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Teacher's Guide

Traits andHeredity

Editors:Brian A. Jerome Ph.D.Stephanie Zak Jerome

Assistant Editors:Louise MarrierHannah Fjeld

Graphics:Fred ThodalHeidi BerryBrian Bliven

Emily MerkertLyndsey Canfield

1-800-453-8481Visual Learning CompanyTraits and Heredity

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Table of Contents2

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A Message from our Company

Viewing Clearances

Use and Copyright

National Standards Correlations

Student Learning Objectives

Assessment

Introducing the Program

Program Viewing Suggestions

Video Script

Answer Key to Student Assessments

Answer Key to Student Activities

Pre-Test

Post-Test

Video Review

Vocabulary

Writing Activity

Dominant and Recessive Traits

Acquired vs Inherited Traits

Genetic Engineering

The Double Helix


Life Science - Content Standard C:

National Standards CorrelationsBenchmarks for Science Literacy(Project 2061 - AAAS)

The Living Environment - Heredity (5B) Grades 3-5

By the end of the fi fth grade, students should know that:

National Science Education Standards(Content Standards: K-4, National Academy of Sciences)

• Some likenesses between children and parents, such as eye color in human beings, or fruit or fl ower color in plants, are inherited. Other likenesses, such as people’s table manners or carpentry skills, are learned.

• For offspring to resemble their parents, there must be a reliable way to transfer information from one generation to the next.

• Many characteristics of an organism are inherited from the parents of the organism, but other characteristics result from an individual’s interactions with the environment. Inherited characteristics include the color of fl owers and the number of limbs of an animal. Other features, such as the ability to ride a bicycle, are learned through interactions with the environment and cannot be passed on to the next generation.


• Defi ne traits as the distinguishing characteristics of an organism.

• Describe heredity as the process of passing on traits from parents to offspring.

• Differentiate between acquired traits and inherited traits, giving examples of each.

• Identify Gregor Mendel as the father of modern genetics, and defi ne genetics as the study of heredity.

• Briefl y describe Mendel’s Law of Dominance, explaining that the stronger trait is called the dominant trait and the weaker trait is the recessive trait.

• Generally explain that chromosomes are structures in cells that contain genetic material.

• Defi ne DNA as the chemical that determines the genetic code.

• Understand that genetic engineering is a fi eld of scientifi c research that has accomplished many benefi cial things. However, also understand that there is some controversy and concern about the long- term effects of genetic engineering.

Student Learning ObjectivesUpon viewing the video and completing the enclosed student activities, students will be able to do the following:


The Post-Test can be utilized as an assessment tool following student completion of the program and student activities. The results of the Post-Test can be compared against the results of the Preliminary Test to assess student progress.

The Video Review can be used as an assessment tool or as a student activity. There are two sections. The fi rst part contains questions displayed during the program. The second part consists of a fi ve-question video quiz to be answered at the end of the video.

The Preliminary Test is an assessment tool designed to gain an understanding of students’ preexisting knowledge. It can also be used as a benchmark upon which to assess student progress based on the objectives stated on the previous pages.

Assessment

Preliminary Test (p. 14-15):

Video Review (p. 18):

Post-Test (p. 16-17):


Introducing the Program

Program Viewing SuggestionsThe student master “Video Review” is provided (p. 18) for distribution to students. You may choose to have your students complete this Master while viewing the program or do so upon its conclusion.

The program is approximately 14 minutes in length and includes a fi ve-question video quiz. Answers are not provided to the Video Quiz in the video, but are included in this guide on page 12. You may choose to grade student quizzes as an assessment tool or to review the answers in class.

The video is content-rich with numerous vocabulary words. For this reason you may want to periodically stop the video to review and discuss new terminology and concepts.

Before showing the video program to students, ask students to feel their earlobes. Also tell them to look closely at the earlobes of some of their classmates. Explain that there are two different types of earlobes: attached and unattached. Have students observe a classmate who has attached earlobes and one who has unattached earlobes. Next ask students if they can play a musical instrument. Again have them compare the results with the entire class.

Next tell students that these sorts of distinguishing characteristics can also be called traits. Write the word “trait” on the board. Ask students if they can think of any other traits. Make two lists of different traits on the board, separating inherited and acquired traits. Ask students if they notice a difference between the two lists. Introduce the concept of acquired traits which are skills that you learn. Discuss the concept of inherited traits, which are traits you inherit from your parents. Tell students to pay close attention to the video to learn more about traits and heredity.


