12.3 DNA Replication

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12.3 DNA Replication Lesson Objectives Summarize the events of DNA replication. Compare DNA replication in prokaryotes with that of eukaryotes.

Lesson Summary Copying the Code Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. Because each strand can be used to make the other strand, the strands are said to be complementary. DNA copies itself through the process of replication: ▶ The two strands of the double helix unzip, forming replication forks. ▶ New bases are added, following the rules of base pairing (A with T and G with C). ▶ Each new DNA molecule has one original strand and one new strand. ▶ DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA. ▶ During replication, DNA may be lost from the tips of chromosomes, which are called telomeres. Replication in Living Cells The cells of most prokaryotes have a single, circular DNA molecule in the cytoplasm. Eukaryotic cells have much more DNA. Nearly all of it is contained in chromosomes, which are in the nucleus. ▶ Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied. ▶ In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.

Copying the Code 1. Why are the strands of a DNA molecule said to be complementary? Because each strand can be used to make the other strand.

2. What is the first step in eukaryotic DNA replication? The strands of the double helix separate, or unzip.

3. If the base sequence on a separated DNA strand is CGTAGG, what will the base sequence on its complementary strand be? The complementary strand will be GCATCC.

4. What enzyme joins individual nucleotides to produce the new strand of DNA? DNA polymerase

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5. What enzyme makes it less likely that DNA will be lost from telomeres during replication? telomerase

6. How does this enzyme work? It adds short, repeated DNA sequences to telomeres.

7. What is a replication fork? A replication fork is a point in a DNA molecule where the two strands separate during replication.

8. Does DNA replication take place in the same direction along both strands of the DNA molecule that is being replicated? Explain your answer. (Hint: Look at the illustration of DNA replication in your textbook.) No. DNA replication proceeds in opposite directions between replication forks.

9.

Make a sketch of the double helix of DNA. Show how it unzips for replication and how complementary strands are built. Label the nitrogenous bases, replication fork, DNA polymerase, the original strand, and the new strand.

Students’ sketches should resemble the top part of the figure in the textbook. Labels should include nitrogenous bases, replication fork, DNA polymerase, original strand, and new strand. Students should label some pairs of A-T and G-C along the new strand.


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Replication in Living Cells 10. Complete the table to compare and contrast DNA replication in prokaryotes and eukaryotes. Prokaryotes

Eukaryotes

Location of DNA

Singular, circular molecule in the cytoplasm

Packaged in chromosomes in the nucleus

Amount of DNA

Less than eukaryotes

Up to 1000 times more than prokaryotes

Starting Point(s) for Replication

Single

Dozens or hundreds

11. Is DNA replication always a foolproof process? Explain your answer. No. Although many proteins check the DNA for damage or errors, damaged regions can still be replicated. This may result in gene alterations and serious complications for the organism.

12. Why is the pairing of bases during replication essential for the transmission of inherited traits from parent to offspring? The match is (nearly always) perfect between A and T and G and C, so that the code is copied correctly every time. Offspring get the same sequence of bases their parents had.


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Chapter Vocabulary Review For Questions 1–6, match the term with its definition.

Definition C

Term

1. In DNA, the fit between thymine and adenine and the fit between cytosine and guanine.

A. transformation

E

2. An enzyme that joins individual nucleotides to produce a new strand of DNA

C. base pairing

A

3. The process that can change a harmless bacterial strain into a disease-causing strain

B. bacteriophage D. replication E. DNA polymerase F. telomere

F

4. The tip of a chromosome

D

5. The process that copies a DNA molecule

B

6. A kind of virus that infects bacteria

For Questions 7–15, complete each statement by writing in the correct word or words.

7. Each time a chromosome is replicated, some DNA may be lost from the tip of the chromosome, or

telomere

.

8. Griffith’s experiments showed that some chemical compound in cells must be responsible for bacterial transformation . 9. Hershey and Chase studied a bacteriophage that was composed of a DNA core and a protein coat. 10. The center of the DNA strand exhibits base pairing . 11. The enzyme that “proofreads” each new DNA strand so that each molecule is a near-perfect copy of the original is DNA polymerase . 12. In eukaryotic cells, DNA molecule.

replication

can begin at dozens or even hundreds of places on the

13. The double-helix model explains Chargaff ’s rule of base pairing . 14. The DNA molecule separates into two strands during

replication

.

15. The principal enzyme involved in DNA replication is DNA polymerase .

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In the Chapter Mystery, you were introduced to the complex process by which radiation from the sun causes skin cancer. And there’s even more to learn about the links between sun exposure and skin cancer. But you don’t need to be a molecular geneticist to understand how to prevent skin cancer.

