CHAPTER 13DNA Structure and Function

DISCOVERING DNA Methods of

inheritance uncertain Suspected that it was

due to proteins (great variety)

Some evidence of DNA as passing on genes in viruses

1953—James Watson, Francis Crick Described DNA model

for inheritance

BUILDING DNA Deoxyribonucleic acid (DNA) Nucleotide—monomer unit of nucleic

acid5-carbon sugar (ribose or deoxyribose)Nitrogen basePhosphate group

NUCLEOTIDES Adenine Guanine Thymine Cytosine

NUCLEOTIDES ALWAYS match with

specific base pairingA—T, T—A G—C, C—G

Each chain bonded covalently

Base pairs bonded with hydrogen bonds

DNA STRUCTURE Polarity of molecules determine shape

Double helix

DNA REPLICATION Interphase—S phase Makes one copy, doubles amount of

DNA Semiconservative replication—in each

copy, one strand of the helix is carried over from the original strand

DNA REPLICATION--STEPS Step 1—helicase

attaches to DNA Step 2—helicase

“unzips” hydrogen bonds between base pairsThis causes double

helix to “unwind”

DNA REPLICATION--STEPS Step 3—DNA polymerase forms new,

complementary strand of DNA from free nucleotidesMoves along entire length of DNA

DNA REPLICATION--STEPS Step 3b—DNA

ligase binds short stretches of new DNA on “lagging” strandOkazaki

fragments—short sequences of DNA on lagging strand

DNA REPLICATION--STEPS Step 4—helicase separates

from DNATwo new strands now formedSister chromatids!!!

ANIMATIONS! Brief Replication Overview Brief DNA Replication Detailed DNA Replication

DNA PROOFREADING Rare, but sometimes mismatched base

pairs DNA polymerase usually catches these

& corrects If can’t correct mismatch, protective

mechanisms stop cell division (Ch. 9, slide 19)

Repair enzymes can repair some damageCan “snip” out damage or mismatch

CLONING

CLONING

CLONING Isolate DNA from one organism Create another organism from DNA Second organism genetically identical to

original

CLONING Embryo cloning

Fertilized egg separates early in divisionNatural = identical twins

CLONING Adult cloning

1—Start with somatic cell of organism2—Remove nucleus of the cell3—Take mature egg from female4—Remove nucleus from egg 5—Implant donor nucleus into egg (fused by

shock)6—New cell begins to divide to early

embryo7—Implant embryo into surrogate mother8—If goes to full term, clone is born

CLONING

CLONING Theraputic cloning

Somatic cell DNA into stem cellStem cell is undifferentiated, can turn into

almost any tissuePotentially regenerate organs

CLONING Applications

LivestockPetsHumans???Organs

CLONING Risks

Egg doesn’t always accept new DNANew DNA isn’t always used