4 section 2 how dna works - lab ratkos -...

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Interactive Textbook 55 Genes and DNA

SECTION

2 How DNA WorksGenes and DNA

Name Class Date

CHAPTER 4

After you read this section, you should be able to answer these questions:

• What does DNA look like in different cells?

• How does DNA help make proteins?

• What happens if a gene changes?

What Does DNA in Cells Look Like?The human body contains trillions of cells, which carry

out many different functions. Most cells are very small and can only be seen with a microscope. A typical skin cell, for example, has a diameter of about 0.0025 cm. However, almost every cell contains about 2 m of DNA. How can so much DNA fit into the nucleus of such a small cell? The DNA is bundled.

The DNA in the nucleus is part of a material called chromatin. Long strands of chromatin are usually bundled loosely inside the nucleus.

a

BEFORE YOU READ National Science Education StandardsLS 1c, 1e, 1f, 2b, 2c, 2d, 2e, 5c

STUDY TIPCompare After you read this section, make a table comparing chromatin, chro-matids, and chromosomes.

Math Focus1. Convert About how long is the DNA in a cell in inches?1 in. 2.54 cm

TAKE A LOOK 2. Identify In what form is the DNA in the nucleus?

NAT_IT_SCC_C04_NDNA_S02_055 55 7/31/06 4:06:04 PM



SECTION 2

Name Class Date

How DNA Works continued

FITTING DNA INTO THE CELLLarge amounts of DNA can fit inside a cell because the

DNA is tightly bundled by proteins. The proteins found with DNA help support the structure and function of DNA. Together, the DNA and the proteins it winds around make up a chromosome.

DNA’s structure allows it to hold a lot of information. Remember that a gene is made of a string of nucleotides. That is, it is part of the 2 m of DNA in a cell. Because there is an enormous amount of DNA, there can be a large variety of genes.

Chromatin

Proteins DNA

Nucleotide

A single strand of chromatin is made up of a long strand of DNA that is coiled around proteins.

b

Each strand of DNA has two halves that are connected in the middle. The strands are twisted into a double helix.

c

DNA IN DIVIDING CELLSWhen a cell divides, its genetic material is spread

equally into each of the two new cells. How can each of the new cells receive a full set of genetic material? It is possible because DNA replicates before a cell divides.

Remember that when DNA replicates, the strand of DNA splits down the middle. New strands are made when free nucleotide bases bind to the exposed strands. Each of the new strands is identical to the original DNA strand. This is because the DNA bases can join only in certain ways. A always pairs with T, and C always pairs with G.

READING CHECK

3. Identify What are two things that are found in a chromosome?

TAKE A LOOK 4. Describe What is chromatin made of?

Critical Thinking5. Predict Consequences Imagine that DNA did not replicate before cell division. What would happen to the amount of DNA in each of the new cells formed during cell division?




SECTION 2

Name Class Date


Chromatin

When a cell is ready to divide, it has already copied its DNA. The copies stay attached as two chromatids. The two identical chromatids form a chromosome.

Chromatids

How Does DNA Help Make Proteins?Proteins are found throughout cells. They cause most

of the differences that you can see among organisms. A single organism can have thousands of different proteins.

Proteins act as chemical messengers for many of the activities in cells, helping the cells to work together. They also affect traits, such as the color of your eyes and how tall you will grow.

Proteins are made from many subunits called amino acids. A long string of amino acids forms a protein.

The order of bases in DNA is a code. The code tells how to make proteins. A group of three DNA bases acts as a code for one amino acid. For example, the group of DNA bases CAA codes for, or stands for, the amino acid valine. A gene usually contains instructions for making one specific protein.

STANDARDS CHECKLS 2d Hereditary information is contained in the genes, located in the chromosomes of each cell. Each gene carries a single unit of information. An inherited trait of an individual can be determined by one or by many genes, and a single gene can infl uence more than one trait. A human cell contains many thousands of different genes.

6. Identify Where is DNA found in a eukaryotic cell?

Math Focus7. Calculate How many DNA bases are needed to code for fi ve amino acids?




