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Mark schemes for Topic 3 3.1 Define the terms gene and allele and explain how they differ. 4 marks 3.1 Describe the consequences of a base substitution mutation with regards to sickle cell anemia. 7 marks 3.1 Explain the consequences of a base substitution mutation in relation to the processes of transcription and translation. gene is a heritable factor / unit of inheritance gene is composed of DNA gene controls a specific characteristic / codes for a polypeptide / protein allele is a form of a gene alleles of a gene occupy the same gene locus / same position on chromosome alleles differ (from each other) by one / a small number of bases(s)/ base pair(s) the sequence of nucleotide bases in DNA codes for the sequence of amino acids in proteins DNA is transcribed into mRNA, which is translated into amino acids of protein normal (ß chain) hemoglobin gene / DNA produces normal (ß chain) hemoglobin protein / amino acids substitution= the replacement of one (or more) nucleotide base with another caused by a copying mistake during DNA replication as a result of a mutagen / X-rays / chemical / UV radiation / other mutagen mutation in normal (ß chain) hemoglobin gene alters the sequence of nucleotide bases normal nucleotide sequence = CTC altered to CAC resulting in altered mRNA (GAG to GUG) during transcription resulting in altered sequence of amino acids in (ß chain) hemoglobin protein (glutamic acid to valine) during translation causing red blood cells to change shape / sickle under low oxygen conditions causing sickle cells anemia when two copies of the mutated gene are inherited producing a sickle cell carrier when one copy of the mutated gene is inherited sickle cells anemia reduces oxygen flow to organs, leading to their deterioration mutation is a change in the genetic make-up; base substitution mutation occurs when one (nitrogenous) base in DNA chain is replaced by another; this is a gene mutation / change in the base sequence of a gene; effect of mutation ranges from no effect / no change in amino acid sequence to drastic changes; sickle-cell anaemia involves change in gene for one of polypeptides in hemoglobin / Hb / HBA; GAG has mutated to GTG (on DNA); adenine replaced by thymine in DNA; transcription of DNA produces the triplet GUG instead of GAG on mRNA; one codon is different in mRNA; new codon is for valine rather than glutamic acid; tRNA brings amino acid to ribosome during translation; different amino acid placed in polypeptide chain being formed by translation; the two amino acids differ in solubility / have different properties / valine causes S HB to be less soluble; causes red blood cells to become sickle shaped / carry oxygen less efficiently; S HB allele causes sickle-cell anaemia but gives resistance to malaria; 3.2 . Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained. 5 marks 3.2 State the two classes of compounds that compose chromosomes in animal cells. (1) 3.2 Describe how sexual reproduction promotes genetic variation within a species. (4 marks) 3.3 Outline the formation of chiasmata (bivalent) 8 max application of karyotyping {2 max} find gender / test for Down's syndrome / other chromosome abnormality identify sex chromosomes / numbers of chromosome 21 / other chromosomes counted XX = female and XY = male / third chromosome 21 indicates Down's syndrome / other chromosome abnormality (e.g. Klinefelter's syndrome) obtaining chromosomes {3 max} fetal cells obtained from amniotic fluid / amniocentesis / other named source white blood cells obtained cells encouraged to divide cells accumulated / blocked in metaphase prepare slide / chromosomes examined DNA and protein (segregation of alleles involves) meiosis; crossing over / chiasma formation in prophase I / meiosis (do not allow if wrong phase of meiosis given); random orientation / assortment of homologues at metaphase I; fertilization by chance / one of many male gametes; number of different gametes is 2n (ignoring crossing over); genes / alleles combined from two parents; Accept the points below in an appropriately annotated diagram. during crossing over. 5 marks crossing over/chiasmata formed during prophase I of meiosis; pairing of homologous chromosomes/synapsis; chromatids break (at same point); (do not accept chromatids overlap) non-sister chromatids join up/swap/exchange alleles/parts; X-shaped structure formed / chiasmata are X-shaped structures; chiasma formed at position where crossing over occurred; chiasmata become visible when homologous chromosomes unpair; chiasma holds homologous chromosomes together (until anaphase); 5 max 3.3 Explain how an error in meiosis can lead to Down syndrome. 