UV LIGHT Learning

The Sun and Your Skin Even people who don’t understand how radiation from the sun causes skin cancer know they should protect themselves. Even so, only 40 percent of Americans consistently use sunscreen when they’re in the sun. And 20 percent of American adults actually sunbathe—that is, they deliberately expose their skin to solar radiation. This poster presents information everyone should know. What is ultraviolet radiation? ▶ Ultraviolet (UV) rays are an invisible form of radiation. • They make up a part of sunlight. ▶ There are three types of UV rays. • ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC) What are the results of exposure to ultraviolet radiation? ▶ various types of skin cancer ▶ various eye conditions, including cataracts ▶ premature aging ▶ dry, sagging, and wrinkled skin ▶ yellowing of the skin How does UV radiation cause skin cancer? ▶ Phase 1 • UV radiation interferes with the mechanism by which cells repair damage. • These abnormal cells are more vulnerable to injury. ▶ Phase 2 • Normal cells that are overexposed to UV radiation die. • Abnormal cells that are overexposed to UV radiation do not die. • Genetic damage accumulates. How can you protect yourself from UV radiation? ▶ Seek shade, especially from 10:00 a.m. to 4:00 p.m. when UV rays are strongest. ▶ Cover exposed skin with clothing. ▶ Wear a hat with a wide brim that shades your face, head, ears, and neck. ▶ Wear sunglasses. • Wraparounds are best. • They should block as close to 100 percent of both UVA and UVB rays as possible. ▶ Use sunscreen. • Use one with a sun protective factor (SPF) of 15 or higher. • Use one that blocks both UVA and UVB radiation. • Reapply it every two hours, as well as right after you swim or sweat.

These sunscreen ingredients block UVA radiation. ▶ benzophenone ▶ oxybenzone ▶ sulisobenzone ▶ titanium dioxide ▶ zinc oxide ▶ butyl methoxydibenzoylmethane, also called avobenzone, also called Parsol 1789 These sunscreen ingredients block UVB radiation. ▶ Cinnamates, including octyl methoxycinnamate and cinoxate ▶ Salicylates, including homomenthyl salicylate, octyl salicylate, and triethanolamine salicylate ▶ Octocrylene ▶ Ensulizole, or PBSA Some risk factors make you more likely to contract skin cancer. ▶ lighter natural skin, eye, or hair color ▶ family or personal history of skin cancer ▶ exposure to the sun ▶ history of sunburns early in life ▶ skin that burns, freckles, or reddens easily ▶ certain types of moles ▶ a large number of moles Skin cancer is an undeclared epidemic. ▶ It’s the most common of all the types of cancer. ▶ It’s roughly as common as all other cancers combined. ▶ This year a million Americans will develop skin cancer. It’s time to explode some myths. ▶ UV radiation causes damage whether you get it from the sun or from a tanning bed. ▶ Damage done now will not become evident for many years. ▶ More frequent sun exposure at an early age results in a higher risk of skin damage; 80 percent of a person’s lifetime sun exposure is acquired before age 18. ▶ There is no such thing as a healthy tan.

Continued on next page ▶ Chapter 12 • Workbook A • Copyright © by Pearson Education, Inc., or its affiliates. All Rights Reserved.

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Science and Health Literacy

1. How many types of ultraviolet radiation are there? What are they? three; UVA, UVB, and UVC

2. One form of ultraviolet radiation is absorbed by the ozone in the atmosphere and never reaches Earth’s surface. Which one do you think that is? Why? UVC; it’s never mentioned on the poster, while UVA and UVB are.

3. When is the sun’s UV radiation strongest? from 10:00 in the morning to 4:00 in the afternoon

4. Your friend says she’s going sunbathing. She says, “I’ve been in the sun all summer. I’ve tanned, but I haven’t burned. My skin is still soft and it isn’t dry at all. I have nothing to worry about.” Is she right or wrong? Why? She is wrong. The fact that her skin seems fine now is irrelevant because damage done now will not become evident for many years. Besides, more frequent sun exposure now results in a higher risk of skin damage later. There is no such thing as a safe tan.

5. Do you think that skin cancer can be inherited? Why or why not? SAMPLE ANSWER:

No. Skin cancer is caused by damage to the DNA in adult skin cells,

which would not be passed along to any offspring. However, a tendency toward skin cancer might be inherited.

Warning Signs of Skin Cancer The skills used in this activity include creativity and intellectual curiosity, information and media literacy, and social responsibility. The poster on the previous page was intended to educate people about how UV radiation in sunlight damages skin cells and show them how they can avoid exposure to UV. For some people, though, these warnings come too late. Use Internet and library resources to research the warning signs of skin cancer and what a person who detects one or more of these warning signs should do. Compile this information into a booklet that could be distributed in doctors’ offices and drugstores.

Students’ booklets may be illustrated or text-only. Evaluate booklets based on the accuracy of the information, whether it is presented in an easy-to-read and easy-to-understand format, and creativity.

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