Chapter 1 • Modern Genetics for All Students S 33

CHAPTER 1

DNA: The Hereditary Molecule

SECTION D

What DoesDNA Do?

Chapter 1 • Modern Genetics for All Students S 34

DNA Codes For ProteinsPROTEINS DO THE nitty-gritty jobs of every living cell. Proteins are the molecules thatgive structure and shape to living cells and that carry out all of the chemical reactions nec-essary for life. The importance of DNA is that it contains the information that is used tomake all of the proteins on which life depends.

The proteins whose structures are specified by our DNA do more than carry out all of thereactions necessary to keep our cells alive; they also digest our food for us, influence howwe will respond to infections, and determine what color our eyes, hair and skin will be.

Proteins are made of long strings of individual building blocks known as amino acids.Amino acids come in 20 different kinds that are all slightly different from one anotherchemically. It is not important for us to understand the details of the chemical differencesthat distinguish these 20 kinds of amino acids. But it is important to realize that because ofthese differences, the structure and function of every protein depends on the sequence inwhich the various kinds of amino acids are strung together. A protein may contain manyhundred amino acids. But if just one of these amino acid is changed, the function of theprotein might change drastically.

A gene, which is a functional unit of DNA, carries coded information indicating the pre-cise sequence in which amino acids should be strung together to make one particular kindof protein that will play one particular role in the life of the cell.

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The elongated shape of collagenprotein, shown in this molecular

model, allows collagen to providestructural support to cells and

organs.

Antibodies, such as this humanimmunoglobulin, recognize and

attack viruses, bacteria, andother foreign substances.

Chapter 1 • Modern Genetics for All Students S 35

There are several different categories of proteins. Most proteins function as enzymes toregulate the speed of particular chemical reactions. Every cell contains hundreds of differ-ent enzymes that work together to release energy from food molecules and to use energy tobuild new cellular materials. All enzymes are proteins. But not all proteins are enzymes.Structural proteins, as the name implies, provide structural support inside and outsidecells and for the body as a whole. Defense proteins are an assortment of proteins in theblood that recognize and fight off foreign invaders, such as bacteria and viruses.Transport proteins are used to carry various molecules into and out of cells as well asthrough the body. The table on the next page gives a few examples of these and otherkinds of proteins.

Every aspect of life depends on proteins. In the next set of activities, you will learn aboutthe processes a cell uses to convert the coded information in a bit of DNA (a gene) into aparticular kind of protein.

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Chapter 1 • Modern Genetics for All Students S 36

A FEW HUMAN PROTEINS AND THEIR FUNCTIONS

Protein Type What it doeshemoglobin a red, iron-containing, • makes your blood red

transport protein • picks up oxygen from the air in your lungs and releases it to cells elsewhere in your body

• if you don’t make enough hemoglobin, or you make a type that doesn’t carry oxygen well, you will be anemic

insulin a hormone • tells your cells when to remove sugar from your blood after a meal

• if you don’t make enough insulin, or if your cells do not respond to it well, the concentration of sugar in your blood will get too high, and you will have diabetes

antibody a defense protein • fights infections from bacteria and viruses• vaccination triggers production of antibodies

that will fight a particular kind of infection• if you couldn’t make any antibodies, you

would have been dead long ago!

lactase an enzyme in your digestive tract • digests lactose (milk sugar) in milk and dairy products into simpler sugars

• if you don’t make enough lactase, you get diarrhea when you drink milk or eat ice cream or other diary products

• but you can buy lactase pills to swallow before eating dairy products

collagen a structural protein • provides the tough structural framework of your skin, bones, tendons, and cartilage

keratin a structural protein • provides the resistant outer layer of your skin and the tough structural material of your hair and nails

myosin a structural protein • uses energy derived from food — and works that is also an enzyme together with actin — to cause muscles to

contract• you can increase the myosin content of your

muscles with exercise

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Chapter 1 • Modern Genetics for All Students S 37

How DNA Codes For Proteins

Reprinted with permission from Gonick, L. and Wheelis, M. (1991). The Cartoon Guide to Genetics. New York: Harper Collins Publishers

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Chapter 1 • Modern Genetics for All Students S 46

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QUESTIONS ON “HOW DNA CODES FOR PROTEINS”

AFTER YOU HAVE read about protein synthesis in your excerpt from the Cartoon Guideto Genetics, answer the following questions.

