Consider the following statements about DNA : 1. DNA refers to Deoxyribonucleic Acid. 2. It is located in the ribosomes. 3. It is composed of Ribonucleic Acid. 4. It can make a copy of itself. Which of the statements given above is/are correct ?
- (a)1 and 4
- (b)1 and 3
- (c)4 only
- (d)2 and 3
Correct — A, (a) 1 and 4. Two of the four statements are true and they are the first and the last. Statement 1 is simply the expansion of the abbreviation: DNA is deoxyribonucleic acid. Nothing in it can be disputed, and that makes it the anchor of the whole question — any option that leaves statement 1 out is eliminated before the biology begins, which disposes of options (c) and (d) at once. Statement 4 is true as well, and it is the property that makes DNA the hereditary material. DNA copies itself by replication: the double helix unwinds, each strand serves as a template, and the base-pairing rule — adenine with thymine, guanine with cytosine — dictates the sequence of the new partner strand that DNA polymerase assembles along it. Each daughter molecule therefore carries one old strand and one newly made one, which is why the process is called semi-conservative, and it is what allows an exact copy of the genetic instructions to pass to each daughter cell at division. Statement 2 is false. In a eukaryotic cell the great bulk of the DNA is in the nucleus, packed into chromosomes; smaller amounts sit in the mitochondria and, in plants, in the chloroplasts, and in a bacterium the single circular molecule lies free in the cytoplasm as a nucleoid. What ribosomes contain is ribosomal RNA together with protein; they are the workbenches on which protein is assembled, not the store of the instructions. Statement 3 is false, and it contradicts statement 1 head-on. DNA is deoxyribonucleic acid, not ribonucleic acid — the difference the prefix records is the sugar in the backbone. That mutual contradiction is a second and independent route to the answer: statements 1 and 3 cannot both be true, so option (b), which asserts both, must fail, leaving only one option that includes statement 1.
- (b)1 and 3 — It pairs a true statement with one that directly contradicts it. Statement 1 says DNA is deoxyribonucleic acid; statement 3 says it is composed of ribonucleic acid. The two nucleic acids differ in three definite ways: the sugar is deoxyribose in DNA and ribose in RNA, the base thymine in DNA is replaced by uracil in RNA, and DNA is characteristically double-stranded while RNA is usually single-stranded. RNA is made FROM a DNA template during transcription; it is not what DNA is made of.
- (c)4 only — Statement 4 is correctly identified as true, and then a statement of plain fact is thrown away with it. There is nothing to weigh in statement 1 — it is the expansion of the abbreviation in the stem's own first line — so an option that excludes it cannot be right. This is the option for a candidate who reads 'is/are correct' as an invitation to be sparing, when the safer discipline is to test every statement on its own and let the count fall where it falls.
- (d)2 and 3 — It selects precisely the two false statements. Statement 2 misplaces DNA in the ribosomes, which hold ribosomal RNA and protein and are the site of translation, while DNA resides in the nucleus and, in smaller quantities, in mitochondria and chloroplasts. Statement 3 confuses the two nucleic acids outright. An option built entirely of the false statements is a common construction in this format, and it is why the elimination should start from the statement that is certainly true rather than from the ones that sound doubtful.
DNA is the molecule in which the instructions for building and running an organism are stored. Its unit is the nucleotide, and each nucleotide has three parts — a deoxyribose sugar, a phosphate group and one of four nitrogenous bases: adenine, guanine, cytosine and thymine. Sugar and phosphate alternate to form the backbone of a strand, and two strands run in opposite directions and twist around a common axis, the double helix described by Watson and Crick in 1953. The strands are held together by hydrogen bonds between bases, and the pairing is strict: adenine bonds with thymine through two hydrogen bonds, guanine with cytosine through three. Strictness of pairing is what makes each strand a template for the other, and hence what makes replication possible. The wider scheme, the central dogma of molecular biology, runs DNA to RNA to protein: a stretch of DNA is transcribed into messenger RNA, which carries the message to a ribosome, where transfer RNA brings amino acids and the ribosome joins them into a protein in the order the message specifies. Ribosomes are built of ribosomal RNA and protein, which is exactly why the second statement in this question is wrong — the ribosome handles the message, not the archive.
The general science block of the EO/AO paper asks biology at the level of a school textbook, and this item is a good example of the multi-statement format the Commission favours for it: four short assertions, of which some are definitional, some are functional and at least one is a swap of a neighbouring term. The habit rewarded is treating each statement as a separate true-or-false question and starting with the one whose truth is not a matter of knowledge at all — here, the expansion of an abbreviation. Two statements in this set also contradict each other, and noticing that is worth as much as knowing the biology.
- DNA stands for deoxyribonucleic acid; RNA stands for ribonucleic acid.
- A DNA nucleotide has three components: a deoxyribose sugar, a phosphate group and a nitrogenous base.
- The four bases are adenine, guanine, cytosine and thymine; in RNA, uracil replaces thymine.
- Base pairing is adenine with thymine by two hydrogen bonds and guanine with cytosine by three.
- DNA is double-stranded and helical; RNA is usually single-stranded.
- In a eukaryotic cell DNA lies in the nucleus within chromosomes, with smaller amounts in mitochondria and chloroplasts; a bacterium carries it as a nucleoid in the cytoplasm.
- Ribosomes are made of ribosomal RNA and protein and are the site of protein synthesis; they contain no DNA.
- Replication is semi-conservative — each daughter molecule keeps one parental strand and one newly synthesised strand.
- Placing DNA in the ribosome because both are associated with the making of proteins; the ribosome holds ribosomal RNA.
- Accepting two statements that contradict each other, which no correct option can ever do.
- Discarding an obviously true statement as too easy to be part of the answer.
- Forgetting the DNA outside the nucleus, in mitochondria and chloroplasts, which several questions in this family turn on.
Biology in EO/AO papers comes as cell structure and function, human physiology, plant structure, vitamins and deficiency diseases, and a small amount of genetics. The multi-statement format used here recurs across the whole general science block, so the drill is the same each time: assess each statement independently, look for a pair that cannot both be true, and use the code options to confirm rather than to guess.
No directly related past PYQ was found.
- practice — not a real PYQ
In a molecule of DNA, adenine pairs with :
- (a)Guanine
- (b)Cytosine
- (c)Thymine
- (d)Uracil
Answer(c) Thymine
- practice — not a real PYQ
Which one of the following cell organelles is the site of protein synthesis ?
- (a)Ribosome
- (b)Lysosome
- (c)Golgi apparatus
- (d)Centrosome
Answer(a) Ribosome