Download - From Gene to Protein Chapter 17. Concept 17.3: Eukaryotic cells modify RNA after transcription
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From Gene to Protein
Chapter 17
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Concept 17.3:Eukaryotic cells modify RNA after
transcription
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Additions to pre-mRNA: 5’ cap (modified guanine) and 3’ poly-A
tail (50-520 A’s) are added
Help export from nucleus, protect from enzyme degradation, attach to ribosomes
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RNA Splicing
Pre-mRNA has introns (noncoding sequences) and exons (codes for amino acids)
Splicing = introns cut out, exons joined together
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RNA Splicing small nuclear
ribonucleoproteins = snRNPs snRNP = snRNA + protein Pronounced “snurps” Recognize splice sites
snRNPs join with other proteins to form a spliceosome
Spliceosomes catalyze the process of removing introns and joining exons
Ribozyme = RNA acts as enzyme
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Why have introns? Some regulate gene
activity
Alternative RNA Splicing: produce different combinations of exons One gene can make
more than one polypeptide!
20,000 genes 100,000 polypeptides
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Concept 17.4:Translation is the RNA-directed
synthesis of a polypeptide
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Components of Translation1. mRNA = message2. tRNA = interpreter3. Ribosome = site of
translation
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tRNA Transcribed in nucleus Specific to each amino acid Transfer AA to ribosomes Anticodon: pairs with
complementary mRNA codon
Base-pairing rules between 3rd base of codon & anticodon are not as strict. This is called wobble.
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tRNA
Aminoacyl-tRNA-synthetase: enzyme that binds tRNA to specific amino acid
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Ribosomes Ribosome = rRNA +
proteins made in nucleolus 2 subunits
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Ribosomes
Active sites: A site: holds AA to be
added P site: holds growing
polypeptide chain E site: exit site for tRNA
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Translation:1. Initiation
• Small subunit binds to start codon (AUG) on mRNA• tRNA carrying Met attaches to P site• Large subunit attaches
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2. Elongation
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2. Elongation
Codon recognition: tRNA anticodon matches codon in A site
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2. Elongation
Peptide bond formation: AA in A site forms bond with peptide in P site
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2. Elongation
Translocation: tRNA in A site moves to P site; tRNA in P site moves to E site (then exits)
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2. Elongation
Repeat over and over
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3.Termination Stop codon reached and translation stops Release factor binds to stop codon;
polypeptide is released Ribosomal subunits dissociate
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Polyribosomes A single mRNA
can be translated by several ribosomes at the same time
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Protein Folding During synthesis, polypeptide chain coils and
folds spontaneously Chaperonin: protein that helps polypeptide
fold correctly
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Types of Ribosomes Free ribosomes: synthesize proteins that
stay in cytosol and function there Bound ribosomes (to ER): make proteins of
endomembrane system (nuclear envelope, ER, Golgi, lysosomes, vacuoles, plasma membrane) & proteins for secretion Uses signal peptide to target location
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Cellular “Zip Codes” Signal peptide: 20 AA at leading end of
polypeptide determines destination Signal-recognition particle (SRP): brings
ribosome to ER
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Concept 17.5:Point mutations can affect protein
structure and function
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The Central Dogma
Mutations happen here
Effects play out here
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Mutations = changes in the genetic material of a cell Large scale mutations: chromosomal; always
cause disorders or death nondisjunction, translocation, inversions,
duplications, large deletions Point mutations: alter 1 base pair of a gene
1.Base-pair substitutions – replace 1 with another Missense: different amino acid Nonsense: stop codon, not amino acid
2.Frameshift – mRNA read incorrectly; nonfunctional proteins Caused by insertions or deletions
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Substitution = Missense
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Substitution = Nonsense
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Substitution = Silent (no effect)
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Insertion = Frameshift Mutation
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Deletion = Extensive missense, premature termination
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Sickle Cell Disease
SymptomsAnemia
PainFrequent infections
Delayed growthStroke
Pulmonary hypertensionOrgan damage
BlindnessJaundice
gallstones
Life expectancy42 in males 48 in females
Caused by a genetic defect
Carried by 5% of humans
Carried by up to 25% in some regions of Africa
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Sickle-Cell Disease = Point Mutation
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Sickle cell hemoglobin forms long, inflexible chains
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