SL IB Biology Mutations & Gene Editing PDF

Head to www.savemyexams.com for more awesome resources SL IB Biology Your notes Mutations & Gene Editing Contents Gene Mutations Consequences of Gene Mutations Mutations & Genetic Variation...

Head to www.savemyexams.com for more awesome resources SL IB Biology Your notes Mutations & Gene Editing Contents Gene Mutations Consequences of Gene Mutations Mutations & Genetic Variation Page 1 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Gene Mutations Your notes Gene Mutations A gene mutation is a change in the sequence of base pairs in a DNA molecule; this may result in a new allele Mutations occur all the time and at random There are certain points in the cell cycle when mutations are more likely to occur, for example, copying errors when DNA is being replicated (S phase of interphase) As the DNA base sequence determines the sequence of amino acids that make up a polypeptide, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for Most mutations are harmful or neutral (have no effect) but some can be beneficial Inheritance of mutations: Mutations present in normal body cells are not inherited, they are eliminated from the population once those cells die Mutations within gametes are inherited by offspring, possibly causing genetic disease Substitution mutations A mutation that occurs when a nucleotide base in the DNA sequence is randomly swapped for a different base is known as a substitution mutation A substitution mutation will only change the amino acid for the triplet (group of three consecutive bases) where the mutation occurs; it will not have a knock-on effect further along the gene/polypeptide Substitution mutation diagram Page 2 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources An example of a substitution mutation altering the sequence of amino acids in the polypeptide Your notes Insertion mutations A mutation that occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence is known as an insertion mutation An insertion mutation changes the amino acid that would have been coded for by the original base triplet, as it creates a new, different triplet of bases Remember – every group of three bases in a DNA sequence codes for an amino acid An insertion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence This is sometimes known as a frameshift mutation This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function Insertion mutation diagram An example of an insertion mutation Deletion mutations A mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for Like an insertion mutation, a deletion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence Like an insertion mutation, this is sometimes known as a frameshift mutation Page 3 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function Your notes Causes of Gene Mutations Mutagenic agents are environmental factors that increase the mutation rate of cells Radiation can cause chemical changes in DNA, this includes: High-energy radiation such as UV light Ionising radiation such as X-rays, gamma rays and alpha particles Chemical substances can also caused changes to DNA, examples include Benzo[a]pyrene and nitrosamines found in tobacco smoke Mustard gas used as a chemical weapon in World War I Mutagens can also come from inside the cell such as particular enzymes that either break down DNA or produce substrates that are mutagenic Some mutations may be produced at random, this can happen most frequently during DNA replication and repair where errors in the nucleotide sequence are not detected by the proofreading process carried out by DNA polymerase If the polymerase detects that a wrong nucleotide has been added, it will remove and replace the nucleotide before continuing with DNA synthesis Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed As the genetic code is degenerate (more than one triplet code codes for the same amino acid) some mutations will not cause a change in the amino acid sequence Randomness in Mutations Mutations can occur anywhere in the base sequence of a genome on all chromosomes in all organisms This is how new strains of viruses or bacteria can come into existence Some locations of the genome are more likely to mutate than others Uncoiled DNA has a higher probability of encountering mutations than DNA tightly coiled around a histone as it is more exposed Many mutations occur in non-coding regions of DNA such as satellite DNA Mutation hotspots are regions where mutations are more frequent. One hotspot is where the nucleotide cytosine (C) is followed by guanine (G) and is called a CpG site When methylation occurs here, C can mutate into Thymine (T) in a substitution mutation Where this occurs repeatedly it is known as a CpG island and is associated with particular cancers such as colorectal cancer Intentional changes to base sequences No known mechanisms exist where cells are able to intentionally mutate or change their DNA base sequence Proofreading processes exist to change a mutation back into its original sequence but no mechanism exists for making a deliberate change to a base or sequence of bases with the purpose of changing a trait of the organism Page 4 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Consequences of Gene Mutations Your notes Consequences of Base Substitutions Base substitutions are a mutation that occurs when a base in the DNA sequence is randomly swapped for a different base A substitution mutation can only change the amino acid for the triplet in which the mutation occurs; it will not have a knock-on effect on the rest of the sequence A base substitution can result in single nucleotide polymorphisms, frequently called SNPs (pronounced “snips”) These represent a difference in a single DNA nucleotide. E.g. a SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA SNPs occur normally throughout a person’s DNA They occur once in every 300 nucleotides on average, which means there are roughly 10 million SNPs in the human genome SNPs are commonly found in the non-coding regions of DNA between genes They can act as biological markers, helping to locate genes that are associated with disease The effect of SNPs Substitution mutations can take three forms which may or may not change the amino acid of a polypeptide chain: Silent mutations – the mutation does not alter the amino acid sequence of the polypeptide (this is because certain codons may code for the same amino acid as the genetic code is degenerate) Missense mutations – the mutation alters a single amino acid in the polypeptide chain (sickle cell anaemia is an example of a disease caused by a single substitution mutation changing a single amino acid in the sequence) Nonsense mutations – the mutation creates a premature stop codon (signal for the cell to stop translation of the mRNA molecule into an amino acid sequence), causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function (cystic fibrosis is an example of a disease caused by a nonsense mutation, although this is not always the only