CHAPTER 31 NUCLEIC ACIDS AND HEREDITY SOLUTIONS TO REVIEW QUESTIONS. ƒ H. Guanine H O. Uracil

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1 CAPTER 31 UCLEIC ACIDS AD EREDITY SLUTIS T REVIEW QUESTIS 1. The five nitrogen bases found in nucleotides: 3 C 2 Adenine Thymine Guanine Uracil 2 2 Cytosine 2. A nucleoside is a purine or pyrimidine base linked to a sugar molecule, usually ribose or deoxyribose. A nucleotide is a purine or pyrimidine base linked to a ribose or deoxyribose sugar which in turn is linked to a phosphate group. 3. There are three structural differences between DA and RA. (a) In RA the sugar molecule is always ribose. In DA, the sugar molecule is always deoxyribose, which has instead of at carbon number two. (b) Both molecules use a mixture of four nitrogen bases. Both use cytosine, adenine, and guanine. In DA, the fourth base is thymine. In RA, the fourth base is uracil. (c) DA exists as a double helix whereas RA is a single strand of nucleotides. 4. The major function of ATP in the body is to store chemical energy, and to release it when called upon to carry out many of the complex reactions that are essential to most of our life processes. An equilibrium exists with ADP. ATP + 2 energy utilization ERRRRRF energy storage ADP + P i + 35 kj

2 5. The structure of DA as proposed by Watson and Crick is a double-stranded helix. Each strand has a backbone of alternating phosphate and deoxyribose units. Each deoxyribose unit has one of the four nitrogen bases attached, but coming off the backbone, not part of the backbone. These nitrogen bases thus link to their complementary nitrogen base on the other strand of the double helix. 6. The ratio of adenine to thymine in the human liver is This ratio would be expected because adenine and thymine are a complementary base pair in double-stranded DA. That is, for every adenine on one DA strand,there will be a corresponding thymine on the other strand. 7. Genes must code for twenty different amino acids. If a codon consisted of only one nucleotide, then only four different codons would be possible. A codon of two nucleotides allows for 16 different codons (4 4), still not enough codons for twenty different amino acids. In nature, a codon consists of three nucleotides because this allows 64 different codons (4 4 4) more than enough for twenty different amino acids. 8. DA replication starts with one double stranded DA (the template strands) and produces two double stranded DA products. Each product contains one template strand and one newly-synthesized strand. Since each product has conserved one template strand out of two, replication is said to be semiconservative. 9. A brief outline of the biosynthesis of proteins: (a) A DA strand produces a complementary mra strand which leaves the nucleus and travels to the cytoplasm where it becomes associated with a cluster of ribosomes, binding to five or more ribosomes. (b) With the aid of an enzyme, the proper amino acid attaches to a tra molecule by an ester linkage. (c) The amino acids are brought to the protein synthesis site by tra. (d) The initiation of a polypeptide chain always uses the mra codon AUG or GUG, which ties to the tra anticodon UAC. This code brings -formylmethionine for procaryotic cells. The amino group is blocked by the formyl group, leaving the carboxyl group available to react with the amino group of the next amino acid. (e) The next tra, bringing an amino acid, comes in to the mra and links up, anticodon to codon. The peptide linkage is then made between amino acids. (f) The first tra is ejected,and a third enters the ribosome. (g) The polypeptide chain terminates at a nonsense or termination-codon. The protein molecule breaks free

