1022-7954/04/4010- © 2004
Russian Journal of Genetics, Vol. 40, No. 10, 2004, pp. 1179–1181. Translated from Genetika, Vol. 40, No. 10, 2004, pp. 1429–1431.
Original Russian Text Copyright © 2004 by Ivashchenko, Atambaeva.
Genes of viruses, mitochondria, chloroplasts,
prokaryotes, and those of eukaryotic nuclear genome
contain introns . The intron-containing genes are
mostly coding for proteins though introns are also
found in tRNA and rRNA genes . In mitochondrial,
chloroplast, and bacterial genomes, a small number of
intron-containing genes is determined and therefore,
statistical analysis of intron size is problematic [3–8].
The genomes of multicellular eukaryotes contain a sub-
stantial proportion of introns [1, 2, 9]. In fungal, plant,
and insect genomes, genes with introns contain also
exons widely varying in length. In plant genes, the
exons of 100–400 nucleotides are the most frequent,
whereas in fungal and insect genomes, 200- and at least
600-nucleotide exons predominate. The average intron
length signiﬁcantly increases with increasing size of
the eukaryotic genome; in vertebrate, the average intron
length is about 500 nucleotides [1, 2].
The nuclei of higher organisms contain splice-
somes, the specialized structures that remove introns
from pre-mRNA [1, 2]. During splicing, introns assume
a lasso-like shape, which may restrict the length of
introns and exons. Because of speciﬁc features of intron
and exon length, these structures signiﬁcantly differ in
size in different organisms . It is important to deter-
mine whether the length of introns and exons depends
on the number of introns per gene, because this fact
may have an effect on the splicing time and the rate of
gene expression. In this connection, we studied changes
in length of introns and exons in the genes of
, which contain various number of introns
including those differing in length.
The nucleotide DNA sequences of ﬁve chromo-
somes in the
nuclear genome were from the
database found at http://www.arabidopsis.org. Analysis of
all chromosomes of this genome made it possible to form
the following groups: genes without introns (120 genes),
with one (434 genes), two (324), three (273), four (89),
ﬁve (85) introns, and at least 15 introns (65 genes). All
these gene groups were formed from about an equal
number of genes of each chromosome. The sample
included functionally identiﬁed genes. The average
lengths of introns and exons were determined in each
gene group by dividing the total length of introns or
exons on their number in this group. The number of
introns and exons containing 1–20, 21–40, 41–60 and
so on up to 400 nucleotides and over 400 nucleotides
was also determined in each group of genes.
The table shows the relationship between exon and
intron lengths and the number of introns per gene. The
average length of introns and exons tends to decrease
with increasing number of introns per gene. The ratio of
exon to intron length changes substantially in favor of
introns. The proportion of intron length increases from
22% (in genes with one intron) to 48% in genes with at
least 15 introns. The length of all exons in a gene with
one intron is 856 nucleotides increasing to 3195 nucle-
otides in genes with at least 15 introns. The gene length
increases almost sixfold, from 1099 to 6151, because of
changes in exon and intron lengths.
The ﬁgure shows changes in intron and exon lengths
depending on changes in the number of introns per
gene. In genes with a single intron, the proportion of
400-nucleotide introns is 19%, whereas the proportion
of introns composed of 80–100 nucleotides is the high-
est (30%). With increasing number of introns per gene,
those composed of 400 nucleotides decrease in number
whereas the number introns ranging in length from 80
to 100 nucleotides increases. In genes with 15 introns
and above, the 400- and 80–100-nucleotide introns
make up 4 and 37%, respectively. The proportion of
introns ranging in length from 140 to 400 nucleotides is
similar in these groups of genes (26–30%, Fig. 1a).
Most exons in genes with one intron contain at least
400 nucleotides (35%) but with increasing number of
introns per gene, the number of these exons decreases.
Variation in Lengths of Introns and Exons in Genes
A. T. Ivashchenko and Sh. A. Atambaeva
Research Institute of Problems of Biology and Biotechnology, Al-Farabi Kazakh National University,
Alma-Ata, 48078 Kazakhstan; e-mail: firstname.lastname@example.org
Received January 26, 2004
—In the nuclear genes of
, the length of introns and exons was shown to vary
depending on the number of introns. With increasing number of introns per gene, the proportion of introns com-
posed of 80–100 nucleotides increases whereas the proportion of introns with 400-nucleotide length decreases.
Similar changes in exon length in genes result in predominance of exons of 60–120-nucleotides in length.