With respect to chromosomes in eukaryotic cells regarding DNA as genetic material consider the following statements. (a) Histones are small proteins associated with chromosomal DNA of eukaryotic cells. (b) These proteins are rich in basic Amino Acids Argenine and Lysine. (c) These protein lack Trptophan.
- (1)Only first statement is correct.
- (2)All the above three statements are correct.
- (3)Only first and second statement is correct but not third.
- (4)Only third statement is correct.
Correct — option (2): all three printed statements hold. Statement (a) is the definition of a histone. Histones are small, positively charged proteins that bind the DNA of eukaryotic chromosomes and package it: about two metres of DNA has to be folded into a nucleus a few micrometres across, and histones do the first and most important stage of that folding. The repeating unit is the nucleosome, in which roughly 147 base pairs of DNA are wound not quite twice around an octamer built from two copies each of histones H2A, H2B, H3 and H4, with a fifth histone, H1, binding the linker DNA between successive nucleosomes and drawing the string of them into a thicker fibre. Under the electron microscope the unfolded form looks like beads on a string, and the model was proposed in the mid-1970s, chiefly by Roger Kornberg. Statement (b) is right, and it explains why histones can do this job at all. They are rich in the basic amino acids arginine and lysine, whose side chains carry positive charges at cellular pH; DNA's sugar-phosphate backbone carries a negative charge at every phosphate; and the resulting electrostatic attraction is what holds the DNA against the histone core. The paper prints 'Argenine', which is a misspelling of arginine, and the card leaves the stem as printed. Statement (c) is right too, and it is the one candidates hesitate over because it is a negative claim about something absent. Histones contain no tryptophan. This can be checked directly against the sequences in the protein databases: the canonical human core histones H2A, H2B, H3.1 and H4, and the linker histones H1.2 and H1.4, contain not a single tryptophan residue between them, while carrying lysine and arginine in abundance — H4, for example, is 103 amino acids long with eleven lysines and fourteen arginines and no tryptophan at all. The paper prints 'Trptophan' for tryptophan and 'These protein lack' without the plural, and again the card reproduces the stem as it stands. With all three statements sound, the row that accepts them all is option (2).
- (1)Only first statement is correct. — This option accepts the definition of a histone and rejects both statements about its composition, and both rejections fail. The abundance of arginine and lysine is not an incidental feature of histones but the reason they work: their positively charged side chains bind the negatively charged phosphate backbone of DNA, and a histone stripped of that charge could not hold DNA at all. The absence of tryptophan is equally a matter of record in the sequences. A candidate typically chooses this row not from any contrary knowledge but from caution — the first statement is the one every textbook prints in bold, the other two look like specialist biochemistry, and retreating to the safest-looking claim feels prudent. In a three-statement item that caution is expensive, because rejecting a true statement costs exactly what accepting a false one costs. Judge each statement on what you actually know about it, and if you know nothing about a statement, say so to yourself rather than defaulting to rejection.
- (3)Only first and second statement is correct but not third. — This is the option the question is really built to catch, and it is chosen more often than any other wrong row. Its first two limbs are correct, and it then rejects the one statement that most candidates have never seen stated anywhere: that histones lack tryptophan. The claim sounds like an over-specific piece of trivia, and it is natural to suspect a statement whose content one has never met. It is nevertheless true, and it is checkable in the most direct way possible — by reading the amino-acid sequences of the histones themselves, in which no tryptophan appears. There is a general lesson here about negative statements in verification items. A statement asserting that something is ABSENT cannot be confirmed by recalling a fact, only by recalling the whole of a list, so candidates discount such statements far more readily than the evidence warrants. The habit worth building is to ask whether you have any positive reason to think the thing is present, and if you do not, to leave the statement standing rather than striking it out on unfamiliarity alone.
