6.3.1 Replication: Meselson and Stahl

Question 1

The four functions of DNA as the genetic material

Answer 1

1. DNA must carry large quantities of information.
2. It must be able to communicate with the cell and serve as the blueprint for cell products.
3. It must be able to replicate itself faithfully, thereby passing information on to new cells.
4. Mutation must be possible so that new products may arise

Question 2

Mathew Meselson and Franklin Stahl

Answer 2

• devised experiments that would test the way that DNA replicates itself. Watson and Crick’s model of a double-stranded DNA molecule suggested that the pair of nucleotide strands would separate and each would serve as a template for a new strand. The suggested mechanism was called semiconservative replication.
  • Competing with the semiconservative model were conservative and dispersive replication. If conservative replication occurred, the two parent strands would not separate, and replication would result in a DNA molecule consisting of two new strands. According to the model of dispersive replication, parts of both parental strands separate and act as templates for
  the synthesis of the new strands

Question 3

note

Answer 3

• Bacteria were grown in heavy-nitrogen (15N) medium for several generations. Cells were then transferred to
  light-nitrogen (14N) medium and allowed to replicate for either one or two generations in the light medium. The process of density gradient centrifugation was used to analyze the weights of all three types of cells.
• The hypotheses of semiconservative, conservative and
  dispersive replication predicted different weights for first
  and/or second-generation cells. Cells that were subjected to one round of replication had bands that were intermediate in density, meaning that DNA molecules consisted of one parental (heavy) and one new (light) strand. Thus, conservative replication is ruled out. Cells subjected to two rounds had one intermediate and one light band. Thus dispersive replication is ruled out.
• The density of the cells in all three generations matches the pattern expected for semi- conservative replication.

Question 4

DNA replication can be described as

Answer 4

• semiconservative

Question 5

To study DNA replication, you perform the following experiment. First, you grow E. coli cells on nitrogen-15 (the heavy isotope of nitrogen) medium for several generations and then transfer them to nitrogen-14 for two generations (two complete rounds of DNA replication). You extract DNA from these cells and centrifuge them. What density distribution of DNA would you expect in this experiment?

Answer 5

• one low-density and one intermediate band

Question 6

Meselson and Stahl found that DNA replication is semiconservative, meaning that

Answer 6

• each double helix consists of one old and one newly synthesized strand

Question 7

An isotope is a different form of an element that has __________ number of protons and __________ number of neutrons.

Answer 7

• the same; a different

Question 8

Which of the following is not a function of DNA as the genetic material?

Answer 8

• It must be capable of being hydrolyzed before the beginning of each cell cycle

Question 9

Which of the following is not a requirement of genetic information?

Answer 9

• It must be resistant to change

Question 10

Which of the following is not a hypothesis considered by Meselson and Stahl to describe replication of DNA?

Answer 10

• nonconservative replication

Question 11

Meselson and Stahl predicted that if semiconservative replication were the actual mechanism of DNA replication, after the first generation

Answer 11

• one distinct band would appear in the centrifugation tube, indicating that all the DNA weighed the same