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1.5: The Nuclear Genome

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    27217
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    The C-value of the nuclear genome

    The complete set of DNA within the nucleus of any organism is called its nuclear genome and is measured as the C-value in units of either the number of base pairs or picograms of DNA. There is a general correlation between the nuclear DNA content of a genome (i.e. the C-value) and the physical size or complexity of an organism. Compare the size of E. coli and humans for example in Table \(\PageIndex{2}\).

    Table \(\PageIndex{2}\): Measures of genome size in selected organisms. The DNA content (1C) is shown in millions of basepairs (Mb). For eukaryotes, the chromosome number is the chromosomes counted in a gamete (1N) from each organism. The average gene density is the mean number of non-coding bases (in bp) between genes in the genome.

    DNA content (Mb, 1C)

    Estimated gene number

    Average gene density

    Chromosome number (1N)

    Homo sapiens

    3,200

    25,000

    100,000

    23

    Mus musculus

    2,600

    25,000

    100,000

    20

    Drosophila melanogaster

    140

    13,000

    9,000

    4

    Arabidopsis thaliana

    130

    25,000

    4,000

    5

    Caenorhabditis elegans

    100

    19,000

    5,000

    6

    Saccharomyces cerevisiae

    12

    6,000

    2,000

    16

    Escherichia coli

    5

    3,200

    1,400

    1

    There are, however, many exceptions to this generalization, such as the human genome contains only 3.2 x 109 DNA bases, while the wheat genome contains 17 x 109 DNA bases, almost 6 times as much. The Marbled Lungfish (Protopterus aethiopicus – Figure \(\PageIndex{16}\)) contains ~133 x 109 DNA bases, (~45 times as much as a human) and a fresh water amoeboid, Polychaos dubium, which has as much as 670 x 109 bases (200x a human).

    Fig1.16.png

    Fig 1.16: Marbled Lungfish. (Wikipedia-OpenCage-CC:AS)

    The C-value paradox

    This apparent paradox (called the C-value paradox) can be explained by the fact that not all nuclear DNA encodes genes – much of the DNA in larger genomes is non-gene coding. In fact, in many organisms, genes are separated from each other by long stretches of DNA that do not code for genes or any other genetic information. Much of this “non-gene” DNA consists of transposable elements of various types, which are an interesting class of self-replicating DNA elements discussed in more detail in a subsequent chapter. Other non-gene DNA includes short, highly repetitive sequences of various types.

    Other genomes

    Organelles such as mitochondria and chloroplasts also have their own genomes. These are, compared to the nuclear genome, relatively small and are also circular, like the prokaryotes from which they originated (Endosymbiont hypothesis).


    This page titled 1.5: The Nuclear Genome is shared under a CC BY-SA 3.0 license and was authored, remixed, and/or curated by Todd Nickle and Isabelle Barrette-Ng via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.