Structure of DNA

Structure of DNA - What makes you you!

Deoxyribonucleic acid (DNA) is a molecule encoding the genetic instructions used in the development and functioning of all known living organisms and many viruses. Along with RNA and proteins, DNA is one of the three major macromolecules that are essential for all known forms of life.

Genetic information is encoded as a sequence of nucleotides (guanine, adenine, thymine, and cytosine) recorded using the letters G, A, T, and C. Most DNA molecules are double-stranded helices, consisting of two long polymers of simple units called nucleotides, molecules with backbones made of alternating sugars (deoxyribose) and phosphate groups (related to phosphoric acid), with the nucleobases (G, A, T, C) attached to the sugars.

DNA is well-suited for biological information storage, since the DNA backbone is resistant to cleavage and the double-stranded structure provides the molecule with a built-in duplicate of the encoded information.



The structure of the DNA double helix. The atoms in the structure are colour-coded by element and the detailed structure of two base pairs are shown in the bottom right

Chromosomes
Within cells, DNA is organized into long structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes. Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts.^[1] In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

 

 

What are traits?

BrainPOP on Heredity

14.A
14.C

How would you describe yourself? Do you have curly hair? Do you have dimples when you smile? Are you tall? These are examples of traits. A trait is a characteristic that distinguishes one organism from another organism. An apple tree’s traits might include pink flowers and red apples, while a pine tree’s traits might include flat needles and cones. Each apple and cone also has a unique set of traits—just as you have unique traits that distinguish you from your classmates.

Determining Traits

Traits are determined by genes inside an organism’s cells. A gene (JEEN) is a section of DNA on a chromosome that has genetic information for one trait, as shown in Figure 1. Genes carry coded instructions for making all parts of an organism. Not all of these instructions are contained in just one gene. Different genes contain different codes. The code of just one gene determines some traits.

 

Other more complex traits are determined by the codes of many genes that work together.

Heredity is the passage of genetic instructions from one generation to the next generation.

 

Genes are inherited. That means the instructions they carry are passed in sex cells from parents to offspring. An organism has genes—and some traits—similar to those of its parents.

 

1. Key Concept Check How are genes and traits related?

Traits are determined by genes inside an organism’s cells.

Figure 1 Genes are segments of molecules called DNA. Each gene determines a trait or part of a trait.

Phenotype and Genotype

You can describe an organism’s traits in different ways. Phenotype (FEE nuh tipe) describes an organism’s observable set of traits. The giraffe in Figure 2 has brown spots, thin legs, and a long neck. These traits are parts of the giraffe’s phenotype.

Genotype ( JEE nuh tipe) describes an organism’s complete set of genes. The genotype contains the coded instructions that result in an organism’s phenotype. Scientists say that an organism expresses its genotype in its phenotype. Like all organisms, the giraffe has many different traits. Its genotype contains the instructions for all of these traits.

 

2. Key Concept Check How do phenotype and genotype relate?

Phenotype describes an organism’s observable set of traits, whereas a genotype describes an organism’s entire set of genes.

Anup Shah/Photodisc/Getty Images

Figure 2 The giraffe’s phenotype includes its spots and long neck.

 

11 NOTES

Environmental Influence

Soil

Water

Visual Literacy : Figure 5

Temperature

Visual Literacy : Figure 7

Differentiated Instruction

Differentiated Instruction

Differentiated Instruction

Teacher Demo

Fun Fact

Environmental Influence

Explain that an organism’s genotype stays the same, but its phenotype can change as a result of environmental influences. Have students make a Foldable, as shown at the beginning of the lesson. Use the following questions to gauge students’ understanding of the reading.

Guiding Questions

Name three environmental factors that can affect an organism’s phenotype.

The student can name any three of the five factors discussed in the lesson: diet, soil quality, water, temperature, and social conditions.

Visual Check How might the ability to change color help an octopus survive in its environment?

It can blend into its surroundings so it’s hidden from predators and prey.

What sort of phenotype changes could last a lifetime?

The loss of a limb is an example of a permanent phenotype change. Have students suggest others.

Soil

There are many substances in soil. A deficiency in even one of the substances needed by a plant can cause the plant to grow abnormally. But varying soil composition can also change the phenotypes of healthy plants. Use the scaffolded questions to assess students’ comprehension of the text.