- (4)Only third statement is correct. — This option accepts the least familiar of the three statements and rejects the two that every account of chromatin contains, which reverses the usual pattern of error and is hard to reach by any considered route. Statement (a) is the textbook definition — histones are the small proteins associated with the chromosomal DNA of eukaryotic cells, and they are among the most evolutionarily conserved proteins known, histone H4 differing in only a couple of residues between organisms as distant as a cow and a pea. Statement (b) is the standard description of their composition, rich in the basic amino acids arginine and lysine. Rejecting both while accepting the tryptophan claim means holding a fine detail about a protein while denying what the protein is and what it is made of. Where this row does get chosen, it is usually the result of misreading the option text rather than of any judgment about the statements — which is itself worth guarding against, since the four rows in this set are worded as sentences rather than as symbols and are easy to confuse at speed.
Chromatin is the complex of DNA and protein that makes up a eukaryotic chromosome, and histones are its principal protein component. There are five classes: the four core histones, H2A, H2B, H3 and H4, which assemble into an octamer of two copies each, and the linker histone H1. About 147 base pairs of DNA wrap around each octamer to form a nucleosome, nucleosomes are separated by short stretches of linker DNA, and H1 binds where the DNA enters and leaves, drawing the beads-on-a-string form into a compact fibre which is folded further into loops and eventually into the metaphase chromosome. The packaging is extreme — around two metres of DNA in each human nucleus — and it is achieved by electrostatics: the histones' arginine and lysine residues are positively charged, DNA is negatively charged along its phosphate backbone, and the two bind. Histones are among the most conserved proteins in all of biology, which tells us how little variation this packing job tolerates. They are not merely structural. The N-terminal tails of the core histones project out from the nucleosome and are chemically modified — acetylated, methylated, phosphorylated — and those modifications loosen or tighten the local packing and thereby help decide which genes are available for transcription. That is the material basis of a large part of epigenetics. Prokaryotes have no true histones, their DNA being organised by a different set of nucleoid-associated proteins, though archaea possess histone-like proteins, a resemblance that is one of the several reasons archaea are grouped apart from bacteria.
Molecular biology has become a regular presence in MPSC's general science section, and the Commission tends to test it through multi-statement items rather than single-line recall, because a statement set can probe several layers of one topic at once — what a molecule is, what it is made of, where it is found, what it does. This question does exactly that with histones, and it is unusual in that all three statements are true, which is a configuration candidates find harder than a mixed set. Where two statements are true and one false, a reader has something to find; where all three are true, a reader who is looking for the planted error will manufacture one, and the third statement here is the obvious candidate for manufactured doubt because it asserts an absence. Two features of the printed page deserve recording. The English column carries three misprints in the statements — 'Argenine' for arginine, 'Trptophan' for tryptophan, and 'These protein lack' without the plural — none of which changes the sense, and all of which the card reproduces rather than repairs. And this item does not use the paper's usual options header: it prints 'Now, choose the correct option :' in English and 'खालीलपैकी योग्य पर्यायाची निवड करा :' in Marathi, with the word for 'correct' in bold in both. This paper's option headers vary from question to question, so no single form should be treated as standard.
- Histones are small basic proteins that package the DNA of eukaryotic chromosomes; about 147 base pairs of DNA wrap around an octamer of two copies each of H2A, H2B, H3 and H4 to form a nucleosome, and the linker histone H1 binds the DNA between nucleosomes.
- Histones are rich in the basic amino acids arginine and lysine, whose positively charged side chains bind the negatively charged phosphate backbone of DNA — the electrostatic attraction that holds chromatin together.
- Histones contain no tryptophan: the canonical human sequences of H2A, H2B, H3.1, H4, H1.2 and H1.4 carry not one tryptophan residue between them, while being abundant in lysine and arginine — H4 is 103 residues long with eleven lysines, fourteen arginines and no tryptophan.
- Histones are among the most evolutionarily conserved proteins known, histone H4 differing by only a couple of residues between organisms as distantly related as a cow and a pea.
- The N-terminal tails of the core histones are chemically modified by acetylation, methylation and phosphorylation, and those modifications alter how tightly the chromatin is packed and hence which genes can be transcribed — the material basis of much of epigenetics.