Video Script1. Has anyone ever told you that you have your mother’s or your father’s eyes?2. Or have you ever wondered why the children in a certain family all look so similar?3. These similarities not only occur in people but in plants and animals as well.4. Perhaps you have looked at fl owers in a garden and been amazed at how they all look

very alike.5. Notice how this calf looks so similar to its mother.6. What causes these similarities between offspring and parents?7. Or why in some cases do parents and offspring look different from each other?8. During the next few minutes we are going to explore these questions as we take a look

at some of the factors involved in passing characteristics from parents to offspring.9. And we will discuss some of the major scientifi c discoveries which have given us a

greater understanding of traits and heredity. 10.Graphic Transition – What is Heredity?11. Things such as the color of your hair,...12. ...the color of your eyes,...13. ...the markings on the wings of a butterfl y,...14. ...and the shape of this bird’s beak are all characteristics.15.Another word for characteristics is traits.16.We have thousands of different traits.17.Some of these traits, such as skin color and the shape of our ears are traits we can

easily observe.18.Other traits, such as the shape of a heart valve, or blood type are more diffi cult to see.19.A living thing, whether it be a person,...20. ...animal, or plant has many different traits.21.Some traits, such as hair color, are passed on from parents to offspring, while others

such as athletic ability are not.22. “Heredity” is the process of passing on traits from parents to offspring.23.Let’s take a minute to compare traits that are passed on and those that are not.24.Graphic Transition – Acquired and Inherited Traits25.Think about a skill you had to develop, such as reading...26. ...playing soccer...27. ...or playing a musical instrument.28.You Decide!

Are these skills inherited or learned?29.That’s right, these skills are learned. You are not born with these traits but over time,

and with practice, you acquired these skills. Things that you learn are called “acquired traits.”

30.Some traits such as hair color, eye color, or your height are not acquired. You can’t learn or practice something to make your eyes blue.

31.These types of traits are called inherited traits.32. “Inherited traits” are characteristics that you are born with.33.Let’s now take a closer look at inherited traits.


Video Script34.Graphic Transition – Inherited Traits35.Our understanding of the process of heredity is relatively young, and we are learning new

things all the time.36. It was not until the 1860’s that the fi eld of genetics began to take form.37.“Genetics” is the study of the process of heredity.38. In the 1860’s an Austrian monk by the name of Gregor Mendel performed thousands of

experiments that established the foundation of the science of genetics. 39.Working mainly with pea plants in his garden, Mendel studied a number of different traits

such as pea plant height,...40. ...seed color, seed shape...41. ...and fl ower position to name just a few.42.He became very interested in the way these traits were passed on from parents to offspring

over several generations.43.Mendel himself was also able to control the appearance of traits in offspring by directly

controlling the breeding process.44.Over a period of seven years, Gregor Mendel took careful notes on the 20,000 plants that

he grew.45.From his work, Mendel hypothesized that two factors exist for a given characteristic. 46.For example, in the trait for plant height, one factor for height comes from the father, and

the other factor for height comes from the mother.47.Today scientists call the factors that control traits genes.48. “Genes” are the basic units of heredity.49.Let’s explore how genes can interact with each other.50.Graphic Transition – Dominant and Recessive Traits51.One of Mendel’s most important discoveries dealt with the principle of dominance and

recessiveness.52.He reasoned that in some cases one factor could mask another factor.53.For example, in the case of pea plant height, the trait of tall height masks, or dominates,

the factor for short plant height.54.The stronger trait which masks the weaker trait is called the dominant trait.55.And the weaker trait that seemed to disappear is called the recessive trait.56.This principle is summarized in the Law of Dominance, which states that in a living thing

the dominant trait is expressed and the recessive trait is hidden.57.There are many traits in humans which are dominant and recessive including earlobe

formation, hairline appearance,58. ...and even the ability to roll your tongue.59.The idea of dominance and recessiveness is just one of the many important discoveries

Gregor Mendel made while working with pea plants in his garden.60.Graphic Transition – Chromosomes61.The work of Gregor Mendel went largely unnoticed for over 30 years.62.But in the early 1900’s scientists rediscovered his work, which serves as the foundation for

the modern science of genetics. 63.Scientists agreed with Mendel’s earlier fi ndings that certain traits are inherited from parents. 64.And they also discovered that genes responsible for traits, are found inside cells on small


Video Scriptstructures called chromosomes.

