What you'll learn

• How DNA is organised differently in prokaryotic cells, eukaryotic nuclei, mitochondria and chloroplasts.
• What chromosomes, genes and loci are.
• How DNA triplets relate to amino acids and the genetic code.
• Why much eukaryotic nuclear DNA is non-coding, including introns, exons and repeated sequences.

Starting point: DNA as stored information

DNA stands for deoxyribonucleic acid. It is a biological polymer: a large molecule made from many repeating smaller units called nucleotides. Each DNA nucleotide contains a phosphate group, a deoxyribose sugar and one nitrogen-containing base.

The four DNA bases are adenine (A), thymine (T), cytosine (C) and guanine (G). The base sequence means the order of these bases along the DNA molecule.

The key idea for this topic is that DNA is not just “floating information”. It is physically organised in different ways depending on the cell type and where it is found.

DNA in different types of cells

A prokaryotic cell is a cell without a nucleus, such as a bacterial cell. A eukaryotic cell is a cell with a nucleus and membrane-bound organelles, such as an animal, plant or fungal cell.

This diagram summarises the DNA organisation you need for this section: prokaryotic DNA, eukaryotic nuclear DNA, and the DNA found in mitochondria and chloroplasts.

Prokaryotic DNA

In prokaryotic cells, DNA molecules are:

• short compared with eukaryotic nuclear DNA
• circular, meaning the DNA molecule forms a loop
• not associated with proteins in the AQA specification wording

Prokaryotic cells do not have a nucleus, so their main DNA is in the cytoplasm.

Eukaryotic nuclear DNA

In the nucleus of eukaryotic cells, DNA molecules are:

• very long
• linear, meaning each DNA molecule has two ends
• associated with proteins called histones

Histones are proteins that help package and organise eukaryotic nuclear DNA. Together, one DNA molecule and its associated proteins form a chromosome.

Do not rely only on the familiar “X-shaped” chromosome diagram. That is one condensed form often shown during cell division. For this topic, the exam definition is more important: DNA + associated proteins = chromosome.

Mitochondrial and chloroplast DNA

Mitochondria are organelles involved in aerobic respiration. Chloroplasts are organelles found in plant and algal cells where photosynthesis occurs.

Mitochondria and chloroplasts contain their own DNA. This DNA is like prokaryotic DNA because it is:

• short
• circular
• not associated with protein

From chromosomes to genes

A chromosome contains many sections of DNA. Some of these sections are genes.

An amino acid is a small molecule that can be joined to others. A polypeptide is a chain of amino acids. Polypeptides fold to form proteins, or parts of proteins.

RNA, or ribonucleic acid, is another nucleic acid involved in gene expression. A functional RNA is an RNA molecule that has a job itself rather than being translated into a polypeptide. Ribosomal RNA (rRNA) forms part of ribosomes, and transfer RNA (tRNA) carries amino acids during protein synthesis.

Locus: the gene’s position

A locus is the fixed position of a gene on a particular DNA molecule. Think of it as the gene’s address on a chromosome or other DNA molecule.

The diagram shows how a gene sits at a fixed locus, how exons and introns are arranged within a eukaryotic gene, and how bases are grouped into triplets.

Triplets and the genetic code

A triplet is a sequence of three DNA bases. Each triplet codes for a specific amino acid.

Universal

The genetic code is described as universal because the same triplet codes for the same amino acid in almost all organisms. At A-Level, treat this as a major reason why genes can be compared between different species.

Non-overlapping

The code is non-overlapping because bases are read in separate groups of three. Each base is part of only one triplet.

For example, a sequence of 12 bases would be read as four triplets, not as a sliding set of overlapping groups.

Degenerate

The genetic code is degenerate because most amino acids are coded for by more than one triplet. This does not mean the code is vague.

Coding and non-coding DNA in eukaryotes

In eukaryotes, much of the nuclear DNA does not code for polypeptides. This is a big difference from the simple idea that “DNA is just genes”.

There are non-coding regions between genes, including multiple repeats of base sequences. These are repeated DNA sequences that do not code for amino acid sequences.

There are also non-coding regions within genes. In eukaryotic protein-coding genes:

• exons are sequences that code for amino acid sequences
• introns are non-coding sequences within the gene
• exons are separated by one or more introns

Pulling the hierarchy together

The order of ideas is:

• DNA is the molecule carrying genetic information.
• A chromosome is a DNA molecule with associated proteins.
• A gene is a base sequence of DNA that codes for a polypeptide or functional RNA.
• A locus is the fixed position of a gene on a DNA molecule.
• A triplet is three DNA bases coding for one amino acid.