NEET 2027 · Biology · Class 12 Chapter 5 · File MBI-01
Plain-language explanation, animated diagram, fact sheet, exceptions and scientists for each of the five highest-weightage topics.
What is in this file — the five highest-yield topics in the chapter, together about three quarters of its NEET marks. Work through them in this order; each one depends on the one before it.
DNA is a twisted ladder. The two side rails are made of a sugar and a phosphate joined alternately, over and over. The rungs are pairs of chemical letters called bases. There are only four letters, and they pair in one fixed way: A always faces T, and G always faces C.
That fixed pairing is the single most important fact in the chapter. Because A only ever sits opposite T, if you know one rail you automatically know the other. A molecule that carries its own backup copy is a molecule that can be copied, repaired and inherited. Everything else in this chapter follows from that one property.
There are three joins, and NEET asks for their names.
| Join | What connects to what | Bond name |
|---|---|---|
| base + sugar | nitrogenous base to the 1′ carbon of the sugar | N-glycosidic bond → gives a nucleoside |
| nucleoside + phosphate | phosphate to the 5′ carbon of the sugar | phosphoester bond → gives a nucleotide |
| nucleotide + nucleotide | 3′-OH of one to the 5′-phosphate of the next | 3′–5′ phosphodiester bond |
Because every link is made the same way, the finished chain has two different ends: a free phosphate at the 5′ end and a free hydroxyl at the 3′ end. That asymmetry is what makes direction meaningful in DNA, and it is why so much of this chapter is about arrows.
| Family | Rings | Members | Memory hook |
|---|---|---|---|
| Purines | two fused rings, so bigger | Adenine, Guanine | Pure As Gold |
| Pyrimidines | one ring, so smaller | Cytosine, Thymine (DNA), Uracil (RNA) | CUT the pie |
Every rung is one purine facing one pyrimidine. That is why the ladder has the same width all the way along: big + small, every single time. Two purines would bulge; two pyrimidines would pinch.
Erwin Chargaff measured base amounts in DNA from many organisms before anyone knew the structure, and found the same ratios everywhere.
A = T and G = C | the direct statement |
A + G = T + C | total purines equal total pyrimidines |
(A + G)/(T + C) = 1 | the ratio form, always exactly 1 for double-stranded DNA |
(A + T)/(G + C) | this one is not fixed — it varies between species. Do not assume it equals 1. |
The ratio-reversal trap, and it is the recurring one. Aamirah's most frequent error family across Physics, Chemistry and Biology is writing a ratio the wrong way up under time pressure. This topic is where it shows up in Biology. When a question gives (A+T)/(G+C) = 1.5, write down in words before touching numbers: "A+T is the numerator, so A+T is the bigger one, so A+T = 1.5 × (G+C)." Say which is bigger out loud. Then solve. That one sentence, written down, removes the error.
| DNA | RNA | |
|---|---|---|
| Sugar | deoxyribose — no –OH at the 2′ carbon | ribose — has an –OH at 2′ |
| Bases | A, G, C, T | A, G, C, U |
| Strands | double-stranded, helical | usually single-stranded, but folds back on itself |
| Stability | high | low — that free 2′-OH is reactive |
| Catalysis | no | yes — some RNAs are enzymes (ribozymes) |
| Mutation rate | slow | fast, which is why RNA viruses evolve so quickly |
Thymine is simply uracil with a methyl group attached — thymine is 5-methyl uracil. That extra methyl is one of the things that makes DNA the more durable molecule.
