Anatomy & Physiology: The Unity of Form and Function, 7th Edition

Published by McGraw-Hill Education
ISBN 10: 0073403717
ISBN 13: 978-0-07340-371-7

Chapter 4 - Section 4.3 - Study Guide - Assess Your Learning Outcomes - Page 137: 2

Answer

Semiconservative replication is the process by which DNA is duplicated prior to cell division. In this process, each newly synthesized DNA molecule consists of one old (parental) strand and one newly synthesized (daughter) strand. This mechanism was first proposed by James Watson and Francis Crick and was confirmed by the Meselson-Stahl experiment in 1958. The enzymes and steps involved in semiconservative DNA replication are as follows: 1. **Initiation**: The process begins at a specific site on the DNA molecule called the origin of replication. At this point, enzymes called helicases unwind and separate the double-stranded DNA to create a single-stranded region known as the replication bubble. This unwinding requires energy and is facilitated by ATP. 2. **Primer Synthesis**: Before DNA polymerase can add new nucleotides to the growing DNA strand, a short RNA primer is synthesized by an enzyme called primase. This primer provides a 3'-OH group to which DNA polymerase can attach nucleotides. 3. **DNA Polymerization**: DNA polymerase enzymes are responsible for adding complementary nucleotides to the template strand. DNA polymerase can only extend an existing strand and cannot initiate a new one. It reads the template strand in the 3' to 5' direction (anti-parallel to the template) and synthesizes the new strand in the 5' to 3' direction. This means that one daughter strand (the leading strand) can be synthesized continuously, while the other (the lagging strand) is synthesized in short fragments called Okazaki fragments. 4. **Proofreading and Repair**: DNA polymerases have proofreading capabilities to ensure that the correct nucleotides are added. If an incorrect nucleotide is added, the enzyme will remove it and replace it with the correct one. This proofreading mechanism enhances the fidelity of DNA replication. 5. **Ligation**: After the daughter strand has been synthesized in short fragments on the lagging strand, an enzyme called DNA ligase joins these fragments together to form a continuous, complementary strand. The result of semiconservative replication is two DNA molecules, each of which consists of one old (parental) strand and one newly synthesized (daughter) strand. This is because DNA polymerases synthesize the new strand by adding nucleotides complementary to the template strand. As a result, one of the original strands serves as a template for the synthesis of a new complementary strand, while the other original strand remains intact in each of the two resulting DNA molecules. This semiconservative replication mechanism ensures that the genetic information is faithfully preserved and passed on to the next generation of cells, maintaining the integrity and stability of the DNA.

Work Step by Step

Semiconservative replication is the process by which DNA is duplicated prior to cell division. In this process, each newly synthesized DNA molecule consists of one old (parental) strand and one newly synthesized (daughter) strand. This mechanism was first proposed by James Watson and Francis Crick and was confirmed by the Meselson-Stahl experiment in 1958. The enzymes and steps involved in semiconservative DNA replication are as follows: 1. **Initiation**: The process begins at a specific site on the DNA molecule called the origin of replication. At this point, enzymes called helicases unwind and separate the double-stranded DNA to create a single-stranded region known as the replication bubble. This unwinding requires energy and is facilitated by ATP. 2. **Primer Synthesis**: Before DNA polymerase can add new nucleotides to the growing DNA strand, a short RNA primer is synthesized by an enzyme called primase. This primer provides a 3'-OH group to which DNA polymerase can attach nucleotides. 3. **DNA Polymerization**: DNA polymerase enzymes are responsible for adding complementary nucleotides to the template strand. DNA polymerase can only extend an existing strand and cannot initiate a new one. It reads the template strand in the 3' to 5' direction (anti-parallel to the template) and synthesizes the new strand in the 5' to 3' direction. This means that one daughter strand (the leading strand) can be synthesized continuously, while the other (the lagging strand) is synthesized in short fragments called Okazaki fragments. 4. **Proofreading and Repair**: DNA polymerases have proofreading capabilities to ensure that the correct nucleotides are added. If an incorrect nucleotide is added, the enzyme will remove it and replace it with the correct one. This proofreading mechanism enhances the fidelity of DNA replication. 5. **Ligation**: After the daughter strand has been synthesized in short fragments on the lagging strand, an enzyme called DNA ligase joins these fragments together to form a continuous, complementary strand. The result of semiconservative replication is two DNA molecules, each of which consists of one old (parental) strand and one newly synthesized (daughter) strand. This is because DNA polymerases synthesize the new strand by adding nucleotides complementary to the template strand. As a result, one of the original strands serves as a template for the synthesis of a new complementary strand, while the other original strand remains intact in each of the two resulting DNA molecules. This semiconservative replication mechanism ensures that the genetic information is faithfully preserved and passed on to the next generation of cells, maintaining the integrity and stability of the DNA.
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