Transcript Protein Synthesis

We have looked at DNA. But how is this
genetic code actually used for anything?
Lets
See!
The genetic
code is used
as a blueprint
to make
proteins.
Proteins are widely used in cells to
serve diverse functions. Some
proteins provide the structural
support for cells while others act as
enzymes to catalyze certain
reactions.
But, where do
proteins come
from?
Since the beginning
of evolution, cells
have developed the
ability to synthesize
proteins.
They can produce new proteins either
for reproduction or to simply replace a
degraded one.
To manufacture proteins, cells follow a
very systematic procedure that first
transcribes DNA into mRNA
and then translates the mRNA
into chains of amino acids.
The amino acid
chain then folds
into
specific
proteins.
(Transcribed is a fancy way of saying
“copied.”)
RNA is very similar to DNA except that it:
has only a single strand of
nucleotides instead of two strands
contains a different sugar (ribose
instead of deoxyribose)
contains the nitrogen base uracil
(U) instead of thymine (T)
• Messenger RNA (mRNA) copies
DNA’s code & carries the
genetic information to the
ribosomes
• Ribosomal RNA (rRNA), along
with protein, makes up the
ribosomes
• Transfer RNA (tRNA) transfers
amino acids to the ribosomes
where proteins are synthesized
The “m” in “mRNA”
stands for “messenger,”
because mRNA copies
genetic information from
DNA (which is found in
the nucleus) and carries
it to another part of
the cell (the ribosomes).
Think of the genetic
code the mRNA is
carrying as a series
of three-letter
“words.” Each of
these three-letter
words is called a
codon.
Codon
Codon
Codon
Codon
Codon
Codon
Different codons code
for different amino
acids. For example, the
codon for the amino
acid methionine is AUG
(adenine, uracil,
guanine). The codon
for the amino acid
serine is UCA (uracil,
cytosine, adenine).
Another type of RNA,
called tRNA (“t” stands for
“transfer”) matches the
codons in mRNA to the
correct amino acids.
As the mRNA strand moves along the
ribosome, the amino
acids are joined in the
correct sequence to
form a protein. This
process is called
translation.
U C G U C C G G A
G
Codon
Codon
Codon
Codon
Codon
Codon
The diagram below shows a codon chart. A codon
chart shows which codons code for which amino acids.
Suppose a DNA
mutation led to a
change in a single
mRNA codon. Now
suppose this codon
changed from GCC
to GCG.
By looking at the codon chart, you can see that
both of these codons code for the amino acid
alanine. So even though the DNA and mRNA have
changed, there is no change in the protein!
This chart shows the amino acids coded for by each
of the 64 possible mRNA codons. To find which
amino acid the codon CAA codes for, follow these
steps. (1) Look on the left side of the chart to
find the large row of codons that begin with C. (2)
Move across this row until you get to the column of
codons whose second
base is A. (3) Move
down this column until
you get to the row of
codons whose third
base is A. The codon
CAA codes for the
amino acid glutamine.