Genetic Mutations SDK October 27, 2014

OBJECTIVES By the end of this session the student should be able to:  Define Mutation  Frequency of mutations in normal individuals  Classify different types of mutation  Explain the mechanism of mutation  Explain the role of mutation in biodiversity  Explain how mutations can cause severe diseases  Give examples of deletions, duplications, and insertions in genes  Define trinucleotide repeat expansions and how they cause neurological diseases 2SDK 2012

What is a gene mutation ?  Replacement or change of a nucleotide base with another, in one or both strands, or addition or deletion of a base pair in a DNA molecule.  Mutations are changes in genetic material(Nitrogenous bases) – changes in DNA code – thus a change in a gene(s)  In gene mutations, the DNA code will have a base (or more) missing, added, or exchanged in a codon. 3SDK 2012

Mutations can lead to missing or malformed proteins, and that can lead to disease. SDK Gene mutation out come

Types of Mutations  Germ-line mutations.Mutations that are inherited from parents are called germ-line mutations.  Acquired mutations. Mutations that are acquired during your lifetime are called acquired mutations  Some mutations happen during cell division, when DNA gets duplicated.  Still other mutations are caused when DNA gets damaged by environmental factors, including UV radiation, chemicals, and viruses. SDK 20125

How common are mutations?  Mutations occurs at a frequency of about 1 in every 1 billion base pairs  Everybody has about 5-10 potentially deadly mutations in our genes- in each cell of our body! 6SDK 2012

When everyone has mutations, Why they are not always seen They are not always seen because the mutation may have occurred in a section of DNA that doesn’t make a protein. 7SDK 2012 Most inherited genetic diseases are recessive, which means that a person must inherit two copies of the mutated gene to inherit a disorder. This is one reason that marriage between close relatives is discouraged; two genetically similar adults are more likely to give a child two copies of a defective gene.  Diseases caused by just one copy of a defective gene are not manifested with the exception of  Huntington's disease, which is rare and afflicted carriers are more likely to die before reproducing.

Mutations Outcome  The affected gene may still function.  Mutations may be harmful.  Mutations may be beneficial.  Mutations may have no effect on the organism.  Mutations are a major source of genetic variation in a population increasing biodiversity. 8SDK 2012

1.Basic Structural Components  The HIVirus is consists of two basic components:  Envelop  Genome  Envelop  The HIV envelope has projections known as spikes, which contain specific chemical components (GP120 & GP 41) that may assist the virus when it attaches to other cells.  Genome,  Two SS RNA  P32 Integrase  P 10 Protease  P64 Reverse Transcriptase  A bullet shape protein component that surrounds the genome called a Capsid.  capsid gives the virus its shape and protects the genome.  The capsid is made up of subunits called capsomeres Beneficial Mutations HIV - Virus, Genetics

HIV Binding to the Host Cell  The HIV joins the host cell by spike on HIV envelope and binding site on the host cells.  On the host cell such as T-lymphocyte, macrophage, or brain cell two binding sites are there – A CD4 molecule ( a primary receptor) – Co-receptors (CXCR4 & CCR5) – CCR5 (Beta chemokine receptor) – CXCR4 (alpha chemokine receptor) – These second receptors loops through the cell membrane 7 times and is critical for infection to occur]

SDK About 10% of Caucasians of Western European descent have the mutation for chemokine receptor- CCR5- ▲ 32 making them resistant to HIV. Beneficial Mutations HIV - Virus, Genetics

Mutations a cause of Biodiversity 12SDK 2012

 Only mutations in gametes (egg & sperm) are passed onto offspring(Germline Mutation).  Mutations in somatic cells (body cells) only affect the body in which they occur and are not passed onto offspring. Does all mutations passed on to next generation? 13SDK 2012 NO

Brain Work 1 A mutation may happen in any gene. TRUE OR FALSE? TRUE 14SDK 2012

Spontaneous and Induced Mutations  Spontaneous: Occur spontaneously without obvious reason.  Induced mutations: caused by mutagens.  Mutagens are the agent that causes the DNA code to change (mutate)  X-Ray,  Chemicals,  UV light,  Radiation, etc 15SDK 2012

Which of the following may cause mutations? A.Coffee B.UV light (sun light) C.Hair gel D.Vaccines Brain Work 2 UV Light (Sun Light) 16SDK 2012

Types of Mutations 1.Point mutations. A point mutation is a simple change in one base of the gene sequence. 2. Frame shift mutations. one or more bases are inserted or deleted Original The fat cat ate the wee rat. Point Mutation The fat cat ate the wet ra t. Original The fat cat ate the wee rat. Point Mutation The fat cat ate the wet ra t. Original. The fat cat ate the wee rat Frame Shift The fat caa tet hew eer at Original. The fat cat ate the wee rat Frame Shift The fat caa tet hew eer at 17SDK 2012

Morphological Types of Point mutations 1.Transitions. Transitions occur when a  Purine is converted to a purine (A to G or G to A)  Pyrimide is converted to a pyrimidine (T to C or C to T) 2. Transversion. A transversion results when  Purine is converted to a pyrimidine (A to C or G to T)  Pyrimidine is converted to a purine. (T to A or C to G) 18 SDK 2012

Types of Mutations according to their effects on the protein (or mRNA). Silent Mutations. Mutation in a codons that produce same amino acid. These mutations affect the DNA but not the protein. Therefore they have no effect on the organism’s phenotype. CUU CUC Missense Mutations. Missense mutations substitute one amino acid for another. Example. Hb S, Sickle Cell Hemoglobin, is a change in the beta-globin gene, where a GAG codon is converted to GUG. GAG GUG Nonsense mutations. convert an amino acid into a stop codon. The effect is to shorten the resulting protein. Sometimes this has only a little effect, however, often nonsense mutations result in completely non-functional proteins. UUU UAA\ UGA 19 SDK 2012

