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diploid cell apparatus endoplasmic reticulum mitochondria haploid cell b haploid bacteria parent. But, not all cells in a eukaryotic organism are diploid; a subset of eukaryotic cells are either haploid or polyploid. Haploid cells contain a single copy of each chromosome and are involved in sexual reproduction (for example, sperm and eggs are haploid cells). Polyploid cells have more than two copies of each chromosome. Indeed, some organisms maintain the majority of their adult cells in a polyploid state. In extreme cases there can be hundreds or even thousands of copies of each chromosome. This type of global genome amplification allows a cell to generate larger amounts of RNA and, in turn, protein. For example, megakaryocytes are specialized polyploid cells (-128 copies of each chromosome) that produce thousands of platelets which lack chromosomes but are an essential component of human blood (there are -2.00,000 platelets per milliliter of blood). By becoming polyploid, megakaryocytes can maintain the very high levels of metabolism necessary to produce large numbers of platelets. The segregation of such a large number of chromosomes is difficult, therefore polyploid r olls have almost always stopped dividing. No matter the number, eukaryotic chromosomes are always contained within a membrane-hound organelle called the nucleus (see Figure 7-lr:).

Genome Size Is Related to the Complexity of the Organism

Genome size {the length of DNA associated with one haploid complement of chromosomes) varies substantially between different organisms (Table 7-2). Because more genes are required to direct the formation of more complex organisms (at least when comparing

TABLE 7-2 Comparison oí the Gene Density m Different Organisms' Genomes

Genome size Approximate Gene density

Species (Mb) number of genes* (genes/Mb)*

PROKARYOTES {bacteria}

Mycoplasma 0.58 500 860

genltalium

Streptococcus 2 2 2.300 1,060

pneumonia

Escherichia coli 4.6 4,400 950

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