Every Book Back multiple-choice question from Molecular Genetics (12th Standard Zoology, Samacheer Kalvi) — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Hershey and Chase experiment with bacteriophage showed that
- A. Protein gets into the bacterial cells
- B. DNA is the genetic materialCorrect
- C. DNA contains radioactive sulphur
- D. Viruses undergo transformation
Explanation. Hershey and Chase used radioactive phosphorus and sulfur to label bacteriophages, demonstrating that only viral DNA enters host bacterial cells to direct replication, which proved DNA is the genetic material.
Q2
DNA and RNA are similar with respect to having
- A. Thymine as a nitrogen base
- B. A single-stranded helix shape
- C. Nucleotides containing sugars, nitrogen bases and phosphatesCorrect
- D. The same sequence of nucleotides for the amino acid phenylalanine
Explanation. Both DNA and RNA are polynucleotides comprised of nucleotide subunits. Each nucleotide is composed of a five-carbon pentose sugar, a nitrogenous base, and a phosphate group linked together by phosphodiester bonds.
Q3
An mRNA molecule is produced by
- A. Replication
- B. TranscriptionCorrect
- C. Duplication
- D. Translation
Explanation. An mRNA molecule is synthesized through transcription, where DNA-dependent RNA polymerase copies genetic information from the DNA template strand into a complementary single-stranded RNA molecule.
Q4
The total number of nitrogenous bases in human genome is estimated to be about
- A. 3.5 million
- B. 35000
- C. 35 million
- D. 3.1 billionCorrect
Explanation. The human genome consists of approximately three billion nucleotide bases across all chromosomes, as determined by the global mapping and sequencing efforts of the Human Genome Project.
Q5
E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two generations. What density distribution of DNA would you expect in this experiment?
- A. One high and one low density band
- B. One intermediate density band
- C. One high and one intermediate density band
- D. One low and one intermediate density bandCorrect
Explanation. After two rounds of replication in 14N medium, half of the DNA is hybrid (15N-14N, intermediate density) and the remaining half is completely light (14N-14N, low density), creating two distinct bands.
Q6
What is the basis for the difference in the synthesis of the leading and lagging strands of DNA molecules?
- A. Origin of replication occurs only at the 5' end of the molecules
- B. DNA ligase works only in the 3' to 5' direction
- C. DNA polymerase can join new nucleotides only to the 3' end of the growing strandCorrect
- D. Helicases and single-strand binding proteins work only at the 5' end
Explanation. DNA polymerase only adds nucleotides to the free 3'-hydroxyl end of the growing strand. Because replication is antiparallel, synthesis is continuous on one template and discontinuous on the other.
Q7
Which of the following is the correct sequence of events with reference to the central dogma?
- A. Transcription, Translation, Replication
- B. Transcription, Replication, Translation
- C. Duplication, Translation, Transcription
- D. Replication, Transcription, TranslationCorrect
Explanation. The central dogma represents the unidirectional flow of genetic information where DNA is replicated, transcribed into single-stranded messenger RNA, and finally translated into a polypeptide chain of amino acids.
Q8
Which of the following statements about DNA replication is not correct?
- A. Unwinding of DNA molecule occurs as hydrogen bonds break
- B. Replication occurs as each base is paired with another exactly like itCorrect
- C. Process is known as semi-conservative replication because one old strand is conserved in the new molecule
- D. Complementary base pairs are held together with hydrogen bonds
Explanation. DNA replication relies on complementary base pairing where adenine pairs with thymine and guanine pairs with cytosine. Bases do not pair with identical bases, making the second statement incorrect.
Q9
Which of the following statements is not true about DNA replication in eukaryotes?
- A. Replication begins at a single origin of replicationCorrect
- B. Replication is bidirectional from the origins
- C. Replication occurs at about 1 million base pairs per minute
- D. There are numerous different bacterial chromosomes, with replication occurring in each at the same time
Explanation. Eukaryotic replication begins at multiple origins of replication along the linear chromosomes to ensure timely duplication of the large genome, unlike prokaryotes which use a single origin.
Q10
The first codon to be deciphered was __________ which codes for ________.
- A. AAA, proline
- B. GGG, alanine
- C. UUU, PhenylalanineCorrect
- D. TTT, arginine
Explanation. UUU was the first codon deciphered. Nirenberg and Matthaei demonstrated that a synthetic poly-U RNA homopolymer directs the synthesis of a polypeptide chain consisting solely of phenylalanine in a cell-free system.
Q11
Meselson and Stahl's experiment proved
- A. Transduction
- B. Transformation
- C. DNA is the genetic material
- D. Semi-conservative nature of DNA replicationCorrect
Explanation. Using heavy (15N) and light (14N) isotopes of nitrogen in E. coli, Meselson and Stahl physically demonstrated that DNA replicates semi-conservatively, with each new helix containing one parent and one new strand.
Q12
Ribosomes are composed of two subunits; the smaller subunit of a ribosome has a binding site for _________ and the larger subunit has two binding sites for two __________.
- A. mRNA, tRNACorrect
- B. tRNA, mRNA
- C. tRNA, rRNA
- D. rRNA, tRNA
Explanation. The smaller subunit of the ribosome binds to the messenger RNA (mRNA) to initiate translation, whereas the larger subunit contains specific binding sites (A and P) to accommodate two transfer RNA (tRNA) molecules.
Q13
An operon is a
- A. Protein that suppresses gene expression
- B. Protein that accelerates gene expression
- C. Cluster of structural genes with related functionCorrect
- D. Gene that switches other genes on or off
Explanation. An operon is a cluster of structural genes under the control of a single promoter and operator, allowing coordinated transcription of genes with related metabolic functions into a single polycistronic mRNA.
Q14
When lactose is present in the culture medium
- A. Transcription of lac y, lac z, lac a genes occurs
- B. Repressor is unable to bind to the operator
- C. Repressor is able to bind to the operator
- D. Both (a) and (b) are correctCorrect
Explanation. Lactose acts as an inducer that binds and inactivates the repressor protein. This prevents the repressor from binding to the operator, allowing RNA polymerase to transcribe the lac operon structural genes.
About these Molecular Genetics questions
These are the Book Back multiple-choice questions for Molecular Genetics from the Tamil Nadu State Board (Samacheer Kalvi) 12th Standard Zoology syllabus. Each question shows the correct option and an original, step-by-step explanation so you understand the method, not just the answer. Use the answer key above to jump to any question, then take the practice test to check yourself under exam-like conditions.
Frequently asked questions
How many MCQs are there in Molecular Genetics?
This chapter has 14 book-back multiple-choice questions, each with the correct answer and a step-by-step explanation.
Are these 12th Standard Zoology MCQs free to practise online?
Yes. Every question, answer and explanation here is free, and you can also take them as a timed practice test.
Where can I find the Molecular Genetics book-back answers?
The correct option for each question is highlighted on this page with a worked explanation, plus a quick answer-key summary at the top.
