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Interactive Reader 136 DNA, RNA, and Proteins

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SECTION

2 Replication of DNADNA, RNA, and ProteinsCHAPTER 13

As you read this section, keep these questions in mind:• How does DNA replicate, or make a copy of itself?• What are the roles of proteins in DNA replication?• How is DNA replication different in prokaryotes and eukaryotes?

How Is DNA Copied?When cells divide, each new cell contains an exact

copy of the DNA in the original cell. That happens because DNA consists of two strands of complementary base pairs. If the two strands are separated, each strand serves as a pattern to make a new complementary strand. So, a single DNA molecule can be used as a template, or pattern, to produce two identical DNA molecules. This process of DNA replication involves three steps.

First, the DNA double helix unwinds and forms the Y shapes shown below. These Y shapes are called replication forks. At the replication forks, the complementary DNA strands separate from each other.

Y-shaped replication forks form where two strands of DNA separate.

Next, new nucleotides are added at each fork. The nucleotides form new base pairs according to the base-pairing rules. For example, adenine on an original strand will pair with thymine on a new strand. In this way, the original two strands act as templates for two new strands. Two new double helixes form as more nucleotides are added.

Finally, the two new DNA molecules split apart. Each molecule consists of one strand of the original DNA and one new strand. The nucleotide sequences in both new DNA molecules are identical to each other and to the original DNA molecule.

READING TOOLBOXSummarize As you read this section, underline the main ideas. When you finish reading, write an outline of the section using the underlined ideas.

1. Apply Concepts Aparticular DNA molecule contains 10,000 base pairs. How many base pairs will be in each of the two new strands formed during DNA replication?

2. Identify Label the two replication forks on the figure.

KEY IDEAS



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Replication of DNA continued

How Do Proteins Help to Copy DNA?DNA cannot copy itself. Instead, many different pro-

teins help to copy a DNA molecule. Each protein has a different set of functions.

Proteins called DNA helicases unwind the DNA double helix during replication. These proteins wedge themselves between the two DNA strands and break the hydrogen bonds that hold the strands together. That causes the helix to unwind and form a replication fork. Other proteins keep the two strands separated so that they can be copied.

Proteins called DNA polymerases help to form the new DNA molecules. Starting at the replication fork, DNA polymerases move along each single strand. They add nucleotides that form pairs with each base on the single strands to form two new double helixes.

DNA polymerases also have another job: they act as proofreaders. Sometimes, the wrong nucleotide attaches to the DNA strand. However, DNA polymerases cannot add another nucleotide to the strand if the previous one is paired to the wrong base. If an error occurs, the DNA polymerase removes the incorrectly paired nucleotide and replaces it with the correct one.

Replicationfork

DNAhelicase

DNApolymerases

NewDNA

NewDNA

OldDNA

OldDNA

DNA Replication

DNA helicases separate the two original strands of the DNA molecule.

1 DNA polymerases add complementary nucleotides to each strand.

2 The two DNA molecules that form are identical to the original DNA molecule.

3

READING CHECK

3. Describe What is the function of DNA helicase in DNA replication?

4. Apply Concepts Give an example of an incorrect base pairing.

5. Identify Name two proteins that are important in DNA replication.

SECTION 2



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Replication of DNA continued

DNA REPLICATION IN PROKARYOTIC CELLSRemember that the DNA in cells is packaged in the

form of chromosomes. All cells have chromosomes, but prokaryotes and eukaryotes replicate their chromosomes in different ways.

Prokaryotic cells usually have a single, circular chromosome. It is a closed loop attached to the inner cell membrane. Replication starts at one place along the chromosome. Two replication forks begin at that point. Replication proceeds in opposite directions until the two replication forks meet on the opposite side of the chromosome. At that point, the entire DNA molecule has been copied, as shown in the figure below.

Original DNA New DNA

Original DNA

Replication forks

Prokaryotic DNA Eukaryotic DNA

Replication forks

New DNA

Replication“bubble”

In prokaryotic cells, each circular chromosome has two replication forks. In eukaryotic cells, each linear chromosome may have many replication forks.

DNA REPLICATION IN EUKARYOTIC CELLSMost eukaryotic cells contain several chromosomes.

They stretch out in straight lines, and contain both DNA and protein.

DNA replication in eukaryotic chromosomes starts at several sites at the same time. This process forms the replication “bubbles” shown above. The bubbles get larger as more of the DNA is copied. Eventually, all the bubbles on a particular chromosome meet, leaving two identical DNA molecules.

