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DNA - The Double Helix

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Recall that the nucleus is a small spherical, dense body in a cell. It is often called the "control center"because it controls all the activities ofthe cell including cell reproductior¡ and heredity. Chromosomesare microscopig threadlike sfa¡rds composed of the chemical DNA (short for deoryri'bonucleic acid).In simple terms, DNA controls the production of proteins within the cell. These proteins in turn, formthe structural units of cells and control all chemical processes within the cell. Think ofproteins as thelle building blocks for an organism, proteins make up your skin, your hair, parts ofindividual cells.ÉIow you look is lâigely determined by the proteins that are made. The proteins that are made isdetermined by the Sequence of DNA in the nucleus.

Clr.o-mosomes are composed of genes, which is a segrnent of DNA that codes for a particular proæinwhich in tum codes for a trait. Hence you hear it commonly referred to as the gene^for baldneis or thegene for blue eyes. Meanwhilg DNA is the chemical that genes and chromosomes are made of. DNA iscalled a nucleic acid because it was first found in the nucleus. We now know that DNA is also found inorganelles, the mitochrondria and cbloroplasts, though it is the DNA in the nucleus that achrally controlsthe cell's workings.

In 1953, James Watson and Francis Crick established the structure of DNA. The shape of DNA is adouble helix (color tle title bl¿ck), which is like a twisted ladder. The sides of the ladder are made ofalæmating zugar and phosphate molecr¡les. The sugar is deoryribose. Color all the phosphates pin]x @^(one is labeled with a I'pÍ). Color all the deoxyriboses blue (one is labeled n'fth; "D") .

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The rungs of the ladder are pairs of4 types of niÍogen bases. The bases are known by their codedleuers A, G, T, c. These bases always bond in a certain way. Adenine will only bond to thymine.Guanine will only bond with cytosine. This is known as the "Base-Pair Rule". The bases can occur inany order along a strand ofDNA. The order of these bases is the code the cont¿ins the instructions. Forinstance ATGCACATA wor¡ld code for a different gene tha¡r AATTACGGA. A strand of DNA containsmillions of bases. (For simplicity, the image only cont¿ins a few.) -

Color the thymin", or-g". CColor the adenine* gron. l- JColorthe guanines purpl€. )-]color the cytosines yeliow F

Note that that the bases att¿ch to the sides ofthe ladder ¿t the sugars and not the phosphate.

The DNA helix is acürally made of repeating units called nucleotides. E¿ch nucleotide consists of threemolecules: a sugar (deoxyribose), a phosphate which links the sugars together, and then one of the four!3ses. Tw9 of the bases are purines - adenine and guanine. The pyrimidines are thymine and cytosine.Noûe that the pyrimidines are single ringed and the purines are double ringed. Color fhe nucleotidesusing the same colors as you colored them in fhe double helix.

The two sides of the DNA ladder are held togefher loosely by hydrogen bonds. The DNA can acarally"unzip" when it needs to replicate - or make a copy of itself. DNA needs to copy itself when a celldivides, so that the new cells each contain a copy of tle DNA. Without these instuctions, the new cellswouldnt have the correct information. The hydrogen bonds are represented by smalt circles- Color thehydrogen bonds grey. (Lcu- ,-lr,iþ-l

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l. lVrite out the fifl name for DNA.

2. I{hat is a gene?

3. Whe¡e in ihe cell are chromosomes located?

4. DNA can be found in what two organelles?

5. What two scientists established the structure of DNA?

6. What is the 5h¡Fe of DNA?

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Illessenger RNA

sq now, we know tle nucleus controls the cell's activities through the chemical DNd but how? It isthe sequence of bases that dete¡rniî" *hi;i;t;bir i- iä;" -¿". The sequence is like a code that wecan now interpret The sequence ¿eterminei wtr¡cn froæìos aternade and the proteins determine whichac-tivities witt be performed. And that i; À;; rn"î,rãi",i, i.,re contror *"r*;¡til;il-. The onlyproblem is that the DNA is too big ro g" thtrgh flr;-;;;i"9q por".. so a chemicar is used ro to read rheDNA in the nucleus. Thar chemiðd iJ....-f". ñï. '-Th" ,"r.*g", RNA ('RNA) is smailenough to go through the nucrear pores. It økJs thã,;.lrr.g"" of the DNA to the ribosomes and ,,telrs

them" what proteins are to be mad'e n "¿i

tn"i prit"¡*ä o" uoayt uuilding blocks. rmagine that thecode taken to the ribosomes is tering the ribosori" *luiil *"¡"¿ - like a recipe.

