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DNA MUTATIONS Mutations are any change in the genetic make up of an organism and can occur naturally or can be increased by mutagens. Click on an area for more information Types Diseases Mutagens TYPES OF MUTATIONS Mutations can be spontaneous or natural as a result of errors in DNA replication or gamete production. POINT Click on an area for more information Error rate is 1 in 1,000,000 replications. BLOCK CHROMOSOMAL ABBERATIONS Point mutations •Involves a change to one of the DNA bases that makes up a gene. •Also known as a MISSENSE mutation. •Consequence varies depending on type of change. Change DNA Change amino acid sequence Change protein Three types of point mutation Change organism substitution insertion Click to find out more deletion Base Substitution •Involves one base in the DNA being substituted by another. But… •This mutation only affects one codon in the gene. DNA DNA ATA ATT mRNA UAU UAA Tyr STOP TA C T T C AAA C C G C G T C is replaced by T in the DNA. •Known as NONSENSE mutation •Totally different protein made •Effect on organism can be dramatic Also T A C T T C AAAT C G C G T •End result is a different amino acid in protein. •Generally minor effect on protein/organism. DNA AGA AGG mRNA UCU UCC Ser Ser •Serine is still added •Known as a SILENT mutation Base Insertion/Deletion •Involves one base either being inserted or removed from the DNA sequence. •Both these mutations affect all codons after the insertion or deletion. •The earlier the mutation occurs in the sequence the greater the effect. DNA TA C T T C AAA C C G C G T Dramatic effect on protein made. Known as frame shift mutations. T A C T T C AAAA C C G C G BASE INSERTION INSERTION Trp Arg TA C T T C AA C C G C G TA Block mutations •Involves large scale changes in DNA segments on chromosome. •Only occur during meiosis, formation of gametes (sex cells). •Can affect the functioning of many genes •Severe effect on functioning and characteristics of organism. Change amino acid sequence Change DNA Change protein Four types of block mutation inversions deletions translocations duplications Click to find out more Change organism Deletion •Section of chromosome is lost, anywhere from three bases to large segments. •The DNA breaks due to the effect of mutagens. A B C D E break A B C G H STEP 1 G H STEP 2 genes D A A F D E B C F segment is lost E D E F G DNA ligase rejoins chromosome H F G H STEP 3 Translocation •Section of chromosome breaks and is joined to another chromosome. •Problem when gametes formed-some receive extra genes, some deficient. 1 2 3 4 5 STEP 1 A B C D E F G H break A B C D1 2 E3 F 4 G 5H STEP 2 D E F G H segment removed DNA ligase joins segment A B C D 1 2 D E3 E F4 G 5 H F G STEP 3 H Duplication •Segment of one chromosome is added to the homologous chromosome. •Problem when gametes formed-some have double the genes, some have no genes. A B C D E F G H A B C D E F G H HE F G H E F G H HE F G H E F G H STEP 1 break A B C DA B CE DF G D STEP 2 segment removed A B C DA B CE D F D G STEP 3 Inversion •Section of chromosome falls out and rotates through 180o. •DNA ligase rejoins section but the genes are in reverse order. •There is no loss of genetic information. A B C D F E G H STEP 1 break A segment rotates 180o B C D A B C STEP 2 D F E G H G H E F G H segment rejoins A D C B E F STEP 3 Chromosomal abberations •Loss or gain of entire chromosomes- called aneuploidy. •Main cause is non-disjunction of chromosomes during meiosis. •Gametes end up with extra chromosome(s) or fewer than 23 chromosomes. 2 x 23 Normal diploid (2N) human cell containing 2 sets of chromosomes. <23, >23 Mainly occurs with sex chromosomes. 23 23 Through meiosis haploid (N) gametes are produced, containing only one set of chromosomes. XO XX XXX XY XYY Examples Turner syndrome Normal female Metafemale Normal male Jacob syndrome Mutagens •Chemicals or radiation that increase the likelihood of a mutation occurring. •Rate of mutation induced by mutagen is proportional to the dose of mutagen. •Mutagens cause a decrease in the stability of the DNA molecule. •Thus DNA more susceptible to error during replication or meiosis. •Incorporate into genome •Trigger cancers Ionising radiation •Nuclear •UV rays •X rays Alcohol & dietary factors •High fat, alcohol •Trigger cancers Viruses & micro-organisms Types Environmental poisons/irritants •Benzene, asbestos •Tobacco tar, formaldehyde Diseases Click on a disease to find out more about how it is caused by a mutation. Down syndrome Sickle cell disease Cystic fibrosis Sickle cell disease •Haemoglobin is made up of 4 polypeptide chains (2 alpha & 2 beta chains). •Gene for beta chain is found on chromosome 11 and consists of 438 bases. •A mutation occurs in the gene coding for the beta chain. •The mutation is a substitution where adenine replace thymine on the DNA template strand. •As a result the amino acid valine replaces glutamic acid. •This change the properties of the haemoglobin and results in distorted red blood cells. haemoglobin HBB gene on chromosome 11 normal red blood cell sickle cell This single point mutation has a dramatic effect. Individuals have many health problems, eg weakness, jaundice, anaemia, heart & kidney defects, brain damage, skin lesions and inflamed spleen. Cystic Fibrosis •Inherited disorder and occurs when an individual has both copies of the faulty gene. •The gene (CFTR) is found on chromosome 7 and normally contains 1480 amino acids. •A mutation occurs whereby the 508th triplet (AAA) is deleted resulting in a mutated CFTR protein of 1479 amino acids. CFTR gene on chromosome 7 •The abnormal protein is unable to control chloride ion balance in cells. •The build up of chloride ions in cells results in severe effects. • Individuals have reduced life expectancy, high salt levels, blocked airways, pancreatic deficiencies and males are infertile. triplet deleted •Cystic fibrosis is relatively common and occurs 1 in every 2500 individuals. 506th 507th 508th 509th 510th 506th 507th 508th 509th 510th DNA mRNA amino acids TAA TAG AAA CCA CAA AUU AUC UUU GGU GGU ile ile phe gly val DNA mRNA amino acids AAACCA CAA TAA TAG AAA AUU AUC ile ile GGU GGU gly val Down Syndrome •Arise from having 3 copies of chromosome 21 •known as trisomy-21. •Individuals have characteristic facial features, are relatively short, slowed intellectual and physical development. •Two main types of Down syndrome-97% non-familial, 3% familial. Non-familial 24 Familial 23 One of the parents has only 45 chromosomes. A translocation has occurred where one of no.15 and one of no. 21 have joined together. One of the sex cells contain 23 but one of the 23 is the 15/21 chromosome. One of the parent’s sex cells contains two no. 21 chromosomes, due to nondisjunction during meiosis. 15 Parent 1 Parent 2 Down child 3 no.21 21 15/21