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Chromosome

Fig. 4.19 The size of the 16 yeast chromosomes of the sequenced S. cerevisiae.

important role in defining the genetic map of S. cerevisiae. This physical approach has complemented and extended the traditional techniques of genetic mapping. A detailed analysis of chromosomal composition can be found in Goffeau et al. (1996) and as updated on the MIPS web site. Options here (Heumann et al., 1996) range from a 'top-down' overview of a chromosome to the detailed chromosome sequence and, as noted previously, use of the MIPS genome browser to visualise common sequences on chromosomes. Table 4.16 presents details of chromosome size, number of ORFs, hypothetical proteins and Ty sequences. Data on Ty elements is included as these sequences can be probed for in DNA fingerprinting (Section 4.2.6.1). Ty elements can move from one site in a chromosome to another or move between chromosomes. These transposable elements can trigger genetic rearrangements (Hammond, 1996).

There is a wealth of information to be gleaned from the overview of Mewes et al. (1997). As noted previously, the location of the 5800 proteins is reported systematically and diagrammatically, chromosome by chromosome. Groups of genes that encode proteins with a common function (e.g. amino acid metabolism) are further sub-divided (regulation, transport, degradation and so on) (see Fig. 4.17 ). Although undeniably a remarkable and awe inspiring document, the 'gazetteer' of Mewes et al. (1997) is fascinating but not particularly user-friendly. For the genetic 'tourist', the gazetteer is a milestone document of a milestone project and, as such, will meet their need. However, the professional geneticist will access the interactive web sites of MIPS or the SGD where chromosomes and genes can be browsed and searched (these and other sites are listed in Brown, 1998). As noted above, these extraordinary web sites allow the yeast genome to be searched interactively. For example, searching the entire genome via MIPS for references to the flocculation gene - FLO - delivers a 28 page report detailing 31 ORFs. The level of detail together with the capability to drill down to further strata of complexity is awesome. An edited list of some of the information that can be gleaned from such a search is found in Table 4.17. In addition to the FLO genes, other ORFs whose disruption affects flocculation are reported.

4.3.2.5 Ploidy. The number of'sets' of chromosomes is described as the 'ploidy'. Laboratory strains of S. cerevisiae are usually haploid or diploid. Haploid strains of S. cerevisiae have one set («), diploid strains have two sets (2«), and triploid strains have three sets (3«) and so on. It is generally accepted that 'domesticated' yeasts are polyploid, frequently triploid or tetraploid (4«) (Hammond, 1996). To add a little complication, the number of copies is not necessarily a perfect multiple of the haploid number. This 'aneuploidy' allows for extra or indeed reduced copy numbers of individual chromosomes. Consequently, as noted by Wickner (1991) 'an otherwise diploid strain carrying three copies of chromosome III is trisomic for chromosome III'. Similarly, where a diploid strain lacks a one of a pair of chromosomes it is monosomic in that chromosome. The extent of aneuploidy in domesticated yeasts is revealing. In marked contrast to the above assumptions of tetraploidy, a report by Codon et al. (1998) showed the ploidy of 16 industrial strains to vary widely (wine 1.9«, brewing 1.6-2.2«, distilling 1.3-1.6« and baking 1.3-3«).

Naively, it might be assumed that in polyploid or aneuploid strains, each copy of each chromosome is identical. The reality is somewhat different. The advent of

Table 4.16 Distribution of genes and Ty elements in the sequenced yeast genome (from the MIPS web site: http://www.mips.biochem.mpg.de/mips/yeast/MIPS).

Chromosomal number

Table 4.16 Distribution of genes and Ty elements in the sequenced yeast genome (from the MIPS web site: http://www.mips.biochem.mpg.de/mips/yeast/MIPS).

Chromosomal number

Elements

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