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

Question

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.

UnlockIAS · Accepted Answer

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).

Why the other options are wrong — (a) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2021 Q59 — Science & Technology PYQ with Answer

State PCS 2021 Q59 — Science & Technology PYQ with Answer