SECTION 2

Name Class Date


HELP FROM RNARNA, or ribonucleic acid, is a chemical that helps DNA

make proteins. RNA is similar to DNA. It can act as a tempo-rary copy of part of a DNA strand. One difference between DNA and RNA is that RNA contains the base uracil instead of thymine. Uracil is often represented by U.

How Are Proteins Made in Cells?The first step in making a protein is to copy one side

of part of the DNA. This mirrorlike copy is made of RNA. It is called messenger RNA (mRNA). It moves out of the nucleus and into the cytoplasm of the cell.

Nucleus

Base

Cytoplasm

mRNA

Each group of three bases on the mRNA segment codes for one amino acid.

3

A copy is made of one side of the DNA segment where a par-ticular gene is located. This copy is moved into the cytoplasm.

1

This mirrorlike copy of a DNA segment is called messenger RNA (mRNA). However, in RNA, the base uracil replaces the base thymine.

2

READING CHECK

8. Identify What is one difference between RNA and DNA?

TAKE A LOOK 9. Compare How does the shape of RNA differ from the shape of DNA?




SECTION 2

Name Class Date


RIBOSOMESIn the cytoplasm, the messenger RNA enters a protein

assembly line. The “factory” that runs this assembly line is a ribosome. A ribosome is a cell organelle composed of RNA and protein. The mRNA moves through a ribo-some as a protein is made.

Molecules of transfer RNA (tRNA) deliver amino acids from the cytoplasm to the ribosome.

5The mRNA segment is fed through the ribosome.

4

The genetic code deter-mines the order in which amino acids are brought to the ribosome.

6

The amino acids are joined to make a protein. Usually one pro-tein is produced for each gene.

7 2nd amino acid

1st amino acid

3rd amino acid

4th amino acid

5th amino acid

amino acid

tRNA

mRNA

Ribosome

Cytoplasm

What Happens If Genes Change?Read this sentence: “Put the book on the desk.” Does

it make sense? What about this sentence: “Rut the zook in the tesk.”? Changing only a few letters in a sentence can change what the sentence means. It can even keep the sentence from making any sense at all! In a similar way, even small changes in a DNA sequence can affect the pro-tein that the DNA codes for. A change in the nucleotide-base sequence of DNA is called a mutation.

Critical Thinking10. Explain Proteins are made in the cytoplasm, but DNA never leaves the nucleus of a cell. How does DNA control how proteins are made?

TAKE A LOOK 11. Identify What does tRNA do?

READING CHECK

12. Defi ne What is a mutation?




SECTION 2

Name Class Date


HOW MUTATIONS HAPPENSome mutations happen because of mistakes when

DNA is copied. Other mutations happen when DNA is damaged. Things that can cause mutations are called mutagens. Examples of mutagens include X rays and ultraviolet radiation. Ultraviolet radiation is one type of energy in sunlight. It can cause suntans and sunburns.

Mutations can happen in different ways. A nucleotide may be replaced, added, or removed.

HOW MUTATIONS AFFECT ORGANISMSMutations can cause changes in traits. Some mutations

produce new traits that can help an organism survive. For example, a mutation might allow an organism to survive with less water. If there is a drought, the organism will be more likely to survive.

Many mutations produce traits that make an organ-ism less likely to survive. For example, a mutation might make an animal a brighter color. This might make the ani-mal easier for predators to find.

Some mutations are neither helpful nor harmful. If a mutation does not cause a change in a protein, then the mutation will not help or hurt the organism.

TAKE A LOOK 13. Compare What happens to one strand of DNA when there is a change in a base on the other strand?

Say ItBrainstorm Whether a mutation is helpful or harmful to an organism often depends on the organism’s environment. In a group, discuss how the same mutation could be helpful in one environment but harmful in another.




SECTION 2

Name Class Date


PASSING ON MUTATIONSCells make proteins that can find and fix many muta-

tions. However, not all mutations can be fixed. If a mutation happens in egg or sperm cells, the changed

gene can be passed from one generation to the next. For example, sickle cell disease is caused by a genetic mutation that can be passed to future generations.

