Lec36n - Lecture 36 Genomes Campbell Chapter 19 6th Ed...

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Lecture 36 Genomes Campbell; Chapter 19. 6 th Ed.; 356-361 7 th Ed.; 374-379, 399 8 th Ed.: 432-441 We are now in the era of the genome. Scientists have been conducting genome projects (meaning that the nucleotide sequence of an entire genome is being determined) on several species. In addition to the human genome project, the genomes of important model organisms, including mouse, Drosophila (fruit fly), yeasts, and E. coli have been determined in the last few years. Genomes of important crops (like rice) and pathogens, including bacteria and protozoa, are also on the list. By learning more about our genes and their protein products, we can devise new strategies for fighting diseases that result from defective genes. Genomes of crops are being sequenced with the goal of developing improved strains, while pathogen genomes will hopefully aid in developing improved therapies for the diseases caused by these organisms. Gene numbers and genome sizes The following table lists the numbers of genes, and also the size (in million base pairs or Mbp) of the genomes of several species. Species # of genes Genome size (Mbp) E. coli 4,400 4.6 Baker’s yeast 6,200 13 Drosophila 13,700 180 C. elegans (worm) 20,000 100 Arabidopsis (plant) 25,500 118 Mouse 20,000 2,600 Human 20,500 3,200 This table illustrates the maximum gene number generally found in prokaryotes (i.e., 4,400 genes in E. coli ) and also the range of gene numbers found in eukaryotes (i.e., from around 6,000 genes in unicellular organisms to around 20,000-25,000 in higher eukaryotes.) Several surprising points emerge from this table. First, even though Baker’s yeast (a eukaryote) seems much more complex than E. coli , since it has organelles like our own, it doesn’t have that many more genes. Second, very complex organisms (like us) have only 7 times as many genes as E. coli. Third, plants, which may seem at first glance to be much simpler than animals, have about the same number of genes. A final important point is the lack of correlation between gene number and genome size in different organisms. Comparing E. coli and humans illustrates this. We may have only 7 times more genes than E. coli , but our genome is 5,800 times as large! Simpler multi-celled eukaryotes ( Drosophila and C. elegans ) lie between E. coli and mammals in their gene number:genome size ratio. Explanation for the wide variation in gene number:genome size ratio Some organisms have a lot of DNA that doesn’t code for RNA or protein, while others have very little. At one extreme, more than 90% of the E. coli genome codes for RNA and protein. Much of the remaining DNA is made of promoter sequences and regulatory sequences like operators. Humans are at the other extreme: only 1.5% of our DNA codes for rRNA, tRNA , or protein! Most of our genome consists of “junk DNA” with no known function. We can break the human genome into the following categories: Type of DNA in Human Genome Fraction of Genome
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Exons/coding sequences 1.5% Introns & regulatory sequences 24% Unique non-coding DNA 15% Repetitive DNA 59% Intron size and number can vary greatly between species. For instance, the puffer fish, Fugu , is unusual in having very small introns.
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