"Frontmatter". In: Plant Genomics and Proteomics
Download 1.13 Mb. Pdf ko'rish
|
Christopher A. Cullis - Plant Genomics and Proteomics-J. Wiley & Sons (2004)
P
HYSICAL AND G ENETIC M APS The genetic map is an ordering of the regions of the genome according to the rates of recombination that occur along each of the chromosomes, the genetic distance being the frequency with which recombination occurs between two points on the chromosome. There is not a specific relationship between the genetic distance and the physical distance between the same 5 0 3. S E Q U E N C I N G S T R AT E G I E S Isolate High molecular weight DNA Isolate DNA Partial digest with restriction enzymes or shearing Make large insert BAC libraries Fragmentation, fractionation Rack libraries for automation Clone fragments Isolate BAC DNA Sequence fragments BAC fingerprinting Build contigs Contig building Link to some type of scaffold (EST links, BAC end sequences, physical map) BAC library filter hybridizations (optional) With EST, oligo overgos etc Use of synteny to order regions MTP development Shotgun sequencing of BACs Assembly of sequence reads Build of contig sequences Crossing gaps — PCR, linking libraries Finished sequence Annotation FIGURE 3.1. Flowcharts for various sequencing approaches. two points because the recombination rate is not constant over the length of the chromosomes. A more extreme example of a lack of correspondence between genetic and physical distance is found in humans, where the genetic maps for males and females have the same linear order but do not have the same values for the distances between equivalent points because there is a difference in recombination rate in the two sexes, the rate being higher in females than in males. The density of markers on the genetic map for most organisms has increased rapidly with the use of DNA-based polymor- phisms. The genetic map is generated by the scoring of numerous poly- morphisms in populations segregating for those markers. The larger the number of individuals scored in the population, the more precise the loca- tion of any particular marker becomes and the greater confidence can be placed on the specific order of a series of closely linked markers. The physical map, on the other hand, is the linear order of the sequences linking the ends of each chromosome with overlapping regions of DNA. This is done by fragmenting the whole genome into pieces and then trying to reassemble the pieces into a single continuous molecule for each chromo- some. When complete, each chromosome would consist of a single compos- ite strand made up from a large number of smaller, overlapping fragments. In generation of the physical map, some of the regions will provide special problems. For example, regions that contain long sequences of tandem repeats, such as the genes for the large ribosomal RNAs, cannot be linked in a linear order, because the array is longer than any single subfragment and each repeat is essentially identical. However, a knowledge of the number of repeats and the number of chromosomal sites over which these repeats are spread can facilitate an estimation of the length of the region containing the repeats. Download 1.13 Mb. Do'stlaringiz bilan baham: |
Ma'lumotlar bazasi mualliflik huquqi bilan himoyalangan ©fayllar.org 2024
ma'muriyatiga murojaat qiling
ma'muriyatiga murojaat qiling