8 marks Accept the points below in an appropriately annotated diagram. non-disjunction; chromosomes/chromatids do not separate / go to same pole; non-separation of (homologous) chromosomes during anaphase I; due to incorrect spindle attachment; non-separation of chromatids during anaphase II; due to centromeres not dividing; occurs during gamete/sperm/egg formation; less common in sperm than egg formation / function of parents' age; Down syndrome due to extra chromosome 21; sperm/egg/gamete receives two chromosomes of same type; zygote/offspring with three chromosomes of same type / trisomy / total 47 chromosomes; 3.3 Compare the processes of mitosis and meiosis. 6 marks answers must be pair-wise comparisons to receive any marks. Mitosis: one cell division & Meiosis: two divisions / reduction division Mitosis: chromosome number does not change & Meiosis: converts diploid to haploid cells Mitosis: products genetically identical & Meiosis: products genetically diverse Mitosis: separation of sister chromatids in anaphase & Meiosis: separation of homologous chromosomes in anaphase I and sister chromatids in anaphase II Mitosis: no crossing over & Meiosis: crossing over in prophase I Mitosis: no formation of tetrads / no synapsis & Meiosis: formation of tetrads / synapsis Mitosis: produce cells for growth/repair/asexual reproduction & Meiosis: produce sexual cells / gametes for sexual reproduction Mitosis: two cells produced & Meiosis: four cells produced 3.3 Outline how meiotic division results in almost an infinite genetic variation in the gametes produced. 2marks Mitosis: daughter cells with both copies of chromosomes/random assortment does not occur & Meiosis: random assortment of maternal/ paternal chromosomes Mitosis: replication of DNA in interphase & Meiosis: replication of DNA in interphase I Mitosis: four phases: prophase, metaphase, anaphase, telophase & Meiosis: same four phases twice crossing over / chiasmata; shuffles alleles; random orientation of chromosomes; at metaphase I: at metaphase II; 2 max 3.3 Describe, with the aid of a diagram, the behaviour of chromosomes in the different phases of meiosis. (Total 5 marks) chromosomes condense / coil / become shorter and fatter during prophase I; (homologous) chromosomes pair up in prophase I; crossing over / chiasmata formation in prophase I; movement of pairs of chromosomes / bivalents to the equator in metaphase I; movement of half of the chromosomes to each pole in anaphase I; movement of chromatids to opposite poles in anaphase II; decondensation / uncoiling in telophase II; [4 max] if no diagram is shown. Do not award a mark for a statement if a diagram has been drawn that does not fit in with the statement. For example, if the candidate states that pairs of chromosomes move to the equator in metaphase I but shows single chromosomes, do not award that mark. 3.3 Explain how meiosis random orientation of bivalents / pairs of chromosomes; and fertilization maternal and paternal chromosome could go to either can give rise to pole; genetic variety. 2n combinations; (Total 6 marks) eg over 8 million in humans; crossing over; exchange of material between homologous chromosomes / non-sister chromatids; segregation of alleles in meiosis; combinations of alleles are broken up; fertilization brings together genes / alleles from two different parents; fertilization generates new combinations of genes / alleles; random fertilization / many possible combinations of male and female gamete; eg over 64 million million in humans (ignoring crossing over); [6] 3.4 Outline one example of inheritance involving multiple alleles. 5 marks multiple alleles means a gene has three or more alleles / more than two alleles ABO blood groups / other named example of multiple alleles ABO gene has three alleles / equivalent for other example IA IB and i shown (at some point in the answer) / equivalent for other example accept other notation for alleles if clear any two of these alleles are present in an individual homozygous and heterozygous genotye with phenotypes (shown somewhere) all six genotypes with phenotypes given (shown somewhere) example / diagram of a cross involving all three alleles 3.4 Describe the inheritance of ABO blood groups including an example of the possible outcomes of a homozygous blood group A mother having a child with a blood group O father. 5 marks example of co-dominance multiple alleles / 3 alleles (phenotype) O has (genotype) ii B can be IB IB or IB i A can be IA IA or IA i AB is IA IB (P are) i i x IA IA (gametes) i and IA (F1 genotype) IA i (F1 phenotype) blood group A accept other notations if used consistently and if phenotype and genotype are clearly distinguished 3.4 Outline sex linkage. 