14 FROM DNA TO PROTEIN

INTRODUCTION

This chapter details two processes: transcription, to make RNA copies of D , and translation, to convert the information in the sequence of a nucleic acid into the amino acid sequence of a protein.

FOCAL POINTS

• Figure 14.3 [p.217] compares DNA to RNA. • Figures 14.4,14.5, and 14.6 [pp.218-219] outline the steps of transcription.

• Figure 14.7 [p.220] details the modification of mRNA before it leaves the nucleus.

• Figures 14.8 and 14.9 [pp.220-221] explain the mRNA genetic code.

• Figure 14.12 [p.222] outlines the steps of translation.

INTERACTIVE EXERCISES

Impacts, Issues: Ricin and Your Ribosomes [p.214]

14.1. DNA, RNA, AND GENE EXPRESSION [pp.216-217]

Selected Words: genetic information [p.216], uracil [p.216], "genetic words" [p.216], ribosomes [p.216]' amino acids [p.2l6 ], polypeptide chain [p.2l7]

Boldfaced, Page-Referenced Terms

[p.216 ] transcription___ __~-______ ______- _____ _ ___ ___ _

[p.2161 ribosomal RNA (rRNA) ______ ____________~__________

[p.216] transfer RNA ( tRNA) __________ ___ ___

[p.216] messenger RNA (mRNA) ___ _ _ _ ___ ______ _ _ _ _______ _ ___

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I

[p.2l T] translation _____________ - --___ _____ _ ___________

[p.2l? ] gene expression _____ _______ ________ _______________ _

Fill-in-the-Blanks [pp.2l6-2l ? ]

The (1 ) offour bases contains the genetic information in DNA. The two steps from genes to

proteins are called (2) _ ____ and (3) ._ _ __. In (4) _ ____, single-stranded molecules of RNA are

assembled on DNA templates in the nucleus. In (5) _ _ _ __, th RNA molecules are shipped from the nucleus

into the cytoplasm, wh re they are used as templates for assembling the sequence of (6) _____ of a protein. (7)

_____is the multistep process by which genetic information encoded by a gene is converted into a structural or

functional part of a c II or body. Proteins - (8) ___ __- assemble lipids and complex carbohydrates from

simple building blocks, replicate DNA, and make RNA.

Complete the Table lp.2 161

\). Three types of RN are transcribed from DNA in the nucleus (two are from genes that code only for RNA). Complete the following table, which summarizes information about these molecules .

RNA Mofecule Abbreviation Descriptiol1/Fu Ilclion

a. Ribosomal RNA

b. Messenger RNA

c. Transfer RNA

From DNA lO Prlltein 149

14.2. TRANSCRIPTION: DNA TO RNA [pp.218-219]

Selected Words: complementary [p.2l8], 5' to 3' direction [p.2l9]

Boldfaced, Page-Refererlced Terms

[p.2l8 ] RNA polymerase ______ _______ _ ________________

[p.219] promoter _________________________________

Short Answer [p.218 J

1. List three ways in which a molecule of RNA differs structurally from a molecule of DNA.

2. Cite two similarities between DNA replication and transcription.

3. What are the three key ways in which transcription differs from DNA replication?

14.3. RNA AND THE GENETIC CODE [pp.220-221J

Selected Words: protein-building message [p.22l], post-transcriptional modifications [p.220], "cap" rp.220], poly­A tail [p.220], start codon (p.220 I) stop codons [p.220], peptide bonds [p.22l], polypeptide chain [p.221 ]

Boldfaced, Page-Referenced Terms

[p.220] introns _ __~_~_~ _ _ ___________ _ _ __________

[p.220] exons _____~_ ___ ________ ________________

150 Chapter Fourteen

- ------------- ----------------- - ---------------

[p.22D] codon _ _ _ ______ ______________ _ __________

[p .220] genetic code

[p.221] anticodon ______ _

Short Answer [p.2201

l. In eukaryo tes, what modifications to "pre-mRNA" need to be made before it can be used in translation?

