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Two Cell Types, Split by Whether the DNA Sits in a Nucleus

Definition

Eukaryotic cell

A cell that has its DNA enclosed in a nucleus, along with membrane-bound organelles such as mitochondria; animal and plant cells are eukaryotic.

  1. Every living thing is built from cells, and each cell is either eukaryotic or prokaryotic.
  2. A eukaryotic cell keeps its DNA inside a nucleus.
  3. Animals, plants, fungi and protists are all eukaryotes.
  4. A prokaryotic cell has no nucleus, so its DNA lies free in the cytoplasm.
  5. Bacteria are prokaryotes.
Example
  • You, an oak tree and a yeast cell are all made of eukaryotic cells.
  • A bacterium, such as E. coli, is a single prokaryotic cell.

The Structure of a Prokaryotic Cell

Definition

Prokaryotic cell

A cell with no nucleus, whose DNA floats freely in the cytoplasm as a single loop; bacteria are prokaryotic and are much smaller than eukaryotic cells.

  1. A prokaryotic cell, such as a bacterium, has a simpler structure than a eukaryotic cell.
  2. The cytoplasm fills the cell and is where its chemical reactions happen.
  3. A cell membrane surrounds the cytoplasm and controls what passes into and out of the cell.
  4. A cell wall surrounds the membrane and gives the cell its shape and support.
  5. Ribosomes in the cytoplasm make the cell's proteins.
  6. A prokaryotic cell has no membrane-bound organelles, so no mitochondria and no chloroplasts.
  7. Some prokaryotes also have a flagellum for movement or a slime capsule for protection.

A diagram of a prokaryotic cell (bacterium) showing its internal and external structures, including the nucleoid DNA, cytoplasm, ribosomes, cell wall, capsule, pili, and flagellum.

A diagram of a prokaryotic cell showing the cell wall, cell membrane, slime capsule, flagellum, pili, 70S ribosomes, a single loop of DNA (genophore), and a plasmid.

Note
  • A prokaryotic cell wall is not made of cellulose, so it differs from a plant cell wall.

How a Prokaryote Stores Its Genetic Material

Definition

Plasmid

A small ring of DNA, separate from the main circular chromosome, found in the cytoplasm of a prokaryotic cell.

  1. A prokaryotic cell has no nucleus to enclose its DNA.
  2. Its main genetic material is a single loop of DNA floating free in the cytoplasm.
  3. Many prokaryotes also carry plasmids, small rings of extra DNA separate from the main loop.

A diagram of a prokaryotic cell showing the organization of its DNA, including a single circular bacterial chromosome in the nucleoid region and several small rings of DNA called plasmids.

  1. A plasmid can carry extra genes, such as a gene for resistance to an antibiotic.
Common Mistake
  • Do not write that bacteria have no genetic material; they have DNA but no nucleus to hold it.

The Scale and Size of Cells, Written in Standard Form

  1. Prokaryotic cells are much smaller than eukaryotic cells.
  2. A typical bacterium is about 1 to 5 micrometres across.
  3. A typical eukaryotic cell is about 10 to 100 micrometres across.
  4. Cell sizes are measured in micrometres (µm), where 1 mm = 1000 µm and 1 µm = 1000 nm.
  5. To change millimetres to micrometres you multiply by 1000, and to reverse it you divide by 1000.
  6. Very small sizes are written in standard form, a number between 1 and 10 multiplied by a power of ten.
  7. An order of magnitude is a difference in size of a factor of ten.
  8. For example, a 1.5 µm bacterium is 0.0015 mm, which is 1.5×10−31.5 \times 10^{-3}1.5×10−3 mm in standard form.
Self review
  • What is the one key difference between a eukaryotic and a prokaryotic cell?
  • Name three groups of organisms made of eukaryotic cells.
  • Where is the main genetic material of a prokaryotic cell found?
  • What is a plasmid?
  • How many micrometres are there in 1 millimetre?
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1.1.1 Eukaryotes and prokaryotes Revision Guide

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