Guiding Questions

What effects might inadequate nitrogen in soil have on plants growing in the soil?

The plants might be smaller than normal or have yellow leaves.

Reading Check Does soil acidity affect genotype, phenotype, or both in hydrangeas?

phenotype

Water

All organisms need water. Lack of sufficient water—or too much water—can change a plant’s phenotype. Use the questions below to explore students’ understanding of water’s role in nature.

Guiding Questions

What effects can too little or too much water have on the leaves of a plant?

Too little water can cause a plant’s leaves to be smaller than normal. Too much water can cause leaves to fall off.

How can plants grow in deserts?

Even most desert regions receive some rainfall every year. The plants that grow in those areas have made adaptations that enable them to survive on much less water than most plants need. But a desert region with absolutely no water cannot support any life.

Visual Literacy: Figure 5

Use the image of the water marigold to discuss another way that water can affect a plant’s phenotype.

Temperature

Explain that the temperature of an organism’s environment can sometimes affect the organism’s phenotype, even its gender. Siamese cats develop darker fur at the colder parts of their bodies. Himalayan rabbits show a similar change in phenotype. Chameleons sometimes change color depending on the temperature of their surroundings as well as their mood. Use the questions below to assess students’ understanding of how the environment can affect an organism's phenotype.

Guiding Questions

What other reptiles mentioned in the text are affected in the same way as crocodiles?

The gender of some turtles is also determined by the temperature of the eggs from which the offspring develop.

Reading Check What determines a crocodile’s gender?

the temperature at which the egg is incubated

Key Concept Check How can the environment change phenotypes?

Surroundings, soil type, water, temperature, and social factors can all change phenotypes. They do not change genotypes.

What is an organism’s social environment?

An organism’s social environment is the group of like organisms with which it shares a living space.

Visual Literacy: Figure 7

Have students study the photographs in Figure 7 and then test their understanding of the images.

Ask: What would happen to the desert locust on the left if it joined the swarm of locusts on the right?

It would become orange like the other locusts. Desert locusts change color depending on their social environments, and when a locust goes from being alone to living in a group its phenotype changes.

Differentiated Instruction

Soil and Water Have students review what they have learned about the effects of soil and water on plant phenotypes. Have them make additions to their Foldables.

Differentiated Instruction

Needed Nutrients Different types of plants have adapted to living in different types of soils. Have students learn more about the nutrients that plants require in addition to nitrogen. Tell them that plant nutrients fall into two main categories: primary nutrients, such as phosphorus and potassium, and secondary nutrients, such as sulfur and magnesium. Instruct students to write a report about what they learn. In the report, have students include a list of species that are adapted to local soils.

Differentiated Instruction

Finding Phenotypes Instruct students to observe plants and animals around their home and school and compose a list of phenotypes. Have students write the phenotypes in English as well as in their native languages.

Teacher Demo

Potting a Plant Bring to class a small plant, a clay pot with a drainage hole, potting soil, and a piece of window screen. Show students how to pot the plant properly, including putting the piece of screen at the bottom of the pot to prevent soil from washing out of the pot. Some sources recommend putting pebbles at the bottom of the pot to facilitate drainage, while others advise against it. You may want to consult with an expert gardener before making this demonstration.

Fun Fact

Comparing Genomes Human beings are complex organisms, so you might think they have the largest genome in nature. However, that isn’t so. Many plants have more genes than the 20,000 to 25,000 in the human genome. For example, Arabidopsis thaliana, a plant in the mustard family, has at least 25,000 genes, and rice plants have more than 37,000. In the animal world, some amphibians have much larger genomes than human beings. And the largest estimated number of genes in the natural world belongs to a parasitic protozoan called Trichomonas vaginalis—nearly 60,000. Scientists have not yet learned why genome sizes vary so widely among organisms and why some simple organisms have so many genes.

Environmental Influence

Genotype does not usually change during an organism’s lifetime. The flamingos pictured at the beginning of this lesson are different colors because of the food they eat. They are pink or white depending on the presence of a certain pigment in their diet. Like all organisms, each cell of a flamingo usually contains the same genes throughout the flamingo’s life. Its genes do not depend on the food the flamingo eats or what environment it lives in.

