Evolution__3rd_Edition

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Figure 2.3
The transfer of information in
a cell.
Much of the DNA does not code for
genes
Chromosome Gene Messenger
RNA
CHAPTER 2 / Molecular and Mendelian Genetics 27
Protein chain
Transfer RNA
2.4 Large amounts of non-coding DNA exist in
some species
Translation Protein
Messenger
RNA
Ribosome
The human genome is about 3,000 million (3 × 10 9 ) nucleotides long. The human
genome project made a preliminary estimate of the number of genes in a human being
as about 30,000 (3 × 10 4 ). The average length of a human gene is about 5,000 (5 × 10 3 )
nucleotides. Thus only about 5% (1.5 × 10 8 /3 × 10 9 ) of human DNA codes for genes.
Even if the preliminary figure of 30,000 genes turns out to underestimate the true figure
by a factor of two, still only 10% of our DNA would code for genes. Most human DNA
is not used to code for proteins, or for molecules that control the production of
proteins. Most human DNA is “non-coding” DNA.
The fraction of non-coding DNA varies between species. Bacteria and viruses contain
little non-coding DNA; bacterial and viral genomes are economically organized. At
the other extreme, some salamanders contain 20 times as much DNA as humans do.
Because it is difficult to believe that salamanders contain many more genes than we do,
we can infer that more than 99% of those salamanders’ DNA is non-coding.
The function of non-coding DNA is uncertain. Some biologists argue that it has no
function and refer to it as “junk DNA.” Others argue that it has structural or regulatory
functions. Something is known about the sequence of non-coding DNA. Most noncoding
DNA is repetitive. Some of it consists of side-by-side (or “tandem”) stretches of
repeats of a short (2–20 nucleotide) unit sequence (for example ...ACCACCACC...).
Some of it consists of repeats of longer (a hundred, or a few hundred nucleotides)
sequences. We can partly understand how non-coding DNA originates after we have
considered our next topic: mutation.
2.5 Mutational errors may occur during DNA replication
When a cell reproduces, its DNA and genes are physically replicated. Normally an exact
copy of the parental DNA is produced, but sometimes a copying error happens. The set
of enzymes that replicate the DNA include proof-reading and repair enzymes. These
enzymes detect and correct most of the copying errors, but some errors persist even
after proof-reading and repair. These errors are called mutations. The new sequence of