Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 70
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2d ed. Washington, DC: American Society for Microbiology.Singer, M., and P. Berg. 1991. Genes & Genomes. Mill Valley, CA: University Science Books.Page 220Variegated kernel color in corn due to interaction between the transposable elements Ac and Ds discoveredby Barbara McClintock. These kernels contain two copies of Ds located in chromosome 9 proximal(toward the centromere) to the locus of Cl, which is reponsible for the purple anthocyanin pigment. Thehomologous chromosome carries an inactive mutant allele cl.
The element Ac is presentelsewhere in the genome. When Ac breaks chromosome 9 at the position of either Ds element, the tip ofchromosome 9 containing the dominant Cl allele is lost, and the portion of the kernel that develops fromsuch a cell is colorless. The colorless patches are large or small depending on whether the breakageoccurred early or late in development.[Courtesy of Clifford Weil and Susan Wessler. From C. F. Weil and S. R. Wessler.
1993. The Plant Cell 5:515.]Page 221Chapter 6—The Molecular Organization of ChromosomesCHAPTER OUTLINE6-1 Genome Size and Evolutionary Complexity6-2 The Supercoiling of DNATopoisomerase Enzymes6-3 The Structure of the Bacterial Chromosome6-4 The Structure of Eukaryotic ChromosomesThe Nucleosome Is the Basic Structural Unit of ChromatinNucleosome Core ParticlesThe Arrangement of Chromatin Fibers in a Chromosome6-5 Polytene Chromosomes6-6 Repetitive Nucleotide Sequences in Eukaryotic GenomesKinetics of DNA RenaturationAnalysis of Genome Size and Repetitive Sequences by Renaturation6-7 Nucleotide Sequence Composition of Eukaryotic GenomesUnique SequencesHighly Repetitive SequencesMiddle-Repetitive Sequences6-8 Transposable Elements6-9 Centromere and Telomere StructureMolecular Structure of the CentromereMolecular Structure of the TelomereChapter SummaryKey TermsReview the BasicsGuide to Problem SolvingAnalysis and ApplicationsChallenge ProblemFurther ReadingGeNETics on the webPRINCIPLES• Prokaryotes and lower eukaryotes have smaller genomes (less DNA) than higher eukaryotes.• Chromosomes consist largely of DNA combined with histone proteins; each chromosome contains a single,usually very long, DNA molecule.• Most eukaryotic genomes, in addition to the "unique" DNA sequences that make up the majority of genes, alsocontain DNA sequences that are highly repetitive or moderately repetitive.• Transposable elements are DNA sequences able to change their location within a chromosome or to movebetween chromosomes.• The centromere is a specialized DNA structure that functions as the center of chromosome movement in celldivision.• The telomere is another specialized DNA structure that serves to stabilize the chromosome tips from shorteningthrough progressive loss of DNA; the telomere is elongated by a special enzyme, telomerase.CONNECTIONSCONNECTION: Her Feeling for the OrganismBarbara McClintock 1950The origin and behavior of mutable loci in maizeCONNECTION: Telomeres—The Beginning of the EndCarol W.
Greider and Elizabeth H. Blackburn 1987The telomere terminal transferase of Tetrahymena is a ribonucleoprotein enzyme with two kinds of primerspecificityPage 222To understand genetic processes requires a knowledge of the organization of the genetic material at the level ofchromosomes. In this chapter, we will see that chromosomes are diverse in size and structural properties and thattheir DNA differs in the composition and arrangement of nucleotide sequences.
The most pronounced differencesin structure and genetic organization are between the chromosomes of eukaryotes and those of prokaryotes. Someviral chromosomes are especially noteworthy in that they consist of one single-stranded (rather than doublestranded) DNA molecule and, in a small number of viruses, of one or more molecules of RNA instead of DNA.Also, eukaryotic cells contain several chromosomes, each of which contains one intricately coiled DNA molecule,whereas prokaryotes contain a single major chromosome (and, occasionally, several copies of one or more small,usually circular, DNA molecules called plasmids).The genetic complement of a cell or virus constitutes its genome. In eukaryotes, this term is commonly used torefer to one complete haploid set of chromosomes, such as that found in a sperm or egg.6.1—Genome Size and Evolutionary ComplexityMeasurement of the nucleic acid content of the genomes of viruses, bacteria, and lower and higher eukaryotes hasled to the following generalization:Genome size increases roughly with evolutionary complexity.This generalization is based on the observations that the single nucleic acid molecule of a typical virus is smallerthan the DNA molecule in a bacterial chromosome; that unicellular eukaryotes, such as the yeasts, contain moreDNA than a typical bacterium; and that multicellular eukaryotes have the greatest amount of DNA per genome.However, among the multicellular eukaryotes, no correlation exists between evolutionary complexity and amountof DNA.
