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Section: Genetics » Organization of Genetic Material » Split, Overlapping & Pseudogenes
 
 
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  Pseudogenes
 
     
 
Content
⇒ Organization of Genetic Material 3.  Split Genes, Overlapping Genes and Pseudogenes
⇒ Split genes or interrupted genes 
⇒ Discovery and nature of split genes
⇒ R-loop mapping and restriction mapping of interrupted genes
⇒ Structure of chicken ovalbumin split gene
⇒ Split genes in fungal mitochondria
⇒ Split genes in chloroplasts
⇒ Intron of one gene may contain exon of another gene
⇒ Exon sequences are conserved, but intron sequences vary
⇒ Introns with coding sequences
⇒ Overlapping genes
⇒ Pseudogenes
⇒ Promiscuous DNA
Pseudogenes (U snRNA series, globin pseudogenes and Alu family)
In muiticellular organisms, a wide variety of DNA sequences are found, which are of no apparent use. Some of these sequences are defective copies of functional genes and are, therefore, called pseudogenes. These pseudogenes have been reported in human beings, mouse and Drosophila. The most popular examples of these pseudogenes include the following, (i) Human α-globin and β-globin pseudogenes (ty), found in each of the two globin gene clusters. Complete nucleotide sequence ofpseudo alpha globin gene is now known and it has been shown that both these genes are non-translatable, since they may have mutations in initiation codon and also frame-shift mutations along their length, (ii) In mouse also there are two alpha globin pseudogenes (ψ), one of them (ψα3) is different from other pseudogenes since it has no introns which are present in functional α-globin genes as well as in other pseudogenes. The pseudogene ψα3 is, therefore, believed to be the reverse transcript of mRNA, obtained due to reverse transcriptase enzyme,
(iii) U snRNA series of pseudogenes in human beings includes U1, U2 and U3 snRNA pseudogenes. The corresponding functional U1, U2 and U3 snRNA genes may be involved in splicing of nucleic acid sequences (see Expression of Gene : Protein Synthesis 3.  RNA Processing (RNA Splicing, RNA Editing and Ribozymes)). (iv) In Drosophila, histone pseudogenes have been discovered, (v) There are other sequences in human genome- which are popularly called Alu family of dispersed repetitive elements, of which there are about 300,000 copies in a haploid human genome. Each sequence is 300 bases long and they are capable of being transcribed by RNA polymerase III (meant for 5SRNA and tRNA), rather than RNA polymerase II (meant for mRNA).

There is a number of characteristic features of these pseudogenes which throw light on their possible origin. These characteristics include the following : (i) Most pseudogenes outnumber their genes and are therefore repetitive sequences, (ii) Most pseudogenes are flanked by short direct repeats, 6-21 bases long, suggesting their subsequent insertion. Such pseudogenes are believed to have originated by one of the following methods. First possibility is that the mRNA or snRNA itself becomes incorporated into DNA or into retrovirus RNA, followed by the incorporation of retrovirus; Second possibility is that reverse transcript of mRNA or snRNA was synthesized as cDNA, which subsequently became integrated. It is this second alternative which is considered to be the more probable mechanism of the origin of pseudogenes. In this hypothesis RNA takes up a configuration favourable for synthesis of DNA at its 3' end copying a part of this RNA as a template to form cDNA. The insertion of this cDNA due to staggered cleavage of genomic DNA at a specific site is shown in Figure 29.14.
 
A model for origin of pseudogenes, in which cDNA (complementary DNA) is synthesized on snRNA (small nuclear RNA) and is inserted in DNA, which undergoes staggered cleavage through restriction endonuclease activity.
Fig. 29.14. A model for origin of pseudogenes, in which cDNA (complementary DNA) is synthesized on snRNA (small nuclear RNA) and is inserted in DNA, which undergoes staggered cleavage through restriction endonuclease activity.
 
     






     
     
 
 
     
 
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