1. What is the relationship between genes and proteins? ___________________________

______________________________________________________________________

2. How does RNA differ from DNA? __________________________________________

______________________________________________________________________

3. What is the molecule that carries the information from a gene to the place

where a protein will be made? _____________________________________________

4. What is the process by which such a molecule is made? _________________________

5. What is the enzyme that mediates the process named above? _____________________

6. What is the structure on which proteins are made? _____________________________

7. How many bases form one “word” of the RNA message? _______________________

8. What is the technical name for such a group of bases found on mRNA? ____________

9. What is another term for protein synthesis? ___________________________________

10. What is the group of molecules that translates the genetic code? __________________

11. What is an anticodon? ___________________________________________________

12. At the tail end of each tRNA molecule, an ________________ attaches the appropriate

____________________________ molecule to the tRNA.

13. What happens when two tRNAs are side by side on a ribosome? __________________

______________________________________________________________________

14. The first codon on an mRNA always is ______________________________________.

15. This codes for the amino acid called ________________________________________.

Name __________________________________________________

Date ____________________________ Hour ________________

Chapter 1 • Modern Genetics for All Students S 47

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Chapter 1 • Modern Genetics for All Students S 48

The Gene Expression DanceAS YOU HAVE READ in your excerpt from the Cartoon Guide to Genetics, three kindsof RNA molecules cooperate to convert the coded information in DNA (a gene) into a pro-tein with a particular sequence of amino acids.

The process in which the nucleotide sequence of a gene (DNA) is used to specify theamino acid sequence of a protein is called gene expression, and it consists of two majorphases: 1) transcription, in which a messenger RNA (mRNA) molecule that is comple-mentary in nucleotide sequence to the gene is synthesized, and 2) translation, in whichthe message carried by that mRNA molecule is used to synthesize the corresponding pro-tein. In this exercise, your class will perform a simulation of these two processes.

MATERIALS

Your teacher will hand out the gene-expression flash cards that you will use in this dance.

PROCEDURE

Your teacher will instruct you how to perform the dance.

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DNA proteincodes

mRNA

GG

G

DNA4

G T T

Methionine

C A U

Isoleucine

G A U

DNA2

GT

G

Glutamine

U U GmRNA

U A A

Histidine

G U G

mRNA

C U A

DNA7

TT

A

Chapter 1 • Modern Genetics for All Students S 49

Paper Proteins: Models forSimulating Gene ExpressionMATERIALS

For each student or pair of students:1 set of paper-protein puzzle pieces

PROCEDURE

Follow the directions below to modelthe processes of transcription and trans-lation, and to make a paper protein.

1. Place the DNA strip on the desk sothat the letters read properly for you(fig. 1).

2. Working from left to right, find themRNA pieces that match the DNAand line them up (fig. 2). What isthe process in which an mRNA mol-ecule that is complementary to aDNA molecule is produced?

____________________________(Answer question 2 here)

3. Separate the mRNA from the DNAto simulate the mRNA moving outof the nucleus to a ribosome in thecytoplasm of the cell. Leave themRNA pieces lined up next to oneanother(fig. 3).

4. Match each of the tRNA pieces tothe amino acid piece that fits with it(fig. 4). Lay them out so that all ofthem are visible.

