CourseCourseQ BankQuestion BankDocsDocuments

A1.2 Nucleic acids

Explore how nucleotide components, bonding, base pairing and sequence give DNA and RNA their structures, information capacity, shared code and experimental significance.

Syllabus
First assessment 2025
Topic
A1.2
Level
SL

Learning objectives

A1.2.1DNA as genetic material• DNA is the genetic material of all living organisms• DNA occurs in chromosomes and also in mitochondria and chloroplasts• Some viruses use RNA, but viruses are not considered living organismsA1.2.2Components of a nucleotide• A nucleotide contains a phosphate group, pentose sugar, and nitrogenous base• DNA nucleotides use deoxyribose; RNA nucleotides use ribose• Bases include purines A/G and pyrimidines C/T/UA1.2.3Sugar-phosphate bonding• Condensation reactions link nucleotides by sugar-phosphate bonds• The sugar-phosphate backbone is a continuous covalent chain• Bases project from the backbone and carry sequence informationA1.2.4Bases form the basis of a code• Genetic information lies in the order of nitrogenous bases• DNA uses A, T, G, C; RNA uses A, U, G, C• Triplet codons specify amino acids in protein synthesisA1.2.5RNA as a polymer• RNA is a single-stranded polynucleotide formed by condensation• RNA contains ribose and the bases A, U, G, and C• mRNA, tRNA, and rRNA have different roles in protein synthesisA1.2.6DNA as a double helix• DNA has two antiparallel polynucleotide strands in a double helix• Complementary bases pair by hydrogen bonding: A-T and G-C• Draw bases attached to sugars, not phosphatesA1.2.7Differences between DNA and RNA• DNA is usually double-stranded; RNA is usually single-stranded• DNA uses deoxyribose and thymine; RNA uses ribose and uracil• RNA molecules are usually much shorter than DNA moleculesA1.2.8Complementary base pairing• A pairs with T in DNA and U in RNA; C pairs with G• Complementary base pairing enables accurate DNA replication• It also enables transcription and translation in gene expressionA1.2.9Diversity of DNA base sequences• DNA can vary in length and in base sequence• Genome size and gene number vary widely between organisms• Base-sequence diversity gives DNA enormous information-storage capacityA1.2.10Conservation of genetic code• The 64 codons have nearly the same meanings across life• Conserved genes for transcription, translation, and ribosomes support common ancestry• Synonymous mutations can preserve amino acid sequences

DNA stores the inherited information of living organisms

In every living organism, DNA is the genetic material. Its base sequence stores information that can be copied before cell division and passed between generations.

In eukaryotic cells, most DNA is organized into chromosomes in the nucleus. Mitochondria and chloroplasts also contain smaller DNA genomes.

Some viruses use RNA as their genetic material. This does not contradict the rule for living organisms because viruses depend on host cells and are not classified as living.

A nucleotide has three covalently connected parts

Every nucleotide contains:

  • a phosphate group
  • a five-carbon pentose sugar
  • a nitrogenous base

The sugar is the central connector: the base and phosphate are both bonded to it. DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose. A base alone is not a nucleotide.

Family Bases Ring structure
Purines adenine (A), guanine (G) two rings
Pyrimidines cytosine (C), thymine (T), uracil (U) one ring

Condensation links nucleotides into a strong backbone

A condensation reaction joins the phosphate of one nucleotide to the sugar of the next, releases water and forms a covalent phosphodiester bond. Repetition produces a long polynucleotide.

Alternating sugar and phosphate form a continuous, unvarying backbone. The bases project from the sugars, so the molecule can keep the same structural support while its base sequence changes.

The diagram shows RNA forming through the condensation of nucleotide monomers.

The changing base sequence carries the message

  • DNA uses: A, T, C and G
  • RNA uses: A, U, C and G

The sugar–phosphate backbone repeats, so it cannot encode a changing message. The order of bases can vary. When an RNA sequence is read in groups of three, each codon specifies an amino acid or a control signal, connecting nucleotide order to polypeptide sequence.

A nucleotide base sequence is divided into three-base codons and connected to an amino-acid sequence.

Four bases produce 64 possible triplets: 4 × 4 × 4. Different codons can specify the same amino acid, so the code is redundant.

One RNA polymer supports three different roles

RNA is usually a relatively short, single-stranded polynucleotide made by condensation of ribonucleotides. It contains ribose and the bases A, U, C and G.

