled protein coats. Both were used to infect bacteria, the cultures were agitated in a blender to knock the phage coats off the bacterial surface, and the mixture was centrifuged.
The radioactivity from the phosphorus-labelled batch was found inside the bacteria. The radioactivity from the sulphur-labelled batch stayed outside, in the liquid. Only the DNA had gone in, and that was enough to produce a new generation of phages.
Exceptions and precision points
How this is asked now. These three experiments used to appear as standalone MCQs. They now mostly arrive as match-the-column items where all three must be recalled to earn one mark, or as assertion–reason pairs. Avery–MacLeod–McCarty's enzyme logic has not appeared for several years and is a reasonable candidate for a return.
| Frederick Griffith | 1928 — transformation, Streptococcus pneumoniae, mice |
| Avery, MacLeod and McCarty | 1933–1944 — DNA is the transforming principle |
| Alfred Hershey and Martha Chase | 1952 — bacteriophage T2, ³²P and ³⁵S |
| The organism in Griffith's work | Streptococcus pneumoniae |
| The organism in Hershey–Chase | bacteriophage T2 infecting E. coli |
The DNA in one human cell is about 2.2 metres long. The nucleus it has to fit into is about a millionth of a metre across. That is like packing two kilometres of thread into a tennis ball — and doing it neatly enough that any particular centimetre can be found and read on demand.
The trick is to wrap the thread around spools. The spools are protein, they carry a positive charge, and DNA carries a negative charge, so the two stick together naturally.
The spool is a histone octamer: two copies each of the histones H2A, H2B, H3 and H4. Histones are basic proteins, and their positive charge comes from being rich in the basic amino acids lysine and arginine. About 200 base pairs of DNA wrap around one octamer, and the resulting structure is a nucleosome.
H1 is the fifth histone. It is not part of the octamer — it sits outside, on the linker DNA between beads.
Nucleosomes repeat along the DNA, giving chromatin its beads-on-a-string appearance under the electron microscope. Chromatin then coils into a thicker fibre, and further-order packaging requires an additional set of proteins called non-histone chromosomal proteins. At metaphase this condenses into the chromosome.
| Euchromatin | Heterochromatin | |
|---|---|---|
| Packing | loose | dense |
| Staining | stains light | stains dark |
| Activity | transcriptionally active | transcriptionally inactive |
Exceptions and traps
| DNA per nucleosome | about 200 bp |
| Histone octamer | 2 each of H2A, H2B, H3, H4 |
| Linker histone | H1, outside the octamer |
| Basic amino acids giving the positive charge | lysine and arginine |
| Length of DNA in a human cell | about 2.2 m |
| Nucleosomes in a stretch of DNA | base pairs ÷ 200 |
If you were designing a molecule to carry inherited instructions, you would want four things from it: it must be able to copy itself, it must be chemically tough, it must be able to change occasionally so evolution has something to work with, and it must be able to actually do something — to show up as a visible character.
DNA and RNA can both copy themselves. Proteins cannot, which is why proteins were never really in the running once the question was framed this way.
| Criterion | RNA | DNA |
|---|---|---|
| Replication | yes | yes |
| Chemical stability | poor — the 2′-OH on every nucleotide is a reactive group | good — no 2′-OH, and thymine in place of uracil adds further stability |
| Structural stability | single-stranded, easily damaged | double-stranded, and a damaged strand can be repaired using the other one as reference |
| Mutation rate | fast — which is why RNA viruses evolve so quickly | slow enough to be reliable, fast enough for evolution |
| Expressing itself | better — RNA can directly direct protein synthesis | does it indirectly, through RNA |
The conclusion in one sentence. RNA is the better doer; DNA is the better store. Life began with RNA doing both jobs badly, and DNA evolved later as a more stable archive, leaving RNA to do the working. That is the RNA world.
The evidence that essential processes still run on RNA is exactly the ribozyme fact from Topic 4: in bacterial ribosomes the peptide bond is formed by 23S rRNA, not by a protein. Splicing and several metabolic steps are also RNA-catalysed. These are leftovers from a time when RNA did everything.
