Nucleic acids: Structures and Roles / Central dogma

A nucleotide consists of a pentose sugar, nitrogenous base and a phosphorus atom.

• True

• False

Answer explanation

Note: A nucleotide consists of a phosphate group (PO₄^3-), which is negatively charged, beside a pentose sugar and a nitrogenous base.

For the pentose sugar, a purine comprise of 2 carbon-nitrogen rings while a pyrimidine comprise of 1 carbon-nitrogen ring.

• True

• False

Answer explanation

Note: Purines and pyrimidines are terms for the nitrogenous base.

Correct statement: For the nitrogenous bases, a purine comprise of 2 carbon-nitrogen rings while a pyrimidine comprise of 1 carbon-nitrogen ring.

Uracil, Thymine and Guanine are pyrimidines.

• True

• False

Answer explanation

Note: Purine = Adenine & Guanine.

Pyrimidine = Cytosine, Uracil & Thymine.

DNA & RNA differ in their chemical group on carbon no. 2 of pentose sugar.

• True

• False

Answer explanation

Note: DNA has –H on carbon 2 while RNA has –OH on carbon 2 of pentose sugar.

The strong covalent bond formed between 2 nucleotides in the same nucleic acid strand / chain is termed as a phosphoester bond.

• True

• False

Answer explanation

Note: It is called “phosphodiester bond”.

The 2 ends of a polynucleotide chain are different.

• True

• False

Answer explanation

Note: Both DNA & RNA have directionality i.e a distinct 3’ (OH) end and 5’ (PO₄^-) end. This direction is important for processes like DNA replication, transcription & translation.

1 RNA molecule consists of a double helix, made up of 2 single stranded anti-parallel RNA strands.

• True

• False

Answer explanation

Note: RNA is a single stranded molecule.

1 DNA molecule consists of a double helix, made up of 2 single DNA strands that are anti-parallel to each other.

3 hydrogen bonds are formed when Adenine base pairs with Thymine (or Uracil). But 2 hydrogen bonds are formed when Guanine base pairs with Cytosine.

• True

• False

Answer explanation

Note: A forms 2 H-bonds with T/U. C forms 3 H-bonds with G.

Adenine base pairs with Thymine (or Uracil) while Guanine base pairs with Cytosine by complementary 3D conformation.

• True

• False

Answer explanation

Note: A forms 2 H-bonds with T/U. C forms 3 H-bonds with G. This is by complementary base pairing.

Complementary base-pairing and complementary 3D conformation are two different concepts. 

For complementary base pairing, this term is ONLY used for the hydrogen-bonding between specific nitrogenous bases.

For complementary 3D conformation, e.g enzyme, it is how the shape of the active site fits the shape of the specific substrate ie shape of active site is complementary in shape to the specific substrate. E.g. proteins, the shape of the binding site is complementary in shape to the specific molecule/substance.

Chargaff’s rules only applies to DNA molecules.

• True

• False

Answer explanation

Note: Chargaff’s rules (Ratio of A to T or U = 1:1 & ratio of G to C = 1:1) applies to double-stranded nucleic acids, e.g double stranded DNA, double-stranded RNA, even DNA bound to RNA).