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U

A

C

MethionineAminoAcid

tRNA

U

A G

C C C

A G G

C

A A A A G

U U UmRNA

CCT

AAA

CTT

GGG

TT

G

C

A

TDNA

mRNA

CCT

AAA

CTT

GGG

TT

G

C

A

TDNA

Figure 1

Figure 2

Figure 3

Figure 4

Name __________________________________________________

Date ____________________________ Hour ________________

5. Search for the tRNA that will basepair with the first codon of themRNA (the one on the left-handend). Move the tRNA with itsattached amino acid into place in themRNA (fig. 5). Continue with thesecond codon, and so forth. What isthe name for this process in which aprotein is produced that has anamino acid sequence specified by anmRNA molecule?

____________________________(Answer question 5 here)

6. Now pull the string of six aminoacids away from the tRNA (fig. 6).These six amino acids represent anew protein. (However, real proteinsalways contain many more aminoacids than this, sometimes morethan a thousand.)

After you have completed these steps, use the model pieces to explain the two componentprocesses of gene expression to another student. Then write out the steps in your ownwords. You may refer to your notes or a book to check for scientific accuracy.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

Name __________________________________________________

Date ____________________________ Hour ________________

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Methionine

U

A

C

Methionine

U

A G

C C C

A G G

C

A A A A G

U U UmRNA

AminoAcid

tRNA

Figure 6

Figure 5

AminoAcid

Using the Genetic Code toTranslate an mRNAAT THE HEART OF the reg-ular and predictable relation-ship between the sequence ofnucleotides in any gene andthe sequence of amino acidspresent in the protein forwhich that gene is said to“code” is a fixed set ofnucleotide-to-amino acidrelationships that is known asthe genetic code. Just as theMorse code can be printed inthe form of a table indicatingwhich letter of the Englishalphabet is specified by eachcombination of dots anddashes, the genetic code isusually printed in the form ofa table indicating what kindof amino acid is specified byeach possible mRNA codon.

To the right is a DNA codingsequence that codes for partof the hemoglobin molecule.Complete the mRNA strandfollowing base-pairing rules.Then use the mRNA GeneticCode Table on the next pageto determine the amino acidsequence for which this pieceof a gene codes.

Name __________________________________________________

Date ____________________________ Hour ________________

Chapter 1 • Modern Genetics for All Students S 51

DNA mRNA Amino AcidTACCAGGTAAACTGTGGGCTTCTCTTTAGTCGCCAT

Congratulations! You have just “synthesized” the first part (12amino acids) of the protein called ß-globin, which is part of thehemoglobin molecules that make your blood red and carryoxygen to cells throughout your body. (Each real ß-globin mol-ecule actually consists of a string of 147 amino acids.)

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Chapter 1 • Modern Genetics for All Students S 52

Amino AcidAbbreviations

ala = alaninearg = arginineasn = asparagineasp = aspartic acid

cys = cysteine

gln = glutamineglu = glutamic acidgly = glycine

his = histidine

ile = isoleucine

leu = leucinelys = lysine

met = methionine

phe = phenylalaninepro = proline

ser = serine

thr = threoninetrp = tryptophantyr = tyrosine

val = valine

mRNA GENETIC CODE TABLE

2nd baseU C A G

U UUU = phe UCU = ser UAU = tyr UGU = cys U

UUC = phe UCC = ser UAC = tyr UGC = cys C

UUA = leu UCA = ser UAA = stop UGA = stop A

UUG = leu UCG = ser UAG = stop UGG = trp G

C CUU = leu CCU = pro CAU = his CGU = arg U

CUC = leu CCC = pro CAC = his CGC = arg C

CUA = leu CCA = pro CAA = gln CGA = arg A

CUG = leu CCG = pro CAG = gln CGG = arg G

A AUU = ile ACU = thr AAU = asn AGU = ser U

AUC = ile ACC = thr AAC = asn AGC = ser C

AUA = ile ACA = thr AAA = lys AGA = arg A

AUG = met ACG = thr AAG = lys AGG = arg G

G GUU = val GCU = ala GAU = asp GGU = gly U

GUC = val GCC = ala GAC = asp GGC = gly C

GUA = val GCA = ala GAA = glu GGA = gly A

GUG = val GCG = ala GAG = glu GGG = gly G

1st b

ase

3rd

base

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