Semi-Conservative DNA Replication – MCAT Biology | MedSchoolCoach

Semi-Conservative DNA Replication

MCAT Biology - Chapter 1 - Section 2.1 - DNA - Semi-Conservative DNA Replication
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Sample MCAT Question - Semi-Conservative DNA Replication

According to which model of DNA replication are completely brand new double-stranded molecules of DNA synthesized?

a) Conservative model

b) Semi-conservative model

c) Dispersive model

d) None of the above

A is correct. Conservative model. According to the conservative model, the parental DNA molecule directs the synthesis of brand new DNA strands, but is left untouched. The daughter DNA molecules are therefore completely new. B is incorrect because according to the semiconservative model, one strand from the parental DNA is incorporated into each of the daughter DNA molecules. C is incorrect because according to the dispersive model, the parental DNA is broken up and distributed throughout the new DNA molecules.

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DNA Replication Intro

DNA replication is the process of producing two identical copies of DNA from one original copy. This process is important because, in order for parent cells to pass on DNA to daughter cells, that DNA must first be copied. DNA replication is therefore an essential aspect of biological inheritance, and all living organisms undergo this process.

Models of DNA Replication: Conservative, Dispersive, and Semi-Conservative

Several models of DNA replication have been proposed throughout the history of science (Figure 1). The conservative model proposed that brand new copies of double-stranded DNA are synthesized without altering the original double-stranded DNA molecules. Another model, the dispersive model, proposed that the original DNA molecules are broken up into several pieces which then get dispersed throughout the newly produced strands of DNA.


Today, however, neither of these models is accepted as correct. Instead, a scientific consensus has formed around a third model of DNA replication, known as the semi-conservative model. As shown in Figure 1, the semi-conservative model proposes that the original double-stranded DNA molecule is separated into two single strands, which then serve as the template for the synthesis of two new strands. These new strands are called complementary strands because their nucleotide base pairs are “complementary” to the original template strand. Thus, adenine and cytosine on one strand will always be paired with a thymine and guanine respectively on the other strand. The result of semi-conservative replication is two daughter DNA molecules with a nucleotide sequence identical to that of the parent molecule and which each have one strand from the parent molecule and one newly synthesized stand. The fact that one strand is conserved from the parent molecule but not both is why this model of DNA replication is labelled “semi-conservative”.

Conservative, Semi-Conservative, and Dispersive Models of DNA Replication - MCAT Biology
Figure 1. Conservative, Semi-Conservative, and Dispersive Models of DNA Replication

Advantages of Semi-Conservative DNA Replication

From a biological perspective, the semi-conservative DNA replication has several key advantages. First of all, because only one new strand is synthesized based on a pre-existing template, semi-conservative DNA replication is fast and accurate. Rarely do mutations, such as insertions or deletions, occur. Second, if mutations do occur, the semi-conservative model allows for easy repair of the DNA. The involvement of the older strand allows the new strand to be “proofread” as DNA synthesis occurs and for any errors to be corrected as they arise.

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