3 10. Initiation of protein synthesis requires a special codon and starts with a single amino acid. In contrast, elongation requires an unfinished protein chain and, depending on the mra, may use any of the standard codons. 11. Termination of protein synthesis occurs when a nonsense or termination codon appears. Since no tra s recognize these codons, protein synthesis stops. In contrast, elongation requires an unfinished protein chain and, depending on the mra, may use any of the standard codons. 12. A codon is a triplet of three nucleotides, and each codon specifies one amino acid. The cloverleaf model of transfer RA has an anticodon loop consisting of seven unpaired nucleotides. Three of these make up the anticodon, which is complementary to, and hydrogen-bonds to the codon on mra. 13. The role of -formylmethionine in procaryotic protein synthesis is to start the polypeptide chain so it goes in the right direction. It can only build from the carboxyl end. After the synthesis, the -formylmethionine breaks loose from the protein. 14. From time to time a new trait appears in an individual that is not present in either parents or ancestors. These traits which are generally the result of genetic or chromosomal changes are called mutations. 15. A DA fingerprint is a pattern of tagged DA fragments on an electrophoretic gel which can be used to identify possible suspects. 16. In genetic engineering, scientists insert specific genes into the genome of a host cell and program it to produce new and different proteins. Protein engineering uses the techniques of genetic engineering in microorganisms to make proteins with new primary structures and, thus, new functions. These new proteins often have an industrial application. 17. sira stands for small interfering ribonucleic acid. This RA is interfering because it hydrogen bonds to, and triggers destruction of, messenger RA. 18. Apoptosis is cellular self-destruction. If a normal cell is not functioning correctly, it can start a coordinated sequence of steps that lead to cell death. Apoptosis maintains an organism s health at the expense of individual cells. ften cancer cells grow because apoptosis has been inactivated. If this programmed cell death could be restarted, the cancer cells would be eliminated. 19. A mutation often causes a defective gene. Gene therapy seeks to replace the defective gene with a normal version, eliminating the mutation

4 SLUTIS T EXERCISES 1. The letters are associated with compound names as follows: (a) A, adenosine (b) AMP, adenosine- -monophosphate (c) dadp, deoxyadenosine- -diphosphate (d) UTP, uridine- -triphosphate 2. The letters are associated with compound names as follows: (a) G, guanosine (b) GMP, guanosine- -monophosphate (c) dgdp, deoxyguanosine- -diphosphate (d) CTP, cytidine- -triphosphate 3. Structural formulas (a) A (b) AMP 2 C 2 P C 2 2 (c) CDP (d) dgmp 2 C

5 4. Structural formulas (a) U (b) UMP C 2 C 2 (c) CTP (d) dtmp 2 5. (a) This molecule is a nucleotide and is named cytidine-5 -monophosphate. (b) This molecule is a nucleoside and is named deoxyadenosine. 6. (a) This molecule is a nucleoside and is named deoxyguanosine. (b) This molecule is a nucleotide and is named thymidine-2 -monophosphate. 7. A nucleoside contains only a nucleic acid base linked to a sugar. 2 C

6 8. A nucleoside contains only a nucleic acid base linked to a sugar. 2 C 2 9. The common pyrimidine base that has a methyl substituent is found in DA. It is named thymine and has the following structure: C The common purine base that has a double-bonded oxygen is found in both RA and DA. It is guanine adenine uracil

7 12. guanine cytosine 13. tra acts as an adapter molecule. It (1) carries a specific amino acid to the ribosome and (2) matches its anticodon to a specific codon on the mra. In this way specific amino acids are added to a growing polypeptide chain following the message encoded in them RA. 14. mra provides the information or message that dictates the protein amino acid sequence (the 1º structure). 15. Replication is the biological process of making DA using a DA template while the transcription process refers to the making of RA using a DA template. 16. Transcription is the process of making RA using a DA template while translation is a process for making protein using a mra template. 17. tra binds specific amino acids and brings them to the ribosome for protein synthesis. mra carries genetic information from DA to the ribosome and serves as a template for protein synthesis. 18. Both mra and rra can be found in the ribosomes. rra serves as part of the ribosome structure while mra serves as a template for protein synthesis. 19. Three nucleotides are required to specify one amino acid. If there are 146 amino acid residues in the beta chain of hemoglobin, then the number of required nucleotides in mra is 3 * 146 = Three nucleotides are required to specify one amino acid. For the 573 amino acid residues in the phosphoglycerate kinase enzyme, the number of required nucleotides in mra is 3 * 573 = For a DA sequence, TCAATACCCGCG, (a) the complementary mra will be: AGUUAUGGGCGC. (b) the sequence of amino acids coded by the DA will be: Ser-Tyr-Gly-Arg

