Introduction

Chapter 4: Organization of Genes in Chromosomes

Concepts

Genome analysis provides many insights into genetic organization.
Bacterial chromosomes are small, circular, densely packed with protein-coding genes, and mostly organized in polycistronic transcription units without introns.
Bacteria show us the minimum genome size necessary for life.
Archaea have characteristics of eubacteria and eukaryotes
Eukaryote chromosomes are large, linear, less densely packed with protein-coding genes, and mostly organized as single transcription units with introns.
The difference between single-cell eukaryotes and multicellular eucaryotes
The gene organization of higher plants.
rRNA and tRNA gene organization is different from mRNA gene organization.
Multigene families include the globin genes, immunoglobulin genes, and histone genes.
How does gene number change with advancing phylogeny?
The human genome project will change our life in the 21st century.

Chromatin Structure

The DNA in a single human cell is approximately two meters in length. Clearly, the DNA must be packaged to fit into the cell, but at the same time the functions of DNA must be preserved.

The strategy that eukaryotes have evolved to solve this complex packaging problem has been well characterized. Eukaryotic DNA exists in the cell bound to very basic proteins called "histones" to form nucleosomes. Nucleosomes are wound into larger structures that form the fibers of chromosomes. For all practical purposes, all eukaryotes share this common mechanism.

The solenoid structure of chromatin is further compacted and restrained by association with the nuclear scaffold to form thick fibers with a diameter greater than 200 nm. The scaffold is composed of non-histone proteins, including topoisomerase, that function to further condense the DNA. These thick fibers are further condensed into the chromosomal structure that is the most recognized form of a chromosome.

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