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Simulation—DNA Replication

The Escherichia coligenome consists of 4.7 million nucleotide pairs. This entire genome is

replicated in 40 min. Proofreading by DNA polymerase I and polymerase III maintains the error

rate at roughly one error per 1000 cells duplicated! View a complete animation of DNA

replication by accessing the Nelson Web site.

SUMMARY DNA Structure and Replication

http://www.science.nelson.com GO

666 Chapter 20 NEL

Section 20.1 Questions

1. Summarize the key physical and chemical properties of
   DNA.
   2.Differentiate between a purine and a pyrimidine.
   3.Copy Table 1into your notebook, fill in the missing
   information, and supply an appropriate title.

5.Define a replication fork.

6.In a double helix, there is a complete turn every 3.4 nm, or

10 nucleotides. Assume that the DNA molecule in a

particular chromosome is 75 mm long. Calculate the

number of nucleotide pairs in this molecule.

7. Copy Table 2into your notebook and complete the
   missing information. Explain how you determined the
   missing values.

Table 1

Enzyme Function

DNA helicase

DNA polymerase I

DNA polymerase III

DNA ligase

4.A molecule of DNA was analyzed and found to contain

20 % thymine. Calculate the percentage of adenine,

guanine, and cytosine in this molecule.

Table 2

Nucleotide Sample A Sample B Sample C

adenine 10 % 20 %

guanine 40 % 15 %

thymine 35 % 20 %

cytosine

Separating the Strands

• DNA helicase unzips the double helix by breaking the hydrogen bonds between
  the complementary bases in the two strands of the parent DNA molecule.


• Proteins attach to the newly exposed DNA strands, preventing the hydrogen
  bonds from re-forming and keeping the strands apart.

Building the Complementary Strands

• DNA polymerase III adds complementary nucleotides to the growing strands,
  using the exposed strands of the parent DNA molecule as a template.


• The leading strand is formed continuously.


• The lagging strand is formed in short fragments, starting from an RNA primer.


• DNA polymerase I cuts out the RNA primers and replaces them with the
  appropriate DNA nucleotides.


• DNA ligase joins the fragments together to form a complete DNA strand.

DNA Repair

• DNA polymerase enzymes cut out incorrectly paired nucleotides and add the
  correct nucleotides in a process called proofreading.