Completion [pp.22D-2211

2. Given the following DNA sequence, deduce the composition of the mRNA transcript.

TAC AAG ATA ACA TTA TTT CCT Ace GTC ATC

(mRNA transcript)

3. From the mRNA transcript in question 2, lIse Figure 14.9 of the text to deduce the composition of the amino acids of the polypeptide sequence.

(amino acids)

From DNA to Protein 151

Labeling and Matching [Figure 14.7, p.220]

Newly transcribed mRNA contains more genetic information than is necessary to code for a chain of amino acids. Before the mRNA leaves the nucleus for its ribosome destination, an editing process occurs as certain portions of nonessential information are snipped out. Identify each indicated part of the following illustration ; use abbreviations for the nucleic acids. Complete the exercise by entering the letter of the description in the parentheses following each label.

4

II transcnption Into pre-mRllA

5--_____\ ~ ;-7 6-+ J ) \ > J +---8

5 l.~"' ~ .,';;;:;;::" 3"

() I ) )

3· ~----"Y~--~

9

4. A. An actual coding portion of pre-mRNA

B. A noncoding portion of the newly trans ribed 5. mRNA

6. C. Mature mRNA transcript

D. Poly-A tail 7.

E. The region of the DNA template strand to be copied8.

F. Cap on the 5' end ofmRNA (the first 9. synthesized)

14.4. TRANSLATION: RNA TO PROTEIN [pp.222-223]

Selected Words: small ribosomal subunit [p.222], large ribosomal subunit [p.222j, initiation complex [p.222]'

methionine [p.222], release factors [p.222j, polysomes [p.222], biosynthetic process [p.222,l, GTP [p.222]

Boldfaced, Page-Referenced Terms

[p.222] initiation _ ___________----­

[p.223] termination _____

152 Chapter Fourteen

Fill-in-the-Blanks [p.222]

(1) occurs in the cytoplasm of all cells and consists of three stages: (2) ____, (3)

____, and (4) ____. A small ribosomal subunit binds to (5) ____ RNA and a special initiator

(6l _____ RNA base-pairs with the first (7) ____ codon. In (8) ____, the ribosome moves

along the mRNA and builds a (9) ____ chain. During the last stage of translation, (10) ____"a STOP

codon in the mR A moves onto the platfo rm , and no t A has a corresponding anticodon. Now proteins called

(11) ____ factors bind to the ribosome. They trigger (12) ____ activity that detaches the mRNA and

the polypeptide ch ':! l11 from the ribosome. When many ribosomes simultaneousl}' translate the same mRNA the}' are

called ( 13) ____. The energy required for translation is mainly in the form of ( 14) ____ transfers

from the RNA nucleotide (15) ____

14.5. MUTATED GENES AND THEIR PROTEIN PRODUCTS [pp.224-225]

Selected Words: mutations lp.224], reading frame [p.224], Barbara t..kClintock [p.22S], harmful environmental agents [p.22S], ionizing radiation [p.22S], free radicals [p.22S], nonion izing radiation [p.22S], UV light [p.22S], thymine dimers [p.2lS ]

Boldfaced, Page-Referenced Terms

[p.224 ] base-pair substitution_________________ _ ___________ _

[p.224] deletion _________ ________________~_______

[p.224) insertion

[p.224] transposable elements _ __________________ _____ ___ ____

From DN to Protein 153

Short Answer [p .224j

Use Figure 14.13 to answer the following questions.

1. Which is usually the more damaging mutation: base substitution or deletion? Why?

2. Would a base substitution mutation in the third position of a codon be more or less likely to cause a serious mutation? Explain. ____________________________________

3. Would an insertion or deletion mutation be more damaging at the beginning of a m R A strand or at the end? Why?

Choice [pp.224-225]

Choose the answer that best fits. Some answers will be used more than once.

