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Download Lecture # 5 Mutations
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Lecture #5 Mutations Unit: Molecular Genetics What is a mutation? Now and then cells make mistakes in copying their own DNA, inserting the wrong base or even skipping a base as a strand is put together. Mutations are heritable changes in genetic information. What causes mutations? Spontaneous Mutations. Some mutations seem to be due a mistake in base pairing during DNA replication. Mutagens- chemical or physical agents in the environment that cause mutations. Examples of chemical mutagens: certain pesticides, a few natural plant alkaloids, tobacco smoke, and environmental pollutants. Examples of physical mutagens: some forms of electromagnetic radiation, such as X-rays and ultraviolet light. If mutagens interact with DNA, they can produce mutations at high rates. Cells can sometimes repair the damage; but when they cannot, the DNA base sequence changes permanently. If a gene in one cell is altered, the alteration can be passed on to every cell that develops from the original one. If the mutation is in a gamete, the alteration will be passed on to every cell in the individual. Effects of Mutations The effects of mutations on genes vary widely. Mutations may be…. 1. Neutral (have little or no effect on the organism) 2. Beneficial (proteins with new or altered functions that can be useful to the organism in a different/ changing environment) 3. Harmful (these mutations may disrupt gene function/ protein function) Beneficial Effects Some of the variation produced by mutations can be highly advantageous to an organism or species. Mutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments. Without mutations, organisms cannot evolve, because mutations are the source of genetic variability in a species. Example of a beneficial mutation - Polyploidy Polyploidy occurs when all the chromosomes are present in three or more copies. Polyploidy is common in plants and rare in animals. Most Crop Species are Polyploid Polyploids (like the one on the left) are often larger and stronger than their diploid ancestors (strawberry on right). Harmful Effects Some of the most harmful mutations are those that dramatically change protein structure or gene activity. The defective proteins produced by harmful mutations can disrupt normal biological activities, and result in genetic disorders. Take 2 minutes to summarize what mutations are, what causes them, and the effects mutations may have on an organism. Types of Mutations All mutations fall into two basic groups 1. Gene Mutations - Mutations that produce changes in a single gene 2. Chromosomal Mutations - Mutations that produce changes in whole chromosomes. 1. Gene Mutations Point mutations - Mutations that involve changes in one or a few nucleotides. • They are called ‘point mutations’ because they occur at a single point in the DNA sequence. • They generally occur during replication. Sickle cell disease is caused by a point mutation Symptoms of the disease include anemia, severe pain, frequent infections, & stunted growth. There are different types of Point mutations a. Substitutions • In a substitution, one base is changed to a different base. • Substitutions usually affect no more than a single amino acid, & sometimes they have no effect at all. b. Insertion – a point mutation in which one base is inserted into the DNA sequence. c. Deletion- a point mutation in which one base is removed from the DNA sequence. Insertion & Deletion point mutations are also called as ‘Frameshift mutations’ Frameshift mutations - mutations that shift the ‘reading frame’ of the genetic message. Frameshift mutations can change every amino acid that follows the point of the mutation and can alter a protein so much that it is unable to perform its normal functions. Analogy for the effect a frameshift mutation can have on the reading frame. Normal: the dog bit the cat After an deletion mutation: the dob itt hec at Mutations can also be classified according to their effects on the protein (or mRNA) produced by the gene that is mutated. a. Missense- code for a different amino acid b. Nonsense- code for a stop, which can shorten the protein c. Silent- code for the same amino acid (AA). d. Sense- mutation changes a stop codon to an amino acid codon, which can lengthen the protein. Take 2 minutes to summarize what you have learned about gene mutations, focusing on the different types of point mutations that can occur and the effect those mutations may have on the protein the gene codes for. 2. Chromosomal Mutations These types of mutations change the number or structure of chromosomes. There are 4 types of chromosomal mutations 1. Deletion This type of mutation involves the loss of all or part of a chromosome. A Boy with Cri-du-Chat Syndrome – a Debilitating Disorder Caused by Chromosome Deletion Cri-du-Chat is Caused by the Loss of the Short Arm of One Copy of Chromosome 5 2. Duplication This type of mutation produces an extra copy of all or part of a chromosome. 3. Inversion This type of mutation reverses the direction of parts of a chromosome. 4. Translocation This type of mutation occurs when part of one chromosome breaks off & attaches to another. Translocations Lead to a Number of Human Cancers In Burkitt’s lymphoma, a chromosome translocation causes a cell cycle-promoting gene to always be active. Aneuploidy occurs when one of the chromosomes is present in an abnormal number of copies. Trisomy and monosomy are two forms of aneuploidy. Down Syndrome is Caused by Trisomy for Chromosome 21 Aneuploidy is remarkably common, causing termination of at least 25% of human conceptions. It is also a driving force in cancer progression (virtually all cancer cells are aneuploid). What causes aneuploidy? Chromosome Non-Disjunction in Meiosis causes Aneuploidy The Frequency of Chromosome Non-Disjunction And Down Syndrome Rises Sharply with Maternal Age The phenomenon is clear – the explanation isn’t. Sex Chromosome Aneuploid Conditions are Common Klinefelter syndrome Take 2 minutes to summarize the different types of chromosome mutations that may occur in an individual. Detection/ Diagnosis There are genetic tests available for many of the human genetic disorders. DNA, RNA, chromosomes or proteins may be analyzed in order to make the diagnosis. Testing may be done for a variety of reasons, such as in order to determine if an individual carries a defective gene that runs in their family. Many genetic disorders can also be diagnosed in early pregnancy. Tests used to search for chromosomal defects in early pregnancy - Amniocentesis and Chorionic Villus Sampling Pre-Implantation Genetic Diagnosis (PGD) Removing a cell for diagnosis from a human embryo. As a part of in vitro fertilization, this technique may be used to search for chromosome and gene defects prior to implanting the zygote into the women’s uterus. Take 2 minutes to summarize how genetic disorders may be diagnosed.