65. “Chromosomes” contain genetic material made up of many, many genes.66.Every cell in the body contains chromosomes.67.Today we know that chromosomes, magnifi ed here many hundreds of times, control the

traits of an organism.68.Graphic Transition – DNA69. In the 1940’s and 1950’s several scientists including Martha Chase, Alfred Hershey,

Edwin Chargaff, Rosalind Franklin, and Maurice Wilkins...70. ...as well as James Watson and Francis Crick added to our understanding of genetic

material.71.They found that the genetic material, which makes up genes, consits of a complex

chemical called deoxyribonucleic acid, or DNA for short.72.You Observe!

Describe the shape of this DNA molecule.73.That’s right, the DNA molecule kind of looks like a ladder that has been twisted.74.Scientists call the shape of the DNA molecule a double helix.75.A close inspection shows that the rungs on the double helix are formed by pairs of

substances called nitrogen bases The DNA ladder may contain hundreds of millions of pairs of nitrogen bases.

76.There are four different types of nitrogen bases in DNA. These bases combine in specifi c ways.

77.The arrangement of the bases determines the genetic code.78.Graphic Transition – Genetic Engineering79.One of the most active fi elds of scientifi c investigation involves studying the genetic code

not only of humans, but of all kinds of living things from...80. ...plants to animals to viruses.81.Genetic engineering involves the amazing process of actually changing the genetic

makeup of a living thing.82.For example, scientists have produced vaccines by genetically changing viruses to

protect us from harmful diseases.83.Genetic engineering has also created crops which are more resistant to disease, and

also produce higher yields.84.Through genetic engineering, microorganisms have even been altered to process

sewage and even clean up oil spills.85.While there have been numerous benefi ts from genetic engineering...86. ...there is concern about the long-term effects that genetically altered organisms may

have on people and the environment.87.Graphic Transition – Summing Up88.During the past few minutes we have briefl y explored some of the aspects of traits and

heredity.89.We began by discussing how traits are characteristics of an organism.90.We observed that some traits, such as eye color, are inherited traits, while others such

as playing an instrument are acquired traits.91.Heredity is the study of the passing of traits from parents to offspring.


Video Script92.Some of the early work of Gregor Mendel was highlighted,...93. ...particularly the concept of dominant and recessive traits.94.We saw that the genetic material is found on chromosomes inside cells.95.The actual material which makes up the genetic code-DNA was also described.96.Finally, we discussed some of the benefi ts and concerns of genetic engineering.97.So the next time you look in the mirror... 98. ...or spend time with family members...99.Think about some of the things we have discussed during the past few minutes. 100. You just might think about traits and heredity a little differently.101. Graphic Transition – Video Assessment102. Fill in the correct word to complete the sentence. Good luck and let’s get started!1. _________ are the characteristics of a living thing.2. Hair color is an example of an __________ trait.3. ________ is the study of heredity.4. __________ are made up of many, many genes.5. ______ is the genetic material found in genes.

Answers can be found on page 12.


1. traits2. inherited3. Genetics4. Chromosomes5. DNA

1. The skills, such as reading, playing soccer, or playing a musical instrument, are learned skills.2. The DNA molecule looks like a lad-der that has been twisted. The shape is called a double helix.

1. c - dominant 2. b - inherited trait 3. c - DNA 4. d - recessive 5. d - acquired trait 6. c - traits 7. a - genetics 8. a - genetic engineering 9. a - heredity10. b - pea plants11. false12. false13. true14. true15. false16. DNA is the complex chemical compound that makes up genetic material.17. When you say a trait is “dominant”, it means that the stronger trait sometimes masks the weaker trait.18. Heredity is the process of passing on traits from parents to offspring.19. Genetics is the study of heredity.20. An example of an acquired trait is the ability to play the piano. An example of an inherited trait is the color of your eyes.

Answer Key to Student AssessmentsPre-Test (p. 14-15) 1. c - traits 2. a - heredity 3. d - acquired trait 4. b - inherited trait 5. a - genetics 6. b - pea plants 7. c - dominant 8. d - recessive 9. c - DNA10. a - genetic engineering11. true12. false13. false14. true15. false16. Heredity is the process of passing on traits from parents to offspring.17. An example of an acquired trait is the ability to play the piano. An example of an inherited trait is the color of your eyes.18. Genetics is the study of heredity.19. When you say a trait is “dominant”, it means that the stronger trait sometimes masks the weaker trait.20. DNA is the complex chemical compound that makes up genetic material.

