Download Protein Synthesis

Protein Synthesis
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The genetic code – the sequence of nucleotides
in DNA – is ultimately translated into the
sequence of amino acids in proteins – gene
expression
in general, one gene encodes information for
one protein (can be structural or enzymatic) –
one-gene, one-protein hypothesis
DNA does not directly synthesize proteins
RNA acts as an intermediary between DNA and
protein – polymer of nucleotides but has several
important differences:
sugar
bases
strands
RNA
DNA
ribose
A,U,C,G
single
deoxyribose
A,T,C,G
double
Protein synthesis occurs in two major steps –
transcription and translation
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transcription – a molecule of mRNA
is made using DNA as a template
translation – the molecule of mRNA
is used to make the protein
Overview of Protein Synthesis
•During transcription, one DNA strand, (template
strand), provides a template for making an RNA
molecule.
– Complementary RNA
molecule is made
using base-pairing
rules, except uracil
pairs with adenine.
• During translation,
blocks of three
nucleotides (codons
are decoded into
a sequence of amino
acids.
Three types of RNA
1. messenger RNA
(mRNA) – the
“copy” of the DNA
that is used to
specify the
sequence of amino
acids in the protein
• mRNA nucleotides
are read in groups
of three called
codons
• each codon codes
for a specific amino
acid
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transfer RNA (tRNA) –
bring amino acids to the
ribosome during protein
synthesis
each tRNA carries a
specific type of amino
acid
each tRNA can
recognize a specific
mRNA codon because
it has a complementary
anticodon (sequence of
three bases that
associates with the
codon by base pairing)
•Each amino
acid is joined
to the correct
tRNA by
aminoacyltRNA
synthetase
•Aminoacyl
tRNA – tRNA
with it’s
amino acid
attached
3. ribosomal RNA (rRNA) – forms part of
the ribosome
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Transcription
synthesis of RNA using DNA as a
template
most RNA is synthesized by DNAdependent RNA polymerases
enzymes that require DNA as a template
similar to DNA polymerases
synthesize RNA in a 5’ to 3’ direction
use nucleotides with three phosphate
groups as substrates (nucleotide
triphosphates), removing two of the
phosphates as the subunits are linked
together (just like DNA synthesis)
the transcibed strand of DNA and the
complementary RNA strand are
antiparallel
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Transcription begins with an RNA
polymerase attaching to a DNA sequence
called the promoter (promoter is not
transcribed) – marks the beginning of the
gene
RNA polymerase unwinds the DNA strand
only one of the strands of DNA is transcribed
– called the transcribed strand, template
strand, or antisense strand
The strand that is NOT transcribed is the
sense strand
RNA polymerase continues down the gene
synthesizing a single strand of mRNA
through base-pairing (A matches with U)
until it reaches a termination signal
Translation – protein synthesis
• In the process of
translation, a cell
interprets a series of
codons along a mRNA
molecule.
• Transfer RNA (tRNA)
transfers amino acids
from the cytoplasm’s
pool to a ribosome.
• The ribosome adds
each amino acid carried
by tRNA to the growing
end of the polypeptide
chain.
Ribosome Structure
• Each ribosome has a large and a small subunit
• These are composed of proteins and ribosomal RNA
(rRNA)
• Each ribosome has a binding site for mRNA and
three binding sites for tRNA molecules.
– The P site holds the tRNA carrying the growing polypeptide
chain.
– The A site carries the tRNA with the next amino acid.
– Discharged tRNAs leave the ribosome at the E site.
• Translation occurs in steps called: initiation,
elongation, and termination
• Step1. Initiation brings together mRNA, a tRNA with
the first amino acid, and the two ribosomal subunits.
– First, a small ribosomal subunit binds with mRNA and a
special initiator tRNA, which carries methionine and attaches
to the start codon.
– in all organisms, protein synthesis begins with the codon
AUG (codes for methionine)
– Initiation factors bring in the large subunit which closes in a
way that the initiator tRNA occupies the P site.
Step 2. Elongation – the addition of amino acids
to the growing polypeptide chain
• initiator tRNA is bound to the P site of the
ribosome
• A site is filled with the next tRNA -specified by
the codon (tRNA anticodon matches with
codon by base-pairing)
• the amino acids are linked together (peptide
bond)
• the tRNA in the P site moves to E site to be
released and the ribosome moves down
freeing up the A site
• the ribosome moves in a 5’ to 3’ direction as
the mRNA is translated
•The genetic code is
series of codons; read
one triplet at a time
•genetic code is
redundant – certain amino
acids are specified by
more than one codon –
64 possible codons but
only 20 amino acids
•61 codons specify amino
acids – three do not
(UAA, UGA, and UAG are
all stop codons – code for
nothing)
Step 3. Termination – ribosome reaches
the termination codon (stop codon) at the
end of the sequence – stop codon does
not code for an amino acid
Transcription and Translation in Eukaryotes
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prokaryotic
mRNAs are
used
immediately
after
transcription
prokaryotes can
transcribe and
translate the
same gene
simultaneously.
• eukaroytic mRNAs must go through further
processing – posttranscriptional modification
and processing:
• At the 5’ end of the pre-mRNA molecule, a
modified form of guanine is added, the 5’ cap.
– This helps protect mRNA from hydrolytic enzymes.
– It also functions as an “attach here” signal for
ribosomes.
• At the 3’ end, an enzyme adds 50 to 250
adenine nucleotides, the poly(A) tail.
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eukaryotic genes have interrupted
coding sequences – they contain long
sequences of bases within the proteincoding sequences that do not code for
amino acids in the final protein
noncoding regions within the genes
are called introns (intervening
sequences)
protein-coding sequences are called
exons (expressed sequences)
a eukaryotic gene may have multiple
introns and exons
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the entire gene that is transcribed as a
large mRNA molecule is called a precusor
mRNA or pre-mRNA – contains both
introns and exons
a functional mRNA may be 1/3 the length of
the pre-mRNA
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In order for a pre-mRNA to become a
function message, it must be capped,
have a poly-A tail added, have the
introns removed, and have the exons
spliced together
excision of introns and splicing of
exons catalyzed by snRNPs (small
nuclear ribonucleoprotein complexes)