DNA and Mutations

How Do Mutations Occur? A section of DNA on a chromosome where instructions for making specific proteins are found is called a gene Differences in genes can affect genetic traits as well as the sequencing in making proteins Given the huge number of base pairs in DNA, there is always room for error when DNA is copied Errors that occur during DNA replication can be affected by the environment, such as ultraviolet, radiation, or x-rays; can also be affected by natural and human made toxins

A change in the nucleotide sequence of a gene is a mutation There are several types of mutations that change the sequence of nitrogen base pairs causing a mutated gene to code for a different protein than a normal gene The overall effects of the mutation depends on where in the DNA sequence the mutation occurs and the type of mutation

Types of Mutations

Chromosome Mutations May Involve: Changing the structure of a chromosome The loss or gain of part of a chromosome

Chromosome Mutations Five types exist: Deletion Inversion Translocation Nondisjunction Duplication

Deletion Due to breakage A piece of a chromosome is lost

Inversion Chromosome segment breaks off Segment flips around backwards Segment reattaches

Duplication Occurs when a gene sequence is repeated

Translocation Involves two chromosomes that aren’t homologous Part of one chromosome is transferred to another chromosome

Nondisjunction Failure of chromosomes to separate during meiosis Causes gamete to have too many or too few chromosomes Disorders: Down Syndrome – three 21st chromosomes Turner Syndrome – single X chromosome Klinefelter’s Syndrome – XXY chromosomes

Chromosome Mutation Animation

Down Syndrome Trisomy 21…extra chromosome on the 21st pair (nondisjunction) Most common genetic disorder; 1 in 691 are born with Down’s

Klinefelter’s Occurs in males who have an extra X chromosome (XXY); affects different stages of physical, language, and social development and most common symptom is infertility (nondisjunction)

Turner’s Occurs in females who have a single X chromosome (X); very rare disease causing short stature, infertility, delayed puberty, hearing loss (nondisjunction)

Gene Mutations Change in the nucleotide sequence of a gene May only involve a single nucleotide May be due to copying errors, chemicals, viruses, etc.

Types of Gene Mutations Include: Point Mutations Frameshift

Point Mutation Change of a single nucleotide Includes the deletion, insertion, or substitution of ONE nucleotide in a gene

Sickle Cell Anemia Not enough healthy red blood cells to carry oxygen to the rest of the body In order for the offspring to have sickle cell anemia, both parents must be a carrier; point mutation

Hemophilia Is carried on the X chromosome (female); occurs more frequently in males than females “Free bleeder” because the blood doesn’t clot for lack of the clotting agent Sometimes referred to as the “royal disease” According to the Mayo Clinic, there are 3 types of hemophilia; A, B, and C Each of these 3 types are determined by the lack of a certain clotting factor