In higher eukaryotes, DNA content is not directly proportional to number of genes.A summary of genome size in a sample of organisms is shown in Table 6.1. Bacteriophage MS2 is one of thesmallest viruses; it has only four genes in a single-stranded RNA molecule containing 3569 nucleotides. SV40virus, which infects monkey and human cells, has a genetic complement of five genes in a circular double-strandedDNA molecule consisting of about 5000 nucleotide pairs. Large DNA molecules are measured in kilobase pairs(kb), or thousands of base pairs. The genome of SV40 is about 5 kb.
The more complex phages and animal viruseshave as many as 250 genes and DNA molecules ranging from 50 to 300 kb. Bacterial genomes are substantiallylarger. For example, the chromosome of E. coli contains about 4000 genes in a DNA molecule composed of about4700 kb.Although the genomes of prokaryotes are composed of DNA, their DNA is not packaged into chromosomes. Truechromosomes are found only in eukaryotes. The number of chromosomes is characteristic of the particular species,as we saw in Chapter 3.
In moving up the evolutionary scale of animals or plants, the DNA content per haploidgenome generally tends to increase, but there are many individual exceptions. The number of chromosomes showsno pattern. One of the smallest genomes in a multicellular animal is that of the nematode worm Caenorhabditiselegans, with a DNA content about 20 times that of the E. coli genome. The D. melanogaster and the humangenomes have about 40 and 700 times as much DNA, respectively, as the E.
coli genome. The genomes of someamphibians and fish are are very large—many times the size of mammalian genomes. Such large genomes aremeasured in megabase pairs (Mb), or millions of base pairs. The human genome is large, but it is by no means thelargest among animals or higher plants. At 3000 Mb, the human genome is only 67 percent the size of that in cornand only 4 percent the size of that in the salamander Amphiuma (Table 6.1).Among higher animals and plants, a large genome size does not imply a large number of genes. For example, the3000-Mb human genome is large enough to con-Page 223Table 6.1 Genome size of some representative viral, bacterial, and eukaryotic genomesGenomeApproximate length inthousands of nucleotidesFormVirusMS24Single-stranded RNASV405Circular double-stranded DNAφX174M13Circular single-stranded DNA;double-stranded replicative form5λHerpes simplexT2,T4,T6SmallpoxLinear double-stranded DNABacteriaMycoplasma hominisEscherichia coli760Circular double-stranded DNA4700EukaryotesHaploid chromosome number13,00016Caenorhabditis elegans (nematode)100,0006Arabidopsis thaliana (wall cress)100,0005Drosophila melanogaster (fruit fly)165,0004Homo sapiens (human being)3,000,00023Zea Mays (maize)4,500,00010Amphiuma sp.
(salamander)76,500,00014Saccharomyces cerevisiae (yeast)tain perhaps 106 genes; however, various lines of evidence suggest that the number of genes is no greater thanapproximately 105. Considering the number and size of proteins produced in human cells, it appears that no morethan about 4 percent of the human genome actually codes for proteins. Similarly, although closely related speciesof salamanders are thought to have about the same number of genes, their genome size can differ by 30-fold in thetotal amount of DNA. Therefore, in higher animals and plants, the actual number of genes is much less than thetheoretical maximum. The reason for the discrepancy is that in higher organisms, most of the DNA has functionsother than coding for the amino acid sequence of proteins.
(This issue is discussed further in Chapter 11).A remarkable feature of the genetic apparatus of eukaryotes is that the enormous amount of genetic materialcontained in the nucleus of each cell is precisely divided in each cell division. A haploid human genome containedin a gamete has a DNA content equivalent to a linear DNA molecule 1 meter (106 µm) in length. The largest of the23 chromosomes in the human genome contains a DNA molecule that is 82 mm (8.2 × 104 µm) long. However, atmetaphase of a mitotic division, the DNA molecule is condensed into a compact structure about 10 µm long andless than 1 µm in diameter. An analogy may be helpful in appreciating the prodigious feat of packaging that suchchromosome condensation represents.
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