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Organisms most frequently copy their Instruction Books by asexual
reproduction. Cells reproducing
asexually divide in two, and each of the new cells gets a copy of the
original cell's Instruction Book. The old cell is called the
parent cell and the two new ones are called daughter
cells. Prokaryotes reproduce
asexually, as do some eukaryotes.
Multicellular eukaryotes grow by
mitosis, a kind of
asexual division.
The essential details of how living things asexually copy their
Instruction Books are the same, despite differences between
prokaryotes, unicellular eukaryotes and multicellular eukaryotes. In
all cases, copying the Instruction Book begins with a worker called a helicase
binding to a special note in the Book and unwinding a short stretch of
its DNA. A complex of eight large
workers then unzips the DNA's double strands, reads the bases on each
strand, and builds a new strand to go with each of the old ones.
The bases that make up each new strand complement the bases in each
old strand, so the new DNA double helices are identical to the
original one. This method of copying the Instruction Book gives each
daughter cell an Instruction Book that consists of one strand of the
parent's DNA and one strand of new DNA.
Actually, the new Instruction Books aren't perfectly identical to to
the parental ones. The workers that copy Instruction Books try very
hard to make perfect copies of them, but once in a long while they
make a mistake. Approximately once every ten thousand bases, the
workers insert the wrong base into a new DNA strand. Since the
Instruction Books for humans has around six billion bases, this would
introduce six hundred thousand mistakes into subsequent copies.
However, repair workers -- working both at the time the Book is
copied and later -- catch almost all of these changes, leaving only one in
a billion bases changed. For humans, this leaves only a handful of
changes overall, usually fewer than ten. These changes (called
point mutations, because they affect DNA at a single spot, or
point) produce changes in the workers made by the Instructions where
they occur. Their significance depends on the genetic code and where
in the Instruction they fall.
The genetic code's
construction prevents many point mutations from changing the
Instruction they occur in. For example, changing the codon CTG to CTA still adds the
same link (L) to the worker made from this Instruction. This kind of
mutation -- one that doesn't even change the link -- is called a
silent mutation. Many other sorts of point mutations change
the link added to the worker, but replace it with one that has similar
properties. Changing CTG to TTG replaces L with I, which is conservative
replacement.
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However, there are some kinds of point
mutations that do change the worker in some way. If these changes
occur in the part of the Instruction that codes for scaffolding, the worker
made by the mutated Instruction will likely have slightly different specificity than
the one made by the original Instruction. Point mutations that change
a worker's active
sites produce greater effects. They may produce large changes in
the worker's specificity. They may change the job the worker does. They may simply
prevent the worker from doing any job at all, although this is rare.
The three alleles for
alcohol dehydrogenase I differ
from each other by single point mutations. These point mutations
change only how fast the workers do their jobs. Alcohol
dehydrogenase IV is about 80 point mutations away from any alcohol
dehydrogenase I; it's much faster than they are, and it works on slightly
different alcohols. Further along the spectrum, alcohol
dehydrogenase II differs from any of the alcohol dehydrogenase I's
by about 160 point mutations, and has a very different specificity.
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Al and alcohol dehydrogenase IV
Alcohol dehydrogenase III differs from I about as much as II does. The
numbers just indicate order of discovery, not similarity.
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One particular type of point mutation is especially bad. If a codon
is mutated to one of the three stop codons, then when the
Instruction is expressed,
the ribosome making the
worker stops translating there and the worker
is truncated. If this occurs near the start of the Instruction, only
a useless fragment of the worker is made. These point mutations are
called nonsense mutations. Nonsense mutations are the cause of
some human genetic diseases. Some kinds of thalassemia (a type
of anemia) are caused by a nonsense mutation in hemoglobin, the worker
in blood that carries oxygen. People who have these diseases produce
only a stub of one of the two workers in the hemoglobin complex. As a
result, their blood doesn't carry oxygen very well.
Since point mutations can occur anywhere in the Instruction Book when
it's copied, they can also affect promoters and other notes in the
Book. Point mutations to promoters can either strengthen the
promoter or weaken it. While this doesn't change the worker produced,
it changes the amount of the worker made by the organism. If a
promoter becomes so weak that the dispatchers no longer recognize it, the
Instruction becomes a psuedogene. Point mutations in
promoters can also change which dispatchers recognize it, causing it
to be expressed under different conditions.
Note that while point mutations are always inherited by the offspring
of unicellular organisms, point mutations in multicellular organisms
are inherited only if they occur in their germ line, or the cells that
develop into it. Point mutations in somatic cells affect only
later growth of the organism, not its offspring.
A point mutation can be undone by another one at the same
spot. The same random process of making mistakes in copying Life's
Instruction Book undoes changes as well as making them. For example,
if a point mutation in a promoter produced a psuedogene, another point mutation
in that promoter during subsequent asexual reproduction may restore
the Instruction to use. This means that even though an organism is stuck
with any mutational difference it inherited from its parents, its offspring
may revert (or change back) to the older Instruction.
While asexual reproduction is adequate for reproducing simple organisms,
complex organisms have developed another method for copying their
Instruction Books: sexual reproduction.
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