Mutations

 MUTATIONS

The genetic information stored in DNA is passed on from one However, genes do not remain unchanged, sometimes unexpected and random changes occur. These changes are referred to as mutations. Only mutations that modify à certa generation to the

characteristic can be perceived, Mutations can be passed onto offspring They can originate spontaneously (spontaneous mutations) or as a t of a mutagen Mutations are essential in order to increase variability between individuals of population. However, sometimes mutations can be harmful too; this is the case with cancer and genetic diseases.

MUTATION TYPES

In order to classify mutations we use the following criteria.

1.Mutations according to the type of genetic alteration.

Genetic or molecular mutations. They affect the chemical structure of genes (sequence of nucleotides in the DNA). This can lead to changes in the sequence of amino acids that form a protein. As a result, it will alter the way the protein functions. For example, sickle-cell anaemia is caused by the change of an amino acid in one of the polypeptide chains that form haemoglobin.

Chromosomal mutations. They affect the structure of chromosomes: duplicate, eliminate or change fragments of themselves. They can be perceived using a microscope and occur in some tumours.

Genomic or numeric mutations. They affect the number of chromosomes. They

can be observed when studying the karyotype and can be of two different types

• Aneuploidy: when there is a lack or excess of chromosomes. Down syndrome is an example of this particular type of mutation. It occurs when an individual has a full or partial extra copy of chromosome 21.

• Euploid: in this case, the diploid set of chromosomes affected individuals with 3n chromosomes, 4n chromosomes... instead of 2n chromosomes.

2. Mutations according to what causes them

The mutations that take place in cells can be spontaneous or induced. Spontaneous mutations are a result of mistakes in the process of DNA replication. The probability of these types of mutations affecting eukaryotic cells is 10-14.

Induced mutations take place when the cell or organism is affected by the action of mutagens. Then, the number of mutations is multiplied.

Mutagens can be physical, chemical or biological. They modify DNA and can cause chromosomal alterations.

• Physical mutagens. They are radiation that can alter DNA. The most important ones are UV rays, gamma rays, beta rays, X-rays and certain ultrasounds.

• Chemical mutagens. They are chemical compounds that alter the sequence of nucleotides found in DNA. We know over 6 million substances capable of producing mutations. Some of the most important ones are many dyes, some pesticides, some pollutants, nicotine and drugs such as LSD.

Biological mutagens. There are two types. The first are some viruses that have genomes with fragments of another cell. These genomes are introduced into the organism they infect, modifying its DNA. The second are transposons, which are mobile segments of DNA that change position, altering original DNA.

MUTATIONS AND EVOLUTION

Not all mutations are harmful. Evolution would not have taken place if mutations did not exist at all.

In order to modify a population, it is important that new versions of genes appear and that they are

distributed amongst the individuals in the population. Mutations are the main source of genetic

variability.

Mutations modify DNA and, sometimes, they can change the characteristics of an organism.

These changes can be positive, if they give individuals a new characteristic that improves conditions,

or negative, if they become inconvenient characteristics. Natural selection will favour, or disfavour,

individuals with a mutation: they will be more, or less, likely to survive and create offspring.

Inconvenient mutations tend to be eliminated through the process of natural selection.

However, negative mutations are produced continuously and a certain number of them are always kept

within a population.

Neutral mutations are neither positive nor negative. Natural selection does not affect neutral mutations,

o they are kept and appear randomly within a population.

Beneficial mutations tend to be kept through the process of natural selection and harmful mutations tend to

disappear in the environmental conditions that the affected populations develop in.

Mutations occur by chance. They are not the response of an organism to adapt to the environment.

1.Germinal mutations

Mutations that take place in gametes are known as germinal mutations. From an

evolutionary point of view, they are the only ones that are relevant, as they are passed

on to offspring.

When a germinal mutation takes place, there are three possible scenarios:

The mutation does not cause changes we can perceive; either because the mutation took placein a segment of DNA that is not transcribed or because the mutation does not alter thesequence of amino acids in the protein.

It causes a small change that we can perceive, but it has no significance.

It causes a very important change. This change can be an important improvement or a serious

disadvantage that can even cause the death of the individual.