6.5.5 Composite Maps for Genomes
It is common to show schematically a composite map for particular regions of the human genome (e.g.,
Figure 6.16), which combines the various maps described above. The seven maps in the figure include a
genetic map (at the top), a physical map representing the gene density predicted from the complete
genome sequence, a BAC contig map, three molecular marker maps and a cDNA map. Each of these can
be linked to each other. For example, the exact location of an SSR marker relative to a cDNA clone on the
genomic sequence can be determined by a database search. Additionally, every molecular marker can be
mapped genetically and placed on the genetic map. Eventually, every gene can be assigned to a recognised
cDNA. This is a crucial requirement, since often-genetic traits give little or no clue to the gene responsible
for them. For example, a predisposition to lung cancer could derive from a multiplicity of genes.
6.6 Copying DNA
Since DNA is the source of genetic information from which every living organism is constructed, it must
be copied faithfully and transmitted into the daughter cells to preserve the genetic integrity of the lineage.
DNA must also be transcribed faithfully into its corresponding RNA products to construct and maintain
the function of the organism.
6.6.1 A Comparison of Transcription with DNA Replication
Both DNA replicationand transcriptionare copying processes and both occur in the nucleus of cells from
the same DNA template, often at the same time. But they are fundamentally different in their mechanisms. In
transcriptiononly a small subset of the genetic information needs to be transcribed into RNA namely just
those genes whose gene products, proteins or structural RNAs, are needed at a particular stage of cell life.
Differentgenes need to be expressed at different levels. One gene’s RNA may perhaps be present as a sin-
gle copy in the cytoplasm of a particular cell, a second gene may be expressed as thousands of RNAs and
a third gene may be totally switched off. Thus, transcription needs to be extremely versatile, to cater for
huge differences in expression profiles of thousands of genes, in hundreds of cell types, as well as the need
Genes and Genomes 223
Figure 6.16 Composite maps for genomics. A genetic map (top) is aligned against a sequence-derived gene density
map, a BAC contig physical map and four physical maps (bottom). Positions of molecular markers and
cDNAs on the maps are shown as vertical lines