Page 111 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

PART II What Genes Are and What They Do
8

chapter

Gene Expression:

The Flow of

Information from

DNA to RNA

to Protein

The ability of an aminoacyl-tRNA synthetase (red) to recognize a
particular tRNA (blue) and couple it to its corresponding amino
acid (not shown) is central to the molecular machinery that con-
verts the language of nucleic acids into the language of proteins.

A DEDICATED EFFORT to determine the complete nucle­ chapter outline
otide sequence of the genome in a variety of organisms
has been underway since 1990. This massive endeavor • 8.1 The Genetic Code
has been more successful than many scientists thought • 8.2 Transcription: From DNA to RNA
possible. By the time of this writing in 2016, the DNA • 8.3 Translation: From mRNA to Protein
sequence in the genomes of more than 8100 different spe­ • 8.4 Differences in Gene Expression Between
cies had already been deposited in databases, and sequenc­ Prokaryotes and Eukaryotes
ing projects for more than 35,000 additional species were • 8.5 The Effects of Mutations on Gene Expression
in progress. With this sequence information in hand, and Function
geneticists can consult the genetic code—the cipher
equating nucleotide sequence with amino acid sequence—
to decide what parts of a genome are likely to be genes.
As a result, modern geneticists can discover the number and amino acid sequences
of all the polypeptides that determine phenotype. Knowledge of DNA sequence thus
opens up powerful new possibilities for understanding an organism’s growth and
development at the molecular level.
In this chapter, we describe the cellular mechanisms that carry out gene expression,
the means by which genetic information can be interpreted as phenotype. As intricate as
some of the details may appear, the general scheme of gene expression is elegant and
straightforward: Within each cell, genetic information flows from DNA to RNA to protein.
This statement was set forward as the Central Dogma of molecular biology by Francis
Crick in 1957. As Crick explained, “Once information has passed into protein, it cannot
get out again.”
The Central Dogma maintains that genetic information flows in two distinct stages
(Fig. 8.1). The conversion of the information in DNA to its equivalent in RNA is
known as transcription. The product of transcription is a transcript: a molecule of
messenger RNA (mRNA) in prokaryotes, a molecule of RNA that undergoes process­
ing to become an mRNA in eukaryotes.
In the second stage of gene expression, the cellular machinery decodes the sequence
of nucleotides in mRNA into a sequence of amino acids—a polypeptide—by

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