14.1: Introduction

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Because of their small size, bacterial genomes have few repetitive DNA sequences. In contrast, repetitive DNA sequences make up a large part of a eukaryotic genome. Much of this repeated DNA consists of identical or nearly identical sequences of varying length repeated many times in a genome. Examples include satellite DNA (minisatellite and microsatellite DNA) and transposons, or transposable elements. Here we look at experiments that first revealed the existence and proportion of repeated DNA in genomes. Next we describe Barbara McClintock’s even earlier (and pretty amazing!) discovery of transposable elements. After we describe the different classes of transposons and different mechanisms of transposition, we tackle the question of why they and other repetitive DNAs even exist. Elsewhere we introduced the notion of junk DNA as DNA sequences that serve no known purpose. Is repeated DNA junk DNA? Are transposable elements junk? We are now learning that transposons and other repetitive DNAs can have specific functions, from regulating gene expression to reshaping genomes to increasing genetic diversity in evolution. So, far from being ‘junk’, much redundant DNA exists in genomes because of evolutionary selection.

Learning Objectives

When you have mastered the information in this chapter, you should be able to:

Compare and contrast renaturation kinetic data.
Explain CoT curves and DNA complexity
List physical and chemical properties of main band and satellite DNAs
Outline an experiment to determine if a given sequence of DNA is repetitive or not.
Summarize how Barbara McClintock revealed the genetics of maize mosaicism.
Outline the experiments suggesting that the Ds gene moves from one lovus to another in the maize genome.
Compare and contrast cut-&-paste and replicative transposition.
Compare the behaviors of autonomous and non-autonomous transposons.
List the difference between Mu phage infection and transposition.
Describe the common structural features of transposons.
Compare the mechanisms of LINE and SINE transposition.
Speculate on how species avoid potentially lethal consequences of transposition.
Speculate on which came first in evolution: DNA transposons, RNA transposons or retroviruses, and explain your reasoning.