The linear sequence of deoxyribonucleotides making
up DNA ultimately dictates the components constituting
proteins, the end product of most genes. The
central question is how such information stored as a nucleic
acid is decoded into a protein. Figure 13–1 gives a simplified
overview of how this transfer of information occurs. In
the first step in gene expression, information on one of the
two strands of DNA (the template strand) is transferred into
an RNA complement through transcription. Once synthesized,
this RNA acts as a “messenger” molecule bearing
the coded information—hence its name, messenger RNA
(mRNA). The mRNAs then associate with ribosomes,
where decoding into proteins takes place.
In this chapter we focus on the initial phases of gene
expression by addressing two major questions. First, how
is genetic information encoded? Second, how does the
transfer from DNA to RNA occur, thus defining the process
of transcription? As you shall see, ingenious analytical
research established that the genetic code is written in units
of three letters—ribonucleotides present in mRNA that
reflect the stored information in genes. Each triplet code
word directs the incorporation of a specific amino acid into
a protein as it is synthesized. As we can predict based on our
prior discussion of the replication of DNA, transcription is
also a complex process dependent on a major polymerase
enzyme and a cast of supporting proteins. We will explore
what is known about transcription in bacteria, and then contrast
this prokaryotic model with the differences found in
eukaryotes. Together, the information in this and the next
chapter provides a comprehensive picture of molecular
genetics, which serves as the most basic foundation for the
understanding of living organisms. In Chapter 14, we will
address how translation occurs and discuss the structure
and function of proteins.
- 13.1 The Genetic Code Exhibits a Number of
Characteristics
- 13.2 Early Studies Established the Basic Operational
Patterns of the Code
- The Triplet Nature of the Code
- 13.3 Studies by Nirenberg, Matthaei, and Others
Deciphered the Code
- Cell-free Polypeptide Synthesis
- The Use of Homopolymers
- Mixed Copolymers
- The Triplet Binding Assay
- Repeating Copolymers
- 13.4 The Coding Dictionary Reveals the Function of
the 64 Triplets
- Degeneracy and the Wobble Hypothesis
- Initiation and Termination
- 13.5 The Genetic Code Has Been Confirmed in Studies
of Phage MS2
- 13.6 The Genetic Code Is Nearly Universal
- 13.7 Transcription Synthesizes RNA on a DNA
Template
- 13.8 RNA Polymerase Directs RNA Synthesis
- Promoters, Template Binding, and the sigma Subunit
- Initiation, Elongation, and Termination of RNA
Synthesis
- 13.9 Transcription in Eukaryotes Differs from
Prokaryotic Transcription in Several Ways
- Initiation of Transcription in Eukaryotes
- Recent Discoveries Concerning RNA Polymerase
Function
- Heterogeneous Nuclear RNA and Its Processing: Caps and Tails
- 13.10 The Coding Regions of Eukaryotic Genes Are
Interrupted by Intervening Sequences
- Splicing Mechanisms: Autocatalytic RNAs
- Splicing Mechanisms: The Spliceosome
- 13.11 RNA Editing Modifies the Final Transcript
