Encyclopedia of Evolution.pdf - Online Reading Center

DNA is an extremely long molecule that consists of two
strands that form a double helix (spiral). Each strand
contains sugars, phosphates, and nitrogenous bases.
Within each strand, sugars ( ) and phosphates ( ) form a
backbone. Nitrogenous bases A, C, T, and G meet in the
middle of the molecule between the two strands and hold
them together by weak bonds. A is always opposite T, and
C is always opposite G. This illustration does not show the
arrangements of atoms.
DNA (raw material of evolution)
molecule can become two, two can become four, and so on.
This occurs only when the appropriate enzymes and raw
materials are present to control the process.
Cells use DNA information in three ways:
• A single cell, such as a fertilized egg cell, can develop into
a multicellular organism. Most of the 70 trillion cells in a
human body contain nearly an exact copy of the DNA that
was in the fertilized egg from which the person developed.
• During the daily operation of each cell, the genetic instructions
in the DNA determine which proteins are made, and
what the cell does.
• DNA can be passed on from one generation to the next.
Usually this involves separating the information into two
sets and then recombining them in sexual reproduction
(see meiosis; sex, evolution of). DNA is a potentially
immortal molecule: Each person’s DNA molecules are the
descendants of an unbroken line of replication going back
to the last universal common ancestor of all life on Earth
(see origin of life).
In eukaryotic cells, DNA is organized into chromosomes
(see eukaryotes, evolution of). The evolutionary origin of
chromosomes has not been explained. One hypothesis, proposed
by John Maynard Smith (see Maynard Smith, John),
is that within a chromosome all of the DNA must replicate at
the same time. This system prevents some segments of DNA
from replicating more often than others, and prevents any of
them from getting lost. If each segment of DNA replicated on
its own, some of them might replicate faster than others and
lead to the death of the cell (see selfish genetic elements).
A nucleus can simultaneously replicate all of its chromosomes,
of which most nuclei have fewer than 50, more easily
than it could simultaneously replicate many separate DNA
segments.
Two important facts result from the fact that genetic
information is passed on by DNA:
• Acquired characteristics cannot be inherited. Lamarck, and
Darwin, were wrong about this. Although various regulatory
molecules can pass from one generation to another
through egg cells, the only characteristics that are transmitted
from one generation to the next are the ones coded by
DNA. One 20th-century fundamentalist preacher said, “If
evolution is true, why are Hebrew babies, after thousands
of years of circumcision, still born uncircumcised?” It was
not only evolution but DNA that this preacher misunderstood.
His question, though ridiculous to modern scientists,
might have bothered Lamarck.
• Traits that seem to have disappeared can reappear in a later
generation. Traits do not blend together like different colors
of paint, as scientists once believed; instead, the traits
are discrete units. A trait may be hidden by other traits,
but its DNA is still there and can reappear under the right
conditions. This was also a problem with which Darwin
wrestled, and which has been solved by an understanding
of DNA and of genetics.
DNA encodes genetic information, allowing it to be
passed on from one generation to the next in almost perfect
form, but with just enough imperfection to allow mutations,