Mutations What Are Mutations Changes in the nucleotide

Mutations

What Are Mutations? • Changes in the nucleotide sequence of DNA • May occur in somatic cells (aren’t passed to offspring, only to descendant cells) • May occur in germ-line cells (eggs & sperm) and be passed to offspring

Are Mutations Helpful or Harmful? • Mutations happen regularly • Almost all mutations are neutral • Chemicals & UV radiation cause mutations • Many mutations are repaired by enzymes

Types of Mutations

Point Mutations • Change in the nucleotide sequence of a one or a few base pairs • May be due to copying errors, chemicals, viruses, etc.

Types of Point Mutations • Include: – Substitutions – Frame-shift Mutations (Insertion, Deletion)

Substitution • One or more nucleotides are changed to a different base. • AAC GCC AGC • AAG GCC AGC • Can cause an amino acid to be changed…or not!

Frame-shift Mutation • Inserting or deleting one or more nucleotides • Changes the “reading frame” like changing a sentence • Proteins built incorrectly

Frameshift Mutation • Original: – The fat cat ate the wee rat. • Frame Shift (“a” added): – The fat caa tet hew eer at.

Amino Acid Sequence Changed

Gene Mutation Animation

How Would the Phenotype Change?

Missense Substitution • Sickle Cell disease is the result of one nucleotide substitution • Occurs in the hemoglobin gene

Nonsense Substitution • Thallasemia is the result of a stop codon substitution that shortens the gene • Occurs in the hemoglobin gene

Silent Mutation • The Genetic Code is repetitive – there are 64 codons that code for 20 amino acids. • A silent mutation makes no change in amino acid sequence:

Deletion • Cystic fibrosis (mucus in the lungs) is the result of one nucleotide deletion • Causes a frameshift!

Insertion • Huntington’s Disease (nerve cell deterioration) is the result of many nucleotide insertions (CAG repeats)

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/Insertion – Duplication – Inversion – Translocation – Nondisjunction

Deletion • Due to breakage • A piece of a chromosome is lost Insertion An additional piece of chromosome is added

Deletion • Cri-du-Chat is caused by a deletion in one of the chromosomes. • Causes an abnormal larynx

Duplication • Occurs when a gene sequence is repeated

Inversion • Chromosome segment breaks off • Segment flips around backwards • Segment reattaches

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

Translocation

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

Chromosome Mutation Animation

Normal Male 30

Normal Female 31

Male, Trisomy 21 (Down’s) 2 n = 47 32

Female Down’s Syndrome 2 n = 47 33

Turner’s Syndrome 2 n = 45 34

Turner’s Syndrome 35

Klinefelter’s Syndrome 36

Klinefelter’s Syndrome 37