The exception that anchors the topic: DNA is the genetic material in most organisms, but some viruses — tobacco mosaic virus, Qβ bacteriophage, HIV and other retroviruses — carry RNA genomes. These are the organisms where the RNA world never quite ended.
The project was an attempt to read out the entire sequence of a human genome, base by base, and write it down. It ran for thirteen years and was described as a mega project, because at three dollars a base the bill for three billion bases comes to roughly nine billion dollars.
To picture the scale: if the sequence were printed in books of a thousand pages, each page holding a thousand letters, the sequence from one human cell would fill about 3300 such books.
| Approach | What it targets |
|---|---|
| Expressed Sequence Tags (ESTs) | only the genes that are actually expressed as RNA — the shortcut |
| Sequence Annotation | sequence the whole genome, coding and non-coding alike, and then work out which stretches do what |
The practical method: DNA was isolated and broken into fragments, the fragments were cloned in vectors — BAC (bacterial artificial chromosome) and YAC (yeast artificial chromosome) — then sequenced on automated machines built on the method developed by Frederick Sanger. Computers then assembled the overlapping fragments back into order and assigned them to chromosomes.
| Size of the human genome | 3164.7 million bp |
| Average gene size | 3000 bases |
| Largest known human gene | dystrophin, 2.4 million bases |
| Estimated number of genes | about 30,000 — far below earlier guesses of 80,000 to 140,000 |
| Bases identical between any two people | 99.9% |
| Genes of unknown function | more than 50% |
| Fraction that codes for protein | less than 2% |
| Chromosome with most genes | chromosome 1, with 2968 |
| Chromosome with fewest genes | the Y chromosome, with 231 |
| Known single-base variation sites (SNPs) | about 1.4 million |
| Duration | 1990 to 2003 |
Model organisms sequenced alongside or since include bacteria, yeast, Caenorhabditis elegans (a free-living, non-pathogenic nematode), Drosophila, rice and Arabidopsis.
How much of this to memorise. This topic and DNA fingerprinting together have produced about four questions in ten years. Learn the eleven numbers in the box above and the two approaches, and stop there. Chromosome-by-chromosome gene counts beyond chromosome 1 and Y are not a good use of revision time.
Any two people share 99.9% of their DNA sequence. Everything that makes one person's DNA identifiable sits in the remaining 0.1%. Much of that difference is not in genes at all but in stretches where a short sequence is repeated over and over — and the number of repeats differs from person to person.
DNA fingerprinting measures those repeat lengths. Cut the DNA up, sort the pieces by size, and the pattern of bands you get is characteristic of one individual.
When genomic DNA is spun in a density gradient, most of it forms one large peak and some small extra peaks appear alongside. Those extra peaks are the satellite DNA — repetitive sequences, classified by base composition, segment length and repeat number into microsatellites and minisatellites.
Satellite DNA does not code for protein, but it is highly polymorphic — highly variable between individuals — and that is exactly what makes it useful. The probe Alec Jeffreys used was a VNTR, a variable number of tandem repeats, which belongs to the minisatellite class and ranges from 0.1 to 20 kb in size.
Why it works for forensics. Every tissue in one person's body carries the same DNA, so blood, hair root and saliva give the same pattern. And because the repeats are inherited from parents, every band in a child must have come from one parent or the other — which is what makes parentage testing possible.
| Step | What happens |
|---|---|
| 1. Isolation | extract DNA from the sample |
| 2. Digestion | cut it with restriction endonucleases |
| 3. Electrophoresis | separate the fragments by size on a gel |
| 4. Blotting | transfer the separated fragments onto a nitrocellulose or nylon membrane — this is Southern blotting |
| 5. Hybridisation | add a labelled VNTR probe, which binds its complementary sequences |
| 6. Autoradiography | detect the bound fragments as a pattern of bands |
Exceptions and traps
| Developed by | Alec Jeffreys |
| Probe used | satellite DNA showing high polymorphism — VNTR |
| VNTR class | minisatellite |
| VNTR size range | 0.1 to 20 kb |
| Sequence shared between any two humans | 99.9% |
| Transfer technique | Southern blotting |
| Detection technique | autoradiography |
| Uses | forensics, parentage testing, population and genetic diversity studies |