Download Topic 14: Protein Synthesis

TOPIC 14: PROTEIN SYNTHESIS (lecture 23)
OBJECTIVES:
1. Understand the overall structure and role of tRNA’s in translation.
2. Be able to describe the various parts of the ribosome and the roles of the P site and
A site in translation; understand the overall process of initiation, elongation and
termination of transcription.
3. Know the major kinds of post-translational modifications that take place after
proteins are synthesized.
4. Know what a point mutation is, how point mutations can be produced, and be able to
differentiate between silent, missense, nonsense and frame shift mutations
Translation: The synthesis of protein. (fig. 17.12, overview)
The mRNA carries a faithful record of the amino acid sequence of the protein as
specified by the gene sequence. How is this used to make protein?
fig. 17.13- transfer RNA (tRNA); specialized RNA molecules that literally are involved in
transferring the appropriate amino acid to the growing polypeptide chain
1. roughly 80 nucleotides long
2. at the 3’ end in a site where a particular amino acid will be attached
3. consists of three loops; the middle of which corresponds to a site known as the
anticodon site; it has base sequence that is complementary to codons on the mRNA
4. there are 41 different tRNA’s ; there are 61 different codons so some tRNA’s can
bind more than one codon. There is a relaxation of the base-pairing rules for the
third position of the condon; this is called wobble.
a. U can pair with either A or G when in the third position
b. if the nucleotide Inosine (I) is present, it can bind to U,C and A in the third
position of the codon
c. see fig. 17.4; note that codons coding for the same amino acid typically only
differ in the nucleotide in the third position
aminoacylsynthetases- these are enzymes which attach a particular amino acid to the
3’ end of the corresponding t-RNA; they are specific for each amino acid (see fig.
17.14); note: ATP is hydrolyzed here!
ribosomes (fig. 17.15) - RNA and protein complex; site of protein synthesis; the RNA is
known as ribosomal RNA (rRNA) and consists of large and small subunits with three
binding sites - P site, A site and mRNA binding site.
Fig. 17.16- atomic structure of ribosome complex
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Fig. 17.12- overview of the process of translation; binding of aminoacyl tRNA to mRNA
followed by peptide formation and then translocation and binding of another aminoacyl
tRNA.
Initiation of translation (fig. 17.17)1. small rRNA subunit binds to a region of the mRNA near the beginning of the region
coding for the polypeptide chain
2. an initiator tRNA with the anticodon for methonine (UAC) binds to the AUG (“start”)
site
3. the large rRNA subunit attaches in such a way that the initiator tRNA is in the P site
and the A site is unoccupied
4. proteins called initiation factors are involved in this process; GTP is hydrolyzed
Elongation (fig. 17.18)1. binding of complementary anticodon of a tRNA to the codon in the A site
2. peptide bond formation
3. the tRNA in the P site is released and the ribosome advances one codon
downstream so that the A site is again unoccupied; this means that the peptidyltRNA is now positioned in the P site
4. another complementary tRNA then binds to the codon in the A site….
Termination (fig. 17.19)1. when the termination codon reaches the A site, elongation will stop
2. a release factor binds to the codon and causes a water molecule to be added to the
polypeptide chain causing the polypeptide to be ultimately released.
Polyribosomes (fig. 17.20)- at any given point in time, many ribosomes may be attached
to a single message
After synthesis the polypeptide will fold and assume its native 3-D structure; this is
sometimes assisted by other proteins known as chaperonins. In addition, if it is a
multiple subunit protein, subunits must come together to make the quaternary structure.
Post-translational chemical modifications may take place:
1. cleavage by specific proteases; for instance, most mitochondrial proteins are
synthesized in the cytoplasm. They have a leader peptide in the N-terminal region
which allows the mitochondrion to recognize it and transport it into the matrix. The
leader peptide is then clipped off and the protein is trapped in the mitochondrion.
2. covalent attachment of other molecules onto particular amino acid residuesphosphate, sugars, lipids
3. removal of N-terminal methionine
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Signal sequences- like mitochondrial proteins, other proteins have signal sequences
which target them for particular places in the cell; see fig. 17.21 for targeting mechanism
into Golgi system.
Mutation- an alteration in the genetic information in a cell; mutations can take place by
large scale changes in DNA that might take place during gamete formation or cell
division involving pieces of chromosomes or by point mutations (point mutation- a
change in a single nucleotide in a gene).
Large scale changes in genes and chromosomes- these mutations can involve drastic
changes in which genes or portions of genes or portions of entire chromosomes are
deleted or moved around to different places.
Point mutations occur naturally during DNA replication; they are rare. However, certain
agents such as ionizing radiation (X-rays etc), UV light and a variety of chemicals may
produce mutations. These agents are known as mutagens.
Ionizing radiation like X-rays and gamma rays- these cause the production of highly
reactive chemical compounds known as free radicals; free radicals can break the
phosphodiester bonds of DNA
UV light- causes the formation of thymidine-thymidine dimers (adjacent T’s become
covalently attached); presence will block DNA replication
Chemical mutagens- may covalently add new carbons to nucleotides causing mispairing
Kinds of point muations (fig. 17.24 a & b):
1. base pair substitution- one nucleotide is replaced by another; can have no effect on
the amino acid sequence (silent); it can change the amino acid (missense) or it can
put an inappropriate stop signal into the sequence (nonsense)
2. insertion or deletion- alters the reading frame of the message and can have
catastrophic effects; these are known as frame shift mutations.
Fig. 17.23- sickle cell anemia is the result of a single nucleotide substitution which
results in a protein have a Val instead of Glu! This substitution produces profound
consequences for the functioning of red blood cells.
Fig. 17.25- Summary of transcription/translation
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