Messenger RNA is simirar to DNA' except that it is a singre strand, and it has no thymine. lnstead ofthvmins 'RNA

contains the base-ûracir. n "¿Jri"i

i"*tiæ ditre;ånce, -;ñÃîär'tiîåîe*

¡ror"*fff to*.nbose. RNA sr¿nd. ro. n¡roou"r"i" Ä;rd. coro, ae-näñÀ å. väîää rn" oNcColor the ribose a DARKER BLIIE, and the uracil brown. GJThe Blueprint of LifeEvery cell in vour bodv has the.same "blueprint" or the same DNA. Like the blueprints of a house tellthe builders how ro consfiucr ¿ horte rh" öñe -bfulp-.ü;

tefls rhe cefl h.* toï;ii'J rh; organism.Yeq how can a hea¡t be so different from a braii if ãiítrt" ""11.

*-ø;rË;;;;äons? Alrhoughmuch work remains in geneticg it has becãme app-""r ti"t " ""lr

r,as tne auirity ìã-n i' offmost genesand onrv work with the genes nece-ssary to do a job. w.il;oo;;;"i;^,ïäffirilii-"otry i.nonsense and codes for nothing_ rhesgregions äfnxÀtnat ao not code for proteins a¡e ca'ed

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8. What a¡e the "rungs" of the DNA ladder made of?

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9. What sugar is found in DNA?

10. F{ow do the bases bond together? A bonds with

1 1. The two purines in DNA are

In RNA?

G bonds with

12. DNA is made of repeating units called

Iry15' How do some cells become biain cells and others become skin cellg when the DNA in ALL the cellsis exacdy the same. In othe¡ words, if the instructions are exactly the samq how does tne cett become abrain cell and another a skin cell?

16. Why is DNA called the "Blueprint of Life'?

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Color the images according to the instructions.

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Skills Worksheet

Active Rea

Section: Replication of DNARead the passage below, Then answerthe questions that follow.

The process of making a copy of DNA is called DNA replication.The process oan be broken down into three st€ps.

Step 1: Before replication can begin, the double helix mustunwind. This is accomplished by enzymqi called DNA helicases,which open up the double helix by breaking the hydrogen bondsttrat link the complementary nitrogenous bases. Once the twosûands ofDNA are separated additional proteins atÞch to eachstrand, holding them apart and preventing them from twisting backinto their double-helical shape. The two areas on either end of tleDNA where the double helix separates are called replic¿tion forksbecause of their Y shape.

Step 2: At the replication fork, enzymes known as DNApolymerases move along each of the DNA stands, addingnucleotides to the exposed nitrogørous bases acoording to thebase-pairing rules.

Step 3: The process of DNA replication produces two DNAmolecules, each composed ofa new and a¡r original stand. Thenucleotide sequences in both ofthese DNA molecules are identicalto each other and to the original DNA molecule.

SKILL: READING EFFECTVELY

Read each questíon, and write your enswer in the space prov¡ded.

l. What is replication?

2. When does replic¿tion occur?

3. What must occur before replication can begin?

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Holt Biology DNA. RNA. and Prob¡ns

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SKILL: INTERPRETING GRAPHICS

4. The figure below shows DNA rçlicating. In the space provided, desc¡ibewhat is occurring at each lettered section of the figure.

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_5. DNA helic¿ses and DNA polymerases are alike in that both are¿. nucleotides.b. nitrogenous bases.

c. enzymes.d. Both (a) and (b)

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Holt B¡ology 10 DNA, RNA, and prob¡ns

Skills Worksheet

Science Skills

lnterpreting lnfomationUse the figure below to answer questions l-3,

Read each question, and write your answer in the space prov¡ded.

L In the space provided, identifu the structures labeled A-E.

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2. Wlnt do the lines connecting úre two sEands represent? Why are there threelines connecting the strands in some instances and only two lines in others?

3. Suppose that a stand of DNA has the base sequence ATT{CG. What is thebase sequence of the complementary sûand?

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Holt Biology 14 DNA RNÀ and Prob¡ns

DNA Replication

Q Proteins called hellcasesseparate the two originalDNA slrands.

O Complêmentary nucleotidesare added to each strandby DNA polymerases.

@ Two DNA molecutes areformed that are identicâl tothê origlnal DNA molecule.

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