- Prokaryotes have no true histones, their DNA being organised by nucleoid-associated proteins, although archaea possess histone-like proteins.
All three statements stand, so the row that accepts them all is option (2). An all-true set is harder to handle than a mixed one: where two statements are true and one false a reader has something to find, and where all three are true a reader looking for the planted error will manufacture one — the third statement being the obvious victim here, because a claim of absence feels riskier to accept than a claim of presence. A negative claim needs a positive REASON before it is struck out, not a feeling. Around the item: histones are among the most evolutionarily conserved proteins known, H4 differing by only a couple of residues between organisms as distant as a cow and a pea; the N-terminal tails of the core histones are acetylated, methylated and phosphorylated in ways that loosen or tighten the local packing and so decide which genes can be transcribed, which is the material basis of much of epigenetics; and prokaryotes have no true histones, their DNA being organised by nucleoid-associated proteins, though archaea possess histone-like proteins. Note finally that this item does not use the paper's usual options header, printing 'Now, choose the correct option :' with the word for 'correct' in bold in both columns — the headers vary from question to question, so no single form should be treated as standard.
- Rejecting a statement that asserts an absence simply because the fact is unfamiliar — a negative claim requires a positive reason before it is struck out
- Assuming that in a multi-statement item at least one statement must be false, and manufacturing an error to satisfy the assumption
- Confusing the basic amino acids arginine and lysine with the aromatic ones, when it is the basic side chains' positive charge that binds DNA
- Mixing up the core histones with the linker histone H1, or misremembering how many copies of each are in a nucleosome octamer
- Reading option rows written as full sentences too quickly, since the wording rather than any symbol distinguishes them in this set
Molecular biology reaches MPSC papers in three shapes. The first is definitional — what a nucleosome is, what chromatin is made of, which nitrogenous bases pair with which — and it is straightforward recall. The second is the multi-statement verification item used here, in which several claims about one molecule are printed together and have to be sorted; the Commission draws these from the standard textbook description, so a candidate who has read the description rather than a summary of it is well placed. The third is applied and current, covering DNA fingerprinting, genome sequencing, gene editing and the vaccines and diagnostics that use these techniques. Preparing the first two means reading the chromatin chapter once properly rather than assembling it from bullet points, because it is precisely the sentences that bullet-point notes drop — such as the absence of tryptophan — that examiners lift into statements.
No directly related past PYQ was found.
- practice — not a real PYQ
A nucleosome consists of DNA wrapped around an octamer made up of two copies each of which histones ?
- (a)H1, H2A, H2B and H3
- (b)H2A, H2B, H3 and H4
- (c)H1, H2A, H3 and H4
- (d)H1, H2B, H3 and H4
Answer(b) H2A, H2B, H3 and H4 — the four core histones, present as two copies each, form the octamer around which about 147 base pairs of DNA are wound to make a nucleosome. H1 is the linker histone and is not part of the octamer: it binds where the DNA enters and leaves the nucleosome and to the linker DNA between successive nucleosomes, drawing the beads-on-a-string fibre into a more compact form. Every option that includes H1 among the octamer is therefore wrong.
- practice — not a real PYQ
Histones bind tightly to DNA chiefly because of which of the following ?
- (a)Hydrogen bonding between histone tails and the nitrogenous bases
- (b)Covalent bonds formed between histone cysteine residues and deoxyribose
- (c)Electrostatic attraction between positively charged lysine and arginine residues and the negatively charged phosphate backbone
- (d)Hydrophobic interactions between aromatic amino acids and the base pairs
Answer(c) Electrostatic attraction between positively charged lysine and arginine residues and the negatively charged phosphate backbone — histones are rich in these two basic amino acids, whose side chains carry a positive charge at cellular pH, while DNA carries a negative charge at every phosphate along its backbone. This is also why chemical modifications that neutralise the positive charge of lysine, such as acetylation, loosen the grip of the histone on the DNA and make the local chromatin more accessible to transcription.