READING CHECK

6. Describe At how many points on a prokaryotic chromosome does DNA replication begin?

7. Compare How does the shape of a prokaryotic chromosome compare to the shape of a eukaryotic chromosome?

SECTION 2



Name Class Date

Section 2 ReviewSECTION VOCABULARY

DNA helicase an enzyme that unwinds the DNA double helix during DNA replication

DNA polymerase an enzyme that catalyzes the formation of the DNA molecule

DNA replication the process of making a copy of DNA

1. Identify Give two functions of DNA polymerase in DNA replication.

2. Describe Fill in the blanks in the flowchart below to show how DNA replication occurs.

separate the two complementary strands of DNA, forming

.

At the , DNA polymerases add bases to each strand, forming two new DNA molecules.

bases continue to be added until the entire DNA strand has been copied.

Two new DNA molecules form. Each is to the other and to the original DNA molecule.

3. Compare Give one difference and one similarity between DNA replication in eukaryotic cells and DNA replication in prokaryotic cells.



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SECTION

3 RNA and Gene ExpressionDNA, RNA, and ProteinsCHAPTER 13

As you read this section, keep these questions in mind:• What is the process of gene expression?• What role does RNA play in gene expression?• What happens during transcription?• How do codons determine the sequence of amino acids that

results after translation?• What are the major steps of translation?• Do traits result from the expression of a single gene?

What Is Gene Expression?Gene expression is the process by which genes

control the traits of an organism. The process of gene expression results in the production of proteins, as shown below.

Gene expression consists of two main steps: transcription and translation. In eukaryotic cells, like the one shown here, transcription occurs in the nucleus, and translation occurs in the cytoplasm.

Nucleus

CytoplasmDNA

Transcription

Translation

RNA

Protein

RNA

Both transcription and translation involve RNA, or ribonucleic acid. Like DNA, RNA consists of nucleotide subunits linked together. RNA also has four bases and carries information. However, RNA differs from DNA in three ways:

• RNA generally consists of a single strand of nucleotides.

• RNA nucleotides contain the five-carbon sugar ribose

instead of the sugar deoxyribose.

• RNA nucleotides contain a base called uracil (U) instead of thymine. Like thymine, uracil is complementary to adenine.There are three main kinds of RNA. Each kind is

described in the table on the next page.

READING TOOLBOXSummarize After you read this section, make a flowchart showing the sequence of steps involved in the production of a protein from a DNA molecule.

1. Identify What are the two main steps in gene expression?

2. Apply Concepts ADNA strand has the base sequence GCCATATTG. What is the complementary RNA sequence?

KEY IDEAS



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SECTION 3 RNA and Gene Expression continued

Type of RNA DescriptionMessenger RNA (mRNA)

produced during transcription; is complementary to a DNA strand

Transfer RNA (tRNA) used during translation; attaches to an amino acid; contains a sequence of bases that are complementary to part of an mRNA strand

Ribosomal RNA (rRNA)

found in ribosomes; helps to bind amino acids together during translation

What Is Transcription?The first stage of gene expression is transcription.

During transcription, information in a particular region of DNA is copied into mRNA. A protein called RNA

polymerase carries out transcription in three stages.

3

1 RNA polymerase binds to a specific part of the gene called the promoter region.

2 The two DNA strands unwind and separate.

3 The RNA polymerase moves along the DNA strand. It adds complementary mRNA nucleotides to a growing mRNA strand as it moves. At the end of transcription, the RNA polymerase has produced an mRNA strand that is complementary to the DNA in the gene.

RNApolymerase

mRNA

Transcription

1 2 3

You can think of a gene as a “sentence.” Each “word” in the sentence consists of a group of three bases called a codon. During transcription, the sentence of DNA codons is translated into a complementary sentence of mRNA codons. After transcription, the mRNA sentence moves from the cell nucleus to ribosomes in the cytoplasm. There, it is translated into proteins.

Each mRNA codon either matches one amino acid or acts as a signal to start or stop translation. This system of paired codons and amino acids is called the genetic

code. The figure at the top of the next page shows how to determine which amino acid a certain codon codes for.

3. Identify Which type of RNA is produced during transcription?

Find Word Roots Lookup the word polymer in a dictionary. In a small group, talk about the most likely reason that proteins that produce DNA and RNA from free nucleotides are called polymerases.

4. Summarize What happens during transcription?

READING CHECK

5. Define What is a codon?



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Find the first base of the mRNA codon in this column.

Follow that row to the column that matches the second base of the codon.

Move up or down in that box until you match the third base of the codon with this column.

