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214 2022S exam 1………………………. Name (I) Indicate whether the following statements are true (T) or false (F). If false explain. (2 points each) (1). The number of chromosomes and genes correlates strongly with the complexity of an organism F; Neither (2). A major component of RNA but not of DNA is thymine F; uracil (3). In the Watson-Crick model for the DNA double helix, there are two equally sized grooves that run up the sides of the helix F; major and minor grooves (4). The ratio A+T/G+C is constant for all natural DNAs. F; A+G=T+C (5). In the Watson-Crick model of DNA structure: both strands
run in the same direction, 5 ' to 3 ' ; they are parallel. F; opposite direction, anti-parallel ( 6 ). In a mammalian cell, DNA repair systems: normally repair more than 99% of the DNA lesions that occur. T ( 7 ). In base-excision repair, the first enzyme to act is: Dam methylase. F: DNA glycosylase ( 8 ). DnaA is binding on oriC during initiation of DNA replication in E. coli. T ( 9 ). In contrast to bacteria, eukaryotic chromosomes need multiple DNA replication origins because: eukaryotic chromosomes cannot usually replicate bidirectional. F; their replication rate is much slower, and it would take too long with only a single origin per chromosome
diameter, with a rise of ____ Å per base pair Rigt; 20; 3.
- In the Watson-Crick model for the DNA double helix, which statement is NOT true? A) The two strands run anti-parallel to one another. B) The base-pairing occurs on the inside of the double helix. C) The double helix is right-handed. D) There are two equally sized grooves that run up the sides of the helix. E) The two strands have complementary sequences. D
- When double-stranded DNA is heated at neutral pH, which change does NOT occur? A) The absorption of ultraviolet (260 nm) light increases. B) The covalent N -glycosidic bond between the base and the pentose breaks. C) The helical structure unwinds. D) The hydrogen bonds between A and T break. E) The viscosity of the solution decreases. B
- Which type of damage to DNA structure is MOST likely to be caused by UV light? A) deamination B) pyrimidine dimers C) depurination D) depyrimidination E) hydrolysis of the phosphodiester bond
B
(III) Short Answer ( 6 0 points; 6 each) (1). the type of bond with the role below:
- links base to pentose in nucleotide N -glycosidic
- joins adjacent nucleotides in one strand, phosphodiester
- joins complementary nucleotides in two strands, hydrogen
- difference between a nucleoside and a nucleotide, phosphate ( 2 ). Draw the structure of (a) a 5’ adenine-thymine 3’ di- nucleotides and (b) a guanine:cytosine base pair (include all hydrogen bonds), as found in the Watson-Crick double-helical structure of DNA.
form) containing 500 base pairs? 3.4 Å X 5 00 = 1750 Å
- Mouse DNA hybridizes more extensively with human DNA than with yeast DNA. Explain by describing the factor or factors that determine extent of hybridization. In general, the more similar the sequences in two DNA molecules are, the more readily they will hybridize. Because the evolutionary distance between mouse and yeast is greater than that between mouse and human, mouse and human DNA sequences are more similar than those of mouse and yeast
- DNA replication in E. coli begins at a site in the DNA called the (a) ___________. At the replication fork the (b) ___________ strand is synthesized continuously while the (c) _________ strand is synthesized discontinuously. On the strand synthesized discontinuously, the short pieces are called (d) ____________ fragments. An RNA primer for each of the fragments is synthesized by an enzyme called (e) __________, and this RNA primer is removed after the fragment is synthesized by the enzyme (f) ___________, using its (g) _____________ activity. The nicks left behind in this process are sealed by the enzyme (h) _____________. (a) origin; (b) leading; (c) lagging; (d) Okazaki; (e) primase; (f) DNA pol I; (g) 5 ' to 3 ' exonuclease; (h) DNA ligase
- Describe briefly how equilibrium density gradient centrifugation was used to demonstrate that DNA replication in E. coli is semiconservative. Equilibrium density gradient centrifugation separates DNA molecules of slightly different buoyant density. For
example, molecules containing 15 N-labeled (“heavy”) DNA are separable from identical molecules containing 14 N (“light”) DNA. Meselson and Stahl grew E. coli for many generations in a medium containing 15 N, producing cells in which all DNA was heavy. These cells were transferred to a medium containing 14 N, and the buoyant density of their DNA was determined (by equilibrium density gradient centrifugation) after 1, 2, 3, etc., generations. After one generation, all DNA was of a density intermediate between fully heavy and fully light, indicating that each double- stranded DNA molecule had one heavy (parental) and one light (newly synthesized) strand; replication was semiconservative. (See Fig. 25-2.)
- In contrast to bacteria, eukaryotic chromosomes need multiple DNA replication origins because: their replication rate is much slower, and it would take too long with only a single origin per chromosome.
