Download Gene expression PPT

Gene expression
Transcription and Translation
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1.
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Important Features: Eukaryotic cells
a. DNA contains genetic template for
proteins.
b. DNA is found in the nucleus
c. Protein synthesis occurs in the
cytoplasm - ribosome.
d. "Genetic information" must be
transferred to the cytoplasm where
proteins are synthesized.
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2. Processes of Protein Synthesis
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a. Transcription - genetic template for a
protein is copied and carried out to the
cytoplasm
b. Translation - template serves as a
series of codes for the amino acid
sequence of the protein
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Cells Use RNA to Make Protein
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The RNA Players – rRNA, mRNA, tRNA
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During polypeptide synthesis, ribosomal
RNA (rRNA) is the site of polypeptide
assembly.
Messenger RNA (mRNA) directs which
amino acids are assembled into
polypeptides.
Transfer RNA (tRNA) transports and
positions amino acids.
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Central Dogma of Gene Expression
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Transcription
– in the nucleus (if you have one)
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DNA sequence is transcribed into RNA
sequence
only one of two DNA strands (template or
antisense strand) is transcribed
non-transcribed strand is termed coding strand
or sense strand same as RNA (except T’s are
U’s)
RNA polymerase unzips and adds the
nucleotides unlike in replication where helicase
unzips.
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Transcription
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Promoter
– Transcription starts at RNA polymerase binding
sites called promoters on DNA template strand.
Transcription factor
– Binds to promoter so that RNA polymerase can
then bind
Initiation
– Other transcription factors bind, assembling a
transcription initiation complex.
– RNA polymerase begins to unwind DNA helix.
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Transcription Bubble
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RNA Processing
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In eukaryotes, RNA is modified after
transcription
DNA sequence specifying a protein is broken
into coding segments (exons) scattered
among longer noncoding segments (introns).
Intron sequences are cut out of primary
transcript before it is used in polypeptide
synthesis - they are not translated
remaining exon sequences are spliced
together to form final processed mRNA
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RNA Processing
•5’ GTP cap – G-P-P-P – protects mRNA from
degradation and serves as an “attach here” sign for
ribosomes
•3’ PolyA tail – A-A-A-A-A – inhibits degradation and
stabilizes mRNA as it moves out of nucleus
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RNA Processing
Each person in group tells one way that RNA
is modified after transcription in eukaryotes
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Now TRANSLATION!!!!
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Translation
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Begins when initial portion of mRNA molecule
binds to rRNA in a ribosome
– mRNA is in triplet code – 3 bases = codon
– tRNA molecule with complimentary anticodon
binds to exposed codon on mRNA. The tRNA
has many more nucleotides, but the three on the
anticodon is what match up to the codon.
– The codon determines which amino acid the
tRNA carries as tRNA with a specific anticodon
always carry the same amino acid.
– AUG is always the start codon – it codes for the
amino acid Methionine (Met)
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Translation
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Elongation
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Once mRNA binds to
the ribosome
A site (first spot on the
ribosome) = where
tRNAs Arrive with the
new amino acid.
P site = where Peptide
bonds are fomed with
existing chain of amino
acids
E site = where tRNAs
Exit after dropping off
the chain.
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Translation
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Termination
– stop signal coded by one of three nonsense codons:
UAA - UAG – UGA
– Polypeptide released from ribosome as these tRNa
don’t have any amino acids so the chain can’t continue.
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Translation
Tell the story of translation with your group –
one person starts: “First….,” and says one
sentence. The next group member picks up
where the first left off, and so on.
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The 64 triplet codes
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60 code for 20 different amino
acids
3 act as "stop" and 1 acts as a
"start”.
GGG
GGU
GGC
GGA
All code for the
amino acid glycine
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PROTEIN SHAPE= FUNCTION
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Remember: The order of amino acids determines
the shape. HOW?
The chemistry of the R-group in the amino acid
determines how it reacts to neighboring amino
acids. The peptide bond is just holding the amino
acids together and is the same in all amino acids so
it lacks information.
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