another plant, the weed Arabidopsis thaliana, has far less intergenic repetitious DNA(Figure 6.9).^18 In
fact, the intergenic DNAof eukaryotes is much more susceptible to change than their genes.
For some genomes such as maize the repetitious DNA can outweigh the DNA devoted to genes, lead-
ing to ‘genomic obesity’.^19 This observation explains an old puzzle for molecular geneticists, namely that
the sizes of eukaryotic genomes are extremely variable and there is no obvious correlation between
genome size and evolutionary level (Figure 6.10). For example, one amphibian may possess 50-fold more
genetic information than another. The puzzle has been called the C-value paradox.
Two major classes of repetitious DNA make up the majority of this so-called ‘junk DNA’.^20 The first is
satellite DNA, which is comprised of seemingly endless tandem repeats of a simple sequence. For example,
the fruit fly Drosophila virilispossesses a huge number of ACAAACT repeats that together add up to almost
a quarter of the entire genome. Such DNA is also found in heterochromatin^21 (Section 6.4.2), particularly at
the centromeresof chromosomes (Section 6.4.5). The second major class of repetitious DNA is transposable
elements, particularly retrotransposons(Section 6.8.3).
6.4 Chromosomes
The DNA of an organism is arranged on one or more chromosomes. The number of chromosomes per species
is invariant but can vary a lot between related species. Each chromosome is comprised of a double-stranded
DNA molecule, packed together with a set of associated proteins and other components into a complex
called chromatin.
6.4.1 Eukaryotic Chromosomes
All eukaryotes contain at least two chromosomes. There is no clear correlation between the chromosome
number and the type of organism. For example, the yeast Saccharomyces cerevisiae has 16 chromosomes
per haploid cell, the fruit fly Drosophila melanogasterhas 4 and the human has 23. Most multi-cellular
eukaryotes contain mostly diploid cells and for these cells the chromosome number is doubled. All
eukaryotic nuclear chromosomes studied to date contain simple linear double-stranded DNA.
6.4.2 Packaging of DNA in Eukaryotic Chromosomes
The DNA of a eukaryotic chromosome must fit into a space far smaller than its total length. For example, a
human chromosome has around 3–10 cm of DNA that must fit into a cell nucleus a thousand times smaller.
216 Chapter 6
Figure 6.10 Variation in genome size across the five kingdoms of life