Organization of the Human genome
50kbp region of the human genome is shown in the middle
segment. Note that the genes make up less than half of
the total region, and that large numbers of repeat
sequences and non-coding intergenic DNA
occur between them. In the expanded view above, there
are four labelled genes, in which exons constitute only a
few percent of the region, although some genes like PKP2
are exon-rich. Introns make up the bulk
of the other three genes, notably SYB1.
Introns in FLJ10143 and CD27
are interuppted by repeat
sequences (cf. locus FLJ10143).
Many of the these dispersed repeats are members of the Alu
family, a 200~300 bp repeat dispersed as
many hundreds of thousands of times across the genome.
Although repetitive DNA is sometimes referred
to as "junk DNA", this probably represents
the current state of unknowledge. For example, it
appears likely that repetitive elements assist in alternative
splicing of exons to produce different
transcripts with different properties from
the same coding region. This might explain how
humans are able to function with so few genes.