Mutation – any change in DNA

Mutations Mutations are defined as “a sudden genetic change in the DNA sequence that affects genetic information”. They can occur at the molecular level (genes) and change a single gene, or at the chromosome level and affect many genes.

Things that can cause mutations are called “mutagens”. Known mutagens are ultraviolet light, cigarette smoking, certain chemicals like PCB’s.

Effects of Mutations Silent mutations - have no effect on the expression of the gene. – Causes for this type of mutation: – It is in a non-coding region (intron) – It does not change the amino acid sequence – The change does not affect the folding of the protein

Heritability of Mutations It depends on where it occurs i.Germ line mutations – occur in gametes. Inheritable (colorblindness, hemophilia) ii.Somatic mutations – affect body cell, not inheritable (cancer)

Types of Mutations – mistakes a)Point Mutations – effects a single gene i. Substitution -Missense -Nonsense ii. Frameshift -Insertion -Deletion a)Chromosomal mutations – most drastic, change in structure or # of chromosomes (affects many genes)

III.Point Mutations a)Substitution – one base exchanges for another, affects 1 amino acid (Ex. GCA-TCA  GCT-TCA

The Effects of Point Mutations ► A point mutation is a change in a single base pair in DNA. – A change in a single nitrogenous base can change the entire structure of a protein because a change in a single amino acid can affect the shape of the protein. (SUBSTITUTION) Gene Mutations Normal Point mutation mRNA Protein Stop mRNA Protein Replace G with A

Effects of substitution mutations Missense mutations – causes a change in the amino acid coded for Nonsense mutations – causes a stop codon to occur prematurely

a)Frameshift – affects several amino acids -Insertion – 1 base is inserted, affects several amino acids Ex. (GCA-TCA  GCA-GTC-A -Deletion – base is removed, affects several amino acids Ex. (GCA-TCA  GCT-CA

Frameshift Mutations ► What would happen if a single base were lost from a DNA strand? ► A mutation in which a single base is added or deleted from DNA is called a frameshift mutation because it shifts the reading of codons by one base. – As a result, every codon after the deleted base would be different. Gene Mutations mRNA Protein Deletion of U

 Chromosome mutations  Nondisjunction – incomplete chromosome division during meiosis; results in gametes with too many or too few chromosomes.  Ex: Down syndrome is caused by an extra #21 chromosome

Aneuploidy Polyploidy

Down’s Syndrome Trisomy 21 1 in 700 births Mental retardation Males are sterile but females are not

Likelihood of chromosomal mutations  1 in 1700 for mothers < 20.  1 in 1400 for mothers >20<30.  1 in 750 for mothers >30<35.  1 in 16 for mothers >45. WHY a positive correlation between risk of chromosomal mutation and maternal age?

Sex chromosome disorders – occurs as a result of nondisjunction of the sex chromosomes Ex: Turner’s syndrome – X-; Klinefelter’s syndrome – XXY.

Klinefelter’s Syndrome XXY 1 in 1,000 Usually sterile because of low sperm count Tall, sparse body hair Suffer from gynecomastia- male breast tissue Testosterone treatments

Turner’s Syndrome XO genotype—Monosomy X 1 in 2,500 births Short, sterile 75% result in non-disjunction from the father

Other Chromosomal Mutations Affect many different genes Caused by errors in meiosis or environmental disturbances

– Translocation – occurs when a piece of one chromosome breaks off and attaches to a nonhomologous chromosome – Inversion – occurs when a piece of one chromosome breaks off, flips, and reattaches to the same chromosome

– Deletion – occurs when a piece of a chromosome breaks off and is lost – Duplication – occurs when a segment of a chromosome is repeated