DNA in Prokaryotes and Eukaryotes
In 1928, Frederick Griffith injected mice with a mixture of heat-killed smooth Streptococcus pneumoniae and live rough S. pneumoniae, neither lethal alone. The mice died, and live smooth bacteria were recovered from them. Griffith called this 'transformation': something in the heat-killed cells had changed the rough bacteria. In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty identified DNA as the transforming principle. Transformation is possible specifically because prokaryotes can take up free DNA fragments from their surroundings and recombine them into their own chromosome, something the eukaryotic nuclear envelope prevents.
Orient yourself
Predict how DNA storage might differ between cell types, then scan the goals and key terms.
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
A student says, "DNA is just DNA, so it should be stored the same way in every organism. A bacterium should have chromosomes in a nucleus just like a human cell."
Before reading on, explain what you think is wrong with that statement. How might DNA organisation differ between a bacterium and a human cell, and why could that matter biologically?
Know
- How DNA is organised in prokaryotes and eukaryotes.
- The difference between circular DNA, plasmids and linear chromosomes.
Understand
- Why nucleus and nucleoid are not the same thing.
- How chromatin packaging helps fit DNA into eukaryotic cells.
Can Do
- Compare bacterial plasmids with human chromosomes in a precise HSC-style response.
- Link DNA organisation to cell function and biotechnology use.
Same molecule, different arrangement
Establish that both cell types use DNA and that the real comparison is how that DNA is organised.
Core Content
Syllabus focus · same molecule, different arrangement
In 1928, Frederick Griffith mixed heat-killed virulent (smooth) Streptococcus pneumoniae with live avirulent (rough) bacteria and injected the mixture into mice. The mice died. From those mice, Griffith recovered live smooth bacteria, bacteria that had never been exposed to a living smooth strain. Something physical had transferred from the dead cells to the live ones and changed their hereditary properties. That 'something' was DNA. The transfer was possible because many bacteria are naturally competent: they can take up free DNA fragments released from broken cells and recombine those fragments into their own chromosome, a process the eukaryotic nuclear envelope prevents. (Plasmids are a separate route by which bacteria move genes between cells, covered in the next card.)
In this lesson, the key idea is not that prokaryotes and eukaryotes use different kinds of hereditary material. Both use DNA. The important comparison is how that DNA is stored, packaged and organised inside the cell.
Prokaryotic cells usually contain one main circular DNA molecule located in the nucleoid region. Eukaryotic cells usually contain multiple linear DNA molecules packaged into chromosomes inside a membrane-bound nucleus.
Both cell types use DNA, only the arrangement differs. Prokaryotes: one main circular DNA molecule in the nucleoid (no membrane). Eukaryotes: multiple linear chromosomes in a membrane-bound nucleus. Use precise terms: circular DNA, nucleoid, linear chromosomes, nucleus.
Pause, copy the highlighted distinction into your book before moving on.
Both prokaryotes and eukaryotes use _____ as their hereditary material (type the molecule's abbreviation).
Circular DNA, the nucleoid and plasmids
See how prokaryotes organise a circular main chromosome plus separate plasmids.
Prokaryotic DNA · nucleoid · plasmids
We just saw that prokaryotes have circular DNA in a nucleoid while eukaryotes have linear chromosomes in a nucleus. That raises a question: what else distinguishes prokaryotic DNA organisation, are there additional DNA elements besides the main chromosome? This card answers it → plasmids: small extra circular DNA molecules in many bacteria.
Most prokaryotes, such as bacteria, do not have a nucleus. Their main chromosome is typically a circular DNA molecule located in the nucleoid region of the cell. The nucleoid is not surrounded by a nuclear membrane.
Many prokaryotes also contain plasmids, which are small circular DNA molecules separate from the main chromosome. Plasmids often carry extra genes, for example genes associated with antibiotic resistance. Because plasmids are separate from the main chromosome, they are important in both natural bacterial gene transfer and biotechnology applications.
Plasmids = small circular DNA molecules separate from the main bacterial chromosome. They often carry extra genes (e.g. antibiotic resistance). Plasmids ≠ the main chromosome. Their separateness makes them useful vectors in biotechnology.
Add the highlighted plasmid definition to your notes before the check below.
Small circular DNA molecules separate from the main bacterial chromosome are called:
Linear chromosomes, histones and chromatin
See how eukaryotes wrap DNA around histones into nucleosomes and chromatin inside the nucleus.
