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Explore a Stretch of Code
Explanations
On/off switch
Almost every cell in your body has a copy of every single
gene your body needs. But you wouldn't want the gene that
specifies for hair growth, for example, to be expressed in
your stomach. That's where on/off switches, or gene
promoters, come in. On/off switches sit upstream of the gene
in question and dictate whether, or to what extent, that
gene is turned on or off. Some switches, such as TATA boxes,
are easy to recognize. They usually have a sequence such as
TATAAA that allows certain proteins in the cell to attach to
that piece of DNA and turn on the gene. Other on/off
switches are harder to find and less well-understood.
Start codon
A codon is a group of three bases - A, T, C, or G - and
codes for a single amino acid. (The amino acids are strung
together to make proteins.) A start codon is made up of the
letters ATG, which codes for the amino acid methionine. When
the machinery of the cells sees that first ATG, it knows to
start making the protein there. The code is always read in
groups of three, so the start codon also gives the cell's
machinery it's so-called reading frame. Each set of three
letters thereafter corresponds to a single amino acid.
Stop codon
A codon is a group of three bases - A, T, C, or G - and
codes for a single amino acid, the building blocks of
proteins. A stop codon tells the cell's machinery that it
has reached the end of the protein and should stop
translating the code. Stop codons come in three different
forms - TGA, TAG, and TAA.
Introns
An intron is a section of DNA within a gene that doesn't
actually code for anything. Introns and exons are
interspersed throughout a gene, although there are some
human genes without any introns. When a gene is copied into
mRNA, both introns and exons are faithfully copied, but all
the introns are cut out before the final mRNA transcript is
made. Less complex organisms such as yeast tend not to have
introns. The function of introns, if any, is unknown,
although geneticists now wonder whether the splicing
together of exons required by the presence of introns allows
the human genome to generate more complexity than its mere
30,000 genes would suggest.
Exons
A gene's exons are separated by long regions of DNA called
introns, which often have no apparent function. When DNA is
transcribed into mRNA, the introns are spliced out, and the
exons are spliced together to make the final mRNA. Some
geneticists believe that it is this splicing step that
allows humans to generate more complexity from their 30,000
genes than, say, a fruit fly could from its 10,000 genes;
splicing allows the cell to vary which exons are
incorporated into mRNA, thereby varying the final amino-acid
composition of the protein.
Hitchhiking code
More than half of our genetic code does not really belong to
us. As long as a billion and a half years ago, foreign
pieces of DNA from other organisms began infecting and
spreading through our early ancestors' genome. Scientists
call these pieces of DNA transposable elements. The code
shown here includes an ALU element, which is often found
bunched up with others of its kind near or even inside of
genes. Because of this, geneticists now suspect that these
ALU sequences may actually have some function, perhaps in
regulating the activity of the genes.
Ancient code
Some sections of the human genome code for proteins that are
basic to the function of cells and have probably remained
the same ever since bacteria first evolved. Some of these
proteins are so similar between yeast and humans that the
human equivalent has been inserted into a yeast and made to
work. Sections of the cyclooxygenase 2 gene shown here are
similar to sequences in the pufferfish. Since the common
ancestor of both pufferfish and humans lived a very long
time ago, it is thought that any shared sequences must be
similarly ancient.
Sites of variation
Comparing code taken from any two humans, 99.9 percent of
the letters are identical. But every 1,000 letters or so, on
average, there is a difference between the two codes. Some
people will have a C, for example, where others will have a
G. About one in every 300 letters is a site where at least 1
percent of the population will have a different letter.
These differences are called single nucleotide polymorphisms
(SNPs). Many of the sites of variation in the cyclooxygenase
2 gene are known because the gene has been closely studied.
A gene
A gene is a stretch of DNA that contains all the information
necessary to make a particular protein. Genes can be as
short as 100 letters, or bases, or as long as a couple of
million bases. The gene revealed here, called cyclooxygenase
2, is about 10,000 bases long. Aspirin, ibuprofen, and other
anti-inflammatory medications work in part by blocking the
action of the protein coded for by this gene. Searching for
genes, figuring out what proteins they code for, and
determining how the body uses those proteins is what
geneticists are focused on now that the human genome has
been decoded.
Watch the Program Here
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Our Genetic Future (A Survey)
Manipulating Genes: How Much is Too Much?
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Understanding Heredity
Explore a Stretch of Code
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Nature vs Nurture Revisited
Sequence for Yourself
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Journey into DNA |
Meet the Decoders
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| Updated April 2001
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