Exceptions to hold on to
| Rise per base pair | 0.34 nm |
| Pitch, one full turn | 3.4 nm |
| Base pairs per turn | 10 |
| Diameter of the helix | 2 nm |
| Hydrogen bonds A=T / G≡C | 2 / 3 |
| φX174 bacteriophage | 5386 nucleotides (single-stranded) |
| Bacteriophage lambda | 48502 bp |
| E. coli genome | 4.6 × 10⁶ bp |
| Human haploid genome | 3.3 × 10⁹ bp |
| Total human DNA length | about 2.2 m |
| Friedrich Miescher, 1869 | first isolated DNA from pus cells and called it nuclein — 84 years before the structure was known |
| Erwin Chargaff | measured the base ratios that made the pairing rule inevitable |
| Maurice Wilkins and Rosalind Franklin | the X-ray diffraction data the model was built on |
| James Watson and Francis Crick, 1953 | the double-helix model |
| Nobel Prize, 1962 | Watson, Crick and Wilkins. Franklin had died in 1958 and the prize is not awarded posthumously. |
To copy DNA, the ladder is unzipped down the middle. Each half-ladder still carries a complete set of letters, so each one can be used as a pattern to build a fresh partner. When the job is finished there are two ladders, and each one is half old and half new.
That is what semiconservative means: half of the original is conserved in each copy. Not both old strands staying together, not the old strands being chopped up and shuffled — one old, one new, every time.
Three models were possible: conservative (the old duplex stays intact and an entirely new one is made), dispersive (old and new pieces scattered through both), and semiconservative. Meselson and Stahl separated them with weight.
They grew E. coli for many generations on a medium where the only nitrogen source was heavy ⁵N ammonium chloride, so all the DNA became heavy. Then they moved the cells to ordinary ¹⁴N and sampled at each generation, spinning the DNA in a caesium chloride gradient where molecules settle by density.
Why the bands settle the question. After one generation every molecule is hybrid — that alone kills the conservative model, which predicts one heavy band and one light band with nothing in between. After two generations, half hybrid and half light kills the dispersive model, which predicts a single band getting steadily lighter and never separating.
The same conclusion was reached in a plant by Taylor and colleagues in 1958, using radioactive thymidine in the root tips of Vicia faba, the faba bean. Chromosomes, not just naked DNA, replicate semiconservatively.
| Enzyme or component | Job |
|---|---|
| Helicase | unwinds and separates the two parent strands at the fork |
| DNA-dependent DNA polymerase | the main enzyme — adds nucleotides against the template |
| DNA ligase | seals the gaps between the short pieces on the lagging strand |
| Deoxyribonucleoside triphosphates (dNTPs) | act as both the raw material and the energy source — the two extra phosphates are released and power the join |
| Origin of replication (ori) | where it starts. One in a bacterium, many in a eukaryotic chromosome. |
The rule everything hangs on. DNA polymerase can only add a nucleotide onto a free 3′-OH. So it can only build a new strand in the 5′→3′ direction, and it must read the template 3′→5′. There is no exception to this anywhere in the syllabus. Every awkward feature of replication exists because of it.
Since the two templates are antiparallel, only one of them can be read smoothly in the direction the fork is opening. On that one the new strand runs continuously — the leading strand. On the other, the polymerase has to keep going back and starting again as more template appears, producing short pieces — the lagging strand, made of Okazaki fragments, which DNA ligase then joins up.
The trap in one line. Both strands are made 5′→3′. The lagging strand is not built backwards. Each of its fragments runs 5′→3′ just like the leading strand; it is only the overall progress of the lagging strand that points away from the fork. Questions that say “synthesised in the 3′→5′ direction” are always false.
The whole molecule does not unwind at once — that would take an impossible amount of energy for a molecule this long. Instead a small opening, the replication fork, travels along.
E. coli has 4.6 × 10⁶ base pairs and finishes replication in about 18 minutes. That works out to roughly 2000 base pairs per second at each fork, because replication runs outward in both directions from the single origin, so the two forks share the job.
