DNA: The Molecular Basis of Mutations
Since mutations are simply changes in DNA, in order to understand how mutations work, let’s review how DNA does its job. Your DNA contains a set of instructions for "building" a human. These instructions are inscribed in the structure of the DNA molecule through a genetic code. The sequence of these bases (A,T,C,G) encodes instructions. Your DNA is made up of genes that carry the instructions for making proteins — which are long chains of amino acids. These proteins help build an organism.
Protein-coding DNA can be divided into codons — sets of three bases that specify an amino acid or signal the end of the protein. The cellular machinery uses these instructions to assemble a string of corresponding amino acids (one amino acid for each three bases) that form a protein. After the protein is built based on the sequence of bases in the gene, the completed protein is released to do its job in the cell. When all the proteins are being made correctly then the organism is healthy and not diseased.
Types of Mutations
Environmental factors and lifestyle can play a significant role in many diseases, but increasingly scientists are discovering that all diseases (or our response to them) probably have a genetic component. One small DNA alteration in a critical gene can lead to a severe inherited disease, predispose us to chronic diseases, even render us more vulnerable to an infectious disease, and sometimes result in beneficial new genes and functions. These small DNA alterations are known as mutations.
Mutations in DNA result in an altered protein. In some instances the protein is normal enough to function but not as well. In other instances that protein may be completely disabled. The health consequences of a particular mutation depend on two factors: (1) how it alters a proteins function (2) how vital that particular protein is to survival.
There are many different ways that DNA can be changed, resulting in different types of mutation. Here is a quick summary of a few of these:
Substitution:
A substitution is a mutation that exchanges one base for another. (i.e, a change in a single “chemical letter” such as switching an A to a G.)
Insertion:
Insertions are mutations in which extra base pairs are inserted into a new place in the DNA. These can contribute to frame shift mutations.
Deletion:
Deletions are mutations in which a section of DNA is lost, or deleted. These can contribute to frame shift mutations.
These mutations can:
(1) change a codon to encode a different amino acid and cause a small change in the protein produced. An example of this is sickle cell anemia where the protein to carry oxygen in the blood is not functioning to its full capability.
(2) change a codon to encode the same amino acid and causes no change to the protein. This is called a silent mutation.
(3) change an amino-acid-coding codon into a “stop” codon and cause an incomplete protein. This can have serious effects since the protein will probably won’t function.
(4) cause frameshifts which can alter the gene so that its message is no longer correctly phrased. For example, consider the sentence, “The fat cat sat.” Each word represents a codon. If we delete the first letter the sentence doesn’t make sense, “hef act ats at”. The same thing happens to DNA if a letter is deleted or inserted. The generated message for the protein doesn’t make sense.
The Causes of Mutations
Mutations happen for several reasons.
- DNA fails to copy accurately.
Most mutations are naturally occurring because when the DNA copies its self for a cell to divide the copy is not quite perfect. Making a difference between the two sets of DNA.
- External influences can create mutations.
Mutations can be caused by exposure to specific chemicals or radiation. Types of radiation include ultraviolet radiation, X-rays, and nuclear radiation. Some toxins or chemicals that may cause mutations are caffeine, lead, saccharin (in some artificial sweeteners), and carbon tetrachloride (in tobacco products). These agents cause the DNA to break down. When DNA breaks down it is normally repaired. If the repair job is not perfect then DNA is slightly difference than the original and hence, a mutation.
Getting Mutations
Gene mutations are either inherited from a parent or acquired.
Mutations that are inherited came from a gene change that existed in the gametes (sex cells) of the parents therefore; the mutation can be passed from generation to generation. These mutations will be present in all body cells of the offspring. Diseases that may be caused by these inherited mutations include cystic fibrosis, Huntington disease, sickle cell anemia, and chromosomal mutations (involving a large part of or the whole chromosome), such as Down’s syndrome.
In contrast to inherited mutations, acquired mutations are gene changes that arise within individual cells and accumulate throughout a person’s lifetime. Acquired mutations are passed only to the direct descendants of those cells. In some cases acquired mutations can be caused by exposure to environmental factors such as radiation or toxins.
Mutations occur all the time in most cells of your body. Cells have the amazing ability to recognize mistakes and fix them before passing them on to descendant cells. However a cell’s DNA repair mechanisms can fail, or become overwhelmed, or become less efficient with age. Over time, mistakes can accumulate. These mistakes are believed to be the basis for many chronic diseases, such as cancer, and diseases of the elderly.
- What can one small DNA alteration in a critical gene lead to?
- What is a mutation?
- What do mutations in DNA result in?
- What two factors influence the health consequences of a particular mutation?
- Name and describe one type of mutation.
- What can a mutation do to the organism’s proteins? (Name one effect)
- What are the two causes of mutations?
- Name five environmental factors that can cause mutations.
- Define an inherited mutation.
- Name three inherited diseases caused by mutations.
- Describe what an acquired mutation is.
- Where do acquired mutations occur?