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Chapter 2 DNA extended response [108 marks] 1a. Describe the genetic code and its relationship to polypeptides and proteins. [5 marks] Markscheme Remember, up to TWO “quality of construction” marks per essay. a. (the genetic code is based on) sets of three nucleotides/triplets of bases called codons; b. bases include adenine, guanine, cytosine and thymine in DNA / adenine, guanine, cytosine and uracil in RNA; (do not accept ATCG) c. each codon is code for one amino acid; d. some codons are (start or) stop codons; e. DNA is transcribed into mRNA by base-pair matching/complementary base pairing; f. mRNA is translated into a sequence of amino acids/polypeptide; g. each gene codes for a polypeptide; h. polypeptides may be joined/modified to form proteins; 1b. Outline the role of proteins in active and passive transport of molecules through membranes. [5 marks] Markscheme Remember, up to TWO “quality of construction” marks per essay. a. channel proteins allow diffusion/osmosis/passive transport; b. large/polar molecules cannot cross the (hydrophobic) membrane freely; c. facilitated diffusion involves moving molecules through proteins down their concentration gradient/without requiring ATP; d. aquaporins (specific integral membrane proteins) facilitate the movement of water molecules/osmosis; e. some proteins (for facilitated diffusion) are specific to molecule/ions; f. active transport involves moving molecules through proteins against their concentration gradient/requiring ATP; g. (some) proteins in the membrane are pumps / pumps perform active transport / sodium potassium pump; 1c. Many cell functions, like synthesis of macromolecules and transport, require energy in the form of ATP. Explain how ATP is generated in animal cells. [8 marks] Markscheme Remember, up to TWO “quality of construction” marks per essay. a. ATP is a form of energy currency/immediately available for use; b. ATP is generated in cells by cell respiration (from organic compounds); c. aerobic (cell respiration) requires oxygen; d. anaerobic (cell respiration) does not require oxygen; e. glycolysis breaks down glucose into pyruvate; f. glycolysis occurs in cytoplasm; g. (by glycolysis) a small amount of ATP is released; h. ADP changes into ATP with the addition of a phosphate group/phosphoric acid / accept as chemical equation; i. in mitochondria/aerobic respiration produces large amount of ATP / 38 mols (for the cell, per glucose molecule); j. oxygen/aerobic respiration is required for mitochondrial production of ATP; k. in mitochondria/aerobic respiration pyruvate is broken down into carbon dioxide and water; 2a. Draw a labelled diagram of a section of DNA showing four nucleotides. [5 marks] Markscheme Award [1] for each labelled item shown correctly connected. 2b. Outline a technique used for gene transfer. [5 marks] Markscheme a. plasmid used for gene transfer/removed from bacteria; b. plasmid is a small/extra circle of DNA; c. restriction enzymes/endonucleases cut/cleave DNA (of plasmid); d. each restriction enzyme cuts at specific base sequence/creates sticky ends; e. same (restriction) enzyme used to cut DNA with (desired) gene; f. DNA/gene can be added to the open plasmid/sticky ends join gene and plasmid; g. (DNA) ligase used to splice/join together/seal nicks; h. recombinant DNA/plasmids inserted into host cell/bacterium/yeast; 2c. Explain how evolution may happen in response to an environmental change. [8 marks] Markscheme a. (genetic) variation in population; b. (variation is) due to mutation / sexual reproduction; c. valid example of variation in a specific population; d. more offspring are produced than can survive / populations over-populate; e. competition / struggle for resources/survival; f. example of competition/struggle for resources; g. survival of fittest/best adapted (to the changed environment)/those with beneficial adaptations / converse; h. example of changed environment and adaptation to it; i. favourable genes/alleles passed on / best adapted reproduce (more) / converse; j. example of reproduction of individuals better adapted to changed environment; k. alleles for adaptations to the changed environment increase in the population; l. example of genes/alleles for adaptations increasing in a population; m. evolution by natural selection; n. evolution is (cumulative) change in population/species over time / change in allele frequency; Suitable examples are antibiotic resistance and the peppered moth but any genuine evidence-based example of adaptation to environmental change can be credited. 3a. Distinguish between RNA and DNA. [3 marks] Markscheme DNA is double-stranded while RNA is single-stranded; DNA contains deoxyribose while RNA contains ribose; the base thymine found in DNA is replaced by uracil in RNA; one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA); 3b. Explain the process of DNA replication. [8 marks] Markscheme occurs during (S phase of) interphase/in preparation for mitosis/cell division; DNA replication is semi-conservative; unwinding of double helix / separation of strands by helicase (at replication origin); hydrogen bonds between two strands are broken; each strand of parent DNA used as template for synthesis; synthesis continuous on leading strand but not continuous on lagging strand; leading to formation of Okazaki fragments (on lagging strand); synthesis occurs in 5'→3' direction; RNA primer synthesized on parent DNA using RNA primase; DNA polymerase III adds the nucleotides (to the 3' end) added according to complementary base pairing; adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.) DNA polymerase I removes the RNA primers and replaces them with DNA; DNA ligase joins Okazaki fragments; as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form bond; Accept any of the points above shown on an annotated diagram. 