Gene Cloning

Genome Organization 15
Box 2.2 Interspersed Transposon-Derived Repeats
Interspersed transposon-derived repeats are found in all eukaryotes. They
are typically the most abundant repeat sequences found within the
genome and account for 45% of the human genome. There are four classes
of transposable elements, LTR retroposons (retrovirus-like elements), long
interspersed elements (LINEs), short interspersed elements (SINEs) and
DNA transposons.
LTR retroposons contain long terminal repeats (LTR elements) and are
relics of retroviruses. They contain the necessary signals to allow transposition
within the genome, a process that requires transcription of the
retroposon element followed by reverse transcription to form a DNA copy
which then integrates at a random site within the genome. LTR retroposons
account for approximately 8% of the human genome.
Long interspersed elements (LINEs) are retroelements, 6–8 kb in length,
that do not have an LTR sequence. The LINE sequence encodes two proteins
that are involved in transposition of the sequence within the genome.
The inefficient mechanism used during transposition often results in truncated
forms of the LINEs being transposed within the genome. LINE elements
make up approximately 20% of the human genome.
Short interspersed elements (SINEs) are short DNA sequences, 100–300 bp
in length, which are not capable of transposition on their own but can do
so using functions provided by LINE elements. The Alu element, a 300 bp
sequence, is the most abundant SINE with more than 1,000,000 copies present
within the human genome representing 10% of the genome. In total,
SINEs make up approximately 13% of the human genome.
Active DNA transposons are 2–3 kb in length and contain a transposase
gene flanked by terminal repeat sequences; inactive transposons contain
the inverted repeats but have lost the transposase gene. DNA transposons
make up 3% of the human genome.
LTR retroposons, LINEs and SINEs all transpose via an RNA intermediate,
whereas DNA transposons spread through the genome via a copy and
paste mechanism.
contained functional genes which therefore leads to redundancy. The
duplicated gene or genes can pick up mutations that either lead to change
or loss of function. So gene duplication is one way in which genes with new
functions can occur and is responsible for the evolution of gene families.
Loss of function will result in a pseudogene, i.e. a sequence on the genome
that looks like a gene but no longer gives rise to a functional protein.