MAKING PROTEINS, OR HOW GENES ARE EXPRESSED

Meade’s method of making antithrombin takes advantage of the fact that genes provide instructions for making proteins. But what are those instructions? How is the antithrombin protein actually made by goat cells?

In order to get from a gene to a protein, cells carry out two major steps: transcription and translation. Transcription is the process of using DNA to make a messenger RNA (mRNA) copy of the gene. Translation is the process of using this mRNA copy as a set of instructions to assemble amino acids into a protein (INFOGRAPHIC 8.7).

Why two separate steps? As the names “transcription” and “translation” imply, the process of gene expression is like copying a text and then converting it into another language. In this case, the text to be translated is a valuable, one-of-a-kind document: DNA. Just as you would be forbidden to borrow a rare manuscript from the library and would instead have to copy the text by hand into your notebook, the cell cannot take DNA out of its “library”–the nucleus. It must first make a copy–the mRNA. The cell can then take this mRNA copy into the cytoplasm, where it is translated into a new language: protein.

Transcription begins in the nucleus of a cell when an enzyme called RNA polymerase binds to DNA at a gene’s regulatory sequence, located just ahead of the coding sequence. At that site, cellular machinery unwinds the DNA double helix and RNA polymerase begins moving along one DNA strand. As it moves, the RNA polymerase “reads” the DNA sequence and synthesizes a complementary mRNA strand according to the rules of base pairing. The same rules of base pairing we discussed in the context of DNA structure apply here, with one difference: RNA nucleotides are made with the base uracil (U) instead of thymine (T). So the complementary base pairs are C with G and A with U (INFOGRAPHIC 8.8).

As its name implies, messenger RNA serves to relay information. Once the mRNA copy is made, it leaves the nucleus and attaches to a complex piece of cellular machinery in the cytoplasm called the ribosome. This is the start of translation.

During translation, the ribosome reads the mRNA transcript and translates it into a chain of amino acids. The mRNA transcript specifies which amino acids should be joined together to form chains. Amino acids are specified by groups of three nucleotides; each group is called a codon. Each codon is like a word: its letters name a particular amino acid (for example, the codon GGU specifies the amino acid glycine).

The actual building blocks of proteins–amino acids–are delivered to the ribosome by another type of RNA, called transfer RNA (tRNA), which physically transports amino acids to the ribosome. Each tRNA molecule is structured like an adaptor: one end binds to an amino acid, the other end binds to mRNA. The part that binds mRNA is called the anticodon because it base-pairs in a complementary fashion with an mRNA codon. When the amino acid-toting tRNA finds its codon match, it releases the amino acid to the ribosome, which adds it to the growing amino acid chain (INFOGRAPHIC 8.9).

The vast majority of mRNA codons specify a specific amino acid, but there are a few with other functions. Start codons are the first codon of a coding sequence; they tell the ribosome to start translating and begin adding amino acids. Stop codons tell the ribosome to stop translating and not add any more amino acids to the growing chain.

Although the human genome codes for many thousands of different proteins, each one is pieced together from a starting set of just 20 amino acids. In the same way that the 26 letters in our alphabet can spell hundreds of thousands of words, the basic set of amino acids can make hundreds of thousands of proteins. The set of rules dictating which mRNA codons specify which amino acid is called the genetic code. You can think of the genetic code as the key that cracks the mRNA code. Much in the same way the Rosetta Stone provided scholars with the key to decipher what Egyptian hieroglyphics meant, the genetic code tells us what amino acid each codon stands for.

There are two important features of the genetic code. One, the code is redundant: multiple codons specify the same amino acid. In many cases, a codon will differ at the third nucleotide position without changing the amino acid that is specified. Two, the genetic code is universal, which means that it is virtually the same in all living organisms. It is because the code is universal that the mammary cells of a goat carrying the human gene for antithrombin can express that gene and produce antithrombin protein in her milk (INFOGRAPHIC 8.10).