6.4.5. 1Centromeres. Centromerescan be found at different regions depending on the particular
chromosome. A metacentric chromosomehas its centromere near the middle of the chromosome and a
telocentric chromosomehas one at the telomere. The centromere has been intensively studied by genetic,
molecular and microscopic analysis. Genetic and molecular analysis in the yeast S. cerevisiaehas identi-
fied a minimal structure necessary for centromere function (Figure 6.13).26,27Two regions, CDE1 and
CDE3 have important conserved sequences necessary for centromere function. The first of these regions
binds to a protein called CBF1 and the second binds to a complex of three proteins, namely CBF3b,
NDC10 and CTF13. The DNA sequence CDE2 that separates these regions is about 80 bp in length, has
approximately 90% A/T content, and binds to the MIF2 protein.The centromeres of more complex eukary-
otes are much larger than this. Additionally, centromeric regions tend to accumulate even more repetitious
DNA than the rest of the genome. Repetitious DNA presents serious problems in sequence determination
and computer-based structural analysis and is often difficult to clone.
6.4.5.2 Telomeres. Telomeresare also essential chromosomal components.^28 In their absence, the
chromosome shortens until essential genes are lost and the cell dies. The extreme ends of chromosomes
do not have complex structures; they are simply double-stranded DNA of repeating sequence, exemplified
by the common sequence shown in Figure 6.14. Telomeres counter their natural tendency to become
shorter at their ends by generating new copies of these repeats. The mechanisms whereby this occurs are
described below (Section 6.6.5).
6.4.6 Viral Genomes
Viruses are parasites that can only replicate inside a host cell.^29 Probably all organisms can act as hosts to
viruses. Viruses can have genomes made up of DNA or RNA (Table 6.1). The simplest virushas only a short
nucleic acid encoding a handful of genes, which is packaged into a protein particle. More complicated
Genes and Genomes 219
Figure 6.13 Structure of the yeast centromere
Figure 6.14 Sequence of the human telomere repeat
Table 6.1 Eukaryotic viruses
Genome Type Example Size (base pairs) Structure
ds DNA Poxvirus Smallpox 250,000 Linear
ssDNA Parvovirus AAV 2000 Circular
ss/ds DNA Hepadnavirus Hepatitis B 3000 Circular
dsRNA Reovirus Reovirus 25,000 Linear
ssRNAstrand Picornavirus Poliovirus 7000 Linear
ssRNAstrand Myxovirus Influenza 12,000 Linear (several pieces)