The following points highlight the three ways by which sequencing of genes in isolation from rest of human genome takes place. The ways are: 1. Complementary DNA (cDNA) Sequencing 2. Using CpG Island of Locate Genes 3. DNA Sequencing for Wheat Genome.
Way # 1. Complementary DNA (cDNA) Sequencing:
cDNA is made by converting the mRNA content of cell is DNA. As the mRNA derives only from the genes, a cDNA library contains copies just of genes, without extra-genic DNA and without introns.
Way # 2. Using CpG Island of Locate Genes:
A useful feature of the human genome is that a short stretch of GC-rich sequence called a CpG island is often located in a region immediately upstream of a gene, but rarely elsewhere.
Locating a CpG island in cloned human DNA is therefore a quick way of finding a gene. TIGR has pioneered the ”genes-only” approach to the human genome and has obtained sequences of several hundred thousand cDNAs.
They call these sequences expressed sequence tags (ESTs), which indicates that many of them are not complete gene sequences but merely short segments of the expressed region of a gene.
Although they are incomplete, the sequence information they contain is enough for those derived from a gene of interest to be identified, especially if the amino acid sequence of the gene product is known. They can therefore be used to probe a genomic library to isolate clones containing desired genes, providing rapid access to the complete sequences of those genes.
Way # 3. DNA Sequencing for Wheat Genome:
Wheat is most important food plant in the human diet, being responsible for approximately 20% of the human calorific intake. Wheat is, therefore, one of the plants that geneticists most AC G T wish to study and possibly manipulate in the quest for improved crops.
Unfortunately the wheat genome is huge 17,000,000 kb, six times larger than even the human genome and it is difficult to imagine that its complete sequence will be available in the foreseenable future. An alternative means of understanding the genes of wheat has therefore been devised.
Wheat and other cereals such as rice, are members of Gramineae family, a large and diverse family of grasses. The rice genome is only 400000 kb and there are probably other grasses with even smaller genomes. Is it possible that the gene orders are conserved in these various species?
The answer is yes; since comparative mapping of the rice and wheat genomes reveal many similarities. The possibility, therefore, exists that genes from wheat genome might be isolated by first mapping the positions of the equivalent genes in a smaller model genome such as that of rice.