Prokaryotes

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Prokaryotic Cells

Prokaryotic cells have some features that are also found in eukaryotic cells. Prokaryotic cells may also contain plasmids, a capsule and one or more flagella.

Prokaryotes vs eukaryotes

Features common to these types of cells are:
- A plasma membrane.
- Cytoplasm.
- DNA.
- Ribosomes (ribosomes are smaller in prokaryotes).
So, the main difference in terms of organelles is that prokaryotes do NOT have any membrane-bound organelles (e.g. nucleus, mitochondria, Golgi apparatus etc.).

DNA in prokaryotes

Prokaryotic DNA is found as a circular molecule in the cytoplasm.
Some prokaryotes have smaller loops of DNA called plasmids that are not part of the main circular DNA molecule.
Bacteria can exchange plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA.
- Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange.

Capsule

The capsule enables prokaryotic cells to attach to surfaces in its environment.

Flagella, pili and fimbriae

Some prokaryotes have flagella, pili, or fimbriae:
- Flagella are tail-like and used for locomotion.
  - Some prokaryotes have none and others have more than one.
- Pili are used to exchange genetic material during a type of reproduction called conjugation.
- Fimbriae are used by bacteria to attach to a host cell.

Binary Fission

Prokaryotes, such as bacteria, replicate by binary fission. For unicellular organisms, cell division is the only method to produce new individuals.

1) Replication of genetic material

Binary fission in prokaryotic cells involves replication of the circular DNA and of plasmids.

2) Migration of genetic material

The two sets of genetic material migrate towards opposite poles.

3) Cytoplasm begins to divide

The cytoplasmic contents must be divided to give both new cells the machinery to sustain life.

4) Formation of daughter cells

Two daughter cells are formed, each with a single copy of the circular DNA and a variable number of plasmid copies.
Daughter cells are genetically identical to the parent cells and identical to each other.
The only difference between the daughter cells is the number of plasmid copies in each cell.

Viruses

Viruses are acellular and non-living. Virus particles include genetic material, a capsid and attachment proteins.

Structure

Viruses are made up of nucleic acids.
The nucleic acids are surrounded by a protein called a capsid.
Viruses are smaller than bacteria.

Living cells

Viruses are NOT considered living because:
- They have no nucleus, plasma membrane, cytoplasm or ribosomes.
- They are not made of cells.
- They cannot reproduce independently.

Function

To replicate, viruses have to invade and hijack the reproductive mechanism of a living cell. These cells are then termed host cells.
Viruses have attachment glycoproteins that bind to complementary receptors on host cells.
They then inject DNA or RNA into the host cell.
The host cell divides as normal and the virus is replicated.