Post-Test (p. 16-17)

Video Review (p. 18)


In Your Own Words (p. 20)

Genetic Engineering (p. 24)

Acquired vs. Inherited Traits (p. 23)

Dominant and Recessive Traits(p. 21-22)

Writing Activity (p. 20)

1. heredity 2. acquired 3. traits 4. chromosomes 5. genetic engineering 6. dominant 7. inherited 8. DNA 9. recessive10. genetics

Vocabulary (p. 19 )

Answer Key to Student Activities

Characteristics of an organism are called traits. The process of passing traits from parents to offspring is known as heredity. Not all traits are passed on by parents to offspring. Skills that you must practice to learn are called acquired traits. The study of acquired traits. The study of acquiredheredity is genetics. Gregor Mendel was one of the fi rst scientists to study genetics. Mendel developed the Law of Dominance. It states that stronger traits often mask weaker traits. The weaker traits are called recessive traits. In the 1950s, scientists discovered DNA, the chemical that makes up genes. Genes are responsible for the traits of an organism. Through the process of genetic engineeringscientists have been able to alter the genetic makeup of a living thing.

1. Heredity is the name for the way that traits are passed from one generation to the next.2. An inherited trait is a trait that is passed on from the parents to offspring. An acquired trait is a trait that is learned or obtained over time.3. The role of DNA is to hold the genetic material of an organism.

1. In his experiments with pea plants, Gregor Mendel controlled the breeding of the parents in order to see exactly how traits were passed from one generation to the next.2. Mendel discovered dominant traits and recessive traits. Dominant traits are the “stronger” traits that usually mask the “weaker” recessive traits.3. From his mother, the child inherited curly hair, unattached earlobes, and dimples.

Height –Using a computer –

Eyecolor –Shoe size –

Driving a car –Skin color –

Reading –Playing the guitar –Hitting a baseball –

Blood type –Shape of your nose –

Speaking French –Pierced ears –

Freckles –

inheritedacquiredinheritedinheritedacquiredinheritedacquiredacquiredacquiredinheritedinheritedacquiredacquiredinherited

1. Genetic engineering is the process that allows scientists to change the genetic makeup of a living thing.2. A GMO is a genetically modifi ed organism, the fi rst GMO was created in 1974.3. Benefi ts of genetic engineering include creating foods that are resistant to disease, and developing medecines. Concerns are that the environment will become less diverse, and that the long term effects are completely unknown.

The Double Helix (p.25)1. The structure of DNA was discovered in 1953 by James D. Watson and Francis Crick.2. The double helix looks like a ladder that has been twisted.3. There are four different types of nitrogen bases in DNA. The arrangement of the bases determines the genetic code.

4. The offspring inherited a widow’s peak and cleft chin from his father.5. Recessive traits that can be seen in the pictures are straight hair, straight hairline, attached earlobes, no dimples, and smooth chin.6. You can assume which traits are recessive because they are the traits that do not appear in the offspring.

Dominant and Recessive Traits cont’d.

d. acid

d. paleontology

d. moss

d. chemistry

d. tasks

Pre-Test

4. If you get a trait from your parents, it is referred to as an (a):

5. What is the science of heredity called?

1. Another word for characteristics of a living thing is:

2. The process of passing on traits from parents to offspring is called:

3. A trait that you learn is an:

Circle the best answer for each of the following questions.

a. markings b. fl owers c. traits

a. simple trait

c. skills

c. hard trait d. acquired trait

a. genetics b. paleontology

c. easy trait d. acquired traitb. inherited trait

b. inherited trait

c. biology

a. heredity

a. applied trait

b. ability d. reproduction

6. Gergor Mendel worked with this organism in his studies of heredity:

a. mice b. pea plants c. rats

10. The process of altering the genetic make-up of a living thing is:

d. recessive

d. powerful

7. The “stronger” trait which tends to mask a “weaker” trait is called:

8. A weak trait that seemingly disappears is referred to as:

9. What substance determines genetic code?

a. fading

c. dominant

a. blood b. marrow

c. losingb. dominant

b. recessive

c. DNA

a. winning

a. genetic engineering b. biochemistry c. physics

Visual Learning Company

Page

14Traits and Heredity

Name

Write a short answer for each of the following.

Gregor Mendel bred horses to study genetics.

Pre-Test

16. What is heredity?

17. List one example each of an acquired trait and an inherited trait.

18. What is genetics?

Hair color is an example of an inherited trait.

