CHAPTER 27
Bacteria and Archaea
YOU MUST KNOW
The key ways in which prokaryotes differ fromeukaryotes with respect to:
Genome;
Membrane bound organelles;
Size;
Reproduction
Mechanisms that contribute to genetic diversity inprokaryotes, including:
Transformation;
Conjugation;
Transduction;
Mutation
Structural & Functional Adaptations
Structural and functional adaptations contribute toprokaryotic success:
Both the domain Bacteria and the domain Archaea are madeup of prokaryotes.
Prokaryotes have NO TRUE NUCLEI or internalcompartmentalization.
The DNA of prokaryotes is concentrated in the nucleoid regionand has little associated protein.
Relative to eukaryotes, prokaryotes have simple, smallgenomes.
In addition to their one major chromosome, prokaryotic cellsmay also possess smaller, circular, independent pieces of DNAcalled plasmids.
Prokaryotic Shapes
In general, prokaryotes exist in three commonshapes:
Cocci (round)
Bacillus (rod-shaped)
Spirilla (spiral)
Prokaryotic Reproduction
Prokaryotes reproduce through an asexual processcalled binary fission, and they continually synthesizeDNA.
Prokaryotic Structure
Outside their cell membranes, most prokaryotespossess a cell wall that contains peptidoglycan.
Gram-positive bacteria have simpler cell walls with morepeptidoglycan.
Gram-negative bacteria have cell walls that are structurallymore complex.
Prokaryotic Structure
The cell wall of many prokaryotes is covered by acapsule, a sticky layer of polysaccharide or protein.
The capsule enables prokaryotes to adhere to other substrateor to other individuals in a colony. Some capsules protectagainst dehydration.
Some prokaryotes stick to their substrate or to oneanother by means of hair-like appendages calledfimbriae.
The ability of some prokaryotes to withstand harshconditions is attributed to endospores, a tough wallthat forms outside the cell wall.
Prokaryotic Motility
Prokaryotes use appendages called pili that adhere to each other or tosurrounding surfaces.
About half of the prokaryotes are motile, because they possess whiplikeflagella.
Genetic Diversity in Prokaryotes
Rapid reproduction, mutation, and geneticrecombination promote genetic diversity inprokaryotes:
While mutation is the major source of genetic variation inprokaryotes, there are three mechanisms by which bacteria cantransfer genetic material between each other:
1.Transformation (the uptake of foreign DNA from thesurrounding environment).
2.Conjugation (the direct transfer of genes from one prokaryote toanother).
3.Transduction (the transfer of genes from one prokaryote toanother via a viral vector).
Lateral Gene Transfer in Prokaryotes
Major Nutritional Modes in Prokaryotes
Oxygen Needs in Prokaryotes
Obligate aerobes cannot growth without oxygenbecause they need oxygen for cellular respiration.
Obligate anaerobes are poisoned by oxygen. Someuse fermentation, whereas others extract chemicalenergy by another form of anaerobic respiration.
Facultative anaerobes use oxygen if it is available,when oxygen is not available, they undergofermentation.
Some prokaryotes can use atmospheric nitrogen as adirect source of nitrogen in a process called nitrogenfixation. They convert N2 to NH4+.
Molecular Systematics & Prokaryotic Phylogeny
Molecular systematics is illuminating prokaryoticphylogeny.
The first prokaryotes that were classified in thedomain Archaea are known as extremophiles andlive in extreme environments:
Extreme Halophiles: live in saline environments.
Extreme Thermophiles: live in very hot environments.
Other Archaea do not live in extreme environments.
Methanogens use carbon dioxide to oxidize H2 and producemethane as a waste product.
Ecological Importance of Prokaryotes
Prokaryotes play crucial roles in the biosphere:
Many prokaryotes are decomposers, breaking down deadorganic matter.
Many prokaryotes are symbiotic, forming crucial relationshipswith other species.
Some prokaryotes are pathogenic and cause illness byproducing poisons. Antibiotics are chemicals that can killprokaryotes. They are NOT effective against viruses.
Many bacterial plasmids contain resistance genes to differentantibiotics.
Human Uses of Prokaryotes
Prokaryotes are used by humans in many ways:
Prokaryotes are used in bioremediation, removing pollutantsfrom soil, air, or water.
Includes sewage treatment, oil spill clean up, and radioactivematerial precipitation.
Prokaryotes are symbionts in the gut, and can be used to makevitamins, and assist in digesting food.
Prokaryotes are used in gene cloning and producing transgenicorganisms.
Prokaryotes are used in a wide variety of food productions:cheese, yogurt, and other products.