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19.25 Genomic characteristics of the mustard plant, Arabidopsis thaliana. (a) Number of chromosomes, genome size, number of genes, and G + C content. (b) Percentages of genes affecting various known and unknown functions.

(2) duplication of individual genes arrayed in tandem through unequal crossing over.

Transposable elements are common in the Arabidopsis genome and make up about 10% of the genome but are much less frequent than in the human genome and in some other plant genomes. Most of these transposable elements are not transcribed, and many are concentrated in the regions surrounding the centromere.

Although Arabidopsis, C. elegans, and Drosophila have similar numbers of proteins, the Arabidopsis genome has more genes. This difference can be explained by the large number of duplicated copies of genes found in the Arabidopsis genome.

Fly genome Drosophila melanogaster, the fruit fly, has a genome of 180 million base pairs of DNA located on four chromosomes (IFigure 19.26). A third of its genome is made up of heterochromatin, which contains few genes. This extensive heterochromatin, consisting mainly of short simple repeats, made sequencing the genome of Drosophila difficult (because the repeats lead to much overlap in

Drosophila melanogaster (fruit fly)

Four pairs of linear chromosomes Genome size: 180 million bp Number of genes: 13,338 G + C content: 41%

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