In eukaryotes replication happens in the S phase of interphase. Replication and cell division must stay coupled: if the DNA is copied but the cell does not go on to divide, the cell ends up polyploid.
| Total DNA molecules after n generations | 2ⁿ |
| Molecules containing an original strand | always exactly 2, no matter how large n grows |
| Percentage of hybrid molecules | (2 / 2ⁿ) × 100 |
| Percentage of fully light molecules | 100 − hybrid % |
| Total strands after n generations | 2ⁿ⁺¹, of which 2 are original |
| Length of a DNA molecule | number of base pairs × 0.34 nm |
| Number of helical turns | number of base pairs ÷ 10 |
Exceptions
| Watson and Crick, 1953 | predicted semiconservative replication in the same paper as the structure |
| Matthew Meselson and Franklin Stahl, 1958 | the ⁵N experiment in E. coli that proved it |
| Taylor and colleagues, 1958 | the same result in Vicia faba using radioactive thymidine |
| Reiji Okazaki | the short lagging-strand fragments carry his name |
The cell does not send its master copy of the instructions out to the workshop. It writes out one page and sends the copy. Transcription is the writing-out: a single gene's stretch of DNA is copied into RNA.
Two things make it different from replication. Only one of the two DNA strands gets copied, and only a short stretch of it — one gene, not the whole molecule.
A transcription unit has three parts: a promoter, the structural gene itself, and a terminator. Of the two DNA strands, the one that is actually read is the template strand, and it always runs 3′→5′. The other one is the coding strand: it is never copied, but it is the one whose sequence matches the RNA.
The shortcut that removes an entire error family. If a question gives you the coding strand and asks for the mRNA, do not go via the template. Copy the coding strand and change every T to U. Done. If a question gives you the template strand, first write its complement (that is the coding strand), then swap T for U. Two steps, never one, and never guess which strand you were handed — check whether the sequence is written 5′→3′ (coding) or 3′→5′ (template).
NCERT gives two reasons, and both make excellent assertion–reason material.
Bacteria have a single RNA polymerase that makes all three kinds of RNA. It needs helpers at each end of the job:
| Factor | Stage | What it does |
|---|---|---|
| Sigma (σ) factor | initiation | lets the polymerase recognise the promoter and start |
| Rho (ρ) factor | termination | lets the polymerase let go at the terminator |
Because a bacterium has no nuclear membrane, the mRNA is exposed to ribosomes as soon as it starts to appear. Transcription and translation happen at the same time, in the same compartment. Bacterial mRNA is also polycistronic: one mRNA carries several genes.
| Polymerase | Product |
|---|---|
| RNA polymerase I | rRNAs — 28S, 18S and 5.8S |
| RNA polymerase II | hnRNA, the precursor of mRNA |
| RNA polymerase III | tRNA, 5S rRNA and snRNAs |
Memory hook for the three polymerases. Count I, II, III and read off r, m, t — ribosomal, messenger, transfer. The two loose ends both hang off III: 5S rRNA and the snRNAs go with tRNA, not with the big rRNAs.
Eukaryotic genes are interrupted. The stretches that survive into the mature RNA are exons (expressed); the stretches removed are introns (intervening). The primary transcript, hnRNA, therefore has to be edited before it can be used.
| Modification | Where | Exactly what is added or removed |
|---|---|---|
| Capping | 5′ end | an unusual nucleotide, methyl guanosine triphosphate |
| Tailing | 3′ end | 200 to 300 adenylate residues, added in a template-independent manner |
| Splicing | throughout | introns cut out, exons joined together |
Only after all three is the molecule called mRNA, and only then is it exported from the nucleus.
Exceptions and precision points
| Poly-A tail length | 200–300 adenylate residues |
| The 5′ cap | methyl guanosine triphosphate |
| Eukaryotic RNA polymerase I | 28S, 18S, 5.8S rRNA |
| Eukaryotic RNA polymerase II | hnRNA |
| Eukaryotic RNA polymerase III | tRNA, 5S rRNA, snRNA |
| Prokaryotic initiation / termination factors | sigma / rho |
| Direction RNA is built | 5′ → 3′ |
| Direction the template is read | 3′ → 5′ |
The mRNA is a sentence written in a four-letter alphabet. Proteins are written in a twenty-letter alphabet of amino acids. Nothing about a base looks anything like an amino acid, so something has to sit in between and do the matching. That something is tRNA, and Francis Crick predicted it must exist before anyone had seen one. He called it the adapter molecule.
The ribosome is the bench where the work happens. It grips the mRNA, holds two tRNAs side by side, joins the two amino acids they carry, shuffles along by exactly three letters, and does it again.
