IGCSE Biology Paper-1: Specimen Questions with Answers 201 - 201 of 208

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Question 201



Given diagram is showing a nucleotide.

Organic Base

Which of the following option labelled is Incorrect?


Choice (4)


Q-sugar, R-sugar, S-organic base


Q-phosphate, R-organic base, S-sugar


Q-phosphate, R-sugar, S-organic base


Q-sugar, R-organic base, S-sugar




Nucleic acids bear different components that are briefly discussed below.

Phosphoric Acid

The acidic nature of nucleic acids is due to the presence of phosphoric acid. Sugar of nucleoside combines with phosphoric acid by a phosphodiester bond formed at 5th or 3rd carbon of the sugar.


It is a five carbon (pentose) sugar. There are two types of pentose sugars-ribose and deoxyribose. Deoxyribose sugar has one oxygen atom less at second carbon. Ribose sugar is present in RNA while deoxyribose sugar occurs in DNA.

Nitrogenous Bases

They have two categories:

  • Pyrimidine: It includes cytosine, thymine, and uracil. Pyrimidine bases are made of only one ring of carbon.
  • purine: It includes Adenine and guanine. Purine bases are made of two rings of carbon and nitrogen bases of DNA contains adenine, guanine, cytosine, and thymine. In RNA, uracil is present in place of thymine.
Organic Base

Developed by: