Download PROTEIN SYNTHESIS

Muma
Bio 6
Protein Synthesis
Study Objectives:
1. Define what a gene is.
2. Describe the steps involved in transcription: initiation, elongation, and termination.
Include enzymes involved and where the process occurs in the cell.
3. Describe the roles of mRNA, rRNA, and tRNA in protein synthesis.
4. Describe the post-transcriptional modifications of mRNA, rRNA, and tRNA
5. Describe the steps involved in translation: initiation, elongation, and termination
Include enzymes involved and where in the cell the process occurs.
6. Describe the post-translational modifications of polypeptides
7. Define mutation and explain how the following mutations may affect the final protein:
silent, point, and frame-shift mutations
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Protein Synthesis Outline
Chapter 4
I.
Gene Expression
a. What is a gene?
i. Gene = a segment of DNA coding for a RNA segment. These
RNA segments will be used to produce a polypeptide
(structural or enzymatic protein)
ii. Each strand of DNA can contain thousands of genes
iii. Each gene has a beginning and an end
b. DNA is used as the blueprint to direct the production of certain proteins
c. Genetic Code
i. The DNA nucleotide sequence codes for the order in which amino
acids are put together to form proteins
ii. Every three nucleotides (codon) on the mRNA codes for a
specific amino acid
iii. There are 20 different amino acids but 64 possible codons
iv. Some redundancy - more then one codon codes for the same amino
acid
v. The genetic code is universal in almost all organisms (See figure
15.5 for a copy of the code)
II.
Protein Synthesis
a. Transcription - information is transferred from DNA to RNA
i. Occurs in the nucleus
ii. All three types of RNA are transcribed from DNA
1. Messenger RNA – carries the coded message from the DNA
to the ribosome in the cytoplasm
2. Ribosomal RNA – reads the mRNA
3. Transfer RNA – transfers the correct amino acid to the
ribosome
iii. Overview of Transcription: the segment of DNA that contains the
gene for a specific protein or RNA that the cell wants to produce
will unwind and the complementary RNA strand will be made by
incorporation the complementary RNA nucleotides
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iv. Stages in Transcription
1. Initiation
a. Each gene has a precise beginning known as the
promotor region and an end known as the
termination sequence
b. Transcription factors bind to the promoter region
(TATA box) of the DNA.
c. RNA polymerase then initiates transcription by
binding to the transcription factor. It:
i. Unwinds the DNA
ii. Elongates the RNA segment
2. Elongation of the RNA transcript
a. RNA nucleotides are added in the 5’ to 3’ direction
by RNA polymerase
b. RNA nucleotides form temporary hydrogen bonds
with the DNA template
c. As the DNA helix reforms the RNA peels away
3. Termination
a. At the end of the gene the termination sequence
causes transcription to end
b. The pre-RNA segment dissociates from the DNA
4. Post-transcriptional Modifications to mRNA
a. 5’ cap – a guanine triphosphate is added that signals
for ribosomal attachment in the cytoplasm
b. 3’ poly A tail – polyA polymerase adds ~250 “A”
nucleotides to the end. Protects RNA from being
degraded by nucleases.
c. Splicing - introns are cleaved out by snRNPs, and
exons are spliced together
i. Exons – coding region
ii. Introns – noncoding region
5. Post-transcriptional modifications rRNA
a. rRNA associates with proteins to form two subunits
(40s and 60s)
b. Leaves the nucleus and enters the cytoplasm
6. Post-transcriptional Modifications tRNA
a. Folds into a three dimensional structure (clover
shaped)
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b. Translation - going from the mRNA nucleotide code to amino acid code
i. Occurs in the cytoplasm
ii. Overview: mRNA is read by a ribosome (rRNA) to determine the
sequence of amino acids to produce a polypeptide
iii. Players in Translation
1. mRNA strand
2. ribosomes (rRNA)
a. rRNA has a mRNA binding site and three tRNA
binding sites:
i. A site (amino-acyl binding site)
ii. P site (peptidyl binding site)
iii. E site
3. tRNA
a. Has an anticodon - three base sequence that is
complementary to a codon on the mRNA
b. 3’ end of the tRNA contains a binding site for the
specific amino acid the tRNA carries
iv. Stages of Translation
1. Initiation
a. mRNA binds to the 40s ribosome subunit
b. The initiator tRNA binds to the mRNA start codon
(AUG) at the P site on the ribosome
c. The anticodon on the tRNA matches with the codon
on the mRNA strand
d. The arrival of the 60s subunit completes the initiator
complex
2. Elongation – of the polypeptide chain
a. The next tRNA enters at the A site
b. The enzyme peptidyl transferase forms a peptide
bond between the amino acid on the P site and the
new amino acid on the A site
c. The ribosome then moves down the mRNA (this is
called translocation)
d. The tRNA that was at the A site is now at the P site
and the empty tRNA that was at the P site now exits
at the E site
e. Charging of tRNA
i. Amino acids are floating freely in the
cytoplasm
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ii. The enzyme amino-acyl tRNA synthetase
attaches the amino acids to the 3’end of the
tRNA
iii. Requires ATP
3. Termination of translation
a. Elongation continues until a stop codon on the
mRNA is reached (UAA, UAG, UGA)
b. The polypeptide is then released from the ribosome
by a release factor
v. Polysomes - several ribosomes can simultaneously translate the
same mRNA strand to make multiple copies of the same
polypeptide
vi. Post-translational modifications to the polypeptide
1. The start methionine is removed by the enzyme
aminopeptidase
2. Proteins will under go folding or modifications
a. Cleavage into smaller fragments or joined with
other polypeptides
b. Chemical modifications: addition of carbohydrates
or lipids
c. Transport to its final destination
III.
Mutations and their consequences
a. Mutation = a change in the sequence of bases within a gene
b. Caused by a mistake during DNA replication (rare) or due to
environmental factors called mutagens
c. Mutations can be somatic or germinal
d. Types of Mutations
i. Point mutations (substitutions) – change in a single nucleotide
1. Due to redundancy of the genetic code it may change the
amino acid, it may not “wobble”
2. Silent mutations do not change the protein
ii. Frame-shift mutation – caused by insertion or deletion of a
nucleotide
1. Changes the reading frame of the codons, usually results in
a non-functional protein
e. Are all mutations bad? Although mutations are sometimes harmful…..
i. They are also the source of the rich diversity of genes in the world
ii. They contribute to the process of evolution by natural selection
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