The Size and
Sequence Structure of DNA Molecules
Viral DNA
Molecules Are Small
Bacteria
Contain Chromosomes and Extrachromosomal DNA
Eukaryotic
Cells Contain More DNA than Prokaryotes
Organelles
of Eukaryotic Cells Also Contain DNA
Genes Are
Segments of DNA That Code for Polypeptide Chains
and RNAs
There Are
Many Genes in a Single Chromosome
Eukaryotic
Chromosomes Are Very Complex
Many
Eukaryotic Genes Contain Intervening
Nontranscribed Sequences (Introns)
DNA Supercoiling
Most
Cellular DNA Is Underwound
DNA
Underwinding Is Defined by Topological Linking
Number
Topoisomerases
Catalyze Changes in the Linking Number of DNA
DNA
Compaction Requires a Special Form of
Supercoiling
Chromatin and
Nucleoid Structure
Histones
Are Small, Basic Proteins
Nucleosomes
Are the Fundamental Organizational Units in
Chromatin
Nucleosomes
Are Packed into Successively Higher-Order
Structures
Bacterial
DNA Is Also Highly Organized
Summary
Further Reading
Problems
