Ever wonder why your cat has six toes? Does the latest X-Men movie leave you curious about real mutants? Here’s an explanation of genetic mutations; what they are and and how they occur in nature.
* The Genetic Code *
Genes are units of inheritance that exist within chromosomes; each chromosome a double-stranded molecule of DNA (deoxyribonucleic acid). Along the two strands of a DNA molecule are a series of molecular subunits called bases. This sequence of bases, the order in which they occur, is the actual genetic code.
A mutation is an alteration of a gene’s normal DNA base sequence. Because genes are instructions for building protein molecules, a change in the genetic instructions can result in a critical change to the protein products of DNA. To better understand the relationship between mutations, genes, DNA and proteins, it is helpful to first review what DNA is and how it works.
* Nucleic Acids & Nucleotides *
Deoxyribonucleic acid (DNA) is the genetic material of cells. Nucleic acids are polymers (big molecules) composed of monomers (little molecules) called nucleotides. Nucleotide monomers consist of 3 portions:
1. a pentose sugar
2. one or more phosphate groups
3. one of five cyclic nitrogenous bases
* The Twisted “Ladder” of Nucleic Acid *
The nucleotides of DNA are linked together by chemical bonds between the phosphate of one nucleotide and the sugar of next, creating a phosphate-sugar backbone, with the nitrogenous bases extending from this backbone like the teeth of a comb. In all living things, DNA exists as a double-stranded molecule, with the bases from each strand attracted to each other by hydrogen bonds.
* Nucleotide Bases & the Genetic Code *
The nucleotides of DNA each contain one of four possible nitrogenous bases: Adenine (A) Cytosine (C) Guanine (G) Thymine (T)
The specific base is the only thing that makes one nucleotide differ from another. When nucleotides exist together in a nucleic acid, such as the DNA of our genome, the sequence of these bases is actually the genetic code that makes each of us unique.
* Replication: Copying DNA *
Most of the cells in the human body are frequently dividing. When cells divide, a new copy of DNA must be made, so that each new cell has a complete set of genetic instructions. Sometimes, when a new DNA molecule is built, the wrong base gets inserted, or deleted, changing the genetic code. This is a type of mutation.
* Transcription & Translation: Building Proteins Based on the DNA Blueprint *
Transcription and translation are the processes in which the DNA code is read and the information ultimately used to build protein molecules. First the genetic information in DNA is transferred to another type of nucleic acid called ribonucleic acid (RNA). It is RNA’s job to transport the genetic information out of the cell’s nucleus and bring that genetic blueprint to an area in the cell where the genetic information in the RNA can be translated into protein.
Proteins are large organic molecules that are absolutely vital to the structure and function of our cells. Like nucleic acids, proteins (aka polypeptides) are polymers made of monomer subunits. The subunits of proteins are called amino acids.
Each group of three nucleotides in the nucleic acid blueprint is called a codon and encodes one amino acid. In other words, our cells read the nucleic acid base sequence in segments of three, and build proteins according to these base triplet ‘words.’
* DNA, Genes and Mutations *
Very rarely, when a new DNA molecule is being built, the wrong nucleotide base is inserted. This is a mutation. This type of mistake may result in a three-base codon coding for the wrong amino acid during translation. Using the wrong amino acid to build a protein could then result in a defective protein molecule that doesn’t work properly.
* Environmental Factors That Can Cause Mutation *
Mutations can result from random mistakes during the copying of a DNA molecule, that are then passed on to descendants. For example, a mutation in the gene for the protein hemoglobin has resulted in the genetic disorder Sickle Cell Anemia. Mutations occur more frequently in the presence of certain environmental factors, such as cancer-causing chemicals or ultra-violet radiation. UV radiation (from sunlight or tanning beds) damages DNA, causing some of the bases to pair up incorrectly. This mistake in the genetic instructions can ultimately give rise to skin cancer.
* How Are Mutations Transmitted? *
Mutations are passed on from one generation to the next only if this error occurs in cells that ultimately give rise to sperm and eggs (gametes). Sickle cell is an example of a genetic disorder, meaning that this mutation is passed on through the gametes.
If a mutation happens in somatic cells (cells of the body that are not gametes), the mutation will be limited to the organism that it occurred in. Skin cancer due to UV exposure would represent a mutation that occurs only in the individual and is not passed on to future generations.