Class 12 Molecular Basis of Inheritance Mock Test (CBSE)

Topics in Molecular Basis of Inheritance

• The DNA and its structure
• Packaging of DNA helix
• The search for genetic material
• RNA world
• Replication
• Transcription
• Genetic code
• Translation
• Regulation of gene expression and the lac operon
• Human genome project
• DNA fingerprinting

Molecular Basis of Inheritance practice questions with answers

1. Q1Multiple choice·1 mark
   During DNA replication, Okazaki fragments are synthesized on:

   1. (a)The leading strand in the $5' \rightarrow 3'$ direction
   2. (b)The lagging strand in the $5' \rightarrow 3'$ direction
   3. (c)The leading strand in the $3' \rightarrow 5'$ direction
   4. (d)The lagging strand in the $3' \rightarrow 5'$ direction
   Show answer and step marks

   1. Correct option: (b) Lagging strand in the $5' \rightarrow 3'$ direction+1

   Answer: (b)
2. Q2Multiple choice·1 mark
   Which of the following sequences or regions on a DNA transcription unit is responsible for providing the binding site for RNA polymerase?

   1. (a)Promoter
   2. (b)Structural gene
   3. (c)Terminator
   4. (d)Operator
   Show answer and step marks

   1. Correct option: (a) Promoter+1

   Answer: (a)
3. Q3Short answer·2 marks
   Explain briefly why the genetic code is considered: (i) degenerate (ii) universal
   Show answer and step marks

   1. Degenerate: Some amino acids are coded by more than one codon.+1
   2. Universal: A specific codon codes for the same amino acid in almost all organisms, from bacteria to humans.+1

   Answer: (i) Degenerate: some amino acids are coded by more than one codon. (ii) Universal: a given codon codes for the same amino acid in nearly all organisms, from bacteria to humans.
4. Q4Short answer·2 marks
   State any two salient features of the genetic code.
   Show answer and step marks

   1. First feature, any one of: the codon is a triplet: three nitrogenous bases code for one amino acid; the code is degenerate: some amino acids are coded by more than one codon; the code is unambiguous and specific: one codon codes for only one amino acid; the code is nearly universal: a codon codes for the same amino acid in nearly all organisms; the codons are read on mRNA contiguously, without punctuation (comma-less); AUG has a dual function: it codes for methionine and acts as the initiator codon; 61 codons code for amino acids and 3 codons (UAA, UAG, UGA) are stop codons; or any other valid feature of the genetic code.+1
   2. Second feature, a different one from the same list.+1

   Answer: Any two of: codons are triplets; the code is degenerate; it is unambiguous and specific; it is nearly universal; it is read contiguously without punctuation; AUG codes for methionine and is the initiator codon; 61 codons code for amino acids and 3 (UAA, UAG, UGA) are stop codons.
5. Q5Short answer·3 marks
   Describe the role of lactose as an inducer in the regulation of the lac operon in $E. coli$.
   Show answer and step marks

   1. In the absence of lactose, the active repressor protein binds to the operator region, preventing RNA polymerase from transcribing the structural genes.+1
   2. When lactose is added, a little of it entering the cell is converted by the low basal level of $\beta$-galactosidase into allolactose, the actual inducer; it binds to the repressor, which changes shape and becomes inactive (NCERT describes this as lactose binding the repressor; credit either).+1
   3. The inactive repressor detaches from the operator, allowing RNA polymerase to access the promoter and transcribe the structural genes ($z, y, a$) required for lactose metabolism.+1

   Answer: Without lactose, the repressor binds the operator and blocks transcription. Lactose entering the cell is converted to allolactose (the inducer), which binds the repressor and inactivates it; the repressor leaves the operator, so RNA polymerase transcribes the $z$, $y$ and $a$ genes needed to metabolise lactose.

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