Biology 211 Intro Molecular and Cell Biology
Lectures 19 and 20 Transcription and Translation
Reading: Chap. 17
Outline:
1. DNARNAprotein
2. Prokaryotes versus eukaryotes
3. Transcription
4. Translation
5. Processing and sorting of proteins
Lecture
1. DNARNAprotein
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DNA contains the master instructions (like a cookbook).
RNA contains the information to make one protein (like a recipe). Code is
carried in three nucleotide segments =codons. Proteins carry out many
different functions in the cell.
Process of converting DNA information to RNA = transcription.
Process of using genetic code to make a protein =translation.
2. Prokaryotes vs. eukaryotes
Figure 17.2
Similarities:
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Prokaryotes and eukaryotes all carry genetic information in DNA, which is
transcribed into RNA, then translated into protein.
The genetic code used to determine the messenger RNA triplet code for a
particular amino acid is universal.
Differences:
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In prokaryotes, transcription and translation are "coupled". Genes do not
contain introns.
In eukaryotes, there is a separate compartment for transcription (nucleus)
and translation (cytoplasm). This is because the primary transcripts often
contain introns that must be processed before translation.
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3. Transcription: DNA-directed synthesis of RNA
A. Where does transcription take place? Nucleus
B. What components are needed for transcription?
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DNA template
RNA polymerase=Enzyme that synthesizes RNA
Ribonucleotides
C. What is a gene? A gene is a sequence of DNA that codes for an RNA or
protein. Most genes code for proteins.
D. How is a gene transcribed to make a messenger RNA? A messenger RNA
encodes the instructions for a protein
Three stages
 Initiation
 Elongation (5' 3')
 Termination
Figure 17.6
E. Initiation of transcription is highly regulated; thus mRNAs are not made unless
they are needed.
Initiation requires:
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Promoter region: Region where RNA polymerase binds at the beginning
(5' end) of a gene.
Binding to the promoter is regulated by transcription factors.
F. Eukaryotic cells process RNA "primary transcripts" into mRNAs in the nucleus
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Splicing= Removal of introns; carried out by large complex called a
splicesome (RNA-protein complex)
Addition of 5' cap=Guanine nucleotide added to beginning of mRNA
Addition of 3' poly A tail= About 30-200 adenine nucleotides added to end
of mRNA
G. Why do eukaryotic genes have introns?
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Important in evolution.
Allow for reshuffling of parts of genes to make new kinds of proteins.
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4. Translation: How genetic information flows from mRNA to protein.
A. Where does translation take place? Cytoplasm
B. What components are needed for translation?
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Messenger RNA (mRNA)
Transfer RNA (tRNA)
Amino acids
Ribosomes
Energy source (GTP)
C. Translation: Converting the language of nucleic acids to the language of
proteins.
mRNA: Carries the information as a series of codons
5' AUG GAC GGG ACU GCA UAA 3'
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codon: sequence of 3 bases on mRNA with information for one amino
acid
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genetic code: table of mRNA codons and the amino acids they specify
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information is read from 5' to 3' in mRNA
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AUG serves as the start codon: signal to start translation
tRNAs: Act as interpreters, converting nucleic acid information into a
sequence of amino acids
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small RNA molecules (75-80 nts); cloverleaf shape
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at least one tRNA for every type of amino acid
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have two major functional parts, the anticodon and the amino acid
attachment site
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the anticodon is a sequence of three bases that pair with the codon on
mRNA
5' AUG 3' mRNA codon
3' UAC 5' anticodon
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the amino acid is attached to its specific tRNA by an aminoacyl-tRNA
synthetase. This is called the "charging reaction".
ribosomes: Complexes made of ribosomal RNA and proteins where
protein synthesis takes place.
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Made of two subunits: large and small subunits
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Part of the ribosomal RNA actually catalyzes the formation of the
peptide bonds.
D. How is a mRNA translated into protein?
Process consists of three phases
 Initiation
 Elongation
 Termination
E. Initiation of translation Fig. 17.15
Requires:
mRNA
tRNA for methionine (initiator tRNA)
small subunit of ribosome
GTP provides energy
Then large subunit of ribosome binds
F. Elongation: Process of adding amino acids one by one to the growing
polypeptide chain.
Fig. 17.16
Key features:
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There are two sites on the ribosome for binding tRNAs, the P site and the
A site.
The growing protein chain is attached to the tRNA in the P site.
An incoming charged tRNA binds to the codon of the mRNA in the A site.
The ribosome catalyzes formation of a peptide bond.
Translocation of the ribosome occurs (ribosome moves along the mRNA
by one codon), placing the tRNA linked to the growing protein in the P site.
Elongation factors and GTP (energy source) are also required.
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G. Termination Fig. 17.17
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When a stop codon (UAA, UAG, UGA) is present in the A site, a release
factor binds and releases the polypeptide from the tRNA.
The ribosomal subunits and the mRNA dissociate.
5. Processing and sorting of proteins
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To be functional, the newly synthesized protein must fold into its threedimensional shape.
Additional steps called posttranslational modifications may be required
to make the protein functional.
Examples of modifications are: adding sugars, lipids, or phosphates,
proteolytic cleavages, forming disulfide bridges.
To get to their final locations in the cell and to have the correct groups
removed or added usually requires "sorting signals". These are bits of
information carried in the amino acid sequence that sends the proteins to
the correct locations in the cell.
Example: Signal peptide of hydrophobic amino acids for targeting proteins
to endoplasmic reticulum.
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