Chapter 5: Molecular Basis of Inheritance Short Questions | biology Class 12 CBSE

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Chapter 5: Molecular Basis of Inheritance Short Questions | biology Class 12 CBSE | ByteNotes.in

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Question 1

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What are the three components of a nucleotide? How are two nucleotides joined together in a polynucleotide chain?

A nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group linked via a phosphoester bond to the 5'-C of the sugar.
Two nucleotides are linked through a 3'-5' phosphodiester bond — the phosphate on the 5'-C of one nucleotide is covalently bonded to the 3'-OH of the preceding nucleotide's sugar.
A polynucleotide chain thus has a free phosphate at the 5'-end and a free 3'-OH at the other end, giving it a defined directionality (polarity).

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Question 2

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State Chargaff's rules. How do these rules support the Double Helix model of DNA?

Chargaff's rules state that in any double-stranded DNA, the ratio of adenine to thymine (A/T) and guanine to cytosine (G/C) is always equal to one.
These ratios imply specific base pairing: A always pairs with T (two H-bonds) and G always pairs with C (three H-bonds) across the two strands, forming complementary base pairs.
This complementarity between the two antiparallel strands is a central feature of the Watson-Crick Double Helix model and explains the uniform distance between the two strands of the helix.

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Question 3

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Describe the role of histones in DNA packaging in eukaryotes.

Histones are positively charged basic proteins rich in lysine and arginine; they form an octameric unit (histone octamer) around which the negatively charged DNA wraps to form a nucleosome containing ~200 bp of DNA.
Nucleosomes form the repeating 'beads-on-string' units of chromatin, which further coil and condense with the help of Non-Histone Chromosomal (NHC) proteins into chromatin fibres and ultimately chromosomes.
This hierarchical packaging reduces the 2.2-metre-long DNA in a mammalian cell to fit within the ~10⁻⁶ m nucleus, while also influencing gene expression through chromatin remodelling.

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Question 4

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Why is DNA considered a better genetic material than RNA?

DNA lacks the 2'-OH group present in every RNA nucleotide, making DNA chemically less reactive and structurally more stable, while RNA is easily degradable due to this reactive group.
DNA contains thymine (5-methyl uracil) instead of uracil, which confers additional stability; also, DNA's double-stranded nature enables repair mechanisms that maintain sequence fidelity.
RNA mutates at a faster rate due to its instability, whereas the stability of DNA makes it better suited for long-term storage of genetic information across generations.

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Question 5

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What was the contribution of Avery, MacLeod, and McCarty to establishing DNA as the genetic material?

They biochemically characterised the 'transforming principle' from Griffith's experiment by purifying proteins, DNA, and RNA from heat-killed S-strain bacteria and testing each for transforming ability on live R-strain bacteria.
They found that DNA alone caused transformation of R-strain bacteria into virulent S-strain; DNase (which degrades DNA) inhibited transformation, while proteases and RNases did not.
Their experiments concluded that DNA is the hereditary material responsible for transformation, providing strong biochemical evidence before the Hershey-Chase experiment.

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Question 6

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What is semiconservative DNA replication? Name the scientist who first proposed this scheme.

In semiconservative replication, the two parental DNA strands separate and each acts as a template for the synthesis of a new complementary strand, resulting in two daughter DNA molecules each containing one original (parental) strand and one new strand.
This scheme was proposed by James Watson and Francis Crick in 1953, immediately after they described the Double Helix structure of DNA.
It was experimentally confirmed by Matthew Meselson and Franklin Stahl in 1958 using 15N/14N isotope labelling and CsCl density gradient centrifugation in E. coli.

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Question 7

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Why does replication on one strand of DNA proceed discontinuously during DNA synthesis?

DNA-dependent DNA polymerase can only catalyse polymerisation in the 5'→3' direction; it cannot synthesise in the 3'→5' direction.
The replication fork opens in one direction, so one template strand (3'→5') allows continuous synthesis of the new strand in the 5'→3' direction (leading strand), while the other template strand (5'→3') must be copied in short fragments in the 5'→3' direction (lagging strand), producing Okazaki fragments.
These Okazaki fragments are later joined by DNA ligase, completing the lagging strand synthesis and restoring continuity to the newly synthesised DNA.

