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Figure 4.24.
Organization of the human histone gene cluster on chromosome 6 (after Albig
et al
., 1997a).Solid rectangles: histone genes. Open rectangles: pseudogenes. Orientations of
histone genes are denoted by arrowheads. Restriction sites are shown as short vertical bars
above (
EcoR
I) or below (
Mlu
I) the line.
do not occur as tandem repeats (Heintz
et al
., 1981). A map of part of the major
histone cluster on human chromosome 6 is shown in
Figure 4.24
and it is evident
that the arrangement of the histone genes is fairly random; the only regularity is
the arrangement of the divergently orientated
H2A
and
H2B
gene pairs. For both
main-type and replacement histone types, chromosomal localization, gene num-
ber and gene organization appear to be fairly well conserved between human and
mouse (Albig and Doenecke, 1997; Albig
et al
., 1997). This indicates that the sep-
aration of these two groups of histone genes must have occurred at an early stage
in mammalian evolution. Chromosomal clustering may be a prerequisite for coor-
dinate regulation. An unexpectedly high degree of within-species homogeneity of
tandemly repeated histone genes is explicable in terms of unequal crossing over
and/or gene conversion (Maxson
et al
., 1983).
The residual sequence similarity (pairwise 15-20%) exhibited by the four core
histones together with their structural similarity argues strongly for descent from
a common ancestor, albeit a very ancient one (Ramakrishnan, 1995). A phyloge-
netic analysis of the core histones has suggested that the histones that form
dimers (viz. H2A/H2B and H3/H4) have very similar trees and appear to have co-
evolved (Thatcher and Gorovsky, 1994). H3 and H4 have evolved ~10-fold more
slowly than H2A and H2B and are very highly conserved (Thatcher and
Gorovsky, 1994). The H2A.Z variant arose early and appears to be more highly
conserved than the main-type 2A histone whilst H3.3 variants have arisen inde-
pendently on many occasions (Thatcher and Gorovsky, 1994). By contrast,
H2A.X arose comparatively recently during the evolution of the vertebrates
(Thatcher and Gorovsky, 1994).
Ribosomal RNA genes.
The ribosomal RNA (rRNA) genes occur as 300-400
copies in the human genome. They are organized in tandemly repeated blocks
which are located on the acrocentric chromosomes:
RNR1
(13p12),
RNR2
(14p12),
RNR3
(15p12),
RNR4
(21p12), and
RNR5
(22p12). The 44 kb rDNA
repeat unit contains a 13.3 kb RNA polymerase I-transcribed portion and a 31 kb
