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RNA and Protein Synthesis
RNA Structure
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Like DNA- Nucleic acid- composed of a
long chain of nucleotides (5-carbon
sugar + phosphate group + 4 different
nitrogenous bases)
3 Main differences between DNA/RNA:
– RNA sugar is Ribose (vs. Deoxyribose)
– RNA is single stranded (vs. Double strand)
– RNA contains Uracil (vs. Thymine)
RNA Structure
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BASICALLY: Each RNA molecule is a
working copy of a single gene; A single
DNA sequence is copied to form a RNA
sequence
THEN, the RNA sequence is used for ONE
JOB: PROTEIN SYNTHESIS
Assembly of amino acids into proteins
controlled by RNA
RNA Sequence
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3 types of RNA:
– Messenger RNA (mRNA)
– Transfer RNA (tRNA)
– Ribosomal RNA (rRNA)
– Messenger RNA = Carry copies of the
“instructions” or “messages” to assemble
amino acids into proteins
RNA Sequence
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mRNA carries the “message” from the DNA
(found in the nucleus) to the Ribosomes
(within the cytoplasm) for protein
synthesis to occur
Ribosomes are composed of several
proteins along with a form of RNA called
rRNA
tRNA = transfers amino acids to ribosomes
Transcription
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Transcription- the process involved in
copying DNA to produce an RNA
“transcript;” In other words, the nucleotide
sequence of DNA is copied to produce a
complementary RNA sequence
RNA polymerase- binds to the DNA
sequence, “reads” the template to produce
the RNA “transcript”
Transcription
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How does RNA polymerase “know” where
to start and stop making an RNA copy of
the DNA sequence?
RNA polymerase binds to PROMOTER
regions of DNA (signals on DNA to indicate
where the enzyme should bind to produce
the RNA transcript)
Similar sequences show where enzyme
should stop copying DNA sequence
RNA Editing
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RNA sequences need to be “edited” or
“fine tuned” prior to its readiness during
protein synthesis
rRNA molecules are produced from
larger RNA molecules that are cut and
trimmed to their final sizes
Introns- pieces of RNA sequence
removed during the editing process
RNA Editing
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Exons- remaining portion of RNA
sequences are “expressed sequences”;
exons are spliced together to produce final
RNA sequence
Purpose? Some RNA molecules are cut and
spliced in different ways in different
tissues making it possible for single gene
to produce several different RNA forms
The Genetic Code
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Proteins are produced from the linkage
of amino acids together
Together these strings of amino acids
are called POLYPEPTIDES
Protein properties are determined by
the order of the different amino acids
joined together in polypeptide
production
The Genetic Code
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The Genetic code = the “language” of
mRNA instructions
mRNA made up of 4 “letters” -A, G, C
and U;
CRITICAL THINKING QUESTION:
– a. There are a total of 20 amino acids
– b. How many “letters” define the “word”
for each amino acid sequence?
The Genetic Code
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Codon- three consecutive nucleotides that
specify a single amino acid that is to be
added to the growing polypeptide
sequence
Given there are four different bases, HOW
MANY 3-base codons are possible?
Therefore, there is REDUNDANCY among
the 20 amino acids
The Genetic Code
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AUG = Methionine = Start Codon
UGA, UAA and UAG ALL = Stop Codon
Stop Codon = the period at the end of
a sentence or signify end of polypeptide
sequence
Translation
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Translation = “decoding” of mRNA
message into polypeptide chain (protein)
Ribosome = site of Protein synthesis
Process involves:
– 1. DNA (within nucleus) transcribed into
single strand mRNA sequence
– 2. mRNA sequence shipped out to
cytoplasm
Translation
– 3. In cytoplasm, mRNA attaches to
ribosome
– 4. Each “codon” of mRNA is matched up
with “anticodon” (complimentary)
sequence on tRNA (NOTE: attached to
each tRNA is the appropriate amino acid
that matches the codon-anticodon
recognition sequence)
Translation
– 5. Like an assembly line, the AUG codon on
the mRNA/ribosome is matched up (via a
peptide bond) with the UAC anticodon
sequence on the tRNA (also containing
methionine); the amino acid is added to
the growing peptide sequence and, once
this occurs, the tRNA is discarded to make
room on the ribosome for the next
tRNA/anticodon/attached amino acid
Translation
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6. This codon/anticodon recognition
continues one amino acid at a time
(again, tRNA binds to ribosome at
recognition sequence, adds appropriate
amino acid and then discarded) until
stop codon reached indicating the
completion of the polypeptide sequence