 

While an organism’s genes usually remain the same, an organism’s phenotype can change throughout its lifetime. Phenotype can change when factors in an organism’s environment change. Factors such as soil quality, water, temperature, or social conditions can change an organism’s phenotype without changing its genotype. Some of these factors can cause changes that last a lifetime. Others cause changes that are quickly reversed. For example, the octopus in Figure 3 can change color quickly as it moves, hunts, and hides in its environment.

 

(l, c, r) David Kearnes/SeaPics.com

Figure 3 This octopus changes color in a matter of seconds. No matter its color, its genotype remains the same.

 

1. Visual Check How might the ability to change color help the octopus survive in its environment?

It can blend into its surroundings so it’s hidden from predators and prey.

 

Soil

Just as a flamingo’s color can change as its diet changes, the phenotype of many plants can change depending on the nutrients in the soil in which they grow. Low nitrogen in soil might cause a plant to be smaller than usual, or it might make its leaves yellow.

 

The acidity of soil can affect phenotype, too. In the case of the hydrangea plants shown in Figure 4, soil acidity can determine whether a plant has blue flowers or pink flowers. If grown in basic soil, a plant produces pink flowers. If the same plant is grown in acidic soil, it produces blue flowers. However, the plant’s genotype remains the same no matter what color its flowers are.

2. Reading Check Does soil acidity affect genotype, phenotype, or both in hydrangeas?

phenotype

Hiroshi Kubozuka/Sebun Photo/Getty Images

Figure 4 Variations in soil acidity determine the color of these hydrangea flowers.

 

Water

Have you ever forgotten to water a houseplant?

What happened to it?

The plant might have wilted.

A wilted plant looks different than a plant that is well watered. Water can change a plant’s phenotype. Lack of water over long periods of time can cause some plants to have smaller leaves than usual, or it can cause leaves to curl. Too much water might cause plants to drop some of their leaves. Water can also affect the leaf shape of some plants, such as the water marigold shown in Figure 5. The leaves of the water marigold are thin and branched under the water and broad and wide above the surface.

 

Figure 5 Water affects the shape of water marigold leaves even though both leaves contain the same genes.

 

Temperature

Did you know that the dark areas on a Siamese cat—the nose, ears, and tail—are cooler than the rest of the cat’s body? Changes in temperature affect the color of these areas. If the Siamese cat in Figure 6 lived in a warmer environment for a long period of time, the dark areas would gradually lighten. If the cat moved back to a cool environment, those areas would darken again. However, the cat’s genotype in each environment would be the same.

Temperature can affect the phenotype of other organisms. For example, in some reptiles, such as crocodiles and some turtles, temperature determines offspring gender. If the sandy nest in which a crocodile lays her eggs remains about 32°C, the hatchlings are male. In a slightly warmer or cooler nest, the hatchlings are female.

3. Reading Check What determines a crocodile’s gender?

the temperature at which the egg is incubated

Juniors Bildarchiv/Photolibrary

Figure 6 The dark nose, ears, and tail of this Siamese cat are cooler than the rest of its body.

 

Social Factors

An organism’s social environment is the group of like organisms with which it shares a living space. A change in social environment can cause changes in gender, size, or color. For example, the desert locust, shown in Figure 7, is brown or green when it lives alone. However, in a crowded social environment, the locust is yellow or orange. Despite the color difference, the locust’s genes remain the same.

 

4. Key Concept Check How can the environment changephenotypes?

Surroundings, soil type, water, temperature, and social factors can all change phenotypes. They do not change genotypes.

Insets: Stephen Dalton/Minden Pictures

Figure 7 A desert locust’s color can change depending on whether the locust lives alone or in a crowded environment.

 

7 NOTES

What are mutations ?

Visual Literacy : Figure 8

Differentiated Instruction

Differentiated Instruction

Differentiated Instruction

Teacher Demo

Fun Fact

What are mutations?

Be sure that students understand that an organism’s overall genotype cannot be changed by environmental factors. But individual genes can be changed by mutations, which can be caused by environmental factors. Use the questions below to gauge students’ understanding of the subject.

Guiding Questions

Key Concept Check What is a mutation?

