MUTATIONS

What are Mutations? Mutations are changes in the genetic material. Kinds of Mutations: Mutations that produce changes in the number or structure of chromosomes are known as chromosomal mutations. Mutations that produce changes in a single gene are known as gene mutations.

Chromosomal Mutations Recall that chromosomal abnormalities often result from nondisjunction, the failure of chromosomes to separate properly during meiosis. Nondisjunction results in sex cells that have too many or too few chromosomes. If these abnormal sex cells are used for fertilization, the offspring will have an abnormal chromosome number. Monosomy – missing a chromosome Example: Monosomy X or Turner Syndrome Trisomy – having an extra chromosome Example: Trisomy 21 or Down Syndrome

Turner Syndrome

Down Syndrome

Chromosomal Mutations Recall the 4 types of chromosomal mutations that involve changes in chromosome structure due to breakage: Deletion Duplication Inversion Translocation

Gene Mutations A gene mutation is a change in the nucleotide sequence of a gene. Gene mutations involving a change in one or a few DNA nucleotides are known as point mutations because they occur at a single point in the DNA sequence. Point mutations include substitutions, insertions, and deletions.

Normal Gene Normal gene GGTCTCCTCACGCCA ↓ DNA CCAGAGGAGUGCGGU Codons Pro-Glu-Glu-Cys-Gly Amino acids DNA mRNA Protein

Neutral Substitution Normal gene Substitution mutation GGTCTCCTCACGCCA ↓ CCAGAGGAGUGCGGU Codons Pro-Glu-Glu-Cys-Gly Amino acids Substitution mutation GGTCTTCTCACGCCA ↓ CCAGAAGAGUGCGGU Pro-Glu-Glu-Cys-Gly

Substitution Mutation Normal gene GGTCTCCTCACGCCA ↓ CCAGAGGAGUGCGGU Codons Pro-Glu-Glu-Cys-Gly Amino acids Substitution mutation GGTCACCTCACGCCA ↓ CCAGUGGAGUGCGGU Pro-Arg-Glu-Cys-Gly Substitutions will only affect a single codon. Their effects may not be serious unless they affect an amino acid that is essential for the structure and function of the finished protein molecule (e.g. Sickle Cell Anemia).

Substitution Mutation Normal gene GGTCTCCTCACGCCA ↓ CCAGAGGAGUGCGGU Codons Pro-Glu-Glu-Cys-Gly Amino acids Substitution mutation GGTCTCCTCACTCCA ↓ CCAGAAGAGUGAGGU Pro-Glu-Glu-STOP This substitution mutation changed a normal amino acid codon into a stop codon. Nearly all mutations of this kind lead to non-functional proteins.

Frameshift Mutation The insertion or deletion of a nucleotide results in a frameshift mutation. This changes the “reading frame” like changing a sentence. Example:

Deletion Mutation The loss of a nucleotide causes the incorrect grouping of the remaining codons, making all amino acids downstream change. Normal gene GGTCTCCTCACGCCA ↓ CCAGAGGAGUGCGGU Codons Pro-Glu-Glu-Cys-Gly Amino acids Deletion mutation GGTC/CCTCACGCCA ↓ CCAGGGAGUGCGGU Pro-Gly-Ser-Ala

Insertion Mutation The addition of a nucleotide in a gene also results in a frameshift mutation. Normal gene GGTCTCCTCACGCCA ↓ CCAGAGGAGUGCGGU Codons Pro-Glu-Glu-Cys-Gly Amino acids Addition mutation GGTGCTCCTCACGCCA ↓ CCACGAGGAGUGCGGU Pro-Arg-Gly-Val-Arg

What causes Mutations? Errors in DNA replication Chemicals Radiation Viruses

DNA Replication and Cancer Recall that most errors in DNA replication are repaired. Unrepaired mutations that affect genes that control cell division can cause diseases such as cancer.

Body Cell vs. Sex Cell Mutation Body cell mutations are not passed on to the next generation. Mutations that occur in sex cells are passed on to the organism’s offspring and will be present in every cell of the offspring.