cause) Page 5 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Consequences of Insertions & Deletions Insertions and deletions are two types of point mutations, which are mutations that involve a change in Your notes the DNA base sequence at a single location An insertion occurs when an extra nucleotide is incorporated into the DNA sequence during replication A deletion mutation occurs when a nucleotide is missed or absent from the replicated strand These mutations are often considered more harmful than substitutions, because they impact on the way the rest of the sequence is read by mRNA or the ribosome Insertions and deletions of nucleotides can also have the effect of a frameshift mutation This causes a complete change to the entire amino acid sequence of a protein after the mutation site and can cause the polypeptide to cease to function This happens because of the way the translated mRNA is read by the ribosomes The mRNA is read in codons (groups of 3 nucleotides) so if an additional 1 or 2 nucleotides are added or removed, the sequence is ‘shifted’ The ribosome still reads the sequence of triplet codons along the length of mRNA which means the entire mRNA and resulting protein are completely different The result of frameshift mutation means the entire DNA sequence following the mutation will be incorrectly read. This can result in the addition of the wrong amino acids to the polypeptide chain and/or the creation of a codon that stops the protein from growing longer Although a frameshift mutation during translation is rare (10−5 to 10−7 per codon), the effects are generally catastrophic for the resulting protein The same can be said for large insertions and deletions of nucleotides of the DNA sequence Consequences of Mutations Diagram Page 6 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Insertion and deletion mutations can cause a frameshift in the sequence of bases which can drastically alter the amino acid coded for. The resulting protein may be very different to the original intended. Page 7 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mutations in Germ & Somatic Cells The effect of a mutation can vary depending on whether it occurs in a germ cell or somatic cell Your notes A germ cells use meiosis to produce gametes Somatic cells use mitosis to produce cells all over the body which can grow into tissues and organs Germ cells If a mutation occurs in a germ cell it can be passed on to the offspring and next generation Cells involved in inheritance of genetic information, eggs, sperm and zygote, are know as the germ line A mutation that occurs in sperm cells could potentially affect the zygote of that offspring and all cells developed from that zygote will contain the mutation A female that has inherited a mutation will contain the mutation in the germ cells of their ovaries which will be passed onto future offspring Somatic cells Somatic cell mutations are not inherited by offspring, instead these mutations are associated with cancers Cancers demonstrate how important it is that cell division is precisely controlled, as cancers arise due to uncontrolled mitosis Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells) Cancers start when a mutation occurs in the genes that control cell division If the mutated gene is one that causes cancer it is referred to as an oncogene Mutations are common events and don’t lead to cancer most of the time Most mutations either result in early cell death or result in the cell being destroyed by the body’s immune system As most cells can be easily replaced, these events usually have no harmful effect on the body The mutations that result in the generation of cancerous cells do not result in early cell death or in the cell being destroyed by the body’s immune system Page 8 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mutations & Genetic Variation Your notes Mutations & Genetic Variation Differences exist between organisms of the same species These differences are known as variation Examples of variation include: Coat colour in mammals Body length in fish Flower colour in flowering plants Variation results from small differences in DNA base sequences between individual organisms within a population There are several sources of these differences in DNA base sequences: Mutation Meiosis Random fertilisation during sexual reproduction Mutations The original source of genetic variation is mutation Mutation results in the generation of new alleles which can influence evolution of a species Mutations that take place in the dividing cells of the sex organs lead to changes in the alleles of the gametes that are passed on to the next generation A new allele may be advantageous, disadvantageous or have no apparent effect An advantageous allele is more likely to be passed on to the next generation because it increases the chance that an organism will survive and reproduce A disadvantageous mutation is more likely to die out because an organism with such a mutation is less likely to survive and reproduce Mutations in a species are, in the long term, essential for evolution by natural selection Note that a mutation taking place in a body, or somatic, cell will not be passed on to successive generations, and so will have no impact on natural selection Mutation is the only source of variation in asexually reproducing species NOS: Commercial genetic tests can yield information about potential future health and disease risk. One possible impact is that, without expert interpretation, this information could be problematic There are two types of genetic testing available Clinical or medical genetic testing This is carried out through healthcare providers such as doctors, nurse practitioners, or genetic counselors Healthcare providers determine which test is needed, order the test from a laboratory, collect the DNA sample, send the DNA sample to a laboratory for testing and analysis, and importantly they share the results with the patient and ensure understanding of the test results and the implications to the individual and their families Page 9 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Commercial genetic testing provides genetic tests marketed directly to customers The test kits can be bought online or in stores Customers send the company a DNA sample and receive their results directly from the Your notes genetic company or lab Commercial genetic testing provides people access to their genetic information without necessarily involving a healthcare provider This can pose some problems Commercially available genetic tests are not scientifically validated and can give inaccurate results Unexpected information that a customer receives about their health, family relationships, or ancestry may be stressful or upsetting People may make important decisions about disease treatment based on inaccurate or misunderstood information from their test results Individuals often are not provided with genetic counseling Page 10 of 10 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers