Chapter 3, Problem 1RC

Summary Introduction

To describe: The application of small subunit (SSU) rRNA analysis to the establishment of the three domain classification system proposed by Carl Woese.

Introduction: The tree of life includes three domains: Eukarya, Archaea, and bacteria. This tree of life is mainly based on the small subunit ribosomal ribonucleic acid (SSU rRNA) analysis. The SSU rRNA analysis is proposed by Carl Woese.

Explanation of Solution

The SSU-rRNA is the smallest RNA constituents present in both eukaryotes and prokaryotes.

• The SSU-rRNA analysis is used to study the evolutionary relationship among various organisms.
• The sequences of nucleotides in the genes that encode SSU rRNA from various organisms are aligned and pair-wise comparison made.
• For every pair of SSU-rRNA gene sequences, the number of differences in the nucleotide sequences is calculated.
• The final value serves as a measure of the evolutionary distance between the organisms (the greater differences counted, the more the evolutionary distance).
• To construct the phylogenetic tree, the evolutionary distances from many comparisons are used by computer programs.
• The distance from the tip of one branch to the tip of another is the evolutionary distance between the two organisms.
• Hence, the three domains are classified based on their evolutionary relationship with the sufficient length of these SSU-rRNA gene sequences.