What Is Translation?Translation is the second stage of gene expression.

This process is shown in the figure on the next page. During translation, the “language” of mRNA (codons) is converted into the “language” of proteins (amino acids). Translation requires mRNA, tRNA, and rRNA. The result of translation is a complete polypeptide, or group of amino acids that forms a protein.

How Are Genes Linked to Traits?Genes contain the information to produce proteins.

Proteins, in turn, produce almost all the traits of an organism. However, the relationship between genes and the traits they affect is complex. Variations, mistakes, and other complex interactions can occur at each stage of DNA replication and gene expression. These interactions can cause the effects of a gene to change.

A particular gene does not necessarily cause only one effect. Some genes are expressed only at certain times or under particular conditions. The protein a gene codes for may have many effects on an organism. In addition, a particular trait is not necessarily the result of only one gene. Some traits result from the expression of several genes.

Several factors determine the final outcome of gene expression. They include the cell’s environment, the presence of other cells, and the timing of gene expression.

6. Identify Which amino acid does the codon ACU code for?

7. Identify Give two examples of a stop codon.

READING CHECK

8. Describe What is the end result of translation?

READING CHECK

9. Explain Does a particular trait result from the expression of a single gene?



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The amino acid chain is released, and the ribosome and tRNA move apart.

5

An anticodon is a group of three nucleotides on a tRNA molecule. The anticodon is complementary to an mRNA codon. A tRNA with a particular anticodon carries only the amino acid that corresponds to the complementary codon.

After the mRNA strand leaves the nucleus, it binds to a ribosome and to a tRNA carrying the amino acid methionine. The anticodon on the tRNA binds to the codon on the mRNA.

1

A

A

AA

UUU

U

U

U

U

G

G

G

CCCC

C

Nuclear envelope

Nuclear pore

tRNA

mRNAAnticodon

Codon

Ribosome

Amino acid methionine

The mRNA codon AUG codes for the amino acid methionine. It is also the “start” codon, which tells the ribosome where to begin translation.

The ribosome moves to the next codon on the mRNA. A tRNA with the complementary anticodon binds to the mRNA codon. The amino acid on this tRNA forms a peptide bond to the chain of amino acids attached to the previous tRNA. As the ribosome moves down the mRNA strand, more and more amino acids are added.

3

A new tRNA arrives and binds to the next codon on the mRNA. A type of bond called a peptide bondforms between the methionine and the second amino acid. The bond between the fi rst tRNA and the methionine is broken, and the fi rst tRNA moves away from the ribosome.

2CC

GA

A

U

U

U

U

G

G

G

G

GG

GUU

CC

A

UU

tRNA

Peptide bond

GUUU

U

U

U

U

UU

AA

G

G

G

CCCC

As one ribosome moves further down the mRNA strand, another ribosome can begin to translate the mRNA again. In this way, many molecules of the same protein can be produced from a single strand of mRNA.

When the ribosome reaches a stop codon on the mRNA strand, no more amino acids are added to the chain.

4 UU

C

G

GU

UG

G

GG

G

CC

C

C

UU

A

A

A

A

A

AUUUAA

mRNA

Stop codon

Newly made polypeptide

Large ribosomal subunit

Small ribosomal subunit

10. Explain The fi rst amino acid in most polypeptides is methionine. What is the reason for this?

11. Describe What causes translation to end?

12. Identify Where is the rRNA in the fi gure?

13. Explain How can the same mRNA strand be used to produce more than one polypeptide at the same time?



Name Class Date

Section 3 ReviewSECTION VOCABULARY

codon in DNA and mRNA, a three-nucleotide sequence that encodes an amino acid or signi-fies a start signal or a stop signal

gene expression the manifestation of the genetic material of an organism in the form of specific traits

RNA ribonucleic acid, a natural polymer that is present in all living cells and that plays a role in protein synthesis

transcription the process of forming a nucleic acid by using another molecule as a template; particularly the process of synthesizing RNA by using one strand of a DNA molecule as a template

translation the portion of protein synthesis that takes place at ribosomes and that uses the codons in mRNA molecules to specify the sequence of amino acids in polypeptide chains

1. Define Write a definition of gene expression in your own words.

2. Describe What role does tRNA play in gene expression?

3. Apply Concepts Fill in the chart below to show how a DNA base sequence is converted into a sequence of amino acids.

TAC ACA CGA GGA GGG TCT AAA ATTDNAcodons

mRNAcodons

tRNAanticodons

Amino acids

4. Identify What are two things that can affect the result of gene expression?

Section 3 Review

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