CBE 555 questions & answers graded A+ 2023/2024

CBE 555 "Which of the following statements are correct? Explain your answers. A. A DNA strand has a polarity because its two ends contain different bases. B. G-C base pairs are more stable than A-T base pairs." - correct answer "A. False. The polarity of a DNA strand usually refers to the orientation of its sugar-phosphate backbone: one end contains a phosphate group, the other end contains a hydroxyl group. B. True. G-C: 3 H-bonds vs A-T: only 2 H-bonds " "Histone proteins are among the most highly conserved proteins in eukaryotes. Histone H4 proteins from a pea and a cow, for example, differ in only 2 of 102 amino acids. Comparison of the gene sequences shows many more differences, but only two change the amino acid sequence. These observations indicate that mutations that change amino acids must have been selected against during evolution. Why do you suppose that amino-acid-altering mutations in histone genes are deleterious?" - correct answer The functions of histone proteins must involve nearby all of their amino acids, such that a change in any position (mutation) is deletorious to the cell. "The nucleotide sequence of one DNA strand of a DNA double helix is 5'-GGATTTTTGTCCACAATCA-3'. What is the sequence of the complementary strand? " - correct answer 5'-CCTAAAAACAGGTGTTAGT-3'. In the DNA of certain bacterial cells, 13% of the nucleotides are adenine. What are the percentages of the other nucleotides? - correct answer "%A=%T=13% %G=%C=(100-2*%A)/2=37%" How many possible nucleotide sequences are there for a stretch of DNA that is N nucleotides long, if it is (a) single- stranded or (b) double-stranded? - correct answer "(a) 4^N (b) As someof the 4^N sequence will be complement to another one, less than 4^N If N is odd, every stand will complement another: 0.5*4^N If N is even, some strand are self-complementary: 0.5(4^N+4^(N/2))" Suppose you had a method of cutting DNA at specific sequences of nucleotides. How many nucleotides long (on average) would such a sequence have to be in order to make just one cut in a bacterial genome of 3 × 10^6 nucleotide pairs? How would the answer differ for the genome of an animal cell that contains 3 × 10^9 nucleotide pairs? - correct answer "To specify a unique N nucleotide long sequence we need 4^N>3*10^6, Thus N~11 Animal cell: N~16" A macromolecule isolated from an extraterrestrial source superficially resembles DNA, but closer analysis reveals that the bases have quite different structures (Figure Q5-7). Bases V, W, X, and Y have replaced bases A, T, G, and C. Look at these structures closely. Could these DNA-like molecules have been derived from a living organism that uses principles of genetic inheritance similar to those used by organisms on Earth? - correct answer "V-X // W-Y Hence, yes it could be derived from a living organism using earthlike principles of genetic inheritance" Simply judged by their potential for hydrogen-bonding, could any of these extraterrestrial bases replace terrestrial A, T, G, or C in terrestrial DNA? Explain your answer. ECB4 EQ5.08/Q5.07 - correct answer A pyrimidine needs to pair with a purine to form the double helix. The eight possible combinations are V-A, V-G, W-A, W-G, X-C, X-T, Y-C, Y-T. But because of the positions of the H-bond donor and acceptor groups, none of those are stable enough. Thus no. "The two strands of a DNA double helix can be separated by heating. If you raised the temperature of a solution containing the following three DNA molecules, in what order do you suppose they would "melt"? Explain your answer. A. 5'-GCGGGCCAGCCCGAGTGGGTAGCCCAGG-3' 3'-CGCCCGGTCGGGCTCACCCATCGGGTCC-5' B. 5'-ATTATAAAATATTTAGATACTATATTTACAA-3' 3'-TAATATTTTATAAATCTATGATATAAATGTT-5' C. 5'-AGAGCTAGATCGAT-3' 3'-TCTCGATCTAGCTA-5'" - correct answer "The melting temperature depends on both the size of the strand and the number of H-bonds (the more of them the higher the melting temperature) Hence: C-B-A (from lowest to highest)"