Estimating the Number of Expressed Genes
Estimating the Number of Expressed GenesReassociation kinetics can also estimate the number and abundance of expressed genes. These experiments are performed using high concentrations of RNA and tracer levels of either DNA or cDNA. These experiments are analagous to DNA reassociation experiments except the units are expressed as moles of ribonucleotides per liter per second and the value is called a Rot.
If tracer amounts of DNA are allowed to reanneal only a portion of that DNA will form a duplex because not all the DNA will be expressed in the RNA population to which it is being hybridized. Let's use the example in the Genes V (p.672) to derive an estimate for the number of genes that are being expressed. In this experiment 1.35% of the DNA hybridized to the RNA. Since RNA is single stranded the other anti-sense strand of the DNA would not have a partner with which it could hybridize. Thus actually 2.7% of the DNA is represented in this RNA population. If the genome size is 8.1 X 108 bp and the single-copy sequences represent 75% of the genome then we can estimate the complexity of the expressed DNA.
0.027 x 0.75 x ( 8.1 X 108 bp ) = 1.7 X 107 bp If each gene is about 2000 base pair then the number of genes that is expressed is:
7 X 107 bp / 2000 = 8500 genes
Copyright © 1998. Phillip McClean