Z

DNA and its role in heredity

CHAPTER 13 KEY POINTS

13.1 Experiments Revealed the Function of DNA as Genetic Material

  • Bacterial transformation requires DNA.

  • Bacteriophages inject DNA, not protein, into bacterial hosts.

13.2 DNA Has a Structure That Suits Its Function

  • Chargaff showed that the amount of adenine (A) in DNA = the amount of thymine (T), and that the amount of guanine (G) = the amount of cytosine (C).

  • Franklin determined many of the characteristics of DNA structure from her X-ray images.

  • Watson and Crick proposed that DNA was a double-stranded molecule with each strand composed of a sugar–phosphate backbone with hydrogen bonding between complementary base pairs.

Questions

  1. A sample of DNA from a rabbit consists of 24% adenine nucleotides. What are the frequencies of the other nucleotides?

  2. One strand of a double-stranded DNA molecule has the sequence 5′

    ACGGTTACC 3′. What is the sequence and polarity of the other strand?

  3. How many different sequences

13.3 DNA Is Replicated Semiconservatively

  • Meselson and Stahl used density labeling to show that DNA undergoes semiconservative replication. Each parent strand serves as a template for the synthesis of a new strand.

  • DNA polymerases, which require primers, catalyse the addition of nucleotides to the 3′ end of each new strand.

  • Replication begins at the origins of replication (ori) on both strands and proceeds in the 5′-to-3′ direction, forming replication forks, which have a leading strand and a lagging strand.

  • Telomerase can extend telomeres, overcoming the chromosome shortening that occurs each time DNA is replicated.

Questions

  1. If replication was actually conservative, what results would Meselson and Stahl have obtained after one round of replication on light medium?

  2. A replication fork is moving from left to right. If the top template strand is 5′-to-3′ in the left-to-right direction, which strand is the leading and which is the lagging?

  3. Many cancer cells express telomerase. Why does that make them more able to divide indefinitely?

13.4 Errors in DNA Can Be Repaired

  • DNA polymerases occasionally incorporate the incorrect base in the new strand, which causes mismatches.

  • Mismatches can be repaired by proofreading or mismatch repair.

  • DNA damage is often repaired by excision repair.

Questions

  1. An adenosine nucleotide is on the template strand of a DNA molecule. What are the possible mismatches that could be introduced by a DNA polymerase as it synthesises the new strand?

  2. How can the DNA replication error rate be lower than the DNA polymerase incorporation error rate?

13.5 The Polymerase Chain Reaction Amplifies DNA

  • The polymerase chain reaction (PCR) allows amplification of a particular region of DNA.

Questions

  1. What are the three steps of PCR?

  2. Why is a heat-resistant DNA polymerase important for PCR?

TERMS TO KNOW

adenine: a nitrogen-containing base found in nucleic acids ATP, NAD and other compounds (purine)

base pairs: A-U and C-G

cytosine: a nitrogen-containing based found in DNA and RNA (pyrimidine)

DNA polymerase: any of a group of enzymes that catalyse the formation of DNA strands from a nucleic acid template

excision repair: a DNA repair mechanism that removes damaged DNA and replaces it with the appropriate nucleotide(s)

guanine: a nitrogen-containing base found in DNA, RNA, and GTP (purine)

lagging strand: in a DNA replication fork, the daughter strand that is synthesised in discontinuous stretches

leading strand: in a DNA replication fork the daughter strand that is synthesised continuously

mismatch pair: the base pairs are not properly matched to their counterpart

origins of replication (ori): a DNA sequence at which helices unwinds the DNA double helix and DNA polymerase binds to initiate DNA replication

polymerase chain reaction (PCR): an enzymatic technique for the rapid production of millions of copies of a particular stretch of FNA where only a small amount of the parent molecule is available

primers: a strand of nucleic acid, usually RNA, that is the necessary starting material for the synthesis of a new DNA strand, which is synthesised from the 3’ end of the primer

proofreading: during DNA replication, a mechanism that excises a base that is incorrectly inserted across from the template and inserts the correct base

replication fork: a point at which a DNA molecule is replicating. the fork forms by the unwinding of the parent molecule

semiconservative replication: the way in which DNA is synthesised. each of the two partner strands in a double helix acts as a template for a new partner strand. hence, after replication, each double helix consists of one old and one new strand

telomerase: an RNA-dependent DNA polymerase that catalyses the addition of telomeric sequences lost from chromosomes during DNA replication

telomeres: repeated DNA sequences at the ends of eukaryotic chromosome

template: a molecule or surface on which another molecule is synthesised in complementary fashion, as in the replication of DNA

thymine: a nitrogen-containing single-ringed base found in DNA (pyrimidine)