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4.1.1Genetics, Variation and Interdependence

Prokaryotic DNA — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 46 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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4.1.1 Prokaryotic DNA — AQA A Level Biology 7402 notes: Describe the structure and organisation of DNA in prokaryotic cells, including plasmids, and explain how it differs from eukaryotic DNA.Prokaryotes, such as bacteria, have no nucleus.Their genetic material lies free in the c…

  • not wound around histone proteins
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Supercoiling
  • Conjugation

Section 1 of 9Prokaryotic cells and their genetic material

Prokaryotes, such as bacteria, have no nucleus.

Nucleoid
Their genetic material lies free in the cytoplasm in a region called the nucleoid.
Circular
The main genetic material is a single molecule of DNA that is circular (a closed loop with no free ends),
  • and is often called the bacterial chromosome.
It contains all the genes the cell needs for basic survival, such as those for enzymes involved in respiration and for making proteins.

The DNA is a double helix of two polynucleotide strands held together by hydrogen bonds between complementary bases (A with T, C with G). Each strand has a sugar-phosphate backbone of deoxyribose linked by phosphodiester bonds.

Section 2 of 9No histones: how is the DNA packed?

A bacterial chromosome is much longer than the cell, so it must be compacted.

No histones: how is the DNA packed?
Not wound around histone proteins
However, unlike eukaryotic DNA, prokaryotic DNA is not wound around histone proteins.
Instead, the molecule is supercoiled: enzymes twist the loop tightly on itself,
  • and a small number of DNA-binding proteins help to fold it into loops attached to a central core.
The result is compact enough to fit in the cell, but the DNA remains accessible for the enzymes that carry out transcription and replication.

Because the chromosome is circular and has no ends, there are no telomeres. Replication starts at a single origin and proceeds in both directions until the two forks meet.

Section 3 of 9Plasmids

Many bacteria also contain plasmids: small, circular molecules of double-stranded DNA, separate from the chromosome, which are also not associated with histones.

Plasmids
Plasmids typically carry a few genes.
These are not usually essential for survival under normal conditions but can give the bacterium an advantage in particular conditions, for example genes that confer resistance to antibiotics, genes for toxin production,
  • or for breaking down unusual substrates.
Plasmids
Plasmids replicate independently of the chromosome, and a cell may contain many copies.
Plasmids can be passed between bacteria, even of different species, by conjugation, which is a form of horizontal gene transfer.
  • This is one way that antibiotic resistance spreads rapidly through bacterial populations.
Genetic engineers use plasmids as vectors because they are small, easy to isolate and can carry a foreign gene into a bacterial host.

Section 4 of 9Comparing prokaryotic and eukaryotic DNA

FeatureProkaryotic DNAEukaryotic nuclear DNA
ShapeCircularLinear
Associated with histonesNoYes, forming chromatin and chromosomes
LocationNucleoid in the cytoplasmInside the nucleus
AmountShort, one main chromosomeLong, many chromosomes
Non-coding DNAVery littleA large proportion, including introns
Extra DNAPlasmidsMitochondrial and chloroplast DNA

Section 5 of 9Why this matters

The similarity between prokaryotic DNA and the DNA of mitochondria and chloroplasts is evidence for the endosymbiotic theory, discussed in a later lesson.

Why this matters
Knowing the differences is also essential for understanding why prokaryotic genes can be transcribed and translated at the same time,
  • because there is no nuclear envelope separating transcription from ribosomes,
    • and why bacteria can replicate their DNA and divide rapidly by binary fission.
When a question asks you to 'describe the DNA of a prokaryote', give at least three features: circular, short, no histones, with plasmids present.

The topic also links to bacterial growth and antibiotic resistance, selection, and recombinant DNA technology.

Section 6 of 9Prokaryotic DNA in genetic technology and medicine

The simple organisation of prokaryotic DNA makes bacteria very useful in biotechnology.

Prokaryotic DNA in genetic technology and medicine
Because their genes are not interrupted by introns and their DNA is not bound to histones, bacterial genes are easier to express,
  • and a human gene inserted into a bacterium on a plasmid can be transcribed and translated using the host machinery.
Insulin and growth hormone are produced in this way.
Plasmids are cut with restriction enzymes at a specific recognition sequence, and the foreign gene is joined in using DNA ligase.

The same features explain a medical problem. Genes carried on plasmids, including those for resistance to several antibiotics, spread easily between cells. Hospitals therefore try to limit the use of antibiotics so that there is less selection for plasmid-borne resistance, and patients with resistant infections are isolated.

Finally, remember that in prokaryotes both transcription and translation occur in the cytoplasm. Ribosomes can attach to an mRNA molecule while it is still being made, so protein synthesis can start very quickly, helping bacteria to respond to changing conditions.

Section 7 of 9Exam-style questions

  1. Describe two ways in which the DNA in a bacterial cell differs from the DNA in the nucleus of a human cell. [2]2 marks

  2. Plasmids may carry genes for antibiotic resistance. Explain how this could lead to resistance spreading through a bacterial population. [4]4 marks

  3. A bacterial chromosome contains 4.6 million base pairs and is about 1.5 mm long. The cell is 2 micrometres long. Calculate how many times longer the chromosome is than the cell, and explain how it fits inside. [3]3 marks

Section 8 of 9Key terms

TermMeaning
ProkaryoteAn organism whose cells have no nucleus or membrane-bound organelles.
NucleoidThe region in a prokaryotic cell where the DNA is located.
PlasmidA small circular molecule of DNA separate from the chromosome, found in many bacteria.
HistoneA protein around which eukaryotic DNA is wound.
SupercoilingTwisting of a DNA molecule on itself to compact it.
ConjugationTransfer of DNA, such as a plasmid, between two bacterial cells.
VectorA carrier, such as a plasmid, used to transfer genes into a cell.

Section 9 of 9Exam technique

1
Saying prokaryotes have no DNA because they lack a nucleus. They have a chromosome in the nucleoid.
2
Stating that prokaryotic DNA is 'free of all proteins'. It is not associated with histones, but other DNA-binding proteins help to organise it.
3
Confusing the chromosome and plasmids; plasmids are additional small loops and are not always present.
4
Writing that plasmids are only found in bacteria. They are rare in eukaryotes but do occur in yeast.
5
Saying that a circular chromosome has 'two ends' that are joined by hydrogen bonds. It is a continuous loop with covalent bonds in the backbone.

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