Name Class Date. Summarize the events of DNA replication. Compare DNA replication in prokaryotes with that of eukaryotes.

(1)

187

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 con-tained 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?

2. What is the first step in eukaryotic DNA replication?

3. If the base sequence on a separated DNA strand is CGTAGG, what will the base sequence

on its complementary strand be?

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

Because each strand can be used to make the other strand.

The strands of the double helix separate, or unzip.

The complementary strand will be GCATCC.

(2)

188

Name Class Date

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

6. How does this enzyme work?

7. What is a replication fork?

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.)

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.

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

No. DNA replication proceeds in opposite directions between replication forks. telomerase

It adds short, repeated DNA sequences to telomeres.

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.

(3)

189

Replication in Living Cells

10. Complete the table to compare and contrast DNA replication in prokaryotes and eukaryotes.

Prokaryotes Eukaryotes

Location of DNA

Amount of DNA

Starting Point(s) for Replication

11. Is DNA replication always a foolproof process? Explain your answer.

12. Why is the pairing of bases during replication essential for the transmission of inherited traits from parent to offspring?

Singular, circular molecule in the cytoplasm Packaged in chro-mosomes in the nucleus Less than eukaryotes Up to 1000 times more than prokaryotes

Single Dozens or hundreds

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.

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.

(4)

Name Class Date

190

Chapter Vocabulary Review

For Questions 1–6, match the term with its deﬁ nition.

Definition

1. In DNA, the fit between thymine and adenine

and the fit between cytosine and guanine.

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

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

4. The tip of a chromosome

5. The process that copies a DNA molecule

6. A kind of virus that infects bacteria

Term A. transformation B. bacteriophage C. base pairing D. replication E. DNA polymerase F. telomere

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 .

8. Griffith’s experiments showed that some chemical compound in cells must be responsible

for bacterial .

9. Hershey and Chase studied a that was composed of a DNA core and a

protein coat.

10. The center of the DNA strand exhibits .

11. The enzyme that “proofreads” each new DNA strand so that each molecule is a

near-perfect copy of the original is .

12. In eukaryotic cells, can begin at dozens or even hundreds of places on the DNA molecule.

13. The double-helix model explains Chargaff ’s rule of .

14. The DNA molecule separates into two strands during .

15. The principal enzyme involved in DNA replication is .

C A F D telomere transformation bacteriophage base pairing DNA polymerase replication base pairing replication DNA polymerase E B 013368718X_CH12_179-192.indd 12 1/5/09 12:05:36 PM

(5)

Learning

191

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

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.

(6)

Name Class Date

192

Themes

_{Science and Health Literacy}

1. How many types of ultraviolet radiation are there? What are they?

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?

3. When is the sun’s UV radiation strongest?

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?

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

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.

three; UVA, UVB, and UVC

UVC; it’s never mentioned on the poster, while UVA and UVB are.

from 10:00 in the morning to 4:00 in the afternoon

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.

SAMPLEANSWER: 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.