Normal red blood cell

Sickle-shaped red blood cell

Normal DNA Sequence

Mutated DNA Sequence

mRNA

mRNA

How Can We Use Genetic Knowledge?Scientists use their knowledge of genetics in many

ways. Most of these ways are helpful to people. However, other ways can cause ethical and scientific concerns.

GENETIC ENGINEERINGScientists have learned how to change individual genes

within organisms. This is called genetic engineering. In some cases, scientists transfer genes from one organism to another. For example, scientists can transfer genes from people into bacteria. The bacteria can then make proteins for people who are sick.

GENETIC IDENTIFICATIONYour DNA is unique, so it can be used like a fingerprint

to identify you. DNA fingerprinting identifies the unique patterns in a person’s DNA. Scientists can use these genetic fingerprints as evidence in criminal cases. They can also use genetic information to determine whether people are related.

TAKE A LOOK 14. Identify What kind of mutation causes sickle cell disease: deletion, insertion, or substitution?

READING CHECK

15. Defi ne What is genetic engineering?

Sickle-cell disease is caused by a substitution mutation.




Name Class Date

Section 2 Review NSES LS 1c, 1e, 1f, 2b, 2c, 2d, 2e, 5c

SECTION VOCABULARY

mutation a change in the nucleotide-base sequence of a gene or DNA molecule

ribosome a cell organelle composed of RNA and protein; the site of protein synthesis

RNA ribonucleic acid, a molecule that is present in all living cells and that plays a role in protein production

1. Identify What structures in cells contain DNA and proteins?

2. Calculate How many amino acids can a sequence of 24 DNA bases code for?

3. Explain Fill in the flow chart below to show how the information in the DNA code becomes a protein.

makes a copy of itself called

which moves into the cytoplasm to a

where amino acids are joined to make a

4. Draw Conclusions How can a mutation in a DNA base sequence cause a change in a gene and a trait? What determines whether the mutation is passed on to offspring?

5. Identify Give two ways that genetic fingerprinting can be used.


3. Sex cells do not have homologous chromosomes.

4. to keep the total number of chromosomes the same from one generation to the next

5. When the sex cells joined, the offspring would have twice as many chromosomes as the parent.

6. two times

7. four

8. Mendel did not know how traits were car-ried in the cell. When Sutton saw meiosis happen, he realized that genes must be located on the chromosomes.

9. One; there is only one possible allele from each parent. The offspring will all have the same combination.

10. XY

11. Students should circle the XX combinations.

12. a disorder whose genes are carried on the X chromosome

13. first row, left to right: XX, XY; second row, left to right: XX, XY

14. the male with the X

15. Circled: solid dark shapes; underlined: two-toned shapes

Review 1. gene, chromosome, cell

2. No, meiosis only happens in cells that become sex cells.

3. all divided circles should be circled; female

4. dark-colored squares should have a square around them; male

5. The trait is probably sex-linked. No females in the family are affected, though some are carriers. The only ones who have the trait are males.

Chapter 4 Genes and DNASECTION 1 WHAT DOES DNA LOOK LIKE? 1. They all have a sugar and a phosphate.

2. sugar and phosphate

3. 45 guanines and 55 adenines

4. Thymine pairs with adenine; guanine pairs with cytosine.

5. It replicates.

6. An old strand was part of the original DNA, and a new strand was not.

Review 1. Genes determine inherited traits of organ-

isms; they are located on chromosomes.

2. The four DNA nucleotides differ in which base is present.

3. GAATCCGAATGGT

4. Chromosomes are made up of DNA and proteins.

5. Proteins are responsible for unwinding, copying, and rewinding DNA strands during replication.

6. sugar, phosphate, base

SECTION 2 HOW DNA WORKS 1. about 508 in.

2. chromatin

3. DNA, proteins

4. DNA coiled around proteins

5. If the DNA did not replicate before cell divi-sion, the new cells would have different amounts of DNA than the original cell.

