Copyright © 2009 Pearson Education, Inc. Chapter 14 The Genetic Code and Transcription Copyright ©...

transcript

Chapter 14The Genetic Code and

Transcription

• 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

Evidence for the Triplet Code

• Three bases is the minimum length needed to code for 20 amino acids

• Reading frame studies

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

Evidence for a Commaless and Degnerate Code

• Frameshift would result in many nonsense mutations

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

Polynucleotide Phosphorylase

Figure 14.5

Triplet-Binding Assay

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

Interesting Patterns Among the Codons

• The Wobble Hypothesis

• Ordered code

• Initiation codons

• Termination codons

Figure 14.7

The Code is (Almost) Universal

Overlapping Genes

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

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

Consensus Sequences in Prokaryotic Promoters

• TATAAT

• aka: Pribnow box

• 10 nucleotides upstream from the transcription initiation site

• TTGACA

• These are considered cis-acting elements

• Trans-acting elements bind to the cis elements

Polycistronic mRNA

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

RNA Polymerases

Consensus Sequences in Eukaryotic Promoters

• TATAAAA – The Core Promoter

• aka: TATA box

• GGCCAATCT

• aka: CAAT box

• Others (Enhancers too)

Eukaryotic Trans-acting Factors

• Proteins called transcription factors

• Facilitate RNP II binding

• General transcription factors

• TFIID – aka TATA Binding Protein (TBP)

• Specific transcription factors

Eukaryotic mRNA Processing

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

Heteroduplex

Introns are Categorized

• Group I

• Group II

• Nuclear derived pre-mRNA transcripts

Group I Introns

The Spliceosome

Copyright © 2009 Pearson Education, Inc. Chapter 14 The Genetic Code and Transcription Copyright ©...

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