We have mentioned many times in earlier lectures that DNA itself does not really do anything. It is merely a carrier of information - a cookbook containing recipes for proteins that are the actors of biology. In this section we will learn about how the information contained in DNA is used to synthesize proteins. In summary, this process involves the unpacking and unwinding of the DNA, the transcription of a mRNA copy of one DNA strand, and the translation of the mRNA copy into protein sequence.

Like DNA, RNA is a nucleic acid polymer. Its base sequence represents a copy of one DNA strand, and it is synthesized by RNA polymerase (a protein) in a process that uses base-pairing with the DNA template to determine the sequence of the RNA transcript. It is this mRNA intermediate, then, and not the DNA itself, that serves as the recipe for protein synthesis. See figure 12.4 for a good illustration of the process of transcription. Like replication, transcription uses base pairing between template and new strands to specify sequence. Like replication, bases are added one at a time in the 5' to 3' direction on the new strand. Unlike replication, only certain parts of the DNA (genes) are transcribed and transcription results in a single-stranded product (mRNA). In eukaryotic cells, the mRNA is further processed before export to the cytoplasm - introns are removed and a poly-A tail is added.

Once the mRNA is complete, it can be translated into protein sequence. This process is more complicated than transcription, and involves structures called ribosomes. Ribosomes read the mRNA three bases at a time, and insert an amino acid into the growing protein chain based on the sequence of the mRNA. The genetic code is the translation table that the ribosome uses to specify protein sequence. This reading takes place using tRNA adapter molecules, each of which can base pair with a DNA codon (word) and carried an amino acid monomer. When the adapter molecule (tRNA) is in place (base paired to the mRNA) and in the A site of the ribosome (see Figure 12.10) a covalent bond can be formed between the amino acids in the two sites of the ribosome. In this way, the protein chain is elongated by one amino acid for each three-letter mRNA codon.

Now that you understand the process of gene expression, we can think about what happens when a gene is mutated. Look at the genetic code table on page 188 of your text. Since there are sixty-four possible codons and only twenty amino acids, several different codons sometimes specify the same amino acid (look at the codons for the amino acid valine in the lower left part of the table.) This means that a mutation (defined as a change in the DNA sequence of an organism) might result in an unaltered protein. Alternatively, a base-pair substitution mutation might result in one incorrect amino acid, and in a partially functional protein, as is the case in sickle cell anemia. The consequences of a frameshift mutation usually result in the addition of many incorrect amino acids and in a protein that is not even partially functional.

Since all of the cells in our bodies contain identical DNA sequences, how is it that cells are different from one another? It is hard to believe tat a skin cell and a brain cell are in essence running the same software program. In order to achieve this difference, and to be able to carry out development and adapt to changes in the external environment, cells express differing sets of genes. The genes that are being expressed depend on the type of cell, the developmental state of the organism, and an almost infinite number of environmental cues. These changes in gene expression are mediated by regulatory proteins that interact with the DNA and tell RNA polymerase which genes to express (transcribe into mRNA).

As a simple example of this type of switch, consider the E. coli lac operon shown in Figure 12.15. The genes of this operon specify proteins that allow the bacterial cell to burn lactose for energy. It would be wasteful to express these proteins in the absence of lactose, so the under these conditions, a regulatory protein binds to the DNA upstream of the operon and prevents RNA polymerase from expressing the genes. In the presence of lactose, this protein changes conformation, falls off the DNA, and allows RNA polymerase to transcribe an mRNA transcript.

Furthermore, a second switch also acts on this operon to allow its expression only when glucose (a preferred energy source) is absent. Thus the genes of the lac operon are expressed only in the presence of lactose and the absence of the preferred energy source glucose. This is an example of how a bacterium can use networks of regulatory proteins to make decisions about which genes to express at which times. Remember that regulation of gene expression in eukaryotic cells is more complex, and responds subtly to many different stimuli, but overall obeys the same rules of protein binding preventing or encouraging the expression of sets of genes.