when 100 individuals were tested by a DNA microarray analysis that detects the copy number of DNA sequences throughout the entire length of this chromosome, the indicated distributions of DNA additions (green bars) and DNA losses (red bars) were observed compared with an arbitrary human reference sequence. The shortest red and green bars represent a single occurrence among the 200 chromosomes examined, whereas the longer bars indicate that the addition or loss was correspondingly more frequent. The results show preferred regions where the variations occur, and these tend to be in or near regions that already contain blocks of segmental duplications. many of the changes include known genes. (Adapted from J.l. Freeman et al., Genome Res. 16:949–961, 2006. with permission from Cold Spring Harbor laboratory Press.) effects on human health, physiology, behavior, and physique. A major challenge in human genetics is to recognize those relatively few variations that are functionally important against a large background of variation that is neutral and of no consequence.
Comparisons of the nucleotide sequences of present-day genomes have revolutionized our understanding of gene and genome evolution. Because of the extremely high fidelity of DNA replication and DNA repair processes, random errors in maintaining the nucleotide sequences in genomes occur so rarely that only about one nucleotide in a thousand is altered in every million years in any particular eukaryotic line of descent. Not surprisingly, therefore, a comparison of human and chimpanzee chromosomes—which are separated by about 6 million years of evolution— reveals very few changes. Not only are our genes essentially the same, but their order on each chromosome is almost identical. Although a substantial number of segmental duplications and segmental deletions have occurred in the past 6 million years, even the positions of the transposable elements that make up a major portion of our noncoding DNA are mostly unchanged.
When one compares the genomes of two more distantly related organisms—such as a human and a mouse, separated by about 80 million years—one finds many more changes. Now the effects of natural selection can be clearly seen: through purifying selection, essential nucleotide sequences—both in regulatory regions and in coding sequences (exons)—have been highly conserved. In contrast, nonessential sequences (for example, much of the DNA in introns) have been altered to such an extent that one can no longer see any family resemblance.
Because of purifying selection, the comparison of the genome sequences of multiple related species is an especially powerful way to find DNA sequences with important functions. Although about 5% of the human genome has been conserved as a result of purifying selection, the function of the majority of this DNA (tens of thousands of multispecies conserved sequences) remains mysterious. Future experiments characterizing its functions should teach us many new lessons about vertebrate biology.
Other sequence comparisons show that a great deal of the genetic complexity of present-day organisms is due to the expansion of ancient gene families. DNA duplication followed by sequence divergence has clearly been a major source of genetic novelty during evolution. On a more recent time scale, the genomes of any two humans will differ from each other both because of nucleotide substitutions (single-nucleotide polymorphisms, or SNPs) and because of inherited DNA gains and DNA losses that cause copy number variations (CNVs). Understanding the effects of these differences will improve both medicine and our understanding of human biology.
Which statements are true? explain why or why not.
How many different types of chromatin structure are important for cells? How is each of these structures established and maintained, and which ones tend to be inherited following DNA replication?
why are there so many different chromatin remodeling complexes in cells? what are their essential roles, and how do they get loaded onto chromatin at specific places and at specific times?
How do chromosomal loops form during interphase, and what happens to these loops in condensed mitotic chromosomes?
what genetic changes made us uniquely human? what further aspects of our recent evolutionary development can be reconstructed by sequencing DNA from remains of ancient hominids?
How much of the enormous complexity that we find in cell biology is unnecessary, having evolved by random drift?
4–1 Human females have 23 different chromosomes, served DNA sequences facilitates the search for function-whereas human males have 24. ally important regions.