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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)
A focused answer to the VCE Biology Unit 3 dot point on nucleic acids. Covers nucleotide composition, the antiparallel double helix, complementary base pairing, and how mRNA, tRNA and rRNA differ in structure and role.
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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:
- A pentose sugar (deoxyribose in DNA, ribose in RNA).
- A phosphate group attached to the 5' carbon of the sugar.
- 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
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Double | Single (usually) |
| Location (eukaryote) | Nucleus, mitochondria, chloroplasts | Made in nucleus, used in cytosol or on rough ER |
| Stability | More stable (no 2' OH, no uracil) | Less stable, short-lived |
| Role | Long-term information storage | Information transfer and protein synthesis |
Worked example
A short DNA template strand reads 3' TACGGCATA 5'.
Complementary DNA strand. 5' ATGCCGTAT 3' (A pairs with T, G with C).
mRNA transcribed from this template. 5' AUGCCGUAU 3' (U replaces T; the message is read 5' to 3' by the ribosome).
Reading frame. The first codon AUG codes for methionine, the start codon. CCG codes for proline; UAU codes for tyrosine. The matching tRNA anticodons would be 3' UAC, GGC, AUA 5'.
Common traps
Saying DNA is "single-stranded." DNA is double-stranded in cells. Single-stranded DNA is a transient state during replication or transcription.
Forgetting antiparallel. The two strands run in opposite 5' to 3' directions. Without this detail, replication and transcription cannot be explained.
Mixing up base pairing numbers. A and T have two hydrogen bonds; G and C have three. GC-rich regions are therefore more thermally stable.
Calling rRNA "just structural." rRNA is catalytic; it forms the peptide bond at the peptidyl transferase site.
In one sentence
DNA is an antiparallel double helix of deoxyribose nucleotides held together by specific A-T and G-C base pairing, while the three forms of RNA (mRNA, tRNA and rRNA) are single-stranded ribose polymers that together carry and translate the genetic message into proteins.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2022 VCE3 marksDescribe the structure of a DNA molecule and explain how complementary base pairing supports its function as the genetic material.Show worked answer →
A 3-mark answer needs the nucleotide unit, the antiparallel double helix, and a function link.
Structure. DNA is a double-stranded polymer of nucleotides. Each nucleotide has a deoxyribose sugar, a phosphate group and one of four nitrogenous bases (adenine, thymine, guanine, cytosine). The two strands are antiparallel (one runs 5' to 3', the other 3' to 5') and wind into a double helix, held together by hydrogen bonds between complementary bases.
Complementary base pairing. Adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds). This specificity means one strand acts as a template for the other.
Function link. Because each strand carries the same information in complementary form, DNA can be accurately replicated during the S phase, and one strand can act as a template for mRNA during transcription. Markers reward the explicit link between specificity of base pairing and faithful copying of information.
2023 VCE2 marksState two structural differences between a DNA nucleotide and an RNA nucleotide.Show worked answer →
A 2-mark answer needs two clear structural differences.
- Sugar. DNA nucleotides contain deoxyribose, which lacks a hydroxyl group on the 2' carbon. RNA nucleotides contain ribose, which has a 2' hydroxyl group.
- Base set. DNA nucleotides include thymine. RNA nucleotides have uracil in place of thymine. Adenine, guanine and cytosine appear in both.
A third acceptable point is that DNA is typically double-stranded in cells, while the three main forms of RNA are single-stranded.
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