If you practice and study enough, you can make your eyes blue.

Chromosomes contain the genetic material found in cells.

Genetic engineering has been unsucessful in changing the genes of living things.

11.

12.

13.

14.

15.

Write true or false next to each statement.

19. What does it mean to say a trait is “dominant”?

20. What is DNA?


Page


Name

Circle the best answer for each of the following questions.

Post-Test

d. acid

d. paleontology

d. moss

d. chemistry

d. tasks

2. If you get a trait from your parents, it is referred to as an (a):

7. What is the science of heredity called?

6. Another word for characteristics of a living thing is:

9. The process of passing on traits from parents to offspring is called:

5. A trait that you learn is an:

a. markings b. fl owers c. traits

a. simple trait

c. skills

c. hard trait d. acquired trait

a. genetics b. paleontology

c. easy trait d. acquired traitb. inherited trait

b. inherited trait

c. biology

a. heredity

a. applied trait

b. ability d. reproduction

10. Gergor Mendel worked with this organism in his studies of heredity:

a. mice b. pea plants c. rats

8. The process of altering the genetic make-up of a living thing is:

d. recessive

d. powerful

1. The “stronger” trait which tends to mask a “weaker” trait is called:

4. A weak trait that seemingly disappears is referred to as:

3. What substance determines genetic code?

a. fading

c. dominant

a. blood b. marrow

c. losingb. dominant

b. recessive

c. DNA

a. winning

a. genetic engineering b. biochemistry c. physics


Page


Name

Post-Test

Write a short answer for each of the following.

Gregor Mendel bred horses to study genetics.


20. List one example each of an acquired trait and an inherited trait.

19. What is genetics?

Hair color is an example of an inherited trait.

If you practice and study enough, you can make your eyes blue.

Chromosomes contain the genetic material found in cells.

Genetic engineering has been unsucessful in changing the genes of living things.

11.

12.

13.

14.

15.

Write true or false next to each statement.

17. What does it mean to say a trait is “dominant”?

16. What is DNA?


Page


Name

1. Are these skills inherited or learned?

3. ____________ is the study of heredity.

2. Hair color is an example of an _______________ trait.

1. ___________ are the characteristics of a living thing.

4. ________________ are made up of many, many genes.

5. __________ is the genetic material found in genes.

After you watch the video, test your knowledge with these questions.

While you watch the video, answer these questions:

2. Describe the shape of this DNA molecule.

Video Review

You Observe!

You Decide!


Page


Name

Traits that you are born with.

Small structures in cells that contain genetic material.

Vocabulary

The weaker trait that seemingly disappears.

Use these words to fi ll in the blanks next to the sentences below.

The process of passing traits from parents to offspring.

Traits that are learned or developed over time.

Another word for characteristics of an organism.

The chemical that makes up genetic material.

The stronger trait that sometimes masks the weaker characteristic.

1.

7.

2.

6.

8.

4.

5.

3.

The process of changing the genetic makeup of an organism.

9.

The study of the process of heredity.10.

Wo

rds

DNA genetics inherited heredity recessive

dominant acquired chromosomes traits genetic engineering


Page


Name

2. What is an inherited trait? What is an acquired trait?


Use the correct word from above to complete the sentences in the following paragraph.

Writing Activity

Characteristics of an organism are called _____________ . The process of

passing traits from parents to offspring is known as ______________ . Not all

traits are passed on by parents to offspring. Skills that you must practice to learn

are called ______________ traits. The study of heredity is _______________ .

Gregor Mendel was one of the fi rst scientists to study genetics. Mendel developed

the Law of ________________ . It states that stronger traits often mask weaker

traits. The weaker traits are called ________________ traits. In the 1950s,

scientists discovered _______ , the chemical that makes up genes. Genes are

responsible for the traits of an organism. Through the process of genetic

________________ scientists have been able to alter the genetic makeup of a

living thing.

In Your Own Words

3. What is the role of DNA?

Wo

rds

genetics DNA heredity Dominance traits acquired recessive engineering


Page


Name

Dominant and Recessive TraitsBackground: Gregor Mendel, an Austrian monk, is often credited with beginning the science of genetics in the 1860s. By controlling the breeding of pea plants, Mendel was able to see exactly how traits were passed down from one generation to the next. Through his careful experiments with over 15,000 pea plants Mendel learned a great deal about the process of heredity.

