Ch. 12.4 Mutations Section Objectives: •Categorize the different kinds of mutations that can occur in DNA. •Compare the effects of different kinds of mutations on cells and organisms. Mutations • Any change in DNA sequence is called a mutation. – Mutations can be caused by errors in replication, transcription, cell division, or by external agents. • If mutation occurs in gametes (sex cells) it will be passed on to offspring – A mutation may produceno change, a new trait, or it may result in a protein that does not work correctly. – If the mutation results in a protein that is nonfunctional, and the embryo may not survive. – In some rare cases a gene mutation may have positive effects. Mutations • If mutation takes place in a somatic cell, it is not passed on to organism’s offspring – Damage to a gene may impair the function of the cell – When that cell divides, the new cells also will have the same mutation – Some mutations of DNA in body cells affect genes that control cell division. – This can result in the cells growing and dividing rapidly, producing cancer. Characteristics of Mutations • One of the main sources of genetic variation. – Unique traits are thought to originate through mutations that are passed on • Occur at random • sometimes a mistake in base pairing during DNA replication. • many mutations are caused by factors in the environment • Can be classified as either: – Gene Mutations – Chromosomal Mutations Gene Mutations Changes that affect the nucleotide sequence of a gene – – – – change the DNA changes the mRNA may change protein may change trait DNA TACGCACATTTACGTACG mRNA AUGCGUGUAAAUGCAUGC protein aa aa aa aa aa aa aa trait Types of gene mutations • Changes to the letters (A,C,T,G bases) in the DNA – point (or substitution) mutation • change to ONE letter (base) in the DNA • may cause change to protein, may not – frameshift mutation • addition of a new letter (base) in the DNA sequence • deletion of a letter (base) in the DNA • both of these shift the DNA so it changes how the codons are read • big changes to protein! Point Mutations • One base change – can change the meaning of the whole protein THEFATCATANDTHEREDRATRAN THEFATCARANDTHEREDRATRAN OR THEFATCATENDTHEREDRATRAN Point Mutations • Substitution mutation = may change amino acid AUGCGUGUAUACGCAUGCGAGUGA MetArgValTyrAlaCysGluStop AUGCGUGUAUACGUAUGCGAGUGA MetArgValTyrValCysGluStop Sickle cell anemia- a single substitution • Hemoglobin protein in red blood cells – strikes 1 out of 400 African Americans – limits activity, painful & may die young Sickle Cell Anemia- substitution Does convey resistance to malarial infections when heterozygous Point Mutations • Substitution mutation = may be no change to protein AUGCGUGUAUACGCAUGCGAGUGA MetArgValTyrAlaCysGluStop AUGCGUGUAUACGCUUGCGAGUGA MetArgValTyrAlaCysGluStop Point Mutations • Substitution mutation = may change to STOP AUGCGUGUAUACGCAUGCGAGUGA MetArgValTyrAlaCysGluStop AUGCGUGUAUAAGCAUGCGAGUGA MetArgValStop Frameshift Mutations • Add or delete one or more bases – changes the meaning of the whole protein as it changes how the mRNA codons are read. This change causes all remaining codons to be incorrectly grouped. The change in “reading frame” causes all resulting proteins to be made improperly. THEFATCATANDTHEREDRATRAN THEFATCANTANDTHEREDRATRAN OR THEFATCAANDTHEREDRATRAN Frameshift Mutations • Insertion = add one or more basesAUGCGUGUAUACGCAUGCGAGUGA MetArgValTyrAlaCysGluStop AUGCGUGUAUACGUCAUGCGAGUGA MetArgValTyrValMetArgValA Frameshift Mutations • Deletion = lose one or more bases AUGCGUGUAUACGCAUGCGAGUGA MetArgValTyrAlaCysGluStop AUGCGUGUAUACGAUGCGAGUGA MetArgValTyrAspAlaSerGA Chromosome mutations • Chromosome mutations- changes in the structure of a chromosome or the loss of a chromosome. Often occur during cell division. – Deletion- loss of a piece of a chromosome due to breakage. Information carried by the missing piece is lost. – Inversion- a chromosomal segment breaks off and reattaches in the reverse orientation on the same chromosome. Chromosome mutations cont. – Translocation- a chromosomal segment breaks off and attaches to another, non-homologous chromosome. – Nondisjunction- results from the failure of a chromosome to separate from its homologue during meiosis. During nondisjunction, one gamete receives an extra copy, and the other gamete is missing the chromosome entirely. Patterns of Inheritance • Nondisjunction- failure of the chromosomes to separate correctly during cell division. – Monosomy- and individual is missing one of a pair of particular chromosomes (the total number would be 45 for humans) • Turner syndrome- individual only has 1 X chromosome; results in a female with immature physical development, sterility, and a webbed neck. Chromosomes Don’t Separate Properly during Meiosis Nondisjunction Also Can Change the Number of Sex Chromosomes • Turner syndrome – Missing an X chromosome; XO – Female; webbed neck; no secondary sexual traits at puberty; sterile; may age prematurely • XXX females – Develop normally Turner Syndrome Nondisjunctions cont. • Trisomy- an individual has 3 of a particular chromosome. (total number would be 47 for humans) – Down syndrome- extra chromosome at number 21- individual has mental deficiencies, folded skin above the eyes, weak muscles – Kleinfelter’s syndrome- genotype XXY male- has mental retardation, and low fertility. In Down Syndrome There Are Three Copies of Chromosome 21 Nondisjunction Also Can Change the Number of Sex Chromosomes • Klinefelter syndrome (XXY) – Low fertility; mental retardation; small testes; sparse body hair; enlarged breasts – Testosterone injections may reverse the phenotype • XYY condition – Taller – Normal male phenotype Kleinfelter’s Syndrome Induced Mutations • Mutagen- environmental factor that damages DNA. – – – – – – – – Sunlight (UV light) causing skin cancer. Radiation causing infertility or cancer X-rays Chemicals like pesticides and herbicides Nuclear Radiation Asbestos Formaldehyde Pathogens- frogs with extra legs