By Robert Hazen, George Mason University
A major challenge facing biochemists of the early 20th century was to discover the relationship between genes and the biomolecules that make up an organism. After all, genes somehow have to provide information on how to build chemicals. Genetics provide coded instructions—information about how the chemical laboratory of each cell is to be operated.
Amino Acids and Proteins
By the 1940s, it had been well-established that proteins are the chemical workhorses of life. After all, proteins are made from amino acids, the basic building blocks, and we have lots of amino acids strung together. The amino acids fold up and contort, and make three-dimensional structures which are very complex. And, this three-dimensional shape of the protein is critical to determine function.
It is important to remember that many proteins serve as catalysts, or enzymes, in biological systems. They help synthesize different molecules that are fundamental to the operation of living things. Enzymes digest food in the stomach, for example, enzymes produce heat and energy in cells. They help to assemble all matter of other substances that make up our body’s other structural proteins like skin, muscle, and ligaments.
Protein Vocabulary
These are all parts of the protein vocabulary, if you will. We have blood proteins that transfer oxygen around the body, hemoglobin. We also have the hormones, protein-related compounds that give various chemical signals to other parts of the body, and tell the body when to start manufacturing key chemicals.
In all these capabilities, it’s the shape of the protein molecule that’s paramount. One has to think about the active site, and how it can attract a molecule, how it fits; the size and the shape of that site are critical to the functioning of every protein.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Mutation
Since proteins are constructed from a very specific chain of amino acids, which folds into a compact shape, that sequence of amino acids is critical to the correct functioning of a protein. There are twenty different amino acids one can use. One can choose any one of the twenty in each consecutive place along that protein chain, so the selectivity is extremely important.
Different sequences of amino acids fold and twist and kink differently, creating different, complex three-dimensional shapes that can lock onto other molecules, in these varieties of ways.
A single mistake in an amino-acid sequence is referred to as a mutation. It is a mutation that may cause the protein shape to be altered, and it may fundamentally change the way that protein can follow its function.
Genetic Behavior of Cells
A good example of a mutation is sickle-cell disease. In this mutation, a single amino acid, a valine, substitutes mistakenly for a glutamic acid. It expresses as the protein folding up wrong, causing the hemoglobin to not operate correctly. Thus, as a result, in people with sickle-cell disease the red blood cells are distorted and deformed.
Studies of the genetic behavior of cells, in which certain enzymes were mutated on purpose, revealed that one gene, one set of coded information, corresponds to one specific enzyme or protein. That’s a fundamental idea: that a single piece of genetic information corresponds to one of these molecules that serves as an enzyme, or has a structural role, or works as a hormone. We’re getting closer to the molecular basis of genetics by recognizing that the information passed from one cell to another makes proteins; then proteins make all the other chemicals we need in our body.
Atoms of Inheritance
Similarly, it was the behavior of chromosomes, and understanding by watching chromosomes which illustrated how Gregor Mendel’s ideas of atoms of inheritance were not an abstraction but a real, physical presence. The chromosomes, and segments on the chromosomes, represented these genes, these atoms of inheritance.
As it turns out, each chromosome is a long structure; it’s a long molecule, a long collection of atoms. On that long chain of molecules, there are series of genes. There are series of separate coded instructions, each of which describes how we build a specific protein. This is the reason why Mendel’s fourth law was somewhat in error.
Role of Chromosomes
Strictly speaking, it’s the chromosomes that act independently of each other, not the genes. If we have two genes that lie on the same chromosome, they’re not going to be completely independent of each other, because the chromosomes themselves get passed off, not the individual genes. Mendel was quite fortunate in his selection of pea plants, and in his selection of the traits that he actually used in those experiments.
Chromosomes are now seen to carry the atoms of inheritance that are passed from parents to offspring. Half of the chromosomes come from each parent—that’s part of Mendel’s laws too—yet these studies at the cellular level could only go so far in elucidating the mechanics and the mechanism of genetics. They couldn’t show, for example, how a gene could make a plant tall, or how it could make a plant short. Ultimately, then, scientists had to isolate the molecule that carried genetic information and understand the detailed structure of that molecule.
Common Questions about Amino Acid Sequences and Mutation
Since proteins are constructed from a very specific chain of amino acids, which folds into a compact shape, that sequence of amino acids is critical to the correct functioning of a protein.
A single mistake in an amino-acid sequence is referred to as a mutation. It is a mutation that may cause the protein shape to be altered, and it may fundamentally change the way that protein can follow its function.
Studies of the genetic behavior of cells, in which certain enzymes were mutated on purpose, revealed that one gene, one set of coded information, corresponds to one specific enzyme or protein.
