Class 12 Biology MCQ – Structure of a Polynucleotide Chain


This set of Class 12 Biology Chapter 6 Multiple Choice Questions & Answers (MCQs) focuses on “Structure of a Polynucleotide Chain”.

1. Thymine is present in the place of Uracil in DNA.
a) True
b) False
View Answer

Answer: a
Explanation: The nitrogenous base present in the DNA are Adenine, Guanine, Cytosine and Thymine. In RNA, there will be an Uracil in the place of Thymine. There will be a double hydrogen bond between Adenine and Uracil, whereas, in DNA it will be present between Adenine and Thymine.

2. Through which among the following linkages are the two nucleotides connected through the 3’-5’ end?
a) Phosphodiether linkage
b) Phosphodisulphide linkage
c) Phosphodinitrate linkage
d) Phosphodiester linkage
View Answer

Answer: d
Explanation: When two nucleotides come close to each other, they form a linkage. This linkage is seen from the 3’-5’ end. This type of a linkage is called as the phosphodiester linkage. A phosphodiester linkage between 2 nucleotides forms a dinucleotide.

3. What is chemical name for thymine known as?
a) 5-methoxy uracil
b) 3-methoxy uracil
c) 5-methyl uracil
d) 3-methy uracil
View Answer

Answer: c
Explanation: In RNA structure, Uracil is present in the case of Thymine. Thymine is a nitrogenous base which is achieved, when the Uracil is being methylated at the 5th carbon. Hence the name 5-methyl uracil.
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4. Who considered DNA as a “Nuclein”?
a) James Watson
b) Friedrich Meischer
c) Francis Crick
d) Rosalind Franklin
View Answer

Answer: b
Explanation: Friedrich Meischer had originally called the DNA to be as the “Nuclein”. He observed DNA to be as an acidic substance found in the nucleus. Since technology hadn’t been developed much in the 1860s, he wasn’t able to fully study about the structure of the DNA and isolate such a long polymer. Later in the 1950s, Rosalind Franklin did an X-Ray diffraction study on the DNA using the X-Ray crystallography method. James Watson and Francis Crick on observing the X-Ray pattern of the DNA molecule, gave the molecular structure of the DNA.

5. Which among the following is the exact ratio of guanine to cytosine in a DNA double helical structure?
a) 3:2
b) 2:1
c) 1:3
d) 1:1
View Answer

Answer: d
Explanation: The ratio of the purines (adenine and guanine) to the pyrimidine (thymine and cytosine) should always be equal to 1:1. This is in accordance to the Chargaff’s rule. The ratio of 1:1 is also based on the base pair rule.
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6. Which of the following enzymes are used to transcript a portion of the DNA into mRNA?
a) RNA polymerase
b) DNA polymerase
c) Protein polymerase
d) Hydrolase
View Answer

Answer: a
Explanation: In the central dogma of DNA there are two main processes, they are transcription and translation. During the process of transcription, a portion of the DNA is copied into a new mRNA strand. This is done with the help of the RNA polymerase. In translation, the copied mRNA strand is used to produce proteins.

7. Which is the correct complementary strand for AGAATTCGC?
View Answer

Answer: c
Explanation: Adenine and Thymine form a double hydrogen bond. Likewise, Guanine and Cytosine form a triple hydrogen bond. So, the correct complementary strand for AGAATTCGC is TCTTAAGCG.

8. Which of the following occurs in the process of central dogma reverse?
a) Conversion of a strand of mRNA to DNA
b) Changes in the chromosomal karyotype
c) Gene replication process
d) Protein production process
View Answer

Answer: a
Explanation: Central dogma is the process of conversion of DNA->mRNA->protein. On the contrary, central dogma reverse is the conversion of RNA->DNA. The formed DNA in the process of central dogma reverse or reverse transcription, can undergo the processes of transcription and translation after that.

9. What is name of this nitrogenous base?
Thymine is nitrogenous base with presence of methyl group attached to 5th carbon atom
a) Uracil
b) Thymine
c) Cytosine
d) Guanine
View Answer

Answer: b
Explanation: Thymine and Uracil are similar in their structures. The key difference between them is the presence of a methyl group attached to the 5th carbon atom. Cytosine will have only one amino group in it whereas, guanine will have an attached imidazole group to it.

10. Which of the following ensures the stability of the helical structure of a DNA?
a) Presence of repetitive structures of a DNA code
b) Stacking of one base pair over the other
c) Presence of aneuploidy
d) Occurrence of chromosomal rearrangements
View Answer

Answer: b
Explanation: When one base pair is stacked over the other in a helical fashion, the DNA will be stable. A right-handed curving fashion is seen in the DNA. When repeating structures are present, the DNA will not be stable. Aneuploidy is in relevance to the abnormality in the number of chromosomes. When chromosomal rearrangements occur, deletion, duplication, translocations and inversions may occur. There will not be any stability.

11. Which of the following is not a part of the nucleotide?
a) Phosphate group
b) Nitrogenous base
c) Pentose sugar
d) Guanosine
View Answer

Answer: d
Explanation: Guanosine is the structural component of RNA. Guanosine is a purine adenosine. It has some neuroprotective properties. The other three are the backbone of the nucleotide in the DNA.

12. What will the polarity of this strand be?
The figure shows two strands of DNA run antiparallel to each other for effective bonding
a) Parallel, 3’->5’
b) Antiparallel, 3’->5’
c) Parallel, 5’->3’
d) Antiparallel, 5’->3’
View Answer

Answer: b
Explanation: The two strands of DNA run antiparallel to each other for the effective bonding. The two chains will be having an anti-parallel polarity. If the polarity of one chain is from 3’->5’, then the polarity of other chain will be 5’->3’ and vice versa.

13. What will be the next step in the process of transcription?
DNA -> RNA ->?
a) mRNA
b) tRNA
c) rRNA
d) Protein
View Answer

Answer: d
Explanation: The codons present in the mRNA will code for the respective AA or protein depending on the bases that are present on the strand. mRNA, tRNA and rRNA are the types of RNA. They won’t be formed in the translation process of the central dogma.

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