8 22. A segment of DA strand consists of GCTTAGACCTGA. (a) The order in the complementary mra will be: CGAAUCUGGACU. (b) The sequence of amino acids coded by the DA is Arg-Ile-Trp-Thr. 23. Transcription makes a polymer of nucleotides by forming phosphate ester bonds to connect the nucleotides to each other. The phosphate ester combines a phosphoric acid with an alcohol. 24. Translation makes a polymer of amino acids by forming amide bonds to connect the amino acids to each other. The amide bond combines an amine with a carboxylic acid. 25. Translation termination occurs when the ribosome reaches a nonsense or termination codon along the mra. o tra (in normal cells) has the anticodon to match the termination codon and so no more amino acids are added to the newly synthesized protein chain. The peptidyl-tra connection is broken and the protein chain is released from the ribosome. 26. Translation initiation occurs when the ribosome reaches a special AUG or GUG codon along the mra. Since there is commonly more than one AUG or GUG codon, the ribosome must use other information to choose the special AUG or GUG. This codon is the starting point for protein synthesis and is bound by either a special tra carrying - formylmethionine (in procaryotes) or a tra carrying methionine (in eucaryotes). 27. mra codons and corresponding tra anticodons follow: (a) GUC: anticodon is CAG (c) UUU: anticodon is AAA (b) AGG: anticodon is UCC (d) CCA: anticodon is GGU 28. mra codons and corresponding tra anticodons follow: (a) CGC: anticodon is GCG (c) GAU: anticodon is CUA (b) ACA: anticodon is UGU (d) UUC: anticodon is AAG 29. sira (small interfering ribonucleic acid) and mira (microribonucleic acid) are found in the cytoplasm. Both commonly block translation (protein synthesis). mira stops translation by blocking correct functioning of the ribosome. In contrast, sira stops translation by causing destruction of the mra. 30. Both snra and snora are small RAs that aid posttranscriptional modification of RA in the nucleus. snora specifically helps modify rra in the nucleolus while snra is found in other parts of the nucleus and modifies other RA

9 31. This procedure would best be described as genetic engineering (c). The scientists are seeking to add new genetic material to allow more production of the nerve growth factor protein. 32. (b) protein engineering. This science works to modify proteins by changing individual amino acids in the protein s primary structure. These specific changes are caused by selective changes in the genetic code. 33. The original mra sequence codes for amino acids, Leu-Pro-Thr The mutation changes the first amino acid to Phe, so the sequence becomes Phe-Pro-Thr This mutation can change the function of the protein. 34. The original mra sequence codes for the amino acids Val-Gln-Lys The mutation changes the second codon to a termination or nonsense codon UAA. The protein will end at this point which may cause a major disruption of the protein function. 35. Recombinant means the DA has genes that have been rearranged to contain new or different hereditary information. 36. onsense means the codon does not code for an amino acid. Instead, this codon causes translation to stop. (It is also known as a termination codon.) 37. Termination of protein synthesis means ending the polymerization of amino acids. Polymerization requires a continuous supply of tras carrying specific amino acids. Each tra matches its anticodon to a specific codon on the mra. During termination a codon occurs in the mra that matches no tra. Without a tra the polymerization process cannot continue and protein synthesis ends. 38. The genome is the sum of all hereditary material contained in a cell. The genome contains many genes. Each gene is a segment of DA that controls the formation of a molecule of RA. 39. This thymine isomer now has the appropriate -bonding groups to connect with guanine instead of adenine. 40. A substitution mutation will change one codon. An insertion mutation will shift the sequence position of all bases following the insertion by one. Each base will take the position of its neighbor. For example, if base In general, every codon following an insertion is changed. 41. For the mra segment, UUUCAUAAG, (a) the coded amino acids are: Phe-is-Lys. (b) the sequence of DA for this mra is: AAAGTATTC. is inserted into the sequence Á UUC ACG GCC Á, every codon will change, Á U C U CAC GGC C Á

10 42. In DA fingerprinting, DA polymerase is used to copy very small samples of DA so that enough will be available to separate and visualize on gel electrophoresis. 43. o, RA would not be predicted to have a 1:1 ratio of guanine to cytosine. In DA, these bases have a 1:1; ratio because they are complementary and, thus, are found in equal amounts in the double stranded nucleic acid. Since RA is most often found as a single stranded molecule, there is no requirement that the ratio of guanine to cytosine be set