DNA and RNA Structure & Function (PDF)

6. For the following DNA sequences: (all read 5’ to 3’) Provide the sequence for the opposite strand a) ACGTAT → TGCATA b) AGATCT → TCTAGA c) ATGGTA → TACCAT Indicate whether the sequence is DNA or RNA, and explain how you can determine this. ANSWER: They are DNA sequences b...

6. For the following DNA sequences: (all read 5’ to 3’) Provide the sequence for the opposite strand a) ACGTAT → TGCATA b) AGATCT → TCTAGA c) ATGGTA → TACCAT Indicate whether the sequence is DNA or RNA, and explain how you can determine this. ANSWER: They are DNA sequences because of the presence of thymine rather than thymine. 9. A friend tells you that only four different kinds of bases are found in RNA. What would you say in reply? ANSWER: Four different kinds of bases– adenine, cytosine, guanine, and uracil– make up the preponderant majority of the bases found in RNA, but they are not the only ones. Modified bases occur to some extent, especially in tRNA. 13. Draw a: a) G-C base pair b) A-T base pair See Fig 9.9 14. Which of the following statements is/are true? a) Bacterial ribosomes consist of 40S and 60S subunits b) Prokaryotic DNA is normally complexed with histones c) Prokaryotic DNA normally exists as a closed circle ANSWER: Statements c and d are true. 19. Which of the following statements is/ are true? a) Two strands of DNA run parallel from their 5’ to their 3’ ends b) An adenine- thymine base pair contains three hydrogen bonds c) Positive;y charged counterions are associated with DNA d) DNA base pairs are always perpendicular to the helix axis 20. Define supercoiling, positive supercoil, topoisomerase, and negative supercoil Supercoiling refers to twists in DNA over and above those of the double helix. Positive supercoiling refers to an extra twist in DNA caused by overwinding of the helix before sealing the ends to produce circular DNA. Topoisomerase is an enzyme that induces a single-strand break in supercoiled DNA, relaxes the supercoiling, and reseals the break. Negative supercoiling is the unwinding of the double helix before sealing the ends to produce circular DNA. 21. What is a propeller twist? ANSWER: It is a movement of the two bases on a base pair away from being in the same place. 24. What is the difference between B-DNA and Z-DNA B-DNA is a right-handed helix with specified dimensions (10 base pairs per turn, significant differences between major and minor groove, etc.). Z-DNA is a left-handed double he;lix with different dimensions. (12 base pairs per turn, similar major and minor grooves, etc). 26. Describe the structure of chromatin. Chromatin is a complex consisting of DNA and basic proteins found in eukaryotic cells. See fig 9.17) 29. Explain how DNA gyrase works. It binds to the DNA, forming loops around itself. It then cuts both strands of DNA on one part of the loop, passes the ends across another loop[, and reseals. 31. Would you expect to find adenine-guanine or cytosine-thymine base pairs in DNA? Why? Adenine– guanine base pairs occupy more space than is available in the interior of the double he;lix, whereas cytosine–thymine base pairs are too small to span the distance between the sites to which complementary bases are bonded. One wou;d not normally expect to find such base pairs in DNA. 32. One of the original structures proposed for DNA had all the phosphate groups positioned at the center of a long fiber. Why is this proposal rejected? The phosphate groups in DNA are negatively charged at physiological pH. If they were grouped together closely, as in the center of a long fiber, the result wou;d be considerable electrostatic repulsion in which the structure would be unstable. 33. What is the complete base composition of a double-stranded eukaryotic DNA that contains 22% guanine? The percentage of cytosine equals that of guanine, 22%. This DNA has a 44% G–C content, implying a 56% A–T content. The percentage of adenine equals that of thymine, so adenine and thymine are 28% each. 34. Why was it necessary to specify that the DNA in Q33 is double-stranded? If the DNA were not double stranded, the requirement G=C and A=T would no longer exist. 39. A technology called PCR is used for replicating large quantities of DNA in forensic science (see Chap.15). With this technique, DNA is separated by heating with an automated system. Why is this information about the DNA sequence needed to use this technique? Because any system involving replication of DNA by DNA polymerases must have a primer to start the reaction, the primer can be RNA or DNA, but it must bind to the template strand being read. Thus, enough of the sequence must be known to create the correct primer. 40. Why does DNA with a high A–T content have a lower transition temperature, ™, than DNA with a high G–C content? A–T base pairs have two hydrogen bonds, whereas G–C base pairs have three. It takes more energy and higher temperature to disrupt the structure of DNA rich in G–C base pairs. 41. What are the three primary RNA types? mRNA: messenger RNA tRNA: transfer RNA rRNA: ribosomal RNA 42. What determines the base sequence of all types of RNA? The base sequence of all RNA types is a reflection of the corresponding DNA sequence that leads to the production of th eRNA. It matches the coding strand of the DNA. 43. What is the name of the process that produces RNA from a DNA template? Transcription 44. What is the basic purpose of tRNA? tRNA is bound to an amino acid and brings the correct amino acid to the growing peptide during the process of translation (protein synthesis). 45. The base sequence of which type of RNA is responsible for determining the order of amino acids in a protein? The base sequence of the mRNA. 46. What is the name of the process by which amino acids are linked together in the correct order based on an RNA sequence? Translation 60. Which would be more harmful to a cell, a mutation in DNA or a transcription mistake that leads to an incorrect mRNA? Why? The mistake in the DNA would be more harmful because every cell division would propagate it. A mistake in transcription would lead to one wrong RNA molecule that can be replaced with one wrong RNA molecule that can be replaced with a correct version with the next transcription. 61. Explain what happens to a eukaryotic mRNA before it can be translated to a protein. Eukaryotic mRNA is initially formed in the nucleus by transcription of DNA. th emRNA transcript is then spliced to remove introns of DNA. th emRNA transcript is then spliced to remove introns, a poly-A tail is added at the 3’ end, and a 5’-cap is put on. This is the final mRNA which is then a transporter,in most cases, out of the nucleus for translation by the ribosomes. 62. Why does 50S ribosomal subunit and a 30S ribosomal subunit combine to form a 70S subunit, instead of 80S subunit? The numbers 50S, 30S etc. refer to relative rates of sedimentation in an ultracentrifuge and cannot be added directly. Many things beside molecular weight influence the sedimentation characteristics, such as shape and density.