Session 6 -- Page 1
July 11, 2000

Transcription - 1

Review

Let's review what has been covered so far.

• Polymers are the most important molecules in living systems.
• The polymers that are of prime interest to molecular biologists come in two varieties: informational and operational.
• DNA is an informational polymer; proteins are operational polymers; and some RNA's can serve as informational molecules while others can be operational.
• Proteins function in many ways. But perhaps their most important role is to orchestrate the thousands of chemical reactions that are going on in cells. They do this by serving as highly specific catalysts -- enzymes -- speeding up certain chemical reactions a million or more times.
• Enzymes can distinguish among many different compounds, and bind only to certain ones.
• This specific binding arises from the fact that the proteins have pockets on their surface into which only complementary substances fit.
• To form these complementary pockets, proteins have to assume a highly convoluted shape.
• The shape of a protein is determined by the sequence of its amino acids.

These basic principles of molecular biology have become well established over the last 30 years or so. If you understand them, you should have some additional questions. Here are some that may have occurred to you.

If the shape of a protein is critical to its function, and the shape of a protein is determined by the sequence of its amino acids, how is the sequence of amino acids in a protein determined?

The answer to this question is "DNA". That is, it is DNA that specifies the sequence of amino acids in proteins in an organism. In other words, the order of amino acids in all the proteins in organisms -- organisms as diverse as humans, hazelnut trees, and hagfish -- is dictated by their DNA.

How does DNA specify the sequence of proteins?

The answer to this question appears to be the following: It is the sequence of nucleotides in DNA that dictates the sequence of amino acids in proteins. Somehow -- and in the next session we're going to examine exactly how DNA accomplishes this feat -- DNA sequences are translated into protein sequences.

If it is the DNA sequence that orders the amino acids in proteins, what is it that determines the order of nucleotides in DNA?

The answers to this question are "Evolution" and "Natural Selection". Over billions of years, Nature tries out different sequences of DNA. Those that work well are retained; those that don't are discarded.