5 marks 3.4 Explain, using a named example, why many sexlinked diseases occur more frequently in men than women. 9 marks gene carried on sex chromosome / X chromosome / Y chromosome inheritance different in males than in females males have only one X chromosome therefore, only one copy of the gene mutation on Y chromosome can only be inherited by males women can be carriers if only one X chromosome affected example of sex linked characteristics (e.g. hemophilia / color blindness) example of cross involving linkage named example of sex-linked disease caused by recessive allele on the X chromosome example of pair of alleles (e.g. X H and X h) (reject if alleles do not correspond) females are XX and males are XY females have two alleles of the gene and males have only one allele causing the disease is rare / uncommon probability of females inheriting rare allele twice as low calculation of squaring the gene frequency female would have to inherit the allele from her father who would have suffered from the disease so females can carry the gene but still be normal but males (with the gene) will have the disease non-disjunction 3.4 State how chromosome number can increase in human beings. 1 mark 3.4 Define the term coalleles of gene / pairs of alleles which both affect the dominant alleles. phenotype / both expressed (1) (when present together in an individual / in the heterozygote); 1 Reject: both dominant / both recessive 3.5 Describe the technique for the transfer of the insulin gene using E. coli. (6) 3.5 3.5 Describe the application of DNA profiling to determine paternity and also in forensic investigations. mRNA is extracted; DNA copy of RNA is made using reverse transcriptase; plasmids are cut open with endonucleases (at specific sequences); insulin gene and plasmid are mixed together; addition of “sticky ends” to the DNA copy (so that it will combine with the cut plasmid); DNA ligase will seal the plasmid; recombinant plasmid is inserted into E. coli; E. coli is cultured; E. coli begins to make insulin; 6 max Mark Scheme A. sample of DNA / blood / saliva / semen is obtained; B. satellite DNA / repetitive sequences used for profiling; C. reference samples of DNA are obtained; D. PCR used to amplify / produce more copies of the DNA; E. DNA broken into fragments by restriction enzymes; F. DNA fragments are separated by gel electrophoresis; G. separation according to the length of the fragments; H. pattern of bands obtained / different pattern of bands with DNA from different individuals; I. bands compared between different DNA samples; J. if pattern of bands is the same then DNA is (almost certainly) from same source; K. if some bands are similar then individuals are (almost certainly) related; L. used for criminal investigations / example of use in criminal investigation; M. used to check paternity / who is the father / mother / parent; N. used to check whether two organisms are clones; Outline three outcomes of the sequencing of the complete human genome. 3.5 Discuss the ethical issues for and against the use of transgenic plants. 6 marks Mark Scheme A. may lead to an understanding of genetic / inherited diseases / conditions; B. may lead to the production of gene probes to detect carriers of genetic diseases; C. may lead to the production of pharmaceuticals based on DNA sequences; D. study of similarities / differences between human race / population; E. find location of genes / produce a complete gene map; F. study of human origins / migration / relationships with other species; Responses must have at least two arguments for and two arguments against to receive full marks. arguments for: transgenic plants do not survive long in wild; reduced cost of food production / reduce amount of land used for food production; transfer of genes occurs naturally; DNA and proteins are digested (unlikely to cause problems); longer shelf life for certain products; increased yield / bred for faster growth; better adaptation to certain conditions / eliminate possible plagues; healthier food produced because use of pesticides can be decreased eg Bt corn / increase food quality eg albumin gene has increased sulfur amino acid content of alfalfa used to feed sheep; arguments against: uncertain long-term health effects from eating transgenic crops / may contain animal genes; new proteins in food crops could cause allergies; genes could escape from transgenics to wild plants; create superweeds / damage ecosystems; monopoly (created by large companies) adversely affects