Bacteria and Archaea

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A polysaccharide or protein layer called a capsule covers many prokaryotes.

Some prokaryotes have fimbriae (also called attachment pili), which allow them to stick to their substrate or other individuals in a colony

Sex pili are longer than fimbriae and allow prokaryotes to exchange DNA by conjugation.

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Bacteria Fimbriae - Allow Bacteria to adhere to each other and substrates.

Fimbriae

200 nm

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Motility / Internal Organization

Most motile bacteria propel themselves by flagella that are structurally and functionally different from eukaryotic flagella.

In a heterogeneous environment, many bacteria exhibit taxis, the ability to move toward or away from certain stimuli.

Prokaryotic cells usually lack complex compartmentalization.

Some prokaryotes do have specialized membranes that perform metabolic functions.

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Bacteria flagellum - Structurally different from Eukaryotic flagella

Flagellum

Filament

Hook

Basal apparatus

Cell wall

Plasma

membrane

50 nm

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Bacteria often have Specialized Internal Membranes

(a) Aerobic prokaryote

(b) Photosynthetic prokaryote

Thylakoid

membranes

Respiratory

membrane

0.2 µm

1 µm

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Prokaryotic Genome - No Nucleus

The prokaryotic genome has less DNA than the eukaryotic genome.

Most of the genome consists of a circular chromosome.

Some species of bacteria also have smaller rings of DNA called plasmids.

The typical prokaryotic genome is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region.

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Single Circular Chromosome

Plasmids

smaller DNA rings

1 µm

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Reproduction and Endospores Adaptation

Prokaryotes reproduce asexually quickly by binary fission and can clone / divide every 1–3 hours.

Many prokaryotes form metabolically inactive endospores, which can remain dormant and viable in harsh conditions for centuries.

Prokaryotes can evolve rapidly because of their short generation times.

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Endospores = Adaptation to Survive Dormant in Harsh Environments

Endospore

0.3 µm

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