Nature of the Task

The genome size of an organism, commonly known as its C value, is defined as the content of DNA (measured by weight or numbers of basepairs) in a single copy of the sequence of DNA found within the cells of an organism (i.e., the amount of DNA in a haploid chromosome set, where 1n = 1C) (1). A single sperm of Drosophila melanogaster contains 0.18 pg DNA, which is the C value for this species (2-4). By convention, the DNA content of a diploid somatic cell of this species can then be expressed as 2n = 8 = 2C = 0.36 pg DNA for the male diploid genotype of AAXY (3). And, 2C = 0.40 pg DNA for a diploid Drosophila female somatic cell with the genotype of AAXX because the X chromosome contains a bit more DNA than the Y chromosome in this species (see Table 1). These considerations follow from the "DNA Constancy Hypothesis" formulated in the 1950s (5-8) that the DNA content of nuclei of a eukary-otic species is essentially constant among the individuals within a given species and constitutes the repository of genetic information for that taxon. For example, genome sizes, expressed as C values, are listed in Table 1 for 10 different species of Drosophila.

Although there is relatively little variation in genome size among prokary-otes (1), the genome sizes of eukaryotes vary widely among animals and may vary by several thousand-fold among plants (9-12). The underlying mechanisms that direct these large differences in fundamental genome size for plant and animal species are of increasing interest to students of evolutionary biology, raising questions about the equivalency of genome size, its net genetic information content, and the minimum amount of DNA required for life forms

From: Methods in Molecular Biology, vol. 247: Drosophila Cytogenetics Protocols Edited by: D. S. Henderson © Humana Press Inc., Totowa, NJ

Table 1

Estimates of Genome Size for 10 Different Species of Drosophila Expressed as C Values in pg DNA per Nucleus" and Number of Nucleotide Basepairs (np)

Species

C Value = DNA pg per nucleus

np x 108

D. americana

0.30

2.74

D. arizonensis

0.22

2.01

D. eohydei

0.24

2.19

D. funebris

0.23

2.10

D. hydei

0.20

1.83

D. melanogaster (&)

0.18

1.64

D. melanogaster ($)

0.20

1.82

D. miranda

0.30

2.74

D. neohydei

0.19

1.73

D. simulans

0.12

1.10

D. virilis

0.34

2.89

Chickenb

1.25

11.41

Xenopus laevisb

3.15

28.76

"Computed by assuming 9.13 x 108 nucleotide pairs per picogram of DNA (9).

bC values are shown for red blood cell (RBC) nuclei of two reference standards.

Source: Adapted from T. R. Gregory, www.genomesize.com/insects. For primary citations, see database at this website.

"Computed by assuming 9.13 x 108 nucleotide pairs per picogram of DNA (9).

bC values are shown for red blood cell (RBC) nuclei of two reference standards.

Source: Adapted from T. R. Gregory, www.genomesize.com/insects. For primary citations, see database at this website.

(1,10,13). Also, what is "junque" DNA? And, how do different chromatin domains regulate and modulate gene expression in eukaryotic genomes? (see refs. 1,10,14,15). Because many plant and animal species with relatively small genomes serve as model systems for study by geneticists and developmental biologists, reliable estimates of genome size can provide useful data for these and other current fields of research (12,16).

Of particular interest are those species with strict regulation of total cell number (eutely) that very often display cells and tissues whose nuclei have undergone repeated cycles of DNA endoreduplication without intervening cycles of nuclear and cytoplasmic division (i.e., mitosis), to result in polyp-loidy (increase in the number of copies of the diploid chromosome sets) or polyteny (same number of chromosomes, but each chromosome becomes multistranded)—both manifest in giant somatic cell nuclei that contain very large amounts of DNA (17-29).

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