2018_Euk-Gene-Exp_Organisation_Control_STQ_MS

2018 Gene Expression, Organisation and Control STQ MS2018 / H2 / ACJC PRELIM / P2 Q3 1tRNA molecules have an important role in gene expression. They can be found in the cytoplasm, mitochondria and chloroplasts of eukaryotic cells. Fig. 3.1 shows the structure of a tRNA molecule that is able to carry the amino acid threonine in the cytoplasm of a eukaryotic cell. It is 77 ribonucleotides long. Fig. 3.1(a)(i)With reference to Fig. 3.1, describe how the structure of a tRNA molecule differs from the structure of a mRNA molecule.CategorytRNAmRNA1.Presence of double-stranded regions / Overall structureDouble-stranded regions / clover-leaf structSingle-stranded / Linear structure2.Presence of hydrogen bonds / complementary base pairingPresentAbsent3.3’ end3’ CCA sequence3’ poly-A tail4.5’ modified guanosine capAbsentPresent5.AnticodonHas an anticodonMade up of many codons@1m per valid comparison[2](ii)Explain how these differences allow tRNA and mRNA molecules to perform their roles.tRNAmRNAOverall structure (Presence of hydrogen bonds to 1. Specific 3D conformation / clover-leaf structure is complementary to (active site of) 2. Linear structure allows it to be read by the ribosomes / binding to the small ribosomal subunit;

bring about double-stranded regions)aminoacyl-tRNA synthetase / (A, P-sites of) large ribosomal subunit;3’ end3. 3’ CCA sequence allows binding to the amino acid;4. The 3’ poly-A tail determines the stability of the mRNA / duration of translation of the mRNA;Anticodon5. Anticodon can complementary base-pair to codon for the correct amino acid to be added;(1 tRNA can only carry one amino acid)6. Sequence of codons specify the sequence of amino acids in a polypeptide;@1m per structure-function relationship[2]Fig. 3.2 shows a tRNA molecule that is able to carry the amino acid threonine in mitochondria. It is 62 ribonucleotides long. Fig. 3.2(b)The structures of the tRNA molecules in the mitochondria and the cytoplasm of a eukaryotic cell are different although they are both able to carry the amino acid threonine. Suggest a reason for this difference.1.Mitochondria have 70S ribosomes, while 80S ribosomes are found in the cytoplasm;2.Different tRNA molecules have specific 3D conformation that are complementary to the different A and P sites in mitochondrial and cytoplasmic ribosomes; OR3.They are coded for by the different genes found in the mitochondrial DNA and nuclear DNA respectively;4.Different sequences result in the formation of hydrogen bonds at different locations;5.AVP e.g. evolutionary reasons;[2]

There are 61 unique tRNA molecules in the cytoplasm of a eukaryotic cell. (c)(i)Explain why there are 61 unique tRNA molecules in the eukaryotic cell.1.The codon / anticodon is made up of 3 nucleotides and there are 4 different bases, hence 64 possible unique permutations of triplet codes;2.Stop codons do not code for amino acids/ bind to a release factor;3.*There are 3 stop codons, hence this gives 61 unique codons. 61 tRNA molecul