A mutation is a change in genetic information.

How can genes be mutated?

They can mutate spontaneously or they can be mutated by X-rays, chemicals, or other environmental factors (e.g., ultraviolet light and cosmic rays).

How do inherited mutations cause organisms to change over time?

Mutations can cause phenotype changes that spread within a species. Over generations, the change—if it is beneficial to survival—becomes a characteristic of the entire species and can even lead to the emergence of a new species.

Visual Literacy: Figure 8

14.C

Students should understand that mutations can cause variations in phenotype, such as the extra wings of the fruit fly shown in the Figure 8 photograph. If the mutation occurs in a germ cell (egg or sperm cell), then the fly's offspring could have extra wings if they inherit the mutation. That is how mutations are passed on.

Ask: Was the fruit fly with extra wings in Figure 8 always that way, or could the mutation have occurred in its lifetime?

It was always that way. It inherited the mutation causing the extra wings.

Differentiated Instruction

The Genetic Code Have students review the genetic code. Ask them to write a paragraph explaining what it means for a change to occur in the code of a gene.

Differentiated Instruction

Learning About Mutations Have students learn about the various causes of mutations and the effects that mutations produce. Instruct them to research the difference between somatic-cell mutations and germ-cell mutations. Have them write a report about what they learn.

Differentiated Instruction

Gene Telephone Have student groups sit or stand in a circle. Ask one student to whisper a sentence in English to the next student. That student should whisper it to the next, and so on. When it reaches the last person, her or she should say the sentence out loud and compare it to the original sentence. Have students discuss how this activity models mutations in genes.

Teacher Demo

Making Mutations On an index card, write out a series of a couple dozen As, Gs, Ts, and Cs on a single line. The order of the letters doesn’t matter. The letters represent the bases (adenine, guanine, thymine, and cytosine) along a segment of DNA. Give blank index cards to all of the students. Show your card to one of the students and have the student copy the string of letters onto his or her card. The student will then hold up that card for another student to copy, and so on around the class. Each copying represents a replication (reproduction) of the DNA strand. When the last student has made a copy, have that student present his or her card to you. Place that card under your original card and see if all the letters are the same. The odds are good that there will be at least one replication error—a mutation, in other words. If the final copy is perfect, compliment the students on their copying abilities but explain what the purpose of the exercise was. This process can be repeated. Have students record how many mistakes are made in each round. Does the number of mistakes increase?

Fun Fact

The Molecular Clock Because mutations are assumed to occur at a steady rate within a species over long periods of time, the genetic differences between related species can show scientists how long ago those species diverged from a common ancestor. This measuring system is called the molecular clock. Ask students to recall that heredity is the passage of genetic instructions from one generation to the next generation. Scientists usually study the DNA changes that have occurred in mitochondrial DNA because mitochondria are passed from one generation to the next almost exclusively through the maternal line. The molecular clock can also be used to determine when a disease-causing microbe came into being from an earlier microbe.

Mutations and Phenotype

14.A
14.C

You have read that environmental factors can change an organism’s phenotype without changing its genotype. You also read that the environment cannot change genotype. A genotype can be altered only by changes in the gene’s DNA code.

 

What are mutations?

A change in the DNA code of a gene is called a mutation (myew TAY shun). Think about the last time you typed something. Did your fingers hit a wrong letter? Mutations are similar. Just as one wrong letter changes a word, a mutation changes a gene.

 

Sometimes, a mutation in a gene can also change the trait coded by the gene. When it does, the change can appear in the organism’s pheno type. The fruit fly with extra wings in Figure 8 is the result of a mutation in a gene that codes for the number of wings.

 

1. Key Concept Check What is a mutation?

A mutation is a change in genetic information.

Pascal Goetgheluck/Photo Researchers

Figure 8 The fruit fly with extra wings contains a mutation that appears in its phenotype.

 

Inherited Mutations

Recall that a change to a phenotype that is caused by an environmental factor is not inherited. A pink flamingo’s offspring can be pink or white depending on the offspring’s diet. However, changes to genotype can be inherited. If the fly with extra wings in Figure 8 survives and has offspring, it could pass its wing mutation to future generations. Passing mutations to offspring plays an important role in determining how organisms change over time.

