chapter 27

• Prokaryotes were the earliest organisms on Earth.
• Today, they still dominate the biosphere.
  • Their collective biomass outweighs all eukaryotes combined at least tenfold.
  • More prokaryotes inhabit a handful of fertile soil or the mouth or skin of a human than the total number of people who have ever lived.
• Prokaryotes are wherever there is life.
• They thrive in habitats that are too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote.
  • Prokaryotes have even been discovered in rocks two miles below the surface of the Earth.
• Why have these organisms dominated the biosphere since the origin of life on Earth?
  • Prokaryotes display diverse adaptations that allow them to inhabit many environments.
  • They have great genetic diversity.
• Prokaryotes are classified into two domains, Bacteria and Archaea, which differ in structure, physiology and biochemistry.

Figure 27.2 depicts changes to the amount of DNA present in a recipient cell that is engaged in conjugation with an Hfr cell. Hfr cell DNA begins entering the recipient cell at Time A. Assume that reciprocal crossing over occurs (in other words, a fragment of the recipient's chromosome is exchanged for a homologous fragment from the Hfr cell's DNA). Use Figure 27.2 to answer the following question.

Figure 27.2

What is occurring at Time C that is decreasing the DNA content?

A) crossing over
B) cytokinesis
C) meiosis
D) degradation of DNA that was not retained in the recipient's chromosome
E) reversal of the direction of conjugation

How is the recipient cell different at Time D than it was at Time A?

A) It has a greater number of genes.
B) It has a greater mass of DNA.
C) It has a different sequence of base pairs.
D) It contains bacteriophage DNA.
E) Its membrane-bound DNA pumps are inactive.