Mutations

Page 1

Vocabulary

Mutation: any change

to an organism's genetic material; these can be harmful, beneficial or neutral

Mutagen: physical or

chemical agent that changes the genetic material

Somatic Mutations:

alterations in DNA that occur in body cells and are not able to be passed on

Point Mutations:

single base modification that causes a single nucleotide base change

Gene Alterations:

when the wrong amino acid is placed during protein synthesis

Gene Rearrangements:

when an entire gene moves to another location

Deletion: when a

whole segment of a gene is lost

Insertion: when a

large part of DNA is inserted into a gene

Substitution: type

of mutation where one base pair is replaced by a different base pair

Nondisjunction:

occurs when at least one pair of homologous chromosomes do not separate

Page 2

Pg1 Introduction to

Mutations

A mutation is a

change in the genetic make-up of a cell. this means there is a change in the

sequence of DNA or RNA from the original sequence. Many times, this change

occurs during the DNA replication process or during RNA transcription. There

are many different types of mutations. Mutations can be beneficial (helpful),

benign (neither harmful nor helpful) or malignant (harmful). It is important to

remember that not all mutations are harmful to an organism. It is also

important to remember that mutations contribute to a population's genetic

diversity.

Mutations are often

caused by mutagens. Mutagens are chemical and physical agents found in an

organism's environment that can interact with its DNA and cause a mutation.

Some examples of mutagens are X-rays, UV light, and asbestos. Although mutagens

can cause mutations, some mutations do occur spontaneously. In this lesson we

will explore several of the most common types of genetic mutations.

Page 3

Types of Mutations

Mutations can occur

in many different ways. Think about the complicated steps you learned in the

processes of transcription and translation. At any point in that process, a

little glitch can cause a mutation. Remember, that mutations can also occur

spontaneously. Mutations can also be caused by a mutagen. Mutations are

classified based up how they affect they genetic code. We will look at the

following classifications of mutations:

Point Mutations

Chromosome Level

Mutations

Somatic

Mutations

Point Mutations

A point mutation is

a change in just one nucleotide in a cell. This change can be in the DNA or RNA

of the cell and can occur during DNA replication or RNA transcription. There

are two types of point mutations that can happen in the base-pairs of nucleotides:

substitutions or insertions/deletions.

A base-pair

substitution is when a DNA base pair is replaced with a different pair of

nucleotides. An easy way to understand

substitution is to compare it to spelling.

It is similar to taking the word MONEY and substituting the letter H for

the letter M. The word MONEY becomes

HONEY. It is still a word, just one with

different meaning.

As in the word example above, in a

base-pair substitution the DNA sequence is only changed by one base-pair. An example of this type of mutation in nature

is the sickle-cell mutation. In

individuals afflicted with sickle-cell disorder, a single base-pair in their

DNA that codes for the protein hemoglobin (or blood protein) has been replaced

by a different pair. This affects the

translation process that builds the protein.

The amino acid valine replaces glutamic acid in affected

individuals. This one small change in

the DNA sequence causes the hemoglobin protein to change its shape from a

round, healthy cell to a sickle or C shaped cell.

Most substitution

mutations still code for amino acids and therefore are not usually fatal to the

cells in which they occur. However,

there are instances where the single base-pair changes causes translation to

stop. In this situation, the proteins

are not anywhere near normal and can be harmful or fatal to the cell.

Page 4

Pg3 Chromosome

Mutations

Somatic and Germinal

Mutations

Genes and

chromosomes can mutate in either somatic (body) or germinal (egg and sperm)

tissue. This is important as it determines if a mutation can be genetically

passed on to future generations. Mutations affecting somatic (body) tissue will

only affect the individual while mutations in an egg or sperm cell can be

passed on to offspring.

Mutations that occur

in body cells can not be passed on to offspring. However, if a mutation occurs

in an egg or sperm cell, there is a change that the mutation will be passed on

to offspring.

Somatic Mutations

Not all mutations

are passed down from parent to offspring. If a mutation occurs in a somatic or

body cell, it will only affect that individual. Somatic mutations can be caused

by environmental factors or errors in replication. Examples of things that can

cause these mutations include exposure to radiation and certain chemicals. Once

the mutation occurs, it is carried on to new cells through the process of

mitosis. Cancer is often the result of a somatic mutation.

Germ Cell Mutations

In normal meiosis,

the chromosomes move apart properly and are moved to either end of the cell by

the spindle fibers. When nondisjunction occurs, the homologous (or pair)

chromosomes do not separate correctly. In this situation, one gamete cell

receives two copies of a chromosome and the other receives none. If one of

these gametes happens to unite with another normal cell during fertilization,

the offspring will have an abnormal number of chromosomes. If the offspring

survives it usually has a set of symptoms associated with the extra genes

located on the extra or missing chromosomes.