Chapter 9, Problem 1RE

REFLECT AND APPLY Consider the following in light of the concept of levels of structure (primary, secondary, tertiary, quaternary) as defined for proteins.

(a) What level is shown by double-stranded DNA?

(b) What level is shown by tRNA?

(c) What level is shown by mRNA?

Interpretation Introduction

Interpretation:

Corresponding to the different levels of protein structure, the structural level existing in the given three nucleic acid molecules is to be determined.

Concept introduction:

Proteins are polymers of amino acids that are composed of one or more polypeptide chains. Several amino acids are linked to each other via a peptide bond to form a polypeptide chain. The structure of a protein is thus dependent on the degree of complexity of the polypeptide chains.

Based on the complexity of the polypeptide chains, protein structures can be classified as primary, secondary, tertiary, and quaternary. The primary structure is the simplest structure and describes the linear arrangement of amino acids to form a polypeptide chain. Folding of the polypeptide chains due to interactions between atoms of the backbone form the secondary structure.

Further folding due to the interactions between the R groups of amino acids to form a 3D structure of a protein is referred to as the tertiary structure. Presence of two or more polypeptide chains (subunits) in a 3D conformation within a single protein comprises the quaternary structure.

Answer to Problem 1RE

Solution:

a) Secondary structure

b) Tertiary structure

c) Primary structure

Explanation of Solution

a) The level of double-stranded DNA.

Double-stranded DNA is composed of two strands of opposite polarity that are twisted around each other. The backbone of these strands is made up of alternating deoxyribose and phosphate groups and each deoxyribose is linked with one of the four bases: adenine (A), cytosine (C), guanine (G), or thymine (T). The bonds formed between the bases (A-T and G=C) is responsible for the two strands of the double-stranded DNA being held together.

In the secondary structure of proteins, the polypeptide chains fold and are held together by the interactions between the atoms of the backbone. Similarly, in double-stranded DNA, the two strands are held together by the interaction between bases present on the two strands.

Therefore, double-stranded DNA is considered to have a secondary structure.

b) The level of tRNA.

Transfer RNA, as the name suggests, acts as an adaptor molecule that transfer the amino acid residues to the growing polypeptide chain during the process of translation. tRNA can have a primary structure, clover-shaped secondary structure, or a 3D L-shaped tertiary structure. The L-shaped structure of tRNA is bonded and stabilized by the base pairing between nucleotides in the D loop and the TΨC loop.

During the process of translation, the tertiary structure of the tRNA plays a major role as it fits into the P and A sites of the ribosome and allows the addition of amino acids to the growing polypeptide chain.

Therefore, tRNA is considered to have a tertiary structure.

c) The level of mRNA.

Messenger RNA is the chief RNA molecule that acts as a messenger to transfer the genetic information stored in the DNA to the ribosomes to generate the corresponding protein sequences. mRNA is generated from the nucleotide sequences which are complementary to the template strand of DNA during the process of transcription.

mRNA is single-stranded and also lacks any major folding or secondary structures.

Therefore, mRNA is considered to have a primary structure.