1

1 NOTE

Can you mutate a word?

Prep: 5 min Class: 10 min

Purpose
To simulate possible effects of mutations on gene expression.

Materials
one set of blocks with letters and a paper bag per group, 8 of the blocks have the letters A, D, E, E, F, H, M, and T respectively, ninth block is blank; alternatively, different colored squares of paper with letters

Before You Begin
Review the definitions for genotype, phenotype, and mutation. Phenotype describes an organism’s observable set of traits. Genotype describes an organism’s complete set of genes. A mutation is a change in the DNA code of an organism.

Guide the Investigation
• This activity is set up so that the word head is transformed into feet one letter at a time in four steps.

• During the activity,

Ask: Why does the procedure say to put the block back in the bag if the mutation does not produce a meaningful word?

This is a simulation. Organisms with harmful mutations might not reproduce.

MiniLab: Can you mutate a word?

4.A.xxv4.A.xxix
4.A.lv

Just like genes, making one small change to a word can change its meaning.

Hutchings Photography/Digital Light Source

1. Spell the word head on four construction paper squares.

2. Pull a random letter out of a bag of letters and use it to replace one letter. If you cannot make a meaningful word, place the letter back in the bag, and draw another letter.

3. Repeat step 2 five times, writing down any new, valid words in your Science Journal.

Analyze and Conclude

1. Observe How did your word change?

Answers will vary depending on results, but should indicate that the meaning changed along with the letters.

2. Key Concept Explain how the change to your word is like a mutation to a gene.

A change in words is similar to a change in genes because even a tiny difference can have large repercussions.

1

1 NOTE

WORD ORIGIN

Ask: The word’s origin is the Latin word mutare, which means “to change.” How does that definition relate to the definition of mutation?

The definition of mutation is “a change in the DNA code of a gene,” so both definitions have the term change.

WORD ORIGIN

mutation
from Latin mutare, means “to change”

Lesson Review

Visual Summary

Phenotype is an organism’s observable set of traits. Genotype is an organism’s entire set of genes.

Anup Shah/Photodisc/Getty Images

An organism’s phenotype can be changed by environmental factors.

Genotype can change only by mutations to genes.

Pascal Goetgheluck/Photo Researchers

What do you think NOW?

You first read the statements below at the beginning of the lesson.

 

1. Some organisms can change color as they move from one environment to another.

2. Environmental factors determine whether some organisms are born male or female.

3. Traits acquired during an organism’s lifetime are passed to the organism’s offspring.

 

Did you change your mind about whether you agree or disagree with the statements? Rewrite any false statements to make them true.

Lesson Assessment

14.C

Use Vocabulary

1. Distinguish between phenotype and genotype.

Genotype refers to the genes; phenotype is the outward appearance of an organism.

2. Choose the term that describes an organism’s distinguishing characteristics.

phenotype

3. A change in a gene is called a(n) __________.

mutation

 

Understand Key Concepts

4. If you describe a flower as pink, you are referring to

     A.its genotype.

     B.its genes.

     C.its DNA.

     D.its phenotype.

5. Compare In the fall, an arctic fox begins to grow a white coat. In spring, it sheds its white coat and
     begins to grow a brown coat. Compare the fox’s genotype in winter and in summer.

The fox’s genotype remains the same during the fall and the spring.

6. Describe the location of the instructions that code for traits.

The instructions that code for traits are located in the genetic material found in the genes within chromosomes in the nucleus.

7. Which equation best illustrates how a trait appears?

     A.genotype + environment = phenotype

     B.genotype + phenotype = environment

     C.phenotype + environment = genotype

     D.phenotype + genotype = environment

 

8. The two plants below came from the same parent plants, but the one on the right was watered less.
    What might explain their differences?

     A.Lack of water can alter a plant’s genotype.

     B.Lack of water can alter a plant’s phenotype.

     C.Lack of water can cause a mutation.

     D.Lack of water is an adaptation.

9. Which does NOT change over time?

     A.an individual’s phenotype

     B.an individual’s genotype

     C.populations of organisms

     D.types of adaptations

 

 

Interpret Graphics

 

10. Hypothesize The illustration below shows what happens to the fur color of a Himalayan rabbit after a patch of fur on its back is shaved and an ice pack is placed on it for several days. What might happen if the floor of the rabbit’s cage were warmed but the rest of the cage were kept cool?

