Microcheck 105

Historically prokaryotes have been grouped according to phenotypic attributes. Molecular approaches give greater insights into the relatedness of microorganisms.

■ Describe one drawback of using phenotypic characteristics to classify bacteria.

■ Some scientists that work on bacterial classification are called "lumpers" and others are called "splitters." What do these terms likely indicate?

Table 10.6 Methods Used to Determine the Relatedness of Different Prokaryotes for Purposes of Classification

Methods

Comments

Phenotypic Characteristics

Traditionally, relatedness of different bacteria has been decided by comparing properties such as ability to degrade lactose and the presence of flagella.These characteristics, however, do not necessarily reflect the evolutionary relatedness of organisms.

Numerical taxonomy

A large number of phenotypic traits are examined, resulting in the generation of a similarity coefficient.

Genotypic Characteristics

Differences in DNA sequences can be used to determine the point in time at which two organisms diverged from a common ancestor.

DNA base composition

Determining the G + C content offers a crude comparison of genomes. Organisms with identical G + C contents can be entirely unrelated, however.

DNA hybridization

The extent of nucleotide sequence similarity can be determined by measuring how completely single strands of their DNA will hybridize to one another.

Comparing the sequences of 16S rDNA

This technique has revolutionized classification. Certain regions of the 16S rDNA can be used to determine distant relatedness of diverse organisms; other regions can be used to determine more recent divergence.

Chapter 10 Identification and Classification of Prokaryotes

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