SAJC 2015 H2-Bio-TYS-ANS

2015 UCLES ‘A’ Level H2 Biology Mark Scheme 1 2015 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ)

2015 UCLES ‘A’ Level H2 Biology Mark Scheme 2 PAPER 2 (CORE) QUESTION 1 Fig.1.1 shows a diagram of a hepatocyte, which is a type of cell found in the liver. Fig. 1.1 (a) Identify the structures labelled A and B, as shown in Fig. 1.1, [6] For each structure, state two features that can be seen in Fig. 1.1. structure A: mitochondrion feature 1 : double membrane feature 2 : cristae structure B : rough endoplasmic reticulum feature 1 : cisternae / flattened membrane-bound sacs feature 2 : bound ribosomes Examiner’s comment: The question stated that features of the identified structures should be visible in Fig.1.1. A few candidates mentioned features that could not be seen, e.g. the presence of DNA. A very small number of candidates confused the cristae of the mitochondria with the cisternae of the rough endoplasmic reticulum.

2015 UCLES ‘A’ Level H2 Biology Mark Scheme 3 (b) Describe two functions of the Golgi body. [2] 1 Further post-translational modifications of proteins and lipids received from the ER (rER and sER) e.g. formation of glycoprotein, glycolipid ; 2 Sorting and packaging of proteins before being targeted to various cellular locations ; 3 Synthesis and transport of lipids ; 4 Formation of secretory vesicles (containing enzymes or proteins for release outside cell) or lysosomes ; 5 Involved in carbohydrates synthesis for the formation of cell walls in plants (Ref: 9648/P1/2010) Examiner’s comment: Candidates were able to describe a wide range of valid functions of the Golgi body. (c) Suggest two advantages to eukaryotic cells of having membrane -bound organelles. [2] 1 Provides a boundary between the cytoplasm and specific contents within the membrane-bound organelles so that separate compartments can be formed within a cell ; 2 Increase surface area for reaction to occur ; E.g. Folding of inner mitochondrial membrane results in a greater surface area for attachment of more proteins and enzymes of the electron transport chain and ATP synthase for higher rates of ATP synthesis. Examiner’s comment: Many candidates considered compartmentalisation to be an advantage of having membrane-bound organelle. Since compartmentalisation is only another way of describing this feature, responses citing compartmentalisation required further qualification and relevant details. (d) Explain the role of glycogen in animal cells. [2] 1 Stored in large quantities without having any great effect on the water potential of cells and can be prevented from diffusing out of cells ; 2 Stored in large quantities within the limited space in animal cells ; 3 Ease of breakdown allow glycogen to be a good energy source for respiration and to raise glucose levels quickly when blood sugar level drops in animals. Examiner’s comment: Candidates were largely aware of the role of glycogen in animal cells. [Total: 12]

2015 UCLES ‘A’ Level H2 Biology Mark Scheme 4 QUESTION 2 Fig.2.1 shows a diagram of DNA replication. Fig. 2.1 (a) Describe how replication of the lagging strand template occurs. [3] (Compulsory point) 1 After the DNA unzip at the origin of replication, RNA primers with bases complementary to bases on DNA template strand, were synthesised by DNA primase ; (any two points ; point 2 to 7) 2 DNA polymerase III catalyses addition of free deoxyribonucleoside triphosphates to the free 3’OH end of each RNA primer ; 3 Complementary base pairing occur between newly added nucleotides and those on lagging strand template ; 4 DNA polymerase III catalyses formation of phosphodiester bonds between adjacent deoxyribonucleotides ; 5 DNA polymerase I hydrolyses the RNA primers ; 6 And fills the gaps with complementary deoxyribonucleoside triphosphates / deoxyribonucleotides ; 7 DNA ligase catalyses the formation of a phosphodiester bond between 2 Okazaki fragments. Examiner’s comment: Most candidates understood the significance of the antiparallel strands of DNA to replication of the lagging strand template and were able to provide full responses.

2015 UCLES ‘A’ Level H2 Biology Mark Scheme 5 (b) State two ways in which DNA replication, (i) differs from transcription, [2] Any two: DNA Replication Transcription Template strands Both strands of the entire DNA double helix are used as templates. Only specific regions of one strand of the DNA double helix is used as a template. Enzyme involved in polymerization reaction DNA polymerases are used to add corresponding free deoxyribonucleoside triphosphates (complementary to those on the DNA parental strands) to the growing DNA chain RNA polymerase is used to add corresponding free ribonucleoside triphosphates (complementary to those on the DNA template strand) to the growing RNA chain Requirement for primers to initiate process DNA polymerase III requires a (free 3’OH end on) RNA primer for the initiation of replication RNA polymerase does not require a RNA primer for the initiation of replication Nucleotides incorporated Thymine deoxyribonucleoside triphosphate is used in addition to adenine, cytosine and guanine deoxyribonucleoside triphosphates. Uracil ribonucleoside triphosphate is used in addition to adenine, cytosine and guanine ribonucleoside triphosphates. End-product of process. A DNA molecule is formed. mRNA, tRNA and rRNA are formed. Examiner’s comment: Many candidates provided full and detailed answers that clearly stated how DNA replication (ii) is similar to transcription, [2] 1 Both processes involve complementary base-pairing of nucleotides with a DNA template ; 2 Polymerases were used to catalyse formation of phosphodiester bonds between nucleotides on elongating strand ; 3 Both occur in nucleus. 4 Both involve the unwinding and unzipping of DNA molecule. Examiner’s comment: Most candidates were able to state two similarities between DNA replication and transcription. (c) The symbols below represent the main components of RNA.

2015 UCLES ‘A’ Level H2 Biology Mark Scheme 6 In the space below, draw a short section of mRNA that is made up of two nucleotides, using these symbols to represent the main components. Add lines to show the position of any bonds between the components. Label and name the components and the covalent bond that links the nucleotides. [3] Examiner’s comment: Candidates found this a challenging question. Although candidates were instructed to draw a section of mRNA made up of two nucleotides, many included only one phosphate group. Some drew mRNA as a double-stranded molecule or labelled bases as nucleotides. [Total: 10]

2015 UCLES ‘A’ Level H2 Biology Mark Scheme 7 QUESTION 3 (a)(i) Name the structures S and T, as shown in Fig. 3.1. ………………………………………………………………………………………………..[2] S: promoter T: operator (ii) Identify a structural gene in Fig. 3.1 and explain what is meant by the term, structural gene. ………………………………………………………………………………………………..[1] 1 Structural gene is a region of DNA that codes for proteins that has an enzymatic function in lactose metabolism, e.g. lacZ/ lacY/ lacA (iii) Identify a regulatory gene in Fig. 3.1 and explain what is meant by the term, regulatory gene. ………………………………………………………………………………………………..[1] 1 Regulatory gene, e.g. LacI gene, encodes for a specific protein product, lac repressor, which regulates expression of the structural genes; Examiner’s comments: Some candidates did not link the role of a regulatory gene to the production of a protein or polypeptide. (b) Using Fig. 3.1, describe how the presence of lactose induces a bacterium to use lactose as a respiratory substrate