Biology 12

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 239

2.Use the materials available to construct a short

strand of DNA. Make a note of how each fact on

your list is supported by your model.

3.Write down the nucleotide sequences for each

strand of DNA in your molecule, using the correct

conventions.

4.Now use your model to simulate the process of

DNA replication. Keeping in mind the specific action

of the enzyme DNA polymerase, use your model to

demonstrate:

(a)replication along the leading strand;

(b)replication along the lagging strand; and

(c)the problem created at the ends of linear

chromosomes.

Post-lab Questions

1.Which base pairs in a DNA molecule will be least

resistant to heat? Why?

2.Are there any aspects of DNA replication that your

model cannot illustrate? Explain.

Conclude and Apply

3.Make a list of the key replication enzymes in the

order in which they are involved. For each enzyme,

write a brief description of what would happen if

that enzyme were not present in the replication

medium. (For the purpose of this exercise, assume

that the absence of any one enzyme does not affect

the activity of others.) Compare your findings with

those of another group.

4.Draw a flowchart or concept map relating events at

the molecular level to the observed changes in

chromosomes during cell division. You may wish to

refer to Appendix 4 for a review of cell division.

5.In one of the early models tested by Watson and

Crick, the sugar-phosphate handrails were located

on the inside of the helix while the nitrogenous

bases protruded outward. In what ways is this

model inconsistent with experimental evidence

about the structure of nucleic acids?

Exploring Further

6.In the late 1940s and early 1950s, before the

publication of Watson and Crick’s paper, other

researchers proposed different structures for the

DNA molecule. Conduct research on one of these

early models. Prepare a short written report that

compares this model with Watson and Crick’s.

How did Watson and Crick’s model fit better with

the scientific evidence?

100 000 nucleotides. In fact, the accuracy of the
process is up to 10 000 times better. An additional
process must therefore be involved in ensuring the
accuracy of replication. This function is also
performed by DNA polymerase.
After each nucleotide is added to a new DNA
strand, DNA polymerase can recognize whether or
not hydrogen bonding is taking place between base
pairs. The absence of hydrogen bonding indicates a
mismatch between the bases. When this occurs, the
polymerase excises the incorrect base from the new
strand and then adds the correct nucleotide using
the parent strand as a template. This double check
brings the accuracy of the replication process to a
factor of about one error per billion base pairs.
In total, the process of DNA replication involves
the action of dozens of different enzymes and other
proteins. These substances work closely together
in a complex known as a replication machine.
Figure 7.28 on the following page shows a simplified
version of a replication machine, while Table 7.1
summarizes the roles of the key enzymes.

In this section, you saw how the molecular

structure of DNA contributes to its role as the

material of heredity. In the next section, you will

see how DNA is organized into the functional units

that make up the genetic material of an organism.

Enzyme group Function

helicase

DNA polymerase

DNA ligase

primase

cleaves and unwinds short sections of DNA ahead

of the replication fork

three different functions:

• adds new nucleotides to 3’ end of elongating
  strand


• dismantles RNA primer


• proofreads base pairing
  catalyzes the formation of phosphate bridges
  between nucleotides to join Okazaki fragments
  synthesizes an RNA primer to begin the elongation
  process

Table 7.1

Key enzymes in DNA replication