Chapter 12: DNA
Study Plan
» Getting Started » A Guide to the Reading » Tying it all together
Getting Started
Below are a few questions to consider prior to reading Chapter 12. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.
- In the early 1900s, what molecule did most geneticists believe genes were composed of?
- How did a pneumonia-causing bacterium contribute to our understanding of the molecular basis of the genetic material?
- In terms of genetics, what is transformation?
- Who was Rosalind Franklin, and what was her contribution to the study of DNA?
- In terms of the DNA molecule, what is a complementary strand?
- How is the process of DNA replication similar to word processing on a computer?
- How many rads of radiation energy are required to mortally injure a human?
- Why must people with xeroderma pigmentosum stay out of the sun?
- By what age does a typical person accumulate more than half of their cancer risk as a result of exposure to cancer-causing agents?
A Guide to the Reading
When exploring the content in Chapter 12 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.
Search for the Genetic Material
The experiments of Frederick Griffith described in the textbook (Figure 12.1) highlight the transformation of the pneumonia-causing bacterium Streptococcus pneumoniae. The key to understanding these experiments is to realize how the two different strains used by Griffith affect mice when injected. The “S” strain (smooth) is more deadly than the “R” strain (rough) and can also be rendered harmless through heat-treatment. The critical experiment conducted by Griffith involved mixing living bacteria of the harmless “R” type with the equally harmless heat-killed “S” strain. When the mixture was injected into mice, they unexpectedly died. Based on this result, Griffith proposed that some factor remaining from the heat-killed “S” strain was capable of transforming the living, harmless “R” strain bacteria into a virulent type. This transforming factor was believed to be the genetic material. The exact chemical nature of this factor remained a mystery until the experiments conducted by Hershey and Chase (Figure 12.2) using radioactively labeled virus particles. Since it was possible for Hershey and Chase to selectively label protein and DNA separately, this allowed them to trace which of these two molecules was responsible for transforming infected bacterial cells into tiny virus factories. As seen in the figure, it was determined that DNA was in fact the genetic material.
For more information on this concept, be sure to focus on:
- In Section 12.1, Harmless bacteria can be transformed into deadly bacteria
- In Section 12.1, The genetic instructions of a virus are contained in its DNA
- Figure 12.1, Genetic Transformation of Bacteria
- Figure 12.2, DNA is the Genetic Material
DNA Replication
The key to deciphering how DNA replication occurs came with the discovery of the 3-dimensional structure of the DNA molecule. Recall that the DNA molecule consists of a pair of DNA strands, each strand complementary to the other. The two strands are held together via hydrogen-bonding between the complementary bases. These hydrogen bonds form a base pair. In order for replication to proceed, the two strands must be separated, breaking the hydrogen bonds in the process. Once separate, each of the two strands then contains the required information to serve as a “template” for the construction of a new complementary DNA strand.
For more information on this concept, be sure to focus on:
- In Section 12.2, DNA is a double helix
- Section 12.3, How DNA is Replicated
- Figure 12.5, DNA Replication
DNA Repair
DNA repair is vital to the survival of all organisms. The need for an efficient DNA repair mechanism is due to the many varied ways in which DNA may become damaged. As discussed in the chapter, this can occur when simple mistakes are made during the process of DNA replication or as the result of physical damage caused by radiation or chemicals. In all cases, whenever damage occurs, the cell must recognize the damage, act to remove the damaged segment of DNA and work to replace it with the correct information. This involves the action of several specialized enzymes, all working together to maintain the accuracy of the genetic information.
For more information on this concept, be sure to focus on:
- In Section 12.4, Few mistakes are made in DNA replication
- In Section 12.4, Normal gene function depends on DNA repair
- Figure 12.7, Repair Proteins fix DNA Damage
Tying it all together
Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:
Nucleotides
- Chapter 4 – in Section 4.5, Nucleotides store information and energy
Cancer
- Interlude B, An Interplay of Factors Can Cause Cancer
Inherited Genetic Disorders
- Chapter 11 – Section 11.5, Human Genetic Disorders
The Genetic Code
- Chapter 13 – Section 13.2, How Genes Control the Production of Proteins
- Chapter 13 – Section 13.4, The Genetic Code
Gene Expression
- Chapter 14 – Section 14.4, How Cells Control Gene Expression
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