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Question 8

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Distinguish between the template strand and the coding strand in a transcription unit.

The template strand has 3'→5' polarity and serves as the actual template for RNA polymerase; it is complementary to the RNA synthesised during transcription.
The coding strand has 5'→3' polarity and has the same sequence as the RNA (except it has thymine where RNA has uracil); it is displaced during transcription and does not directly code for RNA.
All positional references for a transcription unit (promoter, terminator) are made with respect to the polarity of the coding strand; switching promoter and terminator positions would reverse the definitions of these two strands.

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Question 9

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What is the role of the promoter in a transcription unit?

The promoter is a specific DNA sequence located upstream (towards the 5'-end of the coding strand) of the structural gene in a transcription unit.
It provides the binding site for RNA polymerase, thereby initiating the process of transcription at the correct position on the DNA.
The presence of a promoter also defines which strand of the DNA will act as the template strand and which will be the coding strand for that transcription unit.

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Question 10

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Explain why both strands of DNA are not used as templates simultaneously during transcription.

If both strands were transcribed simultaneously, they would produce two different RNA molecules with different sequences (they are complementary, not identical), potentially coding for two different and conflicting proteins from the same DNA segment.
Additionally, the two RNA molecules produced would be complementary to each other and would anneal to form double-stranded RNA, preventing either from being translated into protein and making the entire transcription process futile.
Therefore, only one specific strand (template strand) is transcribed by RNA polymerase in a given transcription unit to ensure unambiguous and efficient gene expression.

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Question 11

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What is the difference between exons and introns? Where do introns appear in a transcription unit?

Exons are the coding sequences of a eukaryotic gene; they are expressed (appear) in the mature, processed mRNA after splicing and are ultimately translated into protein.
Introns (intervening sequences) are non-coding sequences present within a eukaryotic gene that are transcribed into the primary transcript (hnRNA) but are removed during splicing before the mRNA is exported for translation.
Introns interrupt the exons within the structural gene of a transcription unit and must be precisely excised and the exons joined in a defined order during RNA processing.

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Question 12

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List any four salient features of the genetic code.

The genetic code is triplet (a codon consists of three nucleotides); 61 codons code for 20 amino acids and 3 codons (UAA, UAG, UGA) are stop codons that do not code for any amino acid.
The code is degenerate: most amino acids are coded by more than one codon (e.g., six codons for leucine); and non-overlapping: each base is read as part of only one codon without overlap.
The code is nearly universal: the same codon specifies the same amino acid in organisms from bacteria to humans, with minor exceptions in mitochondrial codons and some protozoans; AUG has a dual function as the initiator codon (methionine) and a codon within the open reading frame.

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Question 13

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What is the adapter role of tRNA? How is it structurally suited for this function?

The tRNA acts as an adapter molecule by physically linking the genetic code (codons on mRNA) to the appropriate amino acid during translation — it reads the mRNA codon on one end and carries the corresponding amino acid on the other.
Its anticodon loop contains three bases complementary to a specific mRNA codon, allowing correct reading of the code by complementary base pairing; its 3'-amino acid acceptor end binds specifically to the cognate amino acid through aminoacylation.
tRNAs have a cloverleaf secondary structure that folds into a compact inverted-L tertiary structure, and there is at least one specific tRNA for each of the 20 amino acids; initiator tRNA is distinct and used only for starting translation.

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Chapter sections

Chapter 5: Molecular Basis of Inheritance | Short Questions

What are the three components of a nucleotide? How are two nucleotides joined together in a polynucleotide chain?

State Chargaff's rules. How do these rules support the Double Helix model of DNA?

Describe the role of histones in DNA packaging in eukaryotes.

Why is DNA considered a better genetic material than RNA?

What was the contribution of Avery, MacLeod, and McCarty to establishing DNA as the genetic material?

What is semiconservative DNA replication? Name the scientist who first proposed this scheme.

Why does replication on one strand of DNA proceed discontinuously during DNA synthesis?

Distinguish between the template strand and the coding strand in a transcription unit.

What is the role of the promoter in a transcription unit?

Explain why both strands of DNA are not used as templates simultaneously during transcription.

What is the difference between exons and introns? Where do introns appear in a transcription unit?

List any four salient features of the genetic code.

What is the adapter role of tRNA? How is it structurally suited for this function?

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