nucleic acids as information molecules that encode instructions for the synthesis of proteins: the structure of DNA, including nucleotide composition and the role of complementary base pairing, the three main forms of RNA (mRNA, tRNA, rRNA)

What this dot point is asking

VCAA wants you to describe nucleic acids as information molecules, identify the building block (the nucleotide), explain the double helix and complementary base pairing in DNA, and distinguish the three main forms of RNA by structure and role.

The answer

A nucleic acid is a polymer of nucleotides that stores or carries genetic information. The two nucleic acids in cells are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

The nucleotide

Every nucleotide has three components:

1. A pentose sugar (deoxyribose in DNA, ribose in RNA).
2. A phosphate group attached to the 5' carbon of the sugar.
3. A nitrogenous base attached to the 1' carbon.

Nucleotides join through phosphodiester bonds between the 3' hydroxyl of one sugar and the 5' phosphate of the next, building a sugar-phosphate backbone with a directional 5' to 3' polarity.

DNA structure

DNA is a double-stranded helix. The two strands run antiparallel (one 5' to 3', the other 3' to 5') and twist into a right-handed double helix.

Bases

The four DNA bases are the purines adenine (A) and guanine (G), and the pyrimidines thymine (T) and cytosine (C).

Complementary base pairing

A pairs with T through two hydrogen bonds; G pairs with C through three hydrogen bonds. Because pairing is specific, each strand carries the complementary sequence of the other, which allows accurate replication.

Stability and access

The sugar-phosphate backbone is on the outside; bases face inward. Hydrogen bonds between bases are individually weak but collectively stable, while still allowing the strands to separate when enzymes such as helicase unzip the helix.

RNA structure

RNA is usually single-stranded. Its nucleotides contain ribose (with a 2' hydroxyl), and uracil (U) replaces thymine. A pairs with U; G pairs with C. RNA can fold back on itself to form short double-stranded regions held by intramolecular base pairing.

VCAA names three main forms of RNA.

Messenger RNA (mRNA)

A linear single strand that carries the genetic message from DNA to the ribosome. It is read in codons (triplets of bases). In eukaryotes, mature mRNA has a 5' cap, a 3' poly-A tail, and introns removed by splicing.

Transfer RNA (tRNA)

A short folded RNA (about 76 to 90 nucleotides) shaped like an inverted L (cloverleaf in two dimensions). It carries a specific amino acid on its 3' end and presents a three-base anticodon that pairs with a codon on mRNA. There is at least one tRNA for each of the 20 amino acids.

Ribosomal RNA (rRNA)

Combines with proteins to form the two subunits of the ribosome. rRNA does the catalytic work of forming peptide bonds, so the ribosome is described as a ribozyme.

DNA vs RNA at a glance

Examples in context

Example 1. Structural biology at the Australian Synchrotron. The Australian Synchrotron in Clayton (run by ANSTO) hosts macromolecular crystallography beamlines used to resolve DNA-protein complexes at atomic scale. Researchers from Monash University used these beamlines to image the structure of the SARS-CoV-2 spike protein bound to its target receptor, and to determine the structures of DNA-binding transcription factors at sub-2-angstrom resolution. The double-helix dimensions (10 base pairs per turn, 3.4 nm pitch, 2 nm diameter) were originally established by Franklin and Watson-Crick in the 1950s, but synchrotron data refines these to within 0.1 angstrom. VCE students learn the basic structure - antiparallel sugar-phosphate backbones and complementary base pairing - that all subsequent biology builds on.

Example 2. mRNA vaccines and the role of mRNA at Doherty Institute. Doherty Institute virologists explain Pfizer-BioNTech and Moderna vaccines as a clinical application of the three forms of RNA. The vaccine is mRNA encoding the SARS-CoV-2 spike protein, with a 5' cap and poly-A tail. After injection, host ribosomes (built from rRNA) translate the mRNA using tRNAs that bring amino acids matching each codon. The translated spike protein triggers immunity. mRNA is short-lived (degraded within hours), so the vaccine cannot integrate into host DNA. Understanding mRNA, tRNA and rRNA functions allows VCE students to follow modern Australian vaccine science.

Try this

Q1. State the three components of a nucleotide and identify which two differ between DNA and RNA. [3 marks]

• Cue. Phosphate, pentose sugar, nitrogenous base. Sugar (deoxyribose vs ribose) and one base (thymine vs uracil).

Q2. A single strand of DNA reads 5'-ATGCGGTAC-3'. (a) Write the complementary DNA strand with correct polarity. (b) Write the mRNA sequence transcribed from the original strand acting as template. [2+2 marks]

• Cue. (a) 3'-TACGCCATG-5'. (b) 5'-GUACCGCAU-3' (read the template 3' to 5', synthesise mRNA 5' to 3').

Q3. Refer to the three forms of RNA. (a) State the function of mRNA, tRNA and rRNA. (b) Explain why an mRNA vaccine cannot alter host DNA. (c) Predict the effect of a ribonuclease drug on translation. [2+2+2 marks]

• Cue. (a) mRNA carries genetic message; tRNA delivers amino acids; rRNA forms ribosomes and catalyses peptide bond formation. (b) mRNA cannot be reverse-transcribed without a viral reverse transcriptase. (c) RNases degrade mRNA, halting protein synthesis.