(Micron ucJeus) 365 2,500 2,300
thousands (for example, in the macronucleus of the protozoa Tetrahymena, Table 7-1).
Circular and linear chromosomes each pose specific challenges that must be overcome for maintenance and replication of the genome. Circular chromosomes require topoisomerases to separate the daughter molecules after they are replicated. Without these enzymes, the two daughter molecules would remain interlocked, or catenated, with one another after replication. In contrast, the IJKA ends of the linear eukaryotic chromosomes have to be protected from enzymes that normally degrade DNA ends and present a different set of difficulties during DNA replication, as we shall see in Chapter 8.
Every Cell Maintains a Characteristic Number of Chromosomes
Prokaryotic cells typically have only one complete copy of their chromosome(s) that is packaged into a structure called the nucleoid (Figure 7-1 b). When prokaryotic: cells are dividing rapidly, however, portions of the chromosome in the process of replicating are present in two and sometimes even four copies. Prokaryotes also frequently carry one or more smaller independent circular DNAs, called plasmids. Unlike the larger chromosomal DNA, plasmids typically are not essential for bacterial growth. Instead, they carry genes that confer desirable traits to the bacteria, such as antibiotic resistance. Also distinct from chromosomal DNA, plasmids can be present in many complete copies per cell.
The majority of oukaryotic cells are diploid; that is, they contain two copies of each chromosome (see Figure 7-lc), The two copies of a given chromosome are called homology; one is derived from each
FIGURE 7-1 Comparison of typical prokaryotic and euka ryotic cell, (a) The diameter of a typical eukaryotic cell is ~ 10 juo. The typical prokaryotic cell is — 1 fim Jong, (b) Piokaryotic chromosomal QK'A is located in the nucleoid and occupies a substantial portion of the internal region of the ceil. Unlike the eukaryotic nudeus, the nucleoid is riot seperated from the remainder of the eel) by a membrane Plasmid DMA is shown in red (c) Eukaryotic chromosomes are located in the membrane hound nucleus. Haploid (1 copy) and diploid (2 copies) cells are distinguished by the number of copies of each chromosome present in the nucleus. (Source: Adapted from Brawn T.A. 2002. Genomes, 2nd edition, p 32, fig 2.1. © 2002 BiOS Scientific Publishers. Used by permission-www tandf. corn.)
haploid bacteria nucleoid
flagellum prokaryotic t cell
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