3c. Outline how enzymes catalyse reactions. [7 marks] Markscheme they increase rate of (chemical) reaction; remains unused/unchanged at the end of the reaction; lower activation energy; activation energy is energy needed to overcome energy barrier that prevents reaction; annotated graph showing reaction with and without enzyme; substrate joins with enzyme at active site; to form enzyme-substrate complex; active site/enzyme (usually) specific for a particular substrate; enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron transfer); induced fit model / change in enzyme conformation (when enzyme-substrate/ES complex forms); making the substrate more reactive; 4a. Draw a labelled diagram to show how two nucleotides are joined together in a single strand of DNA. [3 marks] Markscheme Award [1] for each labelled item shown above. Award [2 max] if the two nucleotides are not shown in a single strand. 4b. Outline a basic technique for gene transfer. [6 marks] Markscheme plasmid removed from bacteria; plasmid cleaved/cut open by restriction enzymes; desired gene/DNA extracted from donor; DNA from donor cleaved using same restriction enzyme; results in sticky ends; with complementary base sequences; pieces of DNA from two organisms mixed; ligase used to splice pieces (DNA); recombinant plasmids formed; insertion into host cells; 4c. Explain the process of translation. [9 marks] Markscheme translation is the synthesis of proteins/polypeptide chain/specific sequence of amino acids; translation occurs in cytoplasm/ribosomes; uses information on the mRNA; mRNA carries the genetic information of DNA; mRNA binds to ribosome; mRNA contains series of codons/base triplets; tRNA binds with an amino acid and carries it to the ribosome; tRNA has the anticodon that is complementary to the codon on the mRNA; two tRNAs bind to a ribosome/mRNA at the same time; (peptide) bond forms between two amino acids (carried by tRNA molecules to the ribosome); the first tRNA detaches, ribosome moves along mRNA and another tRNA carrying an amino acid binds; process repeats forming chain of amino acids/polypeptides; 5a. Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. [5 marks] Markscheme Award [1] for each pair of statements in the table and [1] for any statement below the table. satellite DNA is repetitive; repetitive sequences are used for profiling; prokaryotes do not (usually) contain repetitive sequences 5b. Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands. [5 marks] Markscheme Award [1] for each of these structures clearly drawn and labelled. four nucleotides shown in diagram with one nucleotide clearly labelled; base, phosphate and deoxyribose (shown as pentagon) connected between the correct carbons and labelled at least once; backbone labelled as covalent bond between nucleotides correctly shown as 3' to 5' bond; two base pairs linked by hydrogen bonds drawn as dotted lines and labelled; two H bonds between A and T and three H bonds between C and G; adenine to thymine and cytosine to guanine; do not accept initials of bases antiparallel orientation shown; 5c. Explain the methods and aims of DNA profiling. [8 marks] Markscheme DNA sample obtained; from hair/blood/semen/human tissue; DNA amplified / quantities of DNA increased by PCR/polymerase chain reaction; satellite DNA/highly repetitive sequences are used/amplified; DNA cut into fragments; using restriction enzymes/restriction endonucleases; gel electrophoresis is used to separate DNA fragments; using electric field / fragments separated by size; number of repeats varies between individuals / pattern of bands is unique to the individual/unlikely to be shared; Award [5 max] for methods forensic use / crime scene investigation; example of forensic use e.g. DNA obtained from the crime scene/victim compared to DNA of suspect / other example of forensic use; paternity testing use e.g. DNA obtained from parents in paternity cases; biological father if one half of all bands in the child are found in the father; genetic screening; presence of particular bands correlates with probability of certain phenotype / allele; other example; brief description of other example; Award [4 max] for aims 6a. Explain why DNA must be replicated before mitosis and the role of helicase in DNA replication. [4 marks] Markscheme two genetically identical nuclei/daughter cells formed during mitosis (so hereditary information in DNA can be passed on); two copies of each chromosome/DNA molecule/chromatid needed; helicase unwinds the DNA/double helix; to allow the strands to be separated; helicase separates the two (complementary) strands of DNA; by breaking hydrogen bonds between bases; 6b. Explain how the base sequence of DNA is conserved during replication. [5 marks] Markscheme DNA replication is semi-conservative; DNA is split into two single/template strands; nucleotides are assembled on/attached to each single/template strand; by complementary base pairing; adenine with thymine and cytosine with guanine / A with T and C with G; strand newly formed on each template strand is identical to other template strand; DNA polymerase used; Marks may be awarded for any of the above points if clearly presented in a well-annotated diagram. 6c. Describe the events that occur during mitosis. [9 marks] Markscheme sequence of stages is prophase → metaphase → anaphase → telophase; chromosomes condense/supercoil/become shorter and fatter in prophase; spindle microtubules grow (from poles to equator) in prophase/metaphase; nuclear membrane breaks down in prophase/metaphase; spindle microtubules attach to the centromeres/chromosomes in metaphase; chromosomes line up at equator in metaphase; centromeres divide / (paired) chromatids separate / chromosomes separate into two chromatids in metaphase/anaphase; (sister) chromatids/chromosomes pulled to opposite poles in anaphase; spindle microtubules disappear in telophase; nuclear membrane reforms around chromosomes/chromatids in telophase; chromosomes/chromatids decondense in telophase; © International Baccalaureate Organization 2017 International Baccalaureate® - Baccalauréat International® - Bachillerato Internacional® Printed for Highland High School