The following points highlight the top three types of multi-gene families. The types are: 1. Pseudo Genes 2. Selfish Genes 3. Split Gene – Exons and Introns.
Multi-Gene Families: Type # 1. Pseudo Genes:
Genes often exist which resemble other genes but their base sequence shows errors that make it impossible to contain biological information. These are pseudo genes which represent genes with errors in their DNA sequence during evolution leading to loss of capacity of coding protein.
Pseudo genes, are thus evolutionary relics. Several globin pseudo genes are present in the globin gene clusters. Pseudo genes are designated by the prefix Ψ (psi) followed by the gene they resemble, e.g., Ψβ (a pseudo gene of the adult p globin gene).
Multi-Gene Families: Type # 2. Selfish Genes:
Selfish genes are those genes which propagate in the cell, despite being detrimental to the organisms that carry them. The natural selection seems to favour them. The segregation distorter gene (SD) in Drosophila has been proved to cause extreme modification of expected segregation ratio of some genes.
This is because a particular class of gametes does not take part in fertilization according to expected meiotic segregation pattern.
Selfish genes also include replicative transposons, mitochondrial male sterile gene in plants, etc. Another term Selfish DNA has been applied by Orgel and Crick to the repeated sequences which exist because of their capacity for duplication in the cell. No evolutionary function has been visualized.
Multi-Gene Families: Type # 3.Split Gene – Exons and Introns:
The genes of higher organisms including plant system are more complex than that of prokaryotes. As compared to the latter, in eukaryotes the genes are made up of essential sequences – the exons, and intervening sequences, which are often considered as unessential – the introns (Fig. 14.16).
As such a single gene which is responsible for a polypeptide are made up of several exons interrupted by intervening sequences – the introns. Such genes are termed as split genes containing informative (exons) and non-informative (introns) sequences lying adjacent to each other. The size of the exons as well as that of introns varies significantly in different species.
Repeat DNAs are abundant in introns. At the time of enzyme protein synthesis by the genes, following transcription, the introns are scissored off and residual exons are joined with each other with the aid of restriction enzyme and ligases respectively.
This scissoring and joining, otherwise known as splicing, which is the characteristic of eukaryotic system and occurs during messenger processing. Therefore, the primary messenger synthesized at the gene level following transcription, undergoes processing before the processed messenger is used in translation, i.e., protein synthesis. The primary transcripts, therefore, are much larger than that of translatable transcript.
Reports of split genes have been obtained from ovalbumin genes of chicken and p globin genes of mice (Chambon et al.), ribosomal genes of Drosophila (Hogness et a!.), in adenovirus (Phillip et al.), tRNA genes of yeast and from other sources.
Chambon and his co-workers isolated mRNA of ovalbumin gene from chicken and subjected to an enzyme reverse-transcriptase that synthesizes cDNA from mRNA in the presence of radioactive nucleotides in medium. This radioactive cDNA was incubated with original nonradioactive DNA with denatured ovalbumin gene, present in one fragment.
It was noticed that radioactive DNA hybridized with the original non-radioactive DNA in 3 fragments instead of complete gene for ovalbumin. This fact showed that genes for ovalbumin lies in 3 pieces. Hybridization between the single stranded DNA having the gene for ovalbumin and its mRNA also showed formation of distinct loops at specific sites (Fig. 14.17).
In Neurospora and yeast where the introns have been studied in detail, show highly repeated sequences. Introns have recently been shown to be essential components in the recombination process. As in higher organisms, the exons form only a fraction of the entire genome and introns occupy much larger segment.
Recombination to a great extent occurs at intron sites leading to exon reshuffling. Intra-exon recombination is rather difficult. In animal system, it has already been shown that recombination within introns help in the rearrangement of exon as in lysozyme of chicken.
Lately, considerable evidences have been derived both in plants and animals to show that introns have originated very early in evolution, as often located at the same site in different genera, such as in mouse and soya-bean.
In the study of the theory of origin of land plants, the ancestry of bryophytes from algal system has been demonstrated though similarity of introns in bryophytes on the one hand and Coleochete and Chara on the other.
