Biology 12

7.3 DNA Replication

232 MHR • Unit 3 Molecular Genetics

The eight cells in the human blastula shown in

Figure 7.20 arose from the single-celled zygote

formed by the merging of sperm and egg. During

the 240-day gestation period, the cells of the

blastula will divide over and over again to produce

about one hundred trillion more cells. These

trillions of cells will differentiate into the hundreds

of different structures and tissues that make up a

human baby, yet each cell will have exactly the

same genetic complement as the original zygote.

The success of this process of development depends

on two factors — the genome must be copied

relatively quickly, and it must be copied accurately.

Figure 7.20The process of DNA replication balances the

need for speed and the need for accuracy.

Imagine you are asked to type out one-letter

codes for each of the six billion base pairs in the

genome of a single human cell. If you type at a rate

equivalent to 60 words per minute and work

without a break, it will take you over 30 years to

complete the sequence. The cell, on the other hand,

needs only a few hours to copy the same material.

The error rate of the cell’s replication process is

about one per billion nucleotide pairs, which is the

equivalent of you making a one-letter error once in

every five years of steady typing. The remarkable

speed and accuracy of the replication process relies

on both the structural features of DNA and the

action of a set of enzymes.

In their landmark paper on the structure of DNA,

Watson and Crick made the passing observation

that the complementarity of the DNA strands

pointed to a means of accurate replication of DNA

molecules. What they meant was that each strand

of the double helix can serve as a template for the

production of a new complementary strand. A

replication process based on this principle was

suggested by Watson and Crick in a second paper

published not long after the first. In this paper,

Watson and Crick proposed that the two strands of

the DNA double helix molecule unwind and

separate, after which each nucleotide chain serves

as a template for the formation of a new companion

chain. The result would be a pair of daughter DNA

molecules, each identical to the parent molecule.

After the publication of Watson and Crick’s

papers, researchers began exploring the question

of how DNA replicates. They identified three main

possibilities, as illustrated in Figure 7.21. The

conservative theoryproposed that replication

involves the formation of two new daughter strands

from the parent templates, with the two new strands

then joining to create a new double helix. The two

original strands would then re-form into the parent

molecule. The semi-conservative theory, the model

suggested by Watson and Crick, proposed that the

EXPECTATIONS

Describe the current model of DNA replication and methods of repair

following an error.

Demonstrate an understanding of the roles played by the key enzymes

involved in the process of replication.

Explain how differences between the molecular structure of DNA in

prokaryote cells and eukaryote cells affect the process of replication.

Relate the key mechanisms for regulating DNA replication to the accurate

transmission of hereditary material during the process of cell division.