4. sickle-cell anemia A. Insertion

5. x-rays B. Deletion

6. LTV radiation C. Base-pair substitution

7. transposable elements D. Ionizing radiation

8. thymine dimer E. Nonionizing radiation

9. free radicals

10. Huntington's disease

Labeling and Matching [p.226]

A summary of the flow of genetic information in protein synthesis is useful as an overview. Identify the numbered parts of the accompanying illustration by filling in the blanks with the names of the app ropriate structures or functions. Choose from the following:

DNA, mRNA, rRNA, tRNA, amino acids, ant icodon, intron, exon, polypeptide,

ribosomal subunits, transcription, translation

Complete the exercise by matching and entering the letter of the d scrip tion in the p arenth ses following each label.

154 Chapter Fourteen

Pr~·IT1RNA

tran, Ip pI Uln!! -

15 17

I

20 ; 19

fOI use in cell or for export

II. ( )

12. ( ) (process)

13.

14.

15.

16.

i 7.

18.

19.

20.

21. ( ) (p rocess)

22. ( )

A. Coding portion of pre-mRNA that will translate into proteins

B. Carries a form of the genetic code from DNA in the nucle us to the cytoplasm

C. Transports amino acids to the ribosome and mRNA

D. The building blocks of polypeptides

E. Noncoding portion of pre-mRNA

F. Combines with proteins to form the riboso ma l subunits

G. loin during the initiation step of protein synthesis

H. Holds the genetic code for protein production

1. Amino acids are joined together

J. RNA synthesized on a DNA template

K. A sequence of three bases that can pair with a specific mRNA codon

L. May serve as a functional protein (enzyme) or structural pr tei n

From DNA to Protein 155

SELF-TEST

I. Transcription . [p .2I6] a. occurs on the surface of a ribosome b. is the final process in the assembly

o f p rotein ON tem plat c. oc urs d uring the sYD,thesis of any

type of RNA by use of a DNA template d. is catalyzed by DNA polymerase

2. carries the actual instructions fo r a protein's sequence to the ribosome. [p.2I6] a. DNA b. m RNA c. rRNA d. tRNA

3. carries amino acids to ribosomes, where amino acids are linked into the primary structure of a polypeptide. [p.2I6] a. mRNA b. tR A c. An intron d. rRN A

4. Transfer RNA differs from other types of RNA because it . [p.221] a. transfers genetic instructions from

cell nucleus to cytoplasm b. specifies the amino acid sequence

of a particular protein c. carries an amino acid at one end d. contains codons

5. catalyzes the process of transcription. [p.2IS ] a. RNA polymerase b. DNA polymerase c. P henylketonuria d. T ransfer RNA

6. is found in RNA but not in DNA. [p.2I6] a. Deoxyribose b. Uracil c. Phosphate d . Thymine

7. Each "word" in the mRNA language consists of letters. [p.220] a. three b. four c. nve d . more than five

S. The genetic code is used to align ____ amino acids. [p.22I] a. four b. sixteen c. nventy d . sixty-four

9. The genetic code is composed of ____ codons. [p.220] a. three b. twenty c. sixteen d. sixty-four

10. The cause of sickle-cell anemia has been traced to . [p.224] a. a mosquito-transmitted virus b. extra bases inserted into DNA c. ionizing radiation d. a DNA mutation in a hemoglobin

chain

CHAPTER OBJECTIVES/REVIEW QUESTIONS

1. State how RNA differs fro m D N in structure and function, and indicate what features RNA has in common with DNA. [p.216]

2. RNA combines with certain proteins to form the ri bosome; RNA carries genetic information for protein construction from the nucleus to the cytoplasm; RNA p icks up specific amino acid ' and moves them to the area of mRNA and the ribosome. Identify which type of R IA d es each of these jobs. [p.2I6]

3. Describe the process of transcription, and indicate three ways in which it d iffers from repli cation lp .218] 4. Transcription starts at a(n) ,a speci fi c sequence of bases o n De fthe two DNA stran that signals

the start of a gene. [p.219] 5. What RNA code would be formed from the following DNA code: TAC- CTC-GTI'-CC - AA? [p.2 9 ] 6. Describe how the three types of RNA participate in the process of t ranslation. [p.222 ] 7. Distinguish introns from exons. [p.220]

156 Chapter Fourteen