Читать книгу Cell Biology - Stephen R. Bolsover - Страница 107

As genomes of more and more organisms were sequenced, the most surprising feature to emerge was just how few genes supposedly complex organisms possess (Table 3.1 on page 39). The first eukaryotic genome to be sequenced was that of the budding yeast, Saccharomyces cerevisiae, the simple unicellular fungus that we use to make bread and beer. S. cerevisiae has 6091 genes. The fruit fly, Drosophila melanogaster, a much more complex organism with a brain, nervous and digestive systems, and the ability to fly and navigate, on the other hand, has 14 133 genes, or roughly twice the number in a yeast. Even more surprising was the finding that humans have only about 19 116 protein‐coding genes. However, humans make many more than 19 116 proteins and it is these that contribute to the complexity of an organism such as ourselves. How is it possible to have so few genes and yet make 100 000s of different proteins? It is the arrangement of human genes into exons and introns (page 59) that provides the solution. Alternative splicing (page 76) allows the cell to “cut and paste” exons in different ways to produce many different mRNAs from the same gene. The most extreme case known is the human gene called SLO, which encodes a protein found in some potassium channels (page 152). This gene has 35 exons, which can produce 40 320 different combinations of exons from a single gene. Estimates are that something like 50% of human genes show alternative splicing with the pattern of splicing (the range of proteins produced) varying from tissue to tissue. Drosophila genes also show alternative splicing but those of yeast, which contain few introns, do not.

Sometimes DNA that encodes RNA is repeated as a series of copies that follow one after the other along the chromosome. Such genes are said to be tandemly repeated. The genes that code for ribosomal RNAs (about 250 copies/cell), transfer RNAs (50 copies/cell), and histone proteins (20–50 copies/cell) are tandemly repeated. The products of these genes are required in large amounts.

This still leaves about 75% of our nuclear genome that lacks a very clearly understood function. A large proportion of this extragenic DNA is made up of repetitive DNA sequences that are repeated many times in the genome. Some sequences are repeated more than a million times and are called satellite DNA. The repeating unit is usually several hundred base pairs long, and many copies are often lined up next to each other in tandem repeats. Most of the satellite DNA is found in a region called the centromere, which plays a role in the physical movement of the chromosomes that occurs at cell division (page 235), and one theory is that it has a structural function.

Our genome also contains minisatellite DNA where the tandem repeat is about 25 bp long. Minisatellite DNA stretches can be up to 20 000 bp in length and are often found near the ends of chromosomes, a region called the telomere. Microsatellite DNA has an even smaller repeat unit of about 4 bp or less. Again, the function of these repeated sequences is unknown but microsatellites, because their number varies between different individuals, have proved very useful in DNA testing (page 130). Other extragenic sequences, known as LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements) occur in our genome. There are about 50 000 copies of LINEs in a mammalian genome and they make up about 17% of the human genome.