6. in the nucleus

7. 15

8. DNA contains thymine, but RNA contains uracil.

9. DNA is a double strand, and RNA is a single strand.

10. DNA makes a copy of itself called messen-ger RNA. mRNA has the same order of bases as one of the DNA strands. mRNA leaves the nucleus and goes to the ribosome in the cytoplasm, where proteins are made.

11. tRNA delivers amino acids from the cytoplasm to the ribosome.

12. a change in the nucleotide-base sequence of DNA

13. The other strand also changes.

14. substitution

15. changing individual genes within organisms

Review 1. Possible answers: chromosomes, chromatin,

chromatids

2. eight

3. DNA, mRNA, ribosome, protein

4. Genes determine traits. DNA mutations can cause changes in proteins that make up the genes. If the genes change, the traits change. The new trait could be inherited if the change is on a sperm or egg cell.

C Cells, Heredity, and Classification Answer Key continued


Interactive Textbook Answer Key 16 Cells, Heredity, and Classification

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5. evidence in criminal cases, determining whether two people are related

Chapter 5 The Evolution of Living ThingsSECTION 1 CHANGE OVER TIME 1. Its bright colors warn predators that it is

poisonous, so it is less likely to be eaten.

2. Possible answer: hiding when predators are nearby

3. the process in which populations change over time

4. Some fossils are the actual remains of organisms. Others are signs that an organ-ism existed.

5. Either fossil is acceptable as an answer. The trilobite is similar to modern crustaceans. The fern is similar to modern ferns.

6. Fossils from different organisms are found in rocks of different ages.

7. Similar bones in each image should be the same color, with a different color for each type of bone.

8. These chemicals are not preserved in fossils.

9. Modern whales are mammals, not fish.

10. Modern whales have no back legs, and they cannot live on land. Both have front limbs and are or were mammals.

11. Fossils or bones of each species share some traits with fossils of earlier species.

12. hippopotamuses

13. camels and llamas

Review 1. Similar bones are found in the arms of

humans and the wings of bats. The similari-ties in forelimb bones suggest that humans and bats had a common ancestor.

2. From top to bottom: orangutans, chimpan-zees, humans, gorillas

3. Species A is probably most closely related to species B, because their DNA is most similar.

SECTION 2 HOW DOES EVOLUTION HAPPEN? 1. about 600 mi

2. South America

3. They were similar to, but not exactly like, the finches in Ecuador. The finches on each island were different from those on other islands.

4. The cactus finch is adapted to eating cacti. Therefore, it probably lives in a dry, desert-like environment where cacti can grow.

5. the process in which well-adapted organisms survive and pass on their traits, but less well-adapted organisms do not

6. They helped Darwin realize that the traits of organisms can change and that traits can spread through populations.

7. starvation and disease

8. It gave evidence that Earth is much older than scientists had previously thought.

9. Selective breeding: helped Darwin realize that traits of organisms can change with time and that traits can spread through populations;

Malthus’s ideas on population growth: helped Darwin realize that not all of an organism’s offspring will survive to reproduce;

Earth is very old: helped Darwin realize that slow changes can produce large differences over a long period of time

10. They have traits that help them avoid dying.

Review 1. Lyell’s book gave evidence that Earth is very

old. Because Earth is very old, there has been enough time for organisms to change slowly over time.

2. The finches have differently shaped beaks based on the type of food they primarily eat.

3. During natural selection, certain individuals survive because they have traits that give them a survival advantage over other individuals. During selective breeding, people breed individ-uals for traits that people find appealing or use-ful, even if the traits have no particular survival advantage.

4. Darwin realized that Malthus’s ideas apply to all species. Most individuals produce more off-spring than can survive. The size of a popula-tion is limited by the availability of resources, starvation, disease, and predation.

5. overproduction, inherited variation, struggle to survive, successful reproduction

SECTION 3 NATURAL SELECTION IN ACTION 1. People hunt tusked elephants.

2. something that kills insects

3. They would all die.

C Cells, Heredity, and Classification Answer Key continued


Interactive Textbook Answer Key 17 Cells, Heredity, and Classification

NA_ITB_SC_FM_AK.indd 17 8/14/06 1:13:16 PM

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