Directions : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
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Assertion : The sugar phosphate backbone of two chains in DNA double helix show anti-parallel polarity.
Reason : The phosphodiester bonds in one strand go from a \(3^{\prime}\) carbon of one nucleotide to a \(5^{\prime}\) carbon of adjacent nucleotide, whereas those in complementary strand go vice versa.
(a)
Assertion: In Griffith’s experiment, a mixture of heat killed virulent bacteria \(R\) and live non-virulent bacteria S , lead to the death of mice.
Reason : ‘Transforming principle’ got transferred from heat-killed \(S\) strain to \(R\) strain and made it virulent.
(d) : When bacteria Streptococcus pneumoniae are grown on a culture plate, some produce smooth shiny colonies (S) while others produce rough colonies \((\mathrm{R})\). This is because the \(S\) strain bacteria have a mucous (polysaccharide) coat, while R strain does not. Mice infected with the \(S\) strain (virulent) die from pneumonia infection but mice infected with the \(R\) strain do not develop pneumonia. In Griffith’s experiment, some ‘transforming principle’, transferred from the heat-killed \(S\) strain, had enabled the \(R\) strain to synthesise a smooth polysaccharide coat and become virulent which must be due to the transfer of the genetic material. This is known as transformation.
Assertion : DNA is considered to be better genetic material than RNA for most organisms.
Reason : \(2^{\prime}\) – OH group present in DNA makes it labile and less reactive.
(c) : \(2^{\prime}-\mathrm{OH}\) group present at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable.
Assertion : The mechanism of DNA replication is semiconservative in nature.
Reason : Each of the complementary strands of the parental double helix is conserved during the process.
(a) : DNA replication is semi-conservative, a type of replication in which one strand of the daughter duplex is derived from the parent while the other strand is formed new. This is carried out by the separation of two strands. The separated strands function as templates. The new strands built up over the templates of old strands will have complementary base pairs (A opposite T and G opposite C). The two daughter DNA molecules thus formed will be carbon copies of the parent molecule but shall have one new strand and one old strand.
Assertion : Synthesis of daughter or new strand occurs continuously along the parent \(3^{\prime} \rightarrow 5^{\prime}\) strand.
Reason : DNA polymerase can polymerise nucleotides in \(3^{\prime} \rightarrow 5^{\prime}\) direction on \(5^{\prime} \rightarrow 3^{\prime}\) strand.
(c) : DNA-polymerase can polymerise nucleotides only in \(5^{\prime}\) \(\rightarrow 3^{\prime}\) direction on \(3^{\prime} \rightarrow 5^{\prime}\) strand because it adds them at the \(3^{\prime}\) end.
Assertion : Template or antisense strand, having \(3^{\prime} \rightarrow 5^{\prime}\) polarity takes part in transcription.
Reason: Non-template or sense strand, having \(5^{\prime} \rightarrow 3^{\prime}\) polarity, does not take part in transcription.
(b)
Assertion : Polycistronic mRNA, found in prokaryotes, specify a number of polypeptides.
Reason : Monocistronic mRNA, found in eukaryotes, specify only a single polypeptide.
(b)
Assertion : Eukaryotic mRNA requires post-transcription processing for formation of functional mRNA.
Reason : Eukaryotic transcripts possess extra nonfunctional segments called introns.
(a) : In eukaryotes, the monocistronic structural genes have interrupted coding sequences, i.e., the genes in eukaryotes are split. The coding sequences or expressed sequences are defined as exons. They are said to be those sequence that appear in mature or processed RNA. The exons are interrupted by introns or intervening sequences which do not appear in mature or processed RNA. Thus, post-transcription processing, which involves splicing of introns and fusion of exons is required to form functional mRNA.
Assertion : A change in nitrogen base at the third position of a codon causes change in the expression of the codon.
Reason : The pairing of the third position bases of the codon may be ambiguous and varies according to the nucleotide present in this position.
(d) : According to the Wobble hypothesis, only the first two position of a triplet codon on mRNA have a precise pairing with the bases of the fRNA anticodon. The pairing of the third position bases of the codon may be ambiguous and varies according to the nucleotide present in this position. Thus, a single IRNA type is able to recognise two or more codons differing only in the third base. The same is called wobble position.
Assertion: tRNA recognises its corresponding codon in mRNA.
Reason : For each codon, there is an individual tRNA.
(c): IRNA has an anticodon loop which has 7 bases, out of which three bases form anticodon for recognising and attaching to the codon of mRNA. Since there are 61 codons specifying amino acids, the cell should contain 61 different tRNA molecules, each with a different anticodon. Actually, however, the number of IRNA molecule types discovered is much less than 61. This implies that the anticodons of some tRNAs read more than one codon on mRNA.
Assertion : Untranslated regions are sequences of RNA before initiation codon and after termination codon.
Reason : Untranslated regions provide stability to mRNA and also increase translational efficiency.
(b)
Assertion : The predominant site for control of gene expression in prokaryotes is transcription initiation.
Reason : The activity of RNA polymerase is regulated by accessory proteins, which affect recognition of start sites.
(a) : In prokaryotes, control of the rate of transcriptional initiation is the predominant site for control of gene expression. In a transcription unit, the activity of RNA polymerase at a given promoter is in turn regulated by interaction with accessory proteins, which affect its ability to recognise start sites. These regulatory proteins can act both positively (activators) and negatively (repressors).
Assertion : Lac operon is a repressible operon.
Reason : Regulation of lac operon by repressor is referred to as negative regulation.
(d) : An operon is a part of genetic material (or DNA) which acts as a single regulated unit having one or more structural genes, an operator gene, a promoter gene, a regulator gene, a repressor and inducer or corepressor (from outside). Lac operon is an inducible operon. An inducible operon system is a regulated unit of genetic material which is switched on in response to the presence of a chemical.
Assertion: Repetitive sequences make up very large portion of human genome.
Reason : Repetitive sequences do not have direct coding functions in the genome.
(b) : Repeated or repetitive sequences make up a large portion of human genome. These sequences are nucleotide sequences that are repeated many times, sometimes hundred to thousand times. They have no direct coding function but provide informations as to chromosome structure, dynamics and evolution.
Assertion: When the DNA sequences of two people are cut using the same restriction enzyme, the length and number of fragments obtained are different for both.
Reason : DNA sequence is arranged randomly in many copy numbers which varies from chromosome to chromosome in an individual, showing high degree of polymorphism.
(a)
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