A ribosome reads an mRNA codon while a tRNA anticodon pairs with it and carries an amino acid.
RNA Distinct contribution to protein synthesis
mRNA carries the codon sequence copied from DNA to a ribosome
tRNA pairs its anticodon with an mRNA codon and delivers the corresponding amino acid
rRNA forms the structural and catalytic core of the ribosome

These molecules share the same type of backbone and bases, but differences in sequence, length and folding give them different functions.

Complementary pairs connect the two DNA strands

DNA contains two polynucleotide strands arranged antiparallel and wound into a double helix. The sugar–phosphate backbones face outward; bases project inward and pair through hydrogen bonds.

  • Adenine pairs with thymine: A–T
  • Cytosine pairs with guanine: C–G
A labeled DNA double helix shows complementary antiparallel strands, purine–pyrimidine base pairs, two hydrogen bonds between A and T, three between C and G, and phosphate–sugar backbones.

In a structural diagram, attach every base to a sugar, alternate sugar and phosphate along each backbone, and show the two strands running in opposite directions. The helical twist is not required.

DNA and RNA differ in sugar, base and strand arrangement

Feature DNA RNA
Pentose sugar deoxyribose ribose
Distinctive base thymine (T) uracil (U)
Usual strand arrangement two antiparallel strands one strand
Typical relative length very long shorter

Both are polynucleotides with a sugar–phosphate backbone and bases A, C and G. Identify the differences using matched features rather than treating them as unrelated molecules.

DNA is shown as two paired strands using deoxyribose and thymine; RNA is shown as one strand using ribose and uracil.

Complementarity lets one strand specify another

Because each base has one complementary partner, the sequence of a template strand determines the sequence that is assembled against it. This is the common molecular logic behind copying and expression.

Process Pairing used Product specified by the template
DNA replication A–T and C–G a new DNA strand
Transcription A–U and C–G an RNA strand
Translation mRNA codon–tRNA anticodon the order of amino acids

Example: a DNA template written 3′-A C G T-5′ produces RNA 5′-U G C A-3′. The strands are complementary and antiparallel, not identical.

Four bases generate an immense sequence space

4n4^n

For a sequence of n positions, each position can contain one of four bases, giving 4ⁿ possible sequences. Changing the length or the order changes the information, so long DNA molecules have enormous storage capacity.

A DNA segment built from four possible bases branches into many different triplet sequences.

Genome size is the total amount of DNA, whereas gene number counts genes. Neither value alone is a simple measure of organism complexity.

A nearly universal genetic code points to common ancestry

Across almost all living organisms, the same codons specify the same amino acids. Genes involved in transcription, translation and ribosome function are also highly conserved.

A complex decoding system shared so widely is best explained by inheritance from a common ancestor. Later evolution changed DNA sequences, but retained the core code and molecular machinery.

The code is nearly, not absolutely, universal. Minor exceptions do not erase the much larger conserved pattern. Redundancy also means that some base substitutions are synonymous and do not change the amino acid.

From nucleotide structure to inherited information

Build and store

  • Phosphate + pentose sugar + base form a nucleotide.
  • Condensation creates a covalent sugar–phosphate backbone.
  • The backbone repeats while base order varies, so the polymer stores information.

Copy and express

Complementary pairing lets one strand specify another. DNA replication preserves information; transcription produces RNA; codon–anticodon pairing helps translate the message into an amino-acid sequence.

Identify from DNA RNA
Sugar deoxyribose ribose
Distinctive base T U
Usual structure double-stranded single-stranded

The same structure-to-expression system operates across life, and the nearly universal code is evidence that living organisms inherited it from a common ancestor.

DNA as genetic material

1 mark

The Human Genome Project completed the sequencing of the human genome by the year 2003. Which could have been a source of the entire genome in humans?

Components of a nucleotide

3 marks

Draw a labelled diagram to show the structure of a DNA nucleotide.

Sugar-phosphate bonding

2 marks

Outline the bonding between DNA nucleotides.

RNA as a polymer

4 marks

Draw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of RNA.

DNA as a double helix

6 marks

Draw a simple labeled diagram to show the structure of a double stranded DNA molecule, comprising four nucleotides.

Differences between DNA and RNA

3 marks

Distinguish between the structures of DNA and RNA.

Complementary base pairing

1 mark

Deduce the base indicated by X on the diagram.

Diversity of DNA base sequences

4 marks

Explain the diversity of possible base sequences in nucleic acids.

Conservation of genetic code

1 mark

Some yeast genes can be replaced by human genes that then continue to produce the same human proteins in the yeast cells. Which statement helps to explain this evidence?