Because the rabbit’s fur turned black when exposed to cold, the rabbit’s paws might turn white when exposed to warmth. The top of the body closest to the coolest parts of the cage might get darker.

 

 

11. Organize Information Copy and fill in the graphic organizer below with three environmental factors that can influence phenotype.

left box: Environmental Influence on Phenotype; right boxes (any order): soil, temperature, social factors

 

Critical Thinking

12. Develop a hypothesis to explain why two organisms with the same genes might look different.

If each were exposed to different environmental factors, such as nutrition, exercise (one might be more muscular than the other), seasonal changes (in which case one might have a thicker coat), and accidents throughout their lifetime, their appearances (phenotypes) might be affected.

 

13. Provide two examples to explain the following statement: The expression of genes is best understood as a combination of the genes and environmental factors.

Answers will vary. Sample answer: Flamingos can change color depending on what food they eat, but their genes remain the same. Hydrangeas change color based on how acidic their soil is, but their genes do not change.

 

14. Draw an Analogy Think of a house as if it were an organism. Explain whether changing its blueprint would be similar to changing genotype or phenotype.

Changing its blueprint would be similar to changing its genotype because you would be making a permanent change in the house plan.

 

Copyright © 2011-2012 The McGraw-Hill Companies. All Rights Reserved.
Texas Essential Knowledge and Skills Copyright © 2011. Texas Education Agency. All Rights Reserved.

REAL LIFE

The Hawksbill Turtle’s Unusual Diet

Eugenia Narco-Maciel/American Museum of Natural History

You might know that a carnivore (KAR nuh vor) is an animal that eats other animals, but have you ever heard of a spongivore (SPUN jih vor)? It is a carnivore that eats sponges. A sponge is a simple animal that has chalky, glasslike spikes that support its body. The hawksbill turtle is a spongivore. Its narrow head has a sharp, curving beak. This enables a hawksbill to remove sponges from small spaces in coral reefs. Hawksbills also have adaptations for digesting and absorbing nutrients from sponges. How did hawksbills evolve as spongivores? Conservation geneticist Eugenia Naro-Maciel of the American Museum of Natural History in New York City is trying to answer to this question.

To understand adaptations of sponge-eating turtles, Naro-Maciel analyzed their DNA. She compared the DNA of sea turtle species living today to learn which are similar. The more similar the DNA of two species, the more closely related they are. Her results revealed that hawksbills and other carnivorous sea turtle species are more closely related than hawksbills and plant-eating sea turtle species. Next, Naro-Maciel wants to gather more data about where hawksbills feed and move. Scientists can use this data to find ways to minimize the effects of human actions on hawksbills.

Visuals Unlimited/CORBIS

Meet the Hawksbill

• Weight: 45–90 kg

• Length: 80–100 cm

• Hawksbills inhabit tropical and subtropical regions of the Atlantic, Pacific, and Indian Oceans.

• In the Caribbean, an adult hawksbill eats an average of 544 kg of sponges per year.

• Every 2–3 years, an adult female hawksbill returns to the beach where she hatched to build her nest. She returns 3–5 times per breeding season to build a nest and lays an average of 130 eggs per nest.

 

1 NOTE

RESEARCH

Have students work in small groups, with each group choosing one species of sea turtle to research and report on. Have groups include the turtle’s average length and weight, physical adaptations, range of habitat, diet, and any other interesting information. When groups are ready, have them report to the rest of the class.

It’s Your Turn

RESEARCH Conduct research about one of the other six sea turtle species. Write a paragraph that describes the turtle’s physical characteristics, its feeding and migration habits, and how these traits help it survive.

Copyright © 2011-2012 The McGraw-Hill Companies. All Rights Reserved.
Texas Essential Knowledge and Skills Copyright © 2011. Texas Education Agency. All Rights Reserved.

   

http://glencoe.mcgraw-hill.com/sites/007874184x/student_view0/unit2/chapter4/standards_review-english.html

http://learn.genetics.utah.edu/content/begin/traits/


http://nature.ca/en/explore-nature/games-activities