small farmers; [6] 3.5 Outline a basic technique used for gene transfer involving plasmids, a host cell (bacterium, yeast or Mark Scheme A. mRNA is extracted; B. DNA copy of RNA is made using reverse transcriptase; C. plasmid is a small piece of circular DNA; D. plasmid removed from (host) cell; other cell), restriction enzymes (endonucleases) and DNA ligase. E. plasmids are cut open with endonucleases (at specific sequences); F. genes / DNA fragments from another organism cleaved by same restriction enzyme; G. gene of interest and plasmid are mixed together; H. addition of “sticky ends” to the DNA copy (so that it will combine with the cut plasmid); I. DNA ligase will seal the plasmid; J. recombinant plasmid is inserted into E. coli/host cell; K. E coli is cultured; L. E coli begins to make protein coded by the gene of interest; Discuss the potential benefits and possible harmful effects of one example of genetic modification. Mark Scheme Possible benefits: A. benefits include more specific (less random) breeding than with traditional methods; B. faster than traditional methods; C. some characteristics from other species are unlikely in the gene pool / selective breeding cannot produce desired phenotype; D. increased productivity of food production / less land required for production; E. less use of chemicals (eg pesticides); F. food production possible in extreme conditions; 3.5 H. eg pharmaceuticals in milk; I. human insulin engineered so no allergic reactions; J. may cure genetic diseases; K. allows use of herbicide on growing crop; L. higher yield due to less weed competition; M. weeds that are very similar to the crop plants can be controlled; Possible harmful effects: N. some gene transfers are regarded as potentially harmful to organism (especially animals); O. release of genetically engineered organisms in the environment; P. can spread and compete with the naturally occurring varieties; Q. some of the engineered genes could also cross species barriers; R. technological solution when less invasive methods may bring similar benefits; S. reduces genetic variation / biodiversity; T. uncertainty about long-term effect; U. uncontrollable superweeds might be produced; V. foreign DNA in the crop plant might cause allergies in humans; W. fewer weeds for wildlife that feed on them; Examples: X. name of organism that was genetically modified; Y. source of the DNA / gene used to modify organism; Z. effect of the gene / characteristic coded for by the gene; AA. reference to gene transfer between species being a natural process (with viral vectors); BB. wheat / maize / other crop plant; CC. Salmonella typhimurium; DD. resistance to glyphosate / roundup herbicide; 3.5 Discuss the ethical issues of therapeutic cloning in humans. genetic screening is testing for the presence or absence of gene / chromosome; screening for chromosomes can involve karyotyping; genetic screening is controversial; advantages: [4 max] parents can choose to avoid having children with disorder; parents can prepare for a child with a disorder; parents can use IVF to select embryos that are normal; can use gene therapy to correct the problem; treatment can start to prevent symptoms; fewer children with the disorder are born; disadvantages: [4 max] frequency of abortion can increase; parents can select embryos for sex of the child; can have harmful side effects such as depression if you know you will develop a disorder later; can create a genetic underclass; health insurance / treatment can be denied if there is genetic predisposition; 8 max 3.5 Outline how the process of meiosis can lead to Down’s Syndrome. (4) Accept any of the points below if clearly drawn and correctly labelled in a diagram. in metaphase homologues in centre of cell / spindles attached; homologues are separating; one pair doesn’t separate / non-disjunction; in telophase cells divide into two; cells have either one more / one less chromosome; can occur in second division of meiosis; sister chromatids fail to separate; fertilization with one gamete / sperm / egg carrying extra chromosome; Down’s syndrome is trisomy of chromosome 21; 4 max 3.5 State two examples of transgenic techniques in agriculture involving animals sheep (milk) produce factor IX for blood clotting; winter flounder fish gene to make tomatoes frost resistant; α-1 antitrypsin in sheep; interferon in cow’s milk; spider silk protein into sheep; 2 max Award any other examples which involve animals as sources of genes or as the genetically modified organism. (5 marks) 3.5 State two procedures used for the preparation of a DNA profile. polymerase chain reaction and electrophoresis