Eukaryotic DNA · packaging into the nucleus
We just saw that bacteria have a circular main chromosome in the nucleoid plus possible plasmids. That raises a question: how does eukaryotic DNA organisation differ, where is it stored and how is it packaged? This card answers it → linear chromosomes in a membrane-bound nucleus, packaged as chromatin.
Eukaryotic cells, such as plant and animal cells, contain DNA inside a membrane-bound nucleus. Their DNA is arranged as multiple linear chromosomes rather than one main circular chromosome.
Eukaryotic DNA is wrapped around packaging proteins called histones. A length of DNA wound around a core of histone proteins forms a repeating unit called a nucleosome, often pictured as "beads on a string". Many nucleosomes coil and fold together to make chromatin, the less-condensed form of the DNA-protein complex. During cell division, this chromatin condenses further into the visible chromosomes. Prokaryotes do not use histones or nucleosomes; their DNA is organised instead by nucleoid-associated proteins. Histone-based packaging is what allows the very large amount of DNA in a eukaryotic cell to fit inside the nucleus while still remaining accessible when genes need to be used.
Genes occupy specific positions on chromosomes, so chromosome organisation matters for inheritance and later for gene expression.
Eukaryotes: multiple linear chromosomes inside a membrane-bound nucleus. DNA wraps around histone proteins to form nucleosomes, which fold into chromatin (less condensed). Chromatin condenses into visible chromosomes during cell division. Prokaryotes have no histones/nucleosomes. Packaging lets large amounts of DNA fit in the nucleus while keeping genes accessible.
Pause, write the highlighted eukaryotic organisation summary into your book.
Eukaryotic DNA is arranged as a single circular chromosome floating in the cytoplasm.
Prokaryotic DNA is typically a single circular chromosome located in the nucleoid region.
Eukaryotic and prokaryotic DNA both associate with histone proteins to form nucleosomes.
Why the organisation matters
Compare the two systems, including organelle DNA, and read the summary diagram.
Comparison · description, gene transfer, biotechnology
We just saw that eukaryotes package linear chromosomes as chromatin inside a nucleus. That raises a question: why does this structural difference matter beyond just description? This card answers it → how the differences in organisation affect gene transfer, biotechnology, and what you can and cannot do with each type of DNA.
Differences in DNA organisation affect how we describe cells, how genes are transferred, and how biotechnology uses DNA. A bacterial plasmid can be isolated and used as a vector. A human chromosome cannot be treated as if it were a small circular DNA ring.
Eukaryotes also carry DNA outside the nucleus. Mitochondria, and in plants and algae the chloroplasts, each contain their own small circular DNA molecules, a reflection of the evolutionary origin of these organelles from once free-living prokaryotes. So a typical plant cell contains DNA in three places: the nucleus, the mitochondria and the chloroplasts.
Prokaryotes
- Main DNA usually circular
- Located in nucleoid, not nucleus
- Often have plasmids
- Useful in gene-transfer biotechnology
Eukaryotes
- DNA arranged as multiple linear chromosomes
- Located inside nucleus
- Packaged with histone proteins as chromatin
- Extra circular DNA in mitochondria and chloroplasts
- Organisation supports complex gene regulation
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Main DNA form | Usually one circular chromosome | Multiple linear chromosomes |
| Location | Nucleoid region | Nucleus |
| Additional DNA | Often plasmids present | Small circular DNA in mitochondria (and chloroplasts in plants) |
| Packaging | Less extensive than eukaryotic chromatin packaging | DNA associated with proteins as chromatin |
Prokaryotes: circular DNA, nucleoid, plasmids, useful in gene-transfer biotechnology (plasmids as vectors). Eukaryotes: linear chromosomes in a nucleus, DNA wrapped on histones as chromatin, plus small circular DNA in mitochondria and chloroplasts. A plasmid can be a vector; a full human chromosome cannot be used the same way.
Add the highlighted comparison summary to your notes before the check below.
Where is the main DNA of a prokaryotic cell located?
Both cell types use DNA, but the form, location and packaging differ.
Consolidate and apply
Build your own labelled comparison, link structure to use, then clear the misconception.
Activities
Compare and Represent
First recall the key facts, then construct your own labelled representation to model the comparison (this is the "construct a representation to compare" skill the syllabus asks for).
1. The main DNA of a bacterium is usually ____ and located in the ____.
2. Additional small circular DNA molecules in some bacteria are called ____.
3. In a eukaryotic cell, DNA is found inside the ____ and arranged as ____ chromosomes.
4. Construct a representation. On paper, draw and label a two-column diagram (prokaryote vs eukaryote) that models: the main DNA form (circular vs linear), where it sits (nucleoid vs nucleus), any additional DNA (plasmids vs mitochondrial/chloroplast DNA), and the packaging proteins (nucleoid-associated proteins vs histones/nucleosomes). Then type a short caption stating the single most important structural difference and one consequence of it.
Link Structure to Use
Explain why plasmids are useful in biotechnology, but why a full human chromosome would not usually be used in the same way as a simple bacterial plasmid.
Core idea
- Prokaryotes and eukaryotes both use DNA, but they organise it differently inside cells.
Mechanism / structure
- Prokaryotes usually have circular DNA in a nucleoid and may contain plasmids. Eukaryotes have linear chromosomes in a nucleus, packaged as chromatin.
Common mistake
- Do not say that bacteria have a nucleus or that plasmids are the same as the main chromosome.
Exam sentence starter
- "DNA in prokaryotes differs from DNA in eukaryotes because it is usually..."
Practise independently
Attempt every response in your own words first. The model answers are for checking, not for copying.
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Outline two differences between DNA organisation in prokaryotic and eukaryotic cells.
AnalyseBand 4(4 marks) 2. Explain why chromatin is an important concept when describing eukaryotic DNA.
EvaluateBand 5–6(5 marks) 3. Evaluate the statement: "Plasmids are a useful example of how DNA organisation in prokaryotes can support biotechnology applications."
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1, Compare and Represent
1. circular DNA molecule; nucleoid.
2. plasmids.
3. nucleus; linear.
Activity 2, Link Structure to Use
Plasmids are useful because they are small, separate circular DNA molecules that can carry inserted genes and replicate inside bacterial cells. A full human chromosome is far larger, organised differently, and not used in the same simple vector role as a bacterial plasmid.
Short Answer Model Responses
Q1 (3 marks): In prokaryotes, the main DNA is usually circular and located in the nucleoid rather than a nucleus [1]. In eukaryotes, DNA is arranged as linear chromosomes [1]. Eukaryotic DNA is enclosed inside a membrane-bound nucleus, unlike prokaryotic DNA [1].
Q2 (4 marks): Chromatin refers to eukaryotic DNA associated with proteins [1]. This packaging helps fit large amounts of DNA into the nucleus [1]. It also allows DNA to exist in a less condensed form when genes need to be accessed [1]. During cell division, chromatin condenses into visible chromosomes [1].
Q3 (5 marks): The statement is valid because plasmids show that prokaryotic DNA organisation includes small DNA molecules separate from the main chromosome [1]. Plasmids are circular and can replicate inside bacterial cells [1]. They are useful in biotechnology because inserted genes can be carried on plasmids into bacteria [1]. This makes plasmids practical vectors for gene transfer and cloning applications [1]. Therefore plasmids are a strong example of how prokaryotic DNA organisation supports biotechnology use [1].
Retrieve and reflect
Use the review session first, then compare how your thinking has changed since the opening prediction.
Prokaryotes
Usually one main circular DNA molecule in a nucleoid, plus possible plasmids.
Eukaryotes
Multiple linear chromosomes inside a nucleus.
Chromatin
DNA associated with proteins in eukaryotic cells.
Exam trap
A nucleoid is not a nucleus, and a plasmid is not the main chromosome.
Rapid-fire questions on circular DNA, plasmids, the nucleoid, linear chromosomes and chromatin. Beat the boss to bank a tier, gold (perfect + fast), silver (80%+), or bronze (cleared).
Griffith's 1928 transformation experiment, confirmed as DNA-mediated by Avery, MacLeod and McCarty in 1944, is explicable only through prokaryotic DNA organisation. Rough Streptococcus bacteria absorbed fragments of DNA from the dead smooth bacteria and incorporated them into their own chromosome, acquiring the smooth capsule gene. This horizontal gene transfer is possible because prokaryotic DNA is a circular chromosome in the nucleoid region (no nuclear envelope barrier) plus small circular plasmids that are freely transferable. Eukaryotic DNA, by contrast, is packaged into linear chromosomes within a membrane-bound nucleus, making the same kind of cell-to-cell gene transfer structurally much more difficult. Understanding these organisational differences explains both the biology of bacterial gene exchange and the principles that modern biotechnology exploits when using plasmids as vectors.