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Frame-shift  In a frameshift mutation one or more bases are inserted, or deleted.  Because our cells read our DNA in three letter words, adding or removing one letter changes each subsequent word.  This type of mutation can make the DNA sequence meaningless.  For example:  Original= THE FAT CAT ATE THE WEE RAT  FRAMESHIFT= THE FAT CAA TET HEW EER AT. 21SDK 2012

Mutations are a natural part of the cellular process reproduction. The cell has tools that catch and repair 99.9% of mutations. TRUE OR FALSE? TRUE Brain Work 3 22SDK 2012

Most mutations are caught and repaired in the cell. TRUE or FALSE? Brain Work 4 TRUE 23SDK 2012

Classical Types of Point mutation Mutations  Point mutation occurs when the base sequence of a codon is changed. (ex. GCA is changed to GAA)  There are 3 types: Also called frameshift mutations Substitution Deletion Insertion 24SDK 2012

Substitution  A substitution is a mutation that exchanges one base for another (i.e. a change in a single “chemical letter” such as switching an A to G.  For example: CTGGAG CTGGGG 25SDK 2012

Normal DNA: CGA – TGC – ATC Substitution Mutations Mutated DNA: CGA – TGC – TTC Alanine – Threonine - stop Alanine – Threonine - Lysine What has happened to the DNA?The adenine was replaced with thymine What will happen to the amino acids? This is a substitution mutation 26SDK 2012

The cat ate the rat The hat ate the rat 27SDK 2012

Analogy 3 letter words because codons are 3 letters THE CAT ATE THE RAT SUBSTITUTION Thc cat ate the rat. May have little effect. You still have the idea like a typo on a test. The hat ate the rat. Changes the thought of the sentence. The effect Depends on where the substitution happens 28SDK 2012

Clinical Examples 29SDK 2012

Sickle Cell Anemia Sickle cell anemia is the result of a (substitution) point mutation in codon 6 of the  -globin gene resulting in the substitution of amino acid glutamic acid by valine 30SDK 2012

 Under conditions of low oxygen tension, such as  Following exercise or  In an atmosphere containing a low oxygen level, The following changes occur:  The haemoglobin agglutinates to form insoluble rod-shaped polymers  Red blood cells become distorted and sickle-shaped  The sickle-shaped cells rupture easily causing haemolytic anaemia  The sickle shaped cells tend to block capillaries interfering with the blood flow to various organs. Sickle Cell Anemia 31SDK 2012

 Thalassaemia  Substitution of C by U in mRNA that is coding  globin chain of 146 amino acid.  Resulted in the formation of a stop signal UAG in place CAG of glutamate in codon number 39.  This result in a shortened globin chain containing only 39 instead of the normal 146 amino acids in the  -globin protein chain.  This protein is functionally useless and is equivalent to absence of  -globin gives clinical symptoms of  thalassaemia, 32SDK 2012

 Thalassaemia 33SDK 2012

 Beta thalassaemia is a genetic disorder in which there is lack of beta globin. It may be the result of:  Deletion of the whole gene so that beta globin cannot not produced (designated  o )  Deletion of the promoter region so that transcription cannot occur (designated  o )  Deletion of a large part of the gene resulting in a grossly abnormal or reduced synthesis functional protein (designated  + )  Thalassemias 34SDK 2012

Clinical Features of  -Thalassaemia  Haemoglobin A (α2  2) cannot be produced  Hb F (α2 g2) is produced even in adults  Hb A2 (α 2 d2) formation is increased  Eerythrocytes are microcytic (small) due to lack of normal haemoglobin  Erythrocytes rupture easily causing severe haemolytic anaemia, requiring repeated blood transfusions  The bone marrow expands trying to compensate by increasing haemopoiesis. 35SDK 2012

 The bones of the face and skull are thickened causing a characteristic facial appearance  The spleen and liver enlarge because haemopoietic tissue forms in them  Excess iron accumulates in the blood and is deposited in the heart, liver, pancreas and other organs (this is because of repeated transfusions while no blood is actually lost from the body)  Children have delayed growth and development and are prone to repeated infections Clinical Features of  -Thalassaemia 36SDK 2012

Point Mutation In alpha-Globin Gene “Elongated α Globin Chain, Haemoglobin Constant spring \Wayne Hb”  Here the stop codon UAA at position 142 in the alpha (  -) globin gene was substituted by the codon for glutamine.  Translation of the protein thus continued until a stop codon was encountered at codon 173.  The  -globin was considerably elongated, resulting in a variant of haemoglobin termed Haemoglobin Constant spring\ Wayne Hb. 37SDK 2012

Elongated α Globin Chain Or Haemoglobin Wayne 38SDK 2012

Insertion  Insertions are mutations in which extra base pairs are inserted into a new place in the DNA. CTGGAG CTGGCCTAG 39SDK 2012

This is an insertion mutation, also a type of frameshift mutation. Normal DNA: CGA – TGC – ATC Insertion Mutations Mutated DNA: CGA – TAG – CAT – C Alanine – Threonine – stop Alanine – Isoleucine – Valine What will happen to the amino acids? An adenine was inserted thereby pushing all the other bases over a frame. What has happened to the DNA? 40SDK 2012

The cca tat eth era t The cat ate the rat Insertion Mutations 41SDK 2012

Analogy Insertion The cat ate the rat. The cca tat eth era t. Inserting the c causes a FRAMESHIFT THE SENTENCE NO LONGER MAKES SENSE!! Insertions may have huge effects. 42SDK 2012