Encyclopedia of Evolution.pdf - Online Reading Center

DNA (raw material of evolution)
The Genetic Code
First Third
base Second base base
A G T C
AAA phe AGA ser ATA tyr ACA cys A
A AAG phe AGG ser ATG tyr ACG cys G
AAT leu AGT ser ATT stop ACT stop T
AAC leu AGC ser ATC stop ACC trp C
G GAA leu GGA pro GTA his GCA arg A
GAG leu GGG pro GTG his GCG arg G
GAT leu GGT pro GTT gln GCT arg T
GAC leu GGC pro GTC gln GCC arg C
T TAA ile TGA thr TTA asn TCA ser A
TAG ile TGG thr TTG asn TCG ser G
TAT ile TGT thr TTT lys TCT arg T
TAC met* TGC thr TTC lys TCC arg C
C CAA val CGA ala CTA asp CCA gly A
CAG val CGG ala CTG asp CCG gly G
CAT val CGT ala CTT glu CCT gly T
CAC val CGC ala CTC glu CCC gly C
There are 64 codons. The three-letter abbreviations are for the 20 kinds
of amino acids found in cells. Three codons cause translation to stop.
*The met (methionine) codon also marks the place where translation
begins.
the raw material of the genetic variability of populations, and
of natural selection.
How Genes Determine Proteins
DNA stores information as a four-letter alphabet (A, C, T,
G) forming three-letter words called codons. Each codon of
three nucleotides specifies one amino acid. Proteins are large
molecules made up of smaller amino acids; therefore 3,000
DNA nucleotide pairs specify the order of amino acids in a
protein that is made of 1,000 amino acids. The DNA that
specifies the structure of one protein or group of related proteins
is called a gene. The proteins do all of the work of the
cell. A human is different from a snail largely because many
of their proteins differ. An organism’s characteristics result
from the work of its proteins; and its proteins are specified
by its DNA.
DNA remains in the nucleus of the cell. Proteins are
manufactured out in the cytoplasm of the cell. Enzymes
copy or transcribe genetic information from the DNA into
messenger RNA; it is the messenger RNA that travels from
the nucleus out to the cytoplasm. Each gene has a group of
nucleotides (see promoter) which identifies it and indicates
where the gene begins. Different groups of genes have different
kinds of promoters. A cell transcribes only the genes that
have promoters that are appropriate for that cell’s functions.
Once the messenger RNA molecule arrives in the cytoplasm,
structures called ribosomes produce proteins whose
amino acid sequence matches the RNA nucleotide sequence.
Small molecules called transfer RNA attach to amino acids
and bring them to the ribosomes. Each transfer RNA molecule
recognizes and attaches only to its particular kind of
amino acid. Each transfer RNA molecule recognizes only
particular codons on the messenger RNA molecule. This
is how the transfer RNA molecule brings the appropriate
amino acid to the right position in the growing protein molecule.
The correspondence between the nucleic acid codons
and the amino acids is called the genetic code. Nearly all
cells use exactly the same genetic code (see table at left).
Mitochondria, and some ciliates, have a slightly different
genetic code. Evolutionary scientists take this as evidence
that the genetic code was established in the common ancestor
of all life-forms now on the Earth. The genetic code
seems not to be an arbitrary coupling of codons and amino
acids. A computer simulation was used to randomly link up
codons and amino acids and produced over a million alternate
genetic codes. The simulation indicated that the genetic
code actually used by cells was one of the most efficient possible
codes. This suggests the possibility that the common
ancestor of all cells was itself the product of a long period of
evolution, during which less efficient genetic codes were tried
and eliminated by natural selection.
DNA stores information digitally, just like a computer.
A computer uses the bits 0 and 1, while DNA uses the four
bases A, C, T, and G. A computer has bits organized into
bytes, which specify letters, just as bases are organized into
codons that specify amino acids. Bytes make up words, just
as codons make up genes.
The processes of transcription and translation are more
complex than here described. In particular, cell components
can transcribe and translate different portions of the DNA,
then modify the resulting protein, so that one gene can
encode several different proteins.
How Genes Determine Characteristics of Organisms
The transcription and translation of genes produces proteins,
which form many structures and do nearly all the work in the
cell. No cell transcribes or translates all of its genes. It transcribes
and translates only the genes for which the promoter
site is open, and which have not been chemically altered:
• In some cases, inhibitor molecules can block a promoter
site. The inhibitor molecule may consist partly of the end
product of the series of reactions that the gene begins.
When the end product is abundant, the end product itself
helps to block the promoter. When the end product is
scarce, the promoter is open. This process helps to keep the
amount of the gene product more or less constant in the
cell. Usually, the interactions of control molecules, most
of them proteins, is very complex, especially in eukaryotic
cells.
• In some cases, the genes can be altered by a process called
methylation. The nucleic acid sequence of the methylated
gene is intact, but the gene cannot be transcribed. In
some cases an entire chromosome can be inactivated, as
with one of the two X chromosomes in female mammals.