One of Gregor Mendel’s most important discoveries was the difference between dominant and recessive traits. Mendel developed a principle called the Law of Dominance. It states that there are two different forms of a trait. The dominant trait is the “stronger” trait, which is usually expressed in the offspring. The recessive trait is the “weaker” trait; it is not usually visible in the offspring. For example, in the case of pea plant height, the trait for tallness dominates that of shortness. In pea plants, tallness is a dominant trait.

People have dominant and recessive traits as well. For example, feel your earlobes. Are they attached directly to your head? Or do they hang free? In the case of earlobes, unattached earlobes are dominant over attached earlobes.

Directions: Look carefully at the pictures of the parents and offspring below. Decide which traits you think are inherited from the mother and which traits are inherited from the father. Finally, answer the questions on the next page.

Father Mother

Offspring

straight hair

widow’s peak

attached earlobes

no dimples

cleft chin

curly hair

straight hairline

unattached earlobes

dimples

smooth chin

curly hair

unattached earlobes

cleft chin

widow’s peak

dimples


Page


Name

Dominant and Recessive TraitsQuestions:1. What did Gregor Mendel control in his experiments with pea plants? Why did he do this?

2. List the two types of traits that Mendel discovered, and describe them.

3. Which traits did the child get from his mother?

4. Which traits did he get from his father?

5. List all of the recessive traits you noticed in the pictures.

6. How did you know which traits were recessive?


Page


Name

Acquired vs. Inherited TraitsBackground: You probably already know that the characteristics of an organism are referred to as traits. There are two different kinds of traits, characteristics you are born with, and those that you learn or obtain over time. Traits that you have from birth are called inherited traits. For example, the color of your hair is an inherited trait. Those traits that you must learn or develop during your life, such as playing the violin, are acquired traits. In this activity you will practice distinguishing between inherited traits and acquired traits.

Directions: For each of the traits listed below, decide whether the trait is acquired or inherited.

Height

Using a computer

Eyecolor

Shoe size

Driving a car

Skin color

Reading

Playing the guitar

Hitting a baseball

Blood type

Shape of your nose

Speaking French

Pierced ears

Freckles


Page


Name

Genetic Engineering

People have controlled the reproduction of some animals and plants for many years. Today, scientists are using techniques that allow them to change the genetic makeup of a living thing. This process is called genetic engineering. It is one of the most active fi elds of scientifi c research today. Using genetic engineering, scientists are able to change an organism’s genetic code. This means that they can create plants and animals with desirable traits without breeding. The organisms that result from genetic engineering are called genetically modifi ed organisms, or GMOs.

The fi rst GMO was created in 1974. Since then, scientists have created many different types of genetically modifi ed organisms.For example, scientists have cloned the genetic code of a sheep. This means that there are two sheep that have the exact same genetic code. Scientists have also used genetic engineering to create a type of corn that repels insects. Some people believe that genetically modifi ed foods will be able to save people who are starving. This would happen because foods like the corn can grow in harsh conditions and provide more nutrients. Insulin is a chemical that people with diabetes need. Scientists can produce insulin using genetic engineering.

However, not everyone believes that genetic engineering is a good thing. Some people say that it is not a good idea to change the natural makeup of a living thing. Others argue that GMOs will make the environment less diverse because GMOs are stronger than natural plants and animals. The long term effects of GMOs are completely unknown. There are both benefi ts and concerns about genetic engineering.

Questions:1. What is genetic engineering?

2. What is a GMO and when was the fi rst one created?

3. What are some benefi ts of genetic engineering? What are some concerns?

Read the information below, and then answer the questions.


Page


Name

The Double Helix

Background:In 1953, two scientists, James D. Watson and Francis Crick, discovered the structure of DNA. They found the shape of the DNA molecule to be a double helix, as pictured to the right. The double helix looks like a ladder that has been twisted. Each of the rungs on the ladder is made up of a pair of nitrogen bases. There are four different types of nitrogen bases in DNA. The sequence of these chemicals creates the unique genetic code of every organism. Today we know that the nitrogen bases always pair up in the following confi guations: cytosine - guanine, adenine - thymine.

Directions:Using four colors, color in the double helix at the right side of this page. Pay close attention to the pairs of nitrogen bases as you color. Use one color to signify each of the four types of nitrogen bases. Then answer the questions below.

Questions:1. When was the structure of DNA discovered, and by who?

2. Describe the appearance of a double helix.

3. How many different types of nitrogen bases are in DNA? What does the arrangement of these bases determine?


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