UNIT 3: Molecular Genetics
Chapter 7: Genes and Proteins Synthesis
pg. 310 -
7.3: Translation: Nucleic Acid to Polypeptide
pg. 325 - 331
tRNA
Anticodon – is the complementary sequence of base pairs on a tRNA that
corresponds to a codon on an mRNA.
Aminoacylation – is the process by which a tRNA molecule is bound to its
corresponding amino acid.
Aminoacyl-tRNA – is a molecule of transfer RNA bound to its associated
amino acid.
The ribosome can not synthesize a polypeptide chain on its own. The
ribosome works in conjunction with transfer RNA to deliver and combine
amino acids together, forming peptide bonds, creating a polypeptide chain.
Transfer RNA is approximately 70 to 90 nucleotides in length and
structurally forms four double helical segments in the shape of a cloverleaf.
On the bottom portion of the cloverleaf an Anticodon is located. The
Anticodon is complementary to the codon found on the mRNA. The
opposite end of the tRNA an amino acid can be attached, which corresponds
to the Anticodon.
mRNA
Codon – 5′AUG3′
tRNA
Anticodon – 3′UAC5′
Amino Acid – MET
There are 61 codons that code for amino acids, three code for a stop, total 64
codons.
When tRNA combines to an amino acid, through the process of
Aminoacylation, it creates an Aminoacyl-tRNA. The energy found in the
Aminoacyl-tRNA drives the formation of the peptide bond.
The ribosome removes the RNA from the amino acids, creating a
polypeptide chain.
Figure 1: the structure of tRNA:
Ribosomes
The ribosome is responsible for protein synthesis by translating mRNA into
polypeptide chains. The ribosome follows the codon message on the mRNA
to produce the appropriate sequence and number of amino acids to be found
in the polypeptide chain (primary protein).
The ribosome is composed of two different sized parts; large and small
ribosomal subunits, made up of a combination of ribosomal RNA and
ribosomal protein. Each ribosome has two binding sites that actively bring
mRNA and Aminoacyl-tRNAs.
The A site (Aminoacyl site) is where the Aminoacyl-tRNA, delivering the
next amino acid to be added to the polypeptide chain, binds to the mRNA.
The Anticodon aligns itself with the mRNA codon (complementary).
The P site (Peptidyl site) is where the tRNA attaches the amino acid to the
adjacent amino acid of the polypeptide chain, adding to the growing
polypeptide chain.
The E site (Exit site) is where the tRNA leaves the ribosome and is released
back into the cytoplasm after releasing its amino acid.
Figure 3: a ribosome assembles amino acids into a polypeptide chain. A tRNA molecule, with an amino
acid bound to it, enters the ribosome on the right (A site). The Anticodon on the tRNA pairs with the codon
in the mRNA. Its amino acid will then be added to the growing polypeptide, which is currently attached to
the tRNA in the middle of the ribosome (P site).
The Process of Translation
The process of translation occurs in three stages; Initiation, Elongation, and
Termination. When the mRNA binds to a ribosome and the start codon is
read (AUG) translation is initiated. AUG codes for the amino acid
Methionine.
Elongation occurs as more amino acids are attached creating a polypeptide
chain. Three nucleotides code for one amino acid, and is known as a reading
frame.
Termination occurs when the stop codon is read, and translation is stopped.
To speed up production of more then one copy of a protein, more then one
ribosome may read the same mRNA at the same time, simultaneously
translating and making multiple copes of the protein.
Initiating Translation
Reading Frame – is a particular system for separating a base pair sequence
into readable codons.
The mRNA is used as a template during initiation. Translation starts when
the large and small ribosomal sub-units combine with an mRNA molecule
and the first Aminoacyl-tRNA, to start synthesizing a new protein. At an
AUG start codon.
Step 1: the initiator Methionine-tRNA (Met-tRNA) forms a complex with
the small ribosomal sub-unit, and binds to the mRNA 5′ cap and then begins
scanning the mRNA for the AUG start codon.
Step 2: Once the start codon has been located, and recognized by the anticodon for the MET-tRNA, the larger ribosomal sub-unit binds to complete
the forming of a ribosome.
Step 3: The initiator Met-tRNA moves to the P-site and initiation is
complete.
Figure 4: The steps in initiation stage of translation in eukaryotes.
1. Met –tRNA forms a complex with the small ribosomal subunit.
2. The complex binds to the 5′ cp of the mRNA and scans along it until it reaches the AUG start
codon.
3. The large ribosomal subunit binds, completing initiation.
From here the translation occurs, reading three nucleotides at a time, from
the mRNA. The tRNA for AUG establishes the correct Reading Frame, the
series of codons for the polypeptide encoded on the mRNA.
Elongating the polypeptide Chain
Elongation follows four steps; using the A, P, and E sites for growing the
polypeptide chain.
Step 1: The initiator tRNA attaches to the P-site, the A-site is empty.
Step 2: A second tRNA, with complementary anti-codon to the MRNA
codon, delivers the amino acid to the A-site of the ribosome. The Met amino
acid (AA1) is cleaved from the tRNA in the P-site and forms a peptide bond
to second amino acid (AA2) by an enzyme called peptidyl transferase.
Step 3: Then moves along one reading frame sequence, with AA1 moving
into the E-site and AA2 into the P-site. An appropriate tRNA delivers the
next amino acid (AA3) to the A-site. Steps 2 and 3 continue along the
mRNA building a polypeptide chain.
Step 4: Each empty tRNA (amino acid has been cleaved) is released from
the ribosome.
Figure 5: The steps in elongation stage of translation.
1. An Aminoacyl-tRNA binds the A site.
2. Peptidyl transferase cleaves the amino acid from the Ps site tRNA and bonds it to the amino acid
on the A site tRNA.
3. The ribosome moves along the mRNA to the next codon, thereby bringing the tRNA with the
growing polypeptide to the P site and moving the empty tRNA to the E site.
4. After the ribosome has moved over one codon, the empty tRNA in the E site is released ad the
cycle is ready to go again.
Termination of Protein Synthesis
When the stop codon (UAA, UAG, and UGA) on the mRNA arrives at the
A-site of the ribosome termination will occur.
The polypeptide chain is released from the tRNA at the P-site.
The ribosomal sub-units will also detach from the mRNA.
Protein Synthesis in Eukaryotes and in Prokaryotes
Polysome – is a complex that is formed when multiple ribosomes attach to
the same mRNA molecule in order to facilitate rapid translation.
Multiple ribosomes can translate a single strand of mRNA at the same time.
This will increase the number of copies of protein produced. In eukaryotes
ribosomes can only attach to mRNA in the cytosol, therefore they have to
wait to the mRNA leaves the nucleus, while the prokaryote does not have a
nucleus some the ribosome can attach immediately to the mRNA as it is
produced. Therefore transcription and translation occurs rapidly in
prokaryotes.
Figure 6: Polysomes consist of a series of ribosomes translating the same mRNA
See table 1: A comparison of Translation in Prokaryotes and Eukaryotes. Pg.
329
Polypeptide to Protein
The primary protein produced is inactive. The protein must be
folded into the correct configuration to become active. (primary,
secondary, tertiary, and quartary).