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Protein Synthesis
Now that we’ve described DNA and RNA, it’s time to take
a look at the process of protein synthesis. The synthesis of proteins
takes two steps: transcription and translation. Transcription takes
the information encoded in DNA and encodes it into mRNA, which heads out
of the cell’s nucleus and into the cytoplasm. During translation,
the mRNA works with a ribosome and tRNA to synthesize proteins.
Transcription
The first step in transcription is the partial unwinding
of the DNA molecule so that the portion of DNA that codes for the
needed protein can be transcribed. Once the DNA molecule is unwound
at the correct location, an enzyme called RNA polymerase helps line up
nucleotides to create a complementary strand of mRNA.
Since mRNA is a single-stranded molecule, only one of the two strands
of DNA is used as a template for the new RNA strand.
The new strand of RNA is made according to the rules of
base pairing:
- DNA cytosine pairs with RNA guanine
- DNA guanine pairs with RNA cytosine
- DNA thymine pairs with RNA adenine
- DNA adenine pairs with RNA uracil
For example, the mRNA complement to the DNA sequence TTGCAC
is AACGUG. The SAT II Biology frequently asks about the sequence
of mRNA that will be produced from a given sequence of DNA. For
these questions, don’t forget that RNA uses uracil in place of thymine.
After transcription, the new RNA strand is released and
the two unzipped DNA strands bind together again to form the double
helix. Because the DNA template remains unchanged after transcription,
it is possible to transcribe another identical molecule of RNA immediately
after the first one is complete. A single gene on a DNA strand can
produce enough RNA to make thousands of copies of the same protein
in a very short time.
Translation
In translation, mRNA is sent to the cytoplasm, where it
bonds with ribosomes, the sites of protein synthesis. Ribosomes
have three important binding sites: one for mRNA and two for tRNA.
The two tRNA sites are labeled the A site and P site.
Once the mRNA is in place, tRNA molecules, each associated
with specific amino acids, bind to the ribosome in a sequence defined
by the mRNA code. tRNA molecules can perform this function because
of their special structure. tRNA is made up of many nucleotides
that bend into the shape of a cloverleaf. At its tail end, tRNA
has an acceptor stem that attaches to a specific amino acid. At
its head, tRNA has three nucleotides that make up an anticodon.
An anticodon pairs complementary nitrogenous bases with
mRNA. For example if mRNA has a codon AUC, it will pair with tRNA’s
anticodon sequence UAG. tRNA molecules with the same anticodon sequence
will always carry the same amino acids, ensuring the consistency
of the proteins coded for in DNA.
The Process of Translation
Translation begins with the binding of the mRNA
chain to the ribosome. The first codon, which is always the start
codon methionine, fills the P site and the second codon fills the
A site. The tRNA molecule whose anticodon is complementary to the
mRNA forms a temporary base pair with the mRNA in the A site. A
peptide bond is formed between the amino acid attached to the tRNA
in the A site and the methionine in the P site.
The ribosome now slides down the mRNA, so that the tRNA
in the A site moves over to the P site, and a new codon fills the
A site. (One way to remember this is that the A site brings new
amino acids to the growing polypeptide at the P site.) The appropriate
tRNA carrying the appropriate amino acid pairs bases with this new
codon in the A site. A peptide bond is formed between the two adjacent
amino acids held by tRNA molecules, forming the first two links
of a chain.
The ribosome slides again. The tRNA that was in the P
site is let go into the cytoplasm, where it will eventually bind
with another amino acid. Another tRNA comes to bind with the new
codon in the A site, and a peptide bond is formed between the new
amino acid to the growing peptide chain.
The process continues until one of the three
stop codons enters the A site. At that point, the protein chain
connected to the tRNA in the P site is released. Translation is
complete.
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