Introns and Exons
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Introns and Exons
The term genome refers to all of the alleles possessed by an organism.
While the amount of DNA for a diploid cell is constant within a species, the differences can be great between species.
Humans have 3.5 X 109; base pairs, Drosophila (the fruit fly) has 1.5 X 108, toads have 3.32 X 109, and salamanders have 8 X 1010 base pairs per haploid genome.
Much of the DNA in each cell either has no function or has a function not yet known.
Eukaryotes have only 10% of their DNA coding for proteins. Humans may have a little as 1% coding for proteins.
Viruses and prokaryotes use a great deal more of their DNA than eukaryotes. Almost half the DNA in eukaryotic cells are repeated nucleotide sequences. This may be the result of millions of years of 'mistakes'.
Protein-coding sequences are interrupted by non-coding regions.
Non-coding interruptions are known as intervening sequences or introns. Coding sequences that are expressed are exons.
Most, but not all structural eukaryote genes contain introns. Although transcribed, these introns are excised (cut out) before translation.
The number of introns varies with the particular gene, even occurring in tRNAs, rRNAs and viral genes!
Generally, the more complex and recently evolved the organism, the more numerous and larger the introns.
It has been suggested that introns promote genetic recombination (via crossing-over), thus speeding-up the evolution of new proteins.
Exons are also thought to code for different functional regions of proteins.
Gene families are made up of similar, but not identical, genes.
The globin family is one of the best-studied gene groups. Haemoglobin consists, in humans, of:
clustered about a common haem group containing the element Iron (Fe).
Human beta-globin genes are scattered over five different sites or loci on human chromosome 11.
These genes are expressed sequentially (one after the other) during development, and are similar in size with same-length introns in similar positions in each gene.
Some of the genes are inactivated copies; others are functional only during certain phases of development.
Some of the base sequences in the introns and intergenic portions of the genome contain sites that regulate gene expression.
The importance of these sites is that they allow genes to produce various forms of the protein for in different tissues or at different times in the development of the organisms.
There are many repeats of some of these isolated DNA portions, in some cases up to several million copies of the same base sequence.
The function of these is still not understood, and may possibly be redundant, which is why it is called 'junk' DNA.
One explanation may be similar to that experienced in computers when the hard drive stores masses of copies of a particular file, every time the file or program is used. These are not referred to and can be removed at intervals to increase disc space.
Another explanation may be that these are the result of the millions of years of DNA copying that has taken place, with the result that DNA polymerase simply copied too many versions.