Mutations

The word mutation comes from the Latin word mutare or to change The word mutation comes from the Latin word mutare or to change. Appropriately, mutations involve changes in the genetic code often as a result of changes in the bases of DNA or changes in the structure of a chromosome or a change in the number of chromosomes.

Mutations in DNA sequences generally occur through one of two processes: 1. DNA damage from environmental agents such as ultraviolet light (sunshine), nuclear radiation or certain chemicals 2. Mistakes that occur when a cell copies its DNA in preparation for cell division.

Some mutations can be harmful or lethal Some mutations can be harmful or lethal. An example of a lethal (causing death) mutation would be an albino plant. A plant without pigment would neither be able to survive the harmful ultraviolet rays of the sun nor contain chlorophyll for photosynthesis.

Often people incorrectly associate the term mutation with harmful consequences and do not realize they can be beneficial to the organism. In fact, mutations can be beneficial. However, most mutations cause no change in gene function. The claim that "almost all mutations are harmful" is false. Mutations provide a population with genetic diversity. According to the Howard Hughes Institute, the average person has inherited hundreds of mutations from their ancestors and will develop 30 new mutations within their lifetime due to errors in reading the genetic code or environmental conditions. Some of these acquired mutations are passed along to the offspring. For more information, go to http://www.hhmi.org/genetictrail/d100.html.

Some mutations may be beneficial Some mutations may be beneficial. For example, a change in coloration may make an organism better able to survive in a changing environment.

Some mutations are passed on to offspring and some are not Some mutations are passed on to offspring and some are not. Germ cell mutations (those taking place in egg or sperm cells) may be passed on to offspring. However, mutations taking place in somatic (body) cells are not passed on to offspring. Video 7 minutes

Gene Mutations

Gene mutations are changes in the bases of DNA. Before a cell divides, it makes an exact copy of each strand of DNA so that new cells will have a complete set of all genes. Sometimes during this process, a mistake is made. These mistakes are called mutations.

Deletions occur when a base is missing. Normal ABCDEFGHI Deletion ABCD GHI

An insertion occurs when additional bases are present. Insertion ABCDEKLMFGHI

A substitution occurs when one or more bases are replaced by others. Substitution ABCKLMGHI

Chromosome Mutations

Chromosome mutations are much more serious than gene mutations as they involve more genetic material.

A chromosome abnormality reflects an abnormality of chromosome number or structure. There are many types of chromosome abnormalities. However, they can be organized into two basic groups:

Numerical Abnormalities: When an individual is missing either a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (trisomy). An example of a condition caused by numerical abnormalities is Down Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two).

Structural Abnormalities: When the chromosome's structure is altered. This can take several forms: Deletions: A portion of the chromosome is missing or deleted.

Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.

Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.

Translocations: When a portion of one chromosome is transferred to another chromosome.

DNA Repair

Although errors in DNA replication (also known as mutations) are essential for evolution, the vast majority of DNA errors are neutral at best and fatal at worst. ·         If DNA replication in humans were only as accurate as one base in a million, about 1,000 genes in every cell would be affected each time the cell divided. ·         To minimize the number of errors, our cells normally have at least three DNA repair mechanisms at their disposal.

A proofreading mechanism corrects errors during the replication process.

A mismatch repair mechanism scans and repairs errors in DNA shortly after replication.

An excision repair mechanism operates over the life of the cell to repair errors that result from chemical or radiation damage.

Aren’t mutations interesting?