MutatioMutationsns

What Are 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 frame-shift!

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

InsertionAn 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 21st chromosomes

– Turner Syndrome – single X chromosome– Klinefelter’s Syndrome – XXY

chromosomes

Chromosome Mutation Animation

Normal MaleNormal Male

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Normal FemaleNormal Female

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Male, Trisomy 21 (Down’s)Male, Trisomy 21 (Down’s)

322n = 47

Female Down’s SyndromeFemale Down’s Syndrome

332n = 47

Turner’s SyndromeTurner’s Syndrome

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2n = 45

Turner’s SyndromeTurner’s Syndrome

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Klinefelter’s SyndromeKlinefelter’s Syndrome

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Klinefelter’s SyndromeKlinefelter’s Syndrome

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