DNA, genes and chromosomesAQA A-Level Biology: Revision notes
Section 1
DNA in prokaryotic cells
In prokaryotes (such as bacteria) the DNA is held in a single short, circular molecule lying free in the cytoplasm. It is not associated with proteins (no histones). There is no nucleus. Many bacteria also carry small circular plasmids, although these are not part of the main chromosome.
Learn the pattern 'short, circular, no histones' for prokaryotes, and 'long, linear, histones' for the eukaryotic nucleus.
Section 2
DNA in the nucleus of eukaryotic cells
In eukaryotes the nucleus contains DNA in very long, linear molecules. Each molecule is wound round proteins called histones, which allows the very long DNA to be packed into the nucleus. A DNA molecule together with its associated proteins forms a chromosome.
Do not say that prokaryotic DNA is 'in a chromosome with histones'. In prokaryotes there are no histones.
Section 3
DNA in mitochondria and chloroplasts
The mitochondria and chloroplasts of eukaryotic cells also contain their own DNA. Like prokaryotic DNA it is short, circular and not associated with proteins.
So a eukaryotic cell contains two kinds of DNA: long, linear, histone-associated DNA in the nucleus, and short, circular DNA in these organelles.
Section 4
Genes and loci
A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or for a functional RNA, which includes ribosomal RNA (rRNA) and tRNAs. A gene occupies a fixed position, its locus, on a particular DNA molecule.
The gene for a polypeptide is transcribed and then translated. The genes for rRNA and tRNA are transcribed, but the RNA made is the final functional product.
Section 5
Non-coding DNA, exons and introns
In eukaryotes much nuclear DNA does not code for polypeptides. Examples are non-coding multiple repeats of base sequences between genes.
Within a gene, only some sequences code for amino acids. These coding sequences are exons. They are separated by one or more introns, which are non-coding sequences within the gene.
Worked example: a gene has exons of 150, 210 and 90 bases and introns of 400 and 1000 bases. The total length is 150 + 400 + 210 + 1000 + 90 = 1850 bases. The exons total 450 bases, which is 450 ÷ 1850 × 100 = 24% of the gene.
Introns are within a gene. Non-coding repeats are between genes. Do not mix the two up.
Section 6
Comparing the DNA
- Prokaryotic cell: short, circular, no histones.
- Eukaryotic nucleus: very long, linear, with histones, forming chromosomes; much is non-coding.
- Mitochondria and chloroplasts: short, circular, no proteins, like prokaryotic DNA.
In a comparison answer, always give both sides in the same sentence (for example 'long and linear in the nucleus but short and circular in a mitochondrion').
That's the notes covered.
Carry on to the next subtopic.
Exam questions on DNA, genes and chromosomes
- A student compares the DNA in a bacterium, Escherichia coli, with the DNA in a human liver cell.Describe how the DNA in the nucleus of the liver cell differs from the DNA in the bacterium.2 marks
- Human cells contain a gene for the polypeptide β-globin, found on chromosome 11. They also contain genes that are transcribed to make ribosomal RNA (rRNA) and transfer RNA (tRNA). Each of these genes is always found at the same position on its chromosome.The rRNA genes are transcribed but their products are never translated into polypeptides. Explain why these sequences are still called genes.2 marks
- A human nuclear gene has three exons and two introns arranged alternately (exon, intron, exon, intron, exon). The exons contain 150, 210 and 90 bases, and the introns contain 400 and 1000 bases. Human cells also contain mitochondria, which have their own DNA.Calculate the percentage of the bases in this gene that are in exons, and state why only the exons affect the amino acid sequence of the polypeptide.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).