A level Biology 6.2 Protein synthesis – Exam style question – Paper 2

Question

Table 5.1 shows the triplets of bases on the template polynucleotide of DNA for some amino acids.

        Fig. 5.1 shows the base sequences in DNA and mRNA for the first seven amino acids of the β chain of haemoglobin.

  (a) (i) Use Table 5.1 to complete Fig. 5.1. [3]

        (ii) State the term used to describe the sequence of amino acids in a polypeptide.[1]

   (b) In sickle cell anaemia, the amino acid at position 6 in the β chain is valine and not glutamic acid.

        Explain how a single change in the DNA triplet for the sixth amino acid of the gene coding for the β chain leads to the production of a different amino acid sequence.[5] [Total: 9]

Answer/Explanation

Ans:

5 (a) (i) GTG ;
                 ACU ;
                 leu ; 

           (ii) primary structure ; 

     (b) 1 mutation ;
             2 base substitution/T→ A in template strand of DNA/AW ;

           transcription
           3 DNA has CAC as 6th triplet ;
           4 (so) mRNA has GUG as (6th) codon ;
           allow one mark for altered mRNA codon if no marks gained for mps 3 and 4

           translation
           5 different tRNA involved/ tRNA specific to val and not glu ;
           6 anticodon on tRNA has CAC (with valine) ;
           7 tRNA brings, incorrect amino acid/ val, to ribosome ;
           8 further detail ; e.g. incorrect amino acid incorporated into growing polypeptide chain

Question

 There are many types of amino acids, but only twenty that are polymerised to make polypeptides and proteins in animals.

    (a) Name the type of chemical reaction that occurs when two amino acids form a dipeptide.[1]

    (b) Fig. 3.1 shows two amino acids, glycine and valine. Use the space below to make a drawing to show what happens when these two molecules join together to form a dipeptide.[4]

  (c) Angiotensinogen is an inactive protein molecule. When blood pressure decreases, part of angiotensinogen is removed to form a short polypeptide, angiotensin that stimulates an increase in blood pressure.

          Fig. 3.2 shows the base sequence within the gene for angiotensinogen that codes for this short polypeptide, the RNA codons and the primary structure of angiotensin.

          (i) Complete Fig. 3.2 to show the missing DNA triplet and the RNA codons. [1]

          (ii) State the full name of the type of RNA shown in Fig. 3.2.[1]

               Table 3.1 shows the blood pressure in the right ventricle and in the pulmonary artery of a person who is in good health.

    (d) Use the information in Table 3.1 to explain why the blood pressure in the pulmonary artery is the same as the pressure in the right ventricle during systole, but higher during diastole.[3]

    (e) People with long-term chronic obstructive pulmonary disease (COPD) usually have blood which is poorly oxygenated during its passage through the lungs. This leads to a constriction of blood vessels in the lungs.
               Suggest the likely effect of this on the heart.[2]

    (f) Describe the signs and symptoms of COPD that help doctors make an early diagnosis of this condition.[2] [Total: 14]

Answer/Explanation

Ans:

3 (a) condensation ; A dehydration

   (b) accept glycine-valine or valine-glycine

         peptide bond drawn correctly ;
         amino and carboxylic acid ends shown ;
         correct R-groups ;
         water eliminated ;

   (c)

         

         (ii) messenger ;

   (d) during systole semi-lunar valve is open ;
          during diastole semi-lunar valve is closed ;
          proximity/AW pulmonary artery to (right) ventricle (so no pressure lost) ;
          elastic recoil of pulmonary artery maintains blood pressure/AW ;
          no/little blood in (right) ventricle, after contraction/during diastole ;
          fills with blood at low pressure ;

   (e) increase in power of contraction ; AW
          increase in (systolic) blood pressure ;
          strain on right ventricle/right ventricle does not function efficiently ;
          growth of muscle in/right ventricle increases in thickness ;
          insufficient oxygen to, heart/cardiac, muscle ;
          heart failure/heart attack ;

   (f) persistent/AW, cough ;
          cough produces much mucus ;
          wheezing ;
          rapid breathing/difficulty breathing/breathlessness ;
          bluish colour to the skin ;
          recurrent chest infections/frequent colds or ‘flu/AW ;
          barrel-shaped chest ;
          chest pains ; R heart pains
          fatigue/weakness, (with exercise) ;

Question

 Red blood cells are formed from cells called reticulocytes. Stem cells in the bone marrow produce reticulocytes which differentiate into red blood cells. During differentiation haemoglobin is produced.

   Fig. 6.1 shows the structure of small sections of DNA and messenger RNA (mRNA) in the nucleus of a reticulocyte during transcription.

   (a) Name the bases P to S.[4]

         P

         Q

         R

         S

   (b) Describe the role of the mRNA molecule shown in Fig. 6.1.[3] [Total: 7]

Answer/Explanation

Ans:

6 (a) P – thymine ; R thiamine/ thiamin/ thyamine
          Q – cytosine ;
          R – guanine ;
          S – uracil ;

    (b) 1 copy of the, DNA/ gene, (coding) for a, polypeptide/ globin ; A protein

          2 travels from, DNA/ nucleus / chromosome, to ribosome ;
                    A mRNA made in nucleus, attached to ribosome so movement is implied

          3 for translation/ for (haemo)globin production ;

          4 mRNA codes for, sequence/ order, of amino acids ; A for primary structure

          5 idea that (nucleotide/base) sequence is a series of codons ;

          6 base pairing/AW, between codon on mRNA and anticodon on tRNA ;
                    e.g. of AW
                    hydrogen bonds between bases
                    examples of base pairing: A–U/C–G
                    R binding between bases

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