| DNA Replication,
Transcription, and Translation
is DNA replication?
When a cell divides, the DNA has to be duplicated, so that each of
the two daughter cells has a complete set of DNA. To do this,
the pairs pull apart (think of it like unzipping a zipper). Each
half of the unzipped chain is used to form a
duplicate. Because A only binds with T and C with G, each newly
formed half becomes an automatically correct copy.
does DNA code for proteins?
To make proteins, the code in the DNA has to be translated to a
language code that can make amino acids line up in the proper order. The process is a little more complicated than it
might seem. DNA does not make proteins directly. There are two major
The first step is to transcribe, or copy, the code in DNA to another
compound called "messenger RNA". You can think of DNA
like a zipper that unzips when it is time to read the code. This is similar to changing
a computer program (DNA code) into a written statement (messenger
RNA). Remember, DNA does nothing unless the code is read. Loose talk
about having certain genes, such as genes for alcoholism,
schizophrenia, sexual orientation, and other conditions is very
misleading. What matters is whether or not genes are "read," and
that depends on many poorly understood aspects of the environment
inside of cells.
RNA (mRNA) is a small molecule (green in diagram to the right) that
copies the message of the DNA
code. The message can be copied because mRNA has base pairs that
preserve the DNA code. mRNA is just like DNA except that a base
called Uracil is substituted for Thymine. Uracil performs the same
function of matching with adenine.
When a gene is actively making protein, it "unzips"
partially so that base pairs separate, and each member can can
match specific base pairs in the mRNA. Thus, mRNA preserves the code
in DNA. The key point is that RNA chains of bases are smaller than
DNA and they can move out of the nucleus into other parts of the
The second step in coding is to translate the code in mRNA to
protein-code language. The "language" of proteins is the sequence of
In stringing together different amino acids to
make a protein, the RNA code must be able to deal with the 20 known
amino acids. If the code were based on only one base compound (A or
U or C or G), then the code could
only translate for four amino acids. If a
PAIR of bases compounds served as the code,
then more amino acids could be specified, but still not enough to
account for all 20 amino acids that appear in proteins. Thus, mathematically we know that
it must take at least three nuclei acids to code 20 amino acids.
Experimentally, that has been demonstrated, and a Nobel Prize was
awarded for that discovery.
Thus short RNA segments of only three nucleotides
can act as carriers for amino acids. Messenger RNA making these three-segment RNAs, known as transfer
RNA (tRNA). The tRNA joins three amino acids together
in the right sequence, as specified by order of the three base
compounds in the tRNA. This process is kept orderly because the
RNA is anchored in membranes, known as endoplasmic reticulum.
The final protein product is typically moved into
another membrane system, an organelle called Golgi apparatus,
which "fine tunes" the proteins by adding certain sugar
to see an animated cartoon of how specific tRNA molecules attach
to specific amino acids and link them together to make proteins.
A neat site that explains these things in a
different way can be found by