copyright © 2009 pearson education, inc. chapter 14 the genetic code and transcription copyright ©...
TRANSCRIPT
Copyright © 2009 Pearson Education, Inc.
Chapter 14The Genetic Code and
Transcription
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc. Figure 14.1
Copyright © 2009 Pearson Education, Inc.
• Written in linear form
• Each word consists of 3 ribonucleotide
letters
• The code is unambiguous
• The code is degenerate
• The code contains 1 start and 3 stop
codons
• The code is commaless
• The code is non-overlapping
• The code is (nearly) universal
Characteristics of the Genetic Code
Copyright © 2009 Pearson Education, Inc.
Evidence for the Triplet Code
• Three bases is the minimum length needed to code for 20 amino acids
• Reading frame studies
Copyright © 2009 Pearson Education, Inc. Figure 14.2
Copyright © 2009 Pearson Education, Inc.
Evidence for a Non-Overlapping Code
• If code was overlapping:
• Amino acid sequences would be restricted
• Base substitutions would affect two adjacent amino acids
• Translation would be too complex to be efficient
Copyright © 2009 Pearson Education, Inc.
Evidence for a Commaless and Degnerate Code
• Frameshift would result in many nonsense mutations
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Deciphering the Code
• Made possible by advancements that:
• Allowed protein synthesis in vitro
• Synthesizing RNA strands in vitro
• First studies utilized homopolymer RNAs
• Advanced to copolymers and heteropolymer RNAs
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Polynucleotide Phosphorylase
Copyright © 2009 Pearson Education, Inc. Table 14.1
Copyright © 2009 Pearson Education, Inc. Figure 14.4
Copyright © 2009 Pearson Education, Inc.Figure 14-5 Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 14.5
Triplet-Binding Assay
Copyright © 2009 Pearson Education, Inc. Table 14.2
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Repeating Copolymers
• Long RNA molecules consisting of short, repeated segments
• Begin with di-, tri-, or tetra- nucleotides
• Connect together using special enzymes
• By comparing data from several experiments, it is possible to assign specific codons to specific amino acids
Copyright © 2009 Pearson Education, Inc. Figure 14.6
Copyright © 2009 Pearson Education, Inc. Table 14.3
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Interesting Patterns Among the Codons
• The Wobble Hypothesis
• Ordered code
• Initiation codons
• Termination codons
Copyright © 2009 Pearson Education, Inc.Figure 14-7 Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 14.7
Copyright © 2009 Pearson Education, Inc. Table 14.4
Copyright © 2009 Pearson Education, Inc. Table 14.5
The Code is (Almost) Universal
Copyright © 2009 Pearson Education, Inc. Figure 14.8
Overlapping Genes
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Observations Suggesting RNA is the Intermediate
• DNA is in the nucleus of eukaryotic cells, but protein synthesis happens on ribosomes in the cytoplasm
• RNA is synthesized in the nucleus
• RNA migrates to the cytoplasm
• Amount of RNA is generally proportional to the amount of protein in the cell
Copyright © 2009 Pearson Education, Inc. Table 14.6
Copyright © 2009 Pearson Education, Inc. Figure 14.3
RNA Polymerase• Same general substrate requirements as
DNA Polymerase
• No primer is needed
• Holoenzyme with several subunites (E. coli)
• β and β’ are catalytic
• σ plays a regulatory function
• Prokaryotes have a single form of RNA pol (with different σ)
• Eukaryotes have three distinct RNA pols
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Consensus Sequences in Prokaryotic Promoters
• TATAAT
• aka: Pribnow box
• 10 nucleotides upstream from the transcription initiation site
• TTGACA
• 35 nucleotides upstream from the transcription initiation site
• These are considered cis-acting elements
• Trans-acting elements bind to the cis elements
Copyright © 2009 Pearson Education, Inc. Figure 14.9a
Copyright © 2009 Pearson Education, Inc. Figure 14.9b
Copyright © 2009 Pearson Education, Inc. Figure 14.9c
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Polycistronic mRNA
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Transcription in Eukaryotes
• Occurs in the nucleus
• 3 different RNA polymerases
• Chromatin remodeling must occur
• Extensive interaction between cis elements and trans factors
• mRNA processing
Copyright © 2009 Pearson Education, Inc. Table 14.7
RNA Polymerases
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Consensus Sequences in Eukaryotic Promoters
• TATAAAA – The Core Promoter
• aka: TATA box
• 35 nucleotides upstream from the transcription initiation site
• GGCCAATCT
• aka: CAAT box
• 80 nucleotides upstream from the transcription initiation site
• Others (Enhancers too)
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Eukaryotic Trans-acting Factors
• Proteins called transcription factors
• Facilitate RNP II binding
• General transcription factors
• TFIID – aka TATA Binding Protein (TBP)
• Specific transcription factors
Copyright © 2009 Pearson Education, Inc. Figure 14.10
Eukaryotic mRNA Processing
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Introns vs Exons
• Not all of a eukaryotic mRNA is translated into proteins
• Intervening sequences – introns
• Are removed during splicing
• Expressed sequences – exons
• Translated into proteins
Copyright © 2009 Pearson Education, Inc. Figure 14.11
Heteroduplex
Copyright © 2009 Pearson Education, Inc. Figure 14.12
Copyright © 2009 Pearson Education, Inc. Table 14.8
Copyright © 2009 Pearson Education, Inc. Figure 14.3
Introns are Categorized
• Group I
• Group II
• Nuclear derived pre-mRNA transcripts
Copyright © 2009 Pearson Education, Inc. Figure 14.13
Group I Introns
Copyright © 2009 Pearson Education, Inc. Figure 14.14
The Spliceosome
Copyright © 2009 Pearson Education, Inc. Figure 14.15