Essential Cell Biology 5th edition

CHAPTER SEVEN
7
From DNA to Protein:
How Cells Read the Genome
Once the double-helical structure of DNA (deoxyribonucleic acid) had
been determined in the early 1950s, it became clear that the hereditary
information in cells is encoded in the linear order—or sequence—of the
four different nucleotide subunits that make up the DNA. We saw in
Chapter 6 how this information can be passed on unchanged from a cell
to its descendants through the process of DNA replication. But how does
the cell decode and use the information? How do genetic instructions
written in an alphabet of just four “letters” direct the formation of a bacterium,
a fruit fly, or a human? We still have a lot to learn about how the
information stored in an organism’s genes produces even the simplest
unicellular bacterium, let alone how it directs the development of complex
multicellular organisms like ourselves. But the DNA code itself has
been deciphered, and we have come a long way in understanding how
cells read it.
FROM DNA TO RNA
FROM RNA TO PROTEIN
RNA AND THE ORIGINS OF LIFE
Even before the code was broken, it was known that the information
contained in genes somehow directed the synthesis of proteins. Proteins
are the principal constituents of cells and determine not only cell structure
but also cell function. In previous chapters, we encountered some
of the thousands of different kinds of proteins that cells can make. We
saw in Chapter 4 that the properties and function of a protein molecule
are determined by the sequence of the 20 different amino acid subunits
in its polypeptide chain: each type of protein has its own unique amino
acid sequence, which dictates how the chain will fold to form a molecule
with a distinctive shape and chemistry. The genetic instructions carried
by DNA must therefore specify the amino acid sequences of proteins. We
will see in this chapter exactly how this happens.