SAJC 2013 H2-Bio-TYS-ANS

2013 UCLES ‘A’ Level H2 Biology Mark Scheme 1 2013 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ)

2013 UCLES ‘A’ Level H2 Biology Mark Scheme 2 PAPER 2 (CORE) QUESTION 1 (a) Describe the processes that occur in the golgi body. [3] 1 Receive proteins and lipids from the ER (rER and sER) and chemically modifies them / Further chemically modifies glycoproteins and glycolipids; e.g. short carbohydrate chains or oligosaccharides are added to the proteins and lipids to form glycoproteins and glycolipids / enzymes of the Golgi body cleave a sugar from a chain or modify one or more of the sugars; 2 Various molecules are sorted and packaged; Then, they are targeted to various cellular locations; or secreted out of the cell; 3 Hydrolytic enzymes of the lysosome will be sorted and packaged together; To form lysosomes; 4 Synthesis of carbohydrates e.g. non-cellulose polysaccharides such as pectin; For formation of plant cell walls; Examiner’s comment: The majority of candidates correctly described protein modification and most went on to provide further details. (b) Describe the roles of the vesicles that fuse with the forming face of the golgi body. [3] 1 The vesicles are made up of membranes; which buds off from the endoplasmic reticulum; 2 These vesicles transport proteins and lipids synthesized in rER and sER ; To be further chemically modified in the Golgi body ; 3 The vesicles has proteins which recognize and bind to specific receptors on the forming face of golgi body; Before membranes of vesicles fuse with membrane of the forming face of golgi body, emptying their contents into the lumen of the Golgi body; Examiner’s comment: Most candidates were able to provide full responses. However, some candidates omitted to mention the involvement of proteins. (c) Outline two roles of the vesicles that are formed from the maturing face of the golgi body. [4] 1 Secretory vesicles migrate toward the plasma membrane ; Membrane of the vesicles fuse with the plasma membrane ; 2 Substances (e.g. hormones, digestive enzymes of the gut) released to the outside of the cell; via exocytosis; 3 Lysosomes contain hydrolytic enzymes; which are introduced to food vesicles produced by phagocytosis or autophagosomes containing worn-out organelles upon fusion of both membranes; 4 Hydrolytic enzymes break down the food molecules / worn-out organelles; Digested substances are then assimilated back into the cell to be re-used; Examiner’s comment: The majority of candidates provided detailed responses. Some candidates confused the enzyme lysozyme with the hydrolytic enzymes found in lysosomes.

2013 UCLES ‘A’ Level H2 Biology Mark Scheme 3 QUESTION 2 (a) State what happens if a cell loses control of the cell cycle. [1] 1 It undergoes uncontrolled cell division and may result in cancer;; Examiner’s comment: A few candidates referred mistakenly to uncontrolled cell growth. (b) With reference to Fig 2.1, suggest how the dysregulation of checkpoints of cell division may occur. [3] 1 Genes coding for M-cyclin and S-cyclin may be mutated; resulting in production of hyperactive M-cyclins and S-cyclins/ M-cyclins and S- cyclins which cannot be degraded; 2 These cyclins bind to and activate their respective CDK proteins permanently;; 3 The permanently activated CDK proteins allow the cell (normal or otherwise) to proceed from G2 to M phase, and from G1 to S phases of the cell cycle without any control;; (This results in continuous DNA replication and mitosis and hence uncontrolled cell divisions) Examiner’s comment: Candidates who considered the information provided in the diagram in the context of loss of control of the cell cycle were able to provide full responses. Not all considered mutations as a possible cause of changes. A significant minority of candidates described changes that would have arrested the cell cycle completely, such as cyclins being unable to bind with CDK. (c) (i) Name one causative agent of cancer. [1] 1 UV irradiation / exposure to carcinogenic chemicals like Ethidium bromide;; (ii) Outline the development of cancer, including the effects of this causative agent. [5] 1 Exposure to UV (or carcinogenic chemicals) can cause gene mutations to occur;; 2 Gain-of-function mutation in at least one proto-oncogene can lead to production of abnormal onco-proteins (e.g. hyperactive Ras); resulting in excessive cell proliferation; 3 Loss-of-function mutations in several tumour suppressor genes can result in production of non-functional gene products; which are unable to trigger cellular processes involved in cell cycle arrest, DNA repair and apoptosis; 4 Cells will be able to progress through the cell cycle checkpoints unchecked; allowing further accumulation of mutations to occur; 5 Activation of telomerase gene; telomerase enzyme prevents the shortening of the chromosome ends / cell can continue to divide indefinitely; 6 angiogenesis / formation of new network of blood vessels to the cancer cells;

2013 UCLES ‘A’ Level H2 Biology Mark Scheme 4 blood vessels provide the cancer cells oxygen and nutrients for growth and to remove any waste products; 7 loss of contact inhibition / density-dependence (and cells do not stop dividing), ability to differentiate and anchorage dependence / loss of cell adhesion;; 8 result in metastasis / tumor break loose and may enter the bloodstream / invade other tissues to form secondary tumor which is now malignant;;

2013 UCLES ‘A’ Level H2 Biology Mark Scheme 5 QUESTION 3 (a) State the name given to this mechanism of DNA replication. [1] 1 Semi-conservative replication;; (b) Describe how these results provided evidence for Watson and Crick’s proposed mechanism. [3] 1 In generation 0, 1 band is formed near the bottom of the tube (heavy density position); indicating that both strands in the DNA molecule contains the heavier 15N isotope (15N-15N); 2 In generation 1, 1 band is formed slightly higher in the tube (intermediate density position); indicating that both DNA strands from generation 0 are templates for the synthesis of new daughter strands, resulting in each DNA molecule containing one 15N and one 14N strand; 3 In generation 2, 1 band corresponds to the band position in generation 1 but half the thickness which indicates that 50% of the DNA is 15N-14N; while the other band occupies the highest position in the tube which indicates the other 50%of DNA is 14N-14N; Examiner’s comments: Some candidates were unclear in the use of terms between DNA molecules, strands and bands. (c) List three ways in which transcription is different from DNA replication. [3] Transcription Replication Product formed is mRNA Product formed is DNA Only specific regions of DNA molecule is transcribed Entire DNA molecule is replicated One DNA strand used as template Both DNA strands used as template Uracil ribonucleotide is used, not thymine Thymine deoxyribonucleotide is used, not uracil RNA polymerase does not need primers to start synthesis of RNA DNA polymerase requires RNA primers to synthesize daughter DNA strands (d) Explain how the information to synthesise polypeptides is coded for by DNA. [3] 1 DNA contains a specific gene sequence of nucleotide bases where the genetic information is transcribed;; 2 The exposed nucleotide bases of the DNA template base pair with the complementary RNA nucleotide bases to form mRNA; Adenine base pairs with uracil, thymine to adenine, cytosine to guanine, and guanine to cytosine; 3 The genetic information on mRNA is read as triplet bases/codon; to give rise to an amino acid sequence of the polypeptide in the translation process; Examiner’s comments: Many candidates give detailed descriptions of polypeptide synthesis without explaining how the information is encoded by DNA.

2013 UCLES ‘A’ Level H2 Biology Mark Scheme 6 QUESTION 4