Download 122 lec 11 revtranslation

Announcements
• Will put quicktime movies on
reserve in library
• Quiz next Thursday (not today!)
Transcription and translation
• Process to produce proteins (AA
polymers) from genes (NA polymers)
• DNA contains information to make a
polypeptide.
• One way flow of information:
DNA --> RNA --> Proteins
Figure 17.3 The triplet code
Interpretation of 17.3
• Each gene codes for a different protein
(one gene-one enzyme hypothesis)
• RNA is transcribed from DNA
– both are NA polymers
– RNA sequence determined by rules of base
pairing
• Proteins are translated from RNA
– Different polymer type
– No chemical rules
One gene-one enzyme
• Metabolism occurs through
pathways of chemical reactions
• Each step catalyzed by a single
enzyme
• Can study pathways by identifying
metabolic needs of mutants
Example: Synthesis of Arginine
Enzyme A
Enzyme B
Enzyme C
Precursor--> ornithine--> citrullene--> arginine
- some mutants require arginine
- others require citrullene or ornithine
hypothesis: each mutant lacks an enzyme
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Figure 17.1 Beadle & Tatum’s evidence for one gene-one enzyme hypothesis
Transcription movie
Figure 17.25 A summary of transcription and translation in a eukaryotic cell
Transcription movie
• Note arrangement of promoter, gene,
and terminator
• Pause to look at template DNA,
coding DNA, and mRNA (similar to
17.6, p. 309)
Figure 17.0 Ribosome
What’s needed for
translation
• mRNA strand- codes for polypeptide
• Ribosome- moves down mRNA
strand to ‘read’ it
• tRNA molecules- add AA’s via base
pairing to mRNA strand
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Figure 17.15 The anatomy of a functioning ribosome
Figure 17.13b The structure of transfer RNA (tRNA)
Translation movie
Translation movie
• Name three stages of translation
• Find the codon and anticodon
Figure 17.17 The initiation of translation
Steps in translation
• Initiation
• Elongation
• Termination
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Figure 17.18 The elongation cycle of translation
Initiation
• Small ribosomal subunit binds to
mRNA
• First tRNA binds
• Large ribosomal subunit binds
Figure 17.18 The elongation cycle of translation
Elongation
• Codon recognition- mRNA binds to
anticodon in tRNA
• Peptide bond formation- between
amino acids on ‘old’ and ‘new’ tRNA
• Translocation- Ribosome moves
down mRNA and releases ‘old’ tRNA
Figure 17.19 The termination of translation
Termination
• Ribosome reaches ‘stop’ codon
• Release factor binds to codon
• Complex dissociates
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Introduction to mutation
• Point mutations (Ch. 17)
– Substitutions
– Insertions or deletions
• Chromosomal mutations (Ch. 15,
pp. 279-281)
– Chromosome structure
– Chromosome number
Significance of mutations
• Source of genetic variation
– Many mutations are disadvantageous
– Some are neutral
– A few are favorable
• Needed for evolution
– Genetic variation required
– Favorable mutations spread through
population
Where do mutations come from?
• Random error when genes passed
to offspring
– DNA replication- point mutations
– Meiosis- chromosomal mutation
• Offspring inherit ‘mutated’ genes
• Mutants ‘spread’ in population
Modified Fig 16.8: Source of a point mutation in
haploid organisms
Original DNA
Wild type
Error in DNA
replication
• Error occurs
during replication
• 1st generation
has ‘mixed DNA
• 2nd generation
produces a
mutant cell
Mix
Mutant cell
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Eukaryote mutations
• Population consists of wild type homozygotes
• Mutant heterozygote appears
• Mutant homozygote appears through hybrid
cross (A+Am X A+A m)
A+A +
A+A +
A+A +
A+A +
A+A +
A+A +
A+A +
A+A +
A+A +
AmA +
A+A +
A+A +
A+A +
AmA +
AmA m
A+A +
A+A +
AmA +
+
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