Chapter 6: DNA Structure, Replication, and Recombination Learning Objectives

Question 1

o Describe the chemical components of DNA

Answer 1

 Polymer of nucleotides joined by phosphodiester bonds

 Nucleotides are made of deoxyribose, phosphate, and one of four nitrogenous bases

Question 2

o Summarize the methods by which DNA was found to be located in chromosomes

Answer 2

 Purifying DNA from nuclein by chemical means, researchers established it contains 4 blocks lined in a long chain
 Feulgen reaction: procedure relies on chemical which stains DNA red; chromosomes are redden while other areas remain colorless (reaction shows DNA is localized almost exclusively within chromosome)

Question 3

o Explain the bacterial transformation experiments of Avery and his colleagues and the results they obtained

Answer 3

 Avery and colleagues showed that a purified DNA preparation from bacteria could transform R bacteria into S form
 Experiment: bacterial transformation occurs in culture medium containing remnants of heat-killed S bacteria; some “transforming principle” from heat killed S bacteria is taken up by live R bacteria converting (transforming) them into virulent S strains (injection of R mutants by themselves don’t cause infections that kill mice, injection of heat-killed S bacteria doesn’t cause lethal infections, lethal infection happens from injection of live R bacteria mixed with heat-killed S strain)

Question 4

o Describe the “blender experiments” of Hershey and Chase and what they revealed about DNA versus protein

Answer 4

 Hershey and Chase grew T2 bacteriophages in the presence of either 35S (labels proteins) or 32P (labels DNA). By using a blender to disrupt bacteriophage attachment, they showed that viral proteins remain outside host cells, while viral DNA is injected into the cells to produce more virus particles

Question 5

o Describe the key features of the Watson-Crick model for DNA structure

Answer 5

 Double helix of two antiparallel strands
 In each of which nucleotides are joined by phosphodiester bonds/Hydrogen bonding
 Complementary bases—A with T, and G with C

Question 6

o Explain what is meant by the antiparallel polarity of the two strands of DNA within the double helix

Answer 6

 Antiparallel means that one strand is oriented in the 5’-to-3’ direction, while the other, complementary strand is oriented in the 3’-to-5’ direction

Question 7

o Distinguish the different structural forms of DNA from one another

Answer 7

 B form DNA: most common form of DNA, molecular configuration spirals to right
 Z DNA: where nucleotide sequences cause structure to assume zigzag shape due to helixes spiraling to left

Question 8

o Explain how DNA stores complex information

Answer 8

 DNA stores information in sequence of its bases

 Base sequence of DNA can be read from a single

Question 9

o Compare the two ways in which the information in DNA may be accessed by proteins

Answer 9

 Proteins “read” info in a single DNA strand by synthesizing a stretch of RNA (transcription) or DNA (replication)
 Specialized proteins can recognize and bind to short base sequences accessible in grooves of double-stranded DNA

Question 10

o Describe the structural differences between DNA and RNA

Answer 10

 DNA: deoxyribose, thymine, usually double-stranded
 RNA: ribose, uracil, usually single-stranded, complex folding pattern (most are single-stranded but flexible so that some regions fold back and form base pairs with other parts)

Question 11

o Describe the key steps in the semiconservative replication of DNA

Answer 11

 The DNA molecule is reproduced by semiconservative replication; the two DNA strands separate and each act as a template for synthesis of a new complementary strand

Question 12

o Summarize the key factors DNA polymerase requires to replicate DNA

Answer 12

 Single stranded DNA template
 Primer that can be RNA or DNA
 Free deoxyribonucleotides (dNTPs)

Question 13

o Outline the steps in the process of DNA replication and how they relate to the requirements of DNA polymerase

Answer 13

 Replication: process of copying DNA accurately, quickly and efficiently; enzymes bring matching (complementary) nucleotide to pair with the bases on each strand = result is two exact copies built on the templates that the unzipped original strands provide
 Initiation: proteins open up double helix and prepare it for complementary base pairing/preparing double helix for use as a template
 Elongation: proteins connect correct sequence of nucleotides on both newly formed DNA double helixes; DNA polymerase synthesizes one new strand (leading strand) continuously while other strand (lagging strand) is synthesized as multiple Okazaki fragments are joined by DNA ligase

Question 14

o Discuss three ways cell preserve the accuracy and integrity of the genetic information in DNA

Answer 14

 Redundancy: either strand of double helix can specify sequence of other
 Remarkable precision of cellular replication machinery
 Enzymes that repair chemical damage to DNA (cell has many enzymes devoted to repair of almost every chemical damage)

Question 15

o Summarize the evidence from tetrad analysis confirming that recombination occurs at the four-strand stage and involves reciprocal exchange

Answer 15

 In tetrad analysis, the existence of Ts and the very low number of NPDs observed establishes that recombination occurs after chromosome replication, when each pair of homologs contains four chromatids. In addition, T and NPD tetrads exhibit equal numbers of both classes of recombinants, indicating reciprocal exchange

Question 16

o Explain how we know that DNA breaks and rejoins during recombination

Answer 16

 Meselson and Weigle’s experiments with bacteriophage lambda indicate that the exchange of chromosome parts during recombination involves the breakage and rejoining of DNA molecules

Question 17

o List the key steps of recombination at the molecular level

Answer 17

 At the molecular level, crossing-over during meiotic prophase entails the formation of heteroduplex DNAs between two Holliday junctions and resolution of the junctions by endonucleases and DNA ligase

Question 18

o Explain why recombination events do not always result in crossing-over

Answer 18

 Recombination events result in crossing-over only part of time because helicases can disrupt D-loop formation

Question 19

o Describe how mismatch repair of heteroduplex regions can lead to gene conversion in fungal tetrads

Answer 19

 Gene conversion, a process whereby one allele in a heterozygote is physically changed into the other, provides evidence for heteroduplex formation during recombination events