Chapter 2
How Genes Work
What are proteins and what do they do?
Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body's tissues and organs.
Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. There are 20 different types of amino acids that can be combined to make a protein. The sequence of amino acids determines each protein's unique 3-dimensional structure and its specific function.
Proteins can be described according to their large range of functions in the body, listed in alphabetical order:
For more information about proteins and their functions:
A discussion of the role of proteins can be found in the NCBI Science Primer in the chapter called What Is A Genome? (http://www.ncbi.nlm.nih.gov/About/primer/ genetics_genome.html). Scroll down to the heading "Proteins."
KidsHealth from Nemours offers a basic overview of proteins (http://kidshealth.org/ kid/stay_healthy/body/protein.html) and what they do.
Illustrations
Immunoglobulin G is a type of antibody that circulates in the blood and recognizes foreign particles that might be harmful.
The functional phenylalanine hydroxylase enzyme is made up of four identical subunits. The enzyme converts the amino acid phenylalanine to another amino acid, tyrosine.
Growth hormone is a messenger protein made by the pituitary gland. It regulates cell growth by binding to a protein called a growth hormone receptor.
Actin filaments, which are structural proteins made up of multiple subunits, help muscles contract and cells maintain their shape.
Ferritin, a protein made up of 24 identical subunits, is involved in iron storage.
How do genes direct the production of proteins?
Most genes contain the information needed to make functional molecules called proteins. (A few genes produce other molecules that help the cell assemble proteins.) The journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps: transcription and translation. Together, transcription and translation are known as gene expression.
During the process of transcription, the information stored in a gene's DNA is transferred to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of nucleotide bases, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.
Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which "reads" the sequence of mRNA bases. Each sequence of three bases, called a codon, usually codes for one particular amino acid. (Amino acids are the building blocks of proteins.) A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a "stop" codon (a sequence of three bases that does not code for an amino acid).
The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology. It is so important that it is sometimes called the "central dogma."
Through the processes of transcription and translation, information from genes is used to make proteins.
For more information about making proteins:
The Genetic Science Learning Center at the University of Utah offers an interactive introduction to transcription and translation (http://learn.genetics.utah.edu/content/ begin/dna/).
For a more detailed description of transcription and translation, refer to the NCBI Science Primer's chapter titled What Is A Genome? (http://www.ncbi.nlm.nih.gov/ About/primer/genetics_genome.html). Scroll down to the heading "From Genes to Proteins: Start to Finish."
The New Genetics, a publication of the National Institute of General Medical Sciences, includes discussions of transcription (http://publications.nigms.nih.gov/ thenewgenetics/chapter1.html#c4) and translation (http://publications.nigms.nih.gov/ thenewgenetics/chapter1.html#c7).
Can genes be turned on and off in cells?
Each cell expresses, or turns on, only a fraction of its genes. The rest of the genes are repressed, or turned off. The process of turning genes on and off is known as gene regulation. Gene regulation is an important part of normal development. Genes are turned on and off in different patterns during development to make a brain cell look and act different from a liver cell or a muscle cell, for example. Gene regulation also allows cells to react quickly to changes in their environments. Although we know that the regulation of genes is critical for life, this complex process is not yet fully understood.
Gene regulation can occur at any point during gene expression, but most commonly occurs at the level of transcription (when the information in a gene's DNA is transferred to mRNA). Signals from the environment or from other cells activate proteins called transcription factors. These proteins bind to regulatory regions of a gene and increase or decrease the level of transcription. By controlling the level of transcription, this process can determine the amount of protein product that is made by a gene at any given time.
For more information about gene regulation:
More information about gene regulation can be found in the NCBI Science Primer. Refer to the chapter called What Is A Genome? (http://www.ncbi.nlm.nih.gov/About/ primer/genetics_genome.html) and scroll down to the headings "Gene Switching: Turning Genes On and Off," "Controlling Transcription," and "Controlling Translation."