Sizes of genomes and cloning vectors

The sizes of genomes of different organisms
vary considerably. In general, genome size reflects
the complexity of the organism. A mammalian
genome (human and mouse are known
best) contains 3!109 base pairs (bp) or 3000
Mb. If each nucleotide pairwere represented by
a 1-mm-wide letter, the text would be more
than 3000km long or take up more than ten
sets of the Encyclopaedia Britannica or 750
megabytes of computer capacity. Thus, finding
all genes, mapping their position, and determining
their structure and function is an
enormous task (see Human Genome Project).
By comparison, the genome of important model
organisms such as Drosophila, the nematode C.
elegans, yeast, and bacteria are much smaller.
The genomes of some important plants such as
maize, rice, and wheat are even larger (5000–
17000 Mb) than mammalian genomes.
Since the size of DNA fragments that can be isolated
and multiplied in cloning vectors for
analysis is relatively small, a huge cloning
capacity is necessary for analysis of a large
genome. Yeast artificial chromosomes (YAC)
can accommodate about 1.4Mb, bacterial artifical
chromosomes (BAC) about 0.5 Mb, whereas
bacteriophages and cosmids