2. • Pseudogenes are nonfunctional segments of DNA that resemble
functional genes. Most arise as superfluous copies of functional
genes, either directly by DNA duplication or indirectly by reverse
transcription of an mRNA transcript.
There are two classes of pseudogenes.
• Class I pseudogenes have arisen by gene duplication and then
have subsequently been inactivated by various mutations
(insertions, substitutions, or deletions) they are often located near
their functional-gene counterpart.
3. • Class Il pseudogenes are processed
sequences (lacking introns and often
containing a vestigial poly A tail) and have
originated during evolution from mRNA that was
copied by reverse transcriptase back into DNA.
• The sequence was then inserted into the
genome by a retrotransposon event. The
footprints of this event are evident in the direct
repeats that flank the pseudogene; the repeata
have facilitated its insertion back into the
genome. Such pseudogenes are usually located
a long way from the functional parent gene
4.
5. • The exact number of pseudogenes in the human genome is unknown,
although some analyses have identified ~2900 regions that probably
represent processed pseudogenes.
• The pattern that has emerged from analysis of the human genome is
that those sequences which tend to give rise to pseudogenes have
shorter than average transcripts and are sequences that are involved in
nuclear regulation and translation. E.g., ribosomal proteins account for
67%; lamin receptors, 10%; and translation elongation factors, 5%. The
common theme here is the increased level of transcription of these
sequences.