Each tRNA has two business ends, at opposite corners of the molecule.
Drawn flat, tRNA looks like a clover leaf. In reality it folds into a compact L shape. There is a special initiator tRNA, and there is no tRNA at all for the stop codons — which is exactly why the chain has to end there.
Loading an amino acid onto its tRNA is called charging or aminoacylation, and it needs ATP. When two charged tRNAs sit next to each other in the ribosome, the peptide bond forms between their amino acids.
A ribosome is two subunits that stay apart until an mRNA arrives. The large subunit contains the groove that the growing polypeptide runs through, and it holds the two tRNA slots.
The most quotable fact in this topic. In bacteria the peptide bond is formed not by a protein enzyme but by the 23S rRNA of the large subunit. An RNA doing an enzyme's job is a ribozyme. This single fact is also the standard molecular evidence for the RNA world, so it sits at the join between this chapter and Evolution.
| Ribosome | Whole | Subunits |
|---|---|---|
| Bacteria (and mitochondria, chloroplasts) | 70S | 50S + 30S |
| Eukaryotic cytoplasm | 80S | 60S + 40S |
The numbers do not add up arithmetically — 50 + 30 is not 70 — because S is a sedimentation coefficient, which depends on shape as well as mass. Questions sometimes test exactly this.
| Stage | What happens |
|---|---|
| Initiation | the small subunit binds the mRNA and the ribosome is positioned at the start codon AUG |
| Elongation | charged tRNAs arrive in turn, peptide bonds form, the ribosome moves on by one codon |
| Termination | a release factor — a protein, not a tRNA — binds the stop codon and the finished polypeptide is let go |
An mRNA is longer than the stretch that gets translated. There is a 5′ UTR before the start codon and a 3′ UTR after the stop codon. They are not translated, but translation cannot run properly without them.
Exceptions and traps
A bacterium that made lactose-digesting enzymes all day long, whether or not there was any lactose around, would be wasting energy constantly. So it keeps those genes switched off by default, with a protein sitting on the DNA like a padlock. When lactose turns up, lactose itself pulls the padlock off. When the lactose is used up, the padlock goes back on.
An operon is a set of genes that share one switch and get transcribed together as a single message.
| Gene or site | Name | Product or role |
|---|---|---|
| i | regulator gene | makes the repressor protein |
| p | promoter | where RNA polymerase attaches |
| o | operator | where the repressor sits when the operon is off |
| z | structural gene | β-galactosidase — splits lactose into galactose and glucose |
| y | structural gene | permease — makes the cell more permeable to β-galactosides |
| a | structural gene | transacetylase |
| Condition | What the repressor does | Result |
|---|---|---|
| No lactose | binds the operator | RNA polymerase is blocked; no transcription; operon off |
| Lactose present | lactose binds the repressor and changes its shape, so it can no longer hold the operator | polymerase runs through; one polycistronic mRNA for z, y and a; operon on |
Because the default state is off and the signal works by removing a block, this is called negative regulation. Lactose acting as the switch-on signal makes it an inducer, and the operon an inducible one.
The three traps, in order of how often they cost marks
The chicken-and-egg point NCERT raises. Lactose has to get inside the cell to act as an inducer, but getting in needs permease, and permease is one of the products the operon only makes once lactose is inside. The resolution is that a very small amount of permease and β-galactosidase is always present, enough to let the first molecules of lactose in and start the cycle.
In prokaryotes, control is almost entirely at the point of initiating transcription. In eukaryotes it can act at four levels: transcription, processing of the transcript, transport of mRNA out of the nucleus, and translation.
| The model | François Jacob and Jacques Monod, 1961 |
| Nobel Prize | 1965, shared with André Lwoff |
| Order on the DNA | i → p → o → z → y → a |
| z gene product | β-galactosidase |
| y gene product | permease |
| a gene product | transacetylase |
| Inducer | lactose |
| Type of control | negative regulation, inducible operon |
| mRNA produced | polycistronic — one message, three genes |