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DNA in Eukaryotic and Prokaryotic Cells

The form of the DNA, and its location within the cell, is different for eukaryotic cells and prokaryotic cells.

DNA in eukaryotic cells

In a eukaryotic cell, the DNA is in the form of linear chromosomes, located in the nucleus.

Diagram of a plant cell (plant cells are eukaryotic) showing the chromosomes in the nucleus. There are 8 chromosomes in the nucleus. These are made up of 4 identical pairs. Within each pair, the two chromosomes appear identical to each other, but they are located at different positions in the nucleus and are oriented in different directions. The nucleus is labelled "Nucleus" and the chromosomes are labelled "Chromosomes".

The DNA in eukaryotic cells is in the form of linear chromosomes, located in the nucleus.

In many eukaryotic cells, including human body cells, the chromosomes come in pairs. The two chromosomes in each pair have the same genes, but they may have different versions of those genes. This will be explained in more detail later in the course.

Diagram of the nucleus of a eukaryotic cell. It contains four pairs of chromosomes. Each chromosome has a coloured box around it. The two chromosomes of each pair have boxes of the same colour and their two boxes are connected by a line. The title at the top of the diagram is "Pairs of chromosomes".

In many eukaryotic cells, the chromosomes come in pairs.

The number of chromosomes in the nucleus varies greatly between different species. Some examples are given in the table below:

SpeciesNumber of chromosomes
Fruit fly4 pairs (8 chromosomes)
Spinach6 pairs (12 chromosomes)
Kangaroo8 pairs (16 chromosomes)
Budding yeast16 pairs (32 chromosomes)
Human23 pairs (46 chromosomes)
Pineapple25 pairs (50 chromsomes)
Platypus26 pairs (52 chromosomes)
Pigeon40 pairs (80 chromosomes)
Grape fern45 pairs (90 chromosomes)
Adders-tongue fern630 pairs (1260 chromosomes)

The diagram below shows all of the chromosomes from the nucleus of a body cell from a male platypus. A platypus has 26 pairs of chromosomes in total, of which 5 pairs are sex chromosomes (humans only have one pair of sex chromosomes).

Each chromosome is an X-shape, indicating that DNA replication has already taken place.

Diagram showing all of the chromosomes in a cell of a male platypus. There are 26 pairs of chromosomes in total. The first 21 pairs are numbered 1 to 21. The remaining chromosomes are labelled X1 and Y1, X2 and Y2, X3 and Y3, X4 and Y4, and X5 and Y5.

Diagram showing all of the chromosomes in a cell of a male platypus. The X and Y chromosomes are sex chromosomes. Image: Rens et al. via Wikimedia Commons (CC BY 2.0 - https://creativecommons.org/licenses/by/2.0/)

Photograph of a wild platypus in water.

A platypus. Image: Klaus via Wikimedia Commons (CC BY-SA 2.0 - https://creativecommons.org/licenses/by-sa/2.0/deed.en)

DNA in prokaryotic cells

Prokaryotic cells do not have a nucleus - instead the DNA is found in the cytoplasm.

Each prokaryotic cell has a single circular chromosome.

In addition to the chromosome, prokaryotic cells often also contain plasmids. The number of plasmids varies between different prokaryotic cells.

Labelled diagram of a prokaryotic cell. It is a rounded rod shape with a thick cell wall around it. On the inside of the cell wall is the cell membrane. Inside that is the cytoplasm. Within the cytoplasm there are ribosomes, one circular chromosome and a few circular plasmids.

Diagram of a prokaryotic cell containing a single circular chromosome and three plasmids. Note that the plasmids are much smaller than the chromosome.

Beyond the Syllabus: Exceptions to the rules

The information above is what you need to know for the GCSE course. However, there are some exceptions to some of the statements made above. If you just want to know what you need for the exams, then you can skip this section. But if you are curious, then read on.

Exceptions to the rules:

  • There are examples of prokaryotic cells that have more than one chromosome.
  • There are examples of prokaryotic cells that have linear chromosomes.
  • Plasmids are sometimes linear.
  • Plasmids are sometimes found in eukaryotic cells - especially unicellular fungi (yeasts).

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

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What form is the DNA of eukaryotic cells in?

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What form is the DNA of prokaryotic cells in?

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