BIOL 112 Lecture 14

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Last Universal Common Ancestor (LUCA)

What did LUCA look like?

Bacteria and Archaea share:

• DNA genome with same genetic code
• No nuclear envelope
• Electron transport to produce ATP (or analogs)
• Proteins
• No organelles

Bacteria and Archaea differ (post-LUCA):

• rDNA gene sequences
• Cell wall composition
• DNA replication enzymes
• Introns (archaea have them, bacteria don't)

Early genome hypothesized to be RNA (not DNA). Why?

• self-replication
• self-cleavage (attaches to self in ribozyme activity)
• fold into complex structures (hairpins and stems)

RNA → DNA switch happened later when reverse transcriptase developed

Enough Speculation Here's some Proof

Oldest fossils are approx. 3.5 billion year-old bacteria (stromatolite)

colonies of photosynthetic cyanobacteria produced O₂ approx 2.75 bya (evidence: rust in rocks)

but O₂ levels were 10% of today's levels

H₂O → O₂ + H + ATP

Bacteria

3 shapes:

• Dots = Cocci
• Rods = Bacilli
• Spirals = Helices / Spirilla

Cell membrane surrounded by cell wall and optionally a lipid outer layer (only gram negative)

Sensitive to high salt and high sugar (hypotonic solution leads to plasmolysis; in other words, they shrivel up like a little raisin)

this is the basic idea behind preservatives

Surface Structures and Membranes

The cell wall is made from peptidoglycans (stained by Gram Stain)

• Gram-positive cells have an exposed cell wall
• Gram-negative cells have a lipopolysaccharide outer membrane around their cell walls

Toxic bacteria are usually gram-negative because:

1. toxicity comes from lipopolysaccharide membrane
2. antibiotics often cannot get through outer membrane (hence many antibiotics target peptidoglycan synthesis