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Making a worker starts when a transcription factor grabs onto a promoter it recognizes and
starts to attract the attention of an RNA polymerase.
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RNA polymerases
copy Instructions from the DNA the Book
is written in to another similar molecule called RNA. They do
this by building up a chain of RNA bases that are the complement of the complementary chain-- which is
to say, a copy of the coding chain. Building up
long molecular chains from individual pieces is called
polymerization, hence the name of the worker doing the job.
Since this RNA chain is a message being passed further along to
specify a worker, it's called messenger RNA or mRNA.
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As an RNA polymerase copies an Instruction onto a chain of mRNA, the chain
trails out behind it. Ribosomes notice the mRNA chain and start reading it
to make the worker to carry out the Instruction it's a copy of. (If
the Instruction contains
introns, a large complex of workers called a
spliceosome removes them, and splices the
exons back together.)
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Although we've been discussing ribosomes as though they were single
workers, they're actually complexes of
55 different workers and some RNA. The RNA that holds them together
is unusual. Not only does it hold the workers together -- most RNA
just carries copies of the Book's Instructions -- but it appears to
help the other ribosomal workers do their jobs by doing a little
molecular origami of its own. Ribosomes are named in part for their
RNA, or ribonucleic acids.
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Retro-viruses get their
name from the backwards way they force infected cells to make their workers. Many
retro-viruses' Instruction Books are written in RNA -- HIV, for
example. This RNA is then copied by a worker called reverse
transcriptase into DNA (which is then pasted into the infected
cell's Instruction Book and copied into mRNA to make workers). This
flow of Instructions from RNA to DNA is backwards compared to living
things. Some retro-viruses, like herpes, go even further and copy a
DNA Instruction Book to RNA, then to DNA, and then again to
mRNA. It seems that there's no job that's so hard that
something can't make it harder.
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The Genetic Code
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| TAT | Y | TAC | Y | TAA | Stop | TAG | Stop |
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| ATT | I | ATC | I
| ATA | I | ATG | M
Start
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Colors indicate links that can be
conservatively
substituted for each other:
- links that like water
- links that don't like water
| long and skinny | flat and bulky |
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This table describes the genetic code that turns Instructions
into workers. The table is organized by the bases that make up each codon.
Reading down, the first base of each codon goes in the order T, C, G, A.
Reading across, the middle base goes in the same order. Finally, within
each box, the last bases are similarly arranged. This organization clarifies
the similarities of the links' properties for similar codons.
Each codon is shown with the link that it codes for.
Since the names are long, we'll use just the link's initial to talk
about it. (Curious readers can read the full names in the table to
the right.) The names also matter much less than their
properties, especially which links are similar to each other.
(Similar links are indicated by color codes in the tables.) Note that
there's one codon marked "Start" and three marked "Stop". These tell
ribosomes where in the mRNA the Instructions for workers begin and
end. Usually, one Instruction fills an mRNA, starting near the mRNA's
beginning and running until almost its end. However, in some organisms the
RNA polymerases copy several Instructions onto a single mRNA, and the
start and stop codons serve to separate them.
The start codon, in addition to telling ribosomes where to start translating
a worker, also codes for the protein link M, which means
that all workers start with this link. Stop codons don't code for any
amino acid; all they do is tell the ribosome to stop translating.
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Formal names of workers' links
| A | Alanine |
| C | Cystine |
| D | Aspartic acid |
| E | Glutamic acid |
| F | Phenylalanine |
| G | Glycine |
| H | Histadine |
| I | Isolucine |
| K | Lysine |
| L | Leucine |
| M | Methonine |
| N | Asparagine |
| P | Proline |
| Q | Glutamine |
| R | Arginine |
| S | Serine |
| T | Threonine |
| V | Valine |
| W | Tryptophan |
| Y | Tyrosine |
Color code same
as left table |
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The last link that does go into a worker -- the one right
before the Stop -- tells Life's recyclers when to recycle the worker.
That last link specifies how likely a recycler is to take apart a
worker when the recycler runs into it. Some final links tell recyclers to
almost always take apart the worker, so workers ending in that link
don't last very long. Other ending links tell the recyclers to generally
leave the worker alone, so that it will last a long time. Usually,
workers that do things the cell always
needs to do last a long time, and workers the cell needs only rarely
exist only briefly.
Now that we've seen how Life decides to recycle its workers, our answer to the
question How does Life read its
Instruction Book? comes to an end. Before we turn to how the Book is
copied for growth and reproduction, however, let's tie off an important
dangling thread: how eukaryotes' two Instruction Books interact to produce a single
set of traits for them.
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The workers bound together into ribosomes perform a number of steps,
and the result of these steps is translating the RNA bases into
protein links. Ribosomes bind
to the mRNA that's being made, move along it, and gather up parts to
make into workers. The important step is what they do with the parts
as they slide along the mRNA. Ribosomes read the mRNA's bases three
at a time. Each set of three bases specifies one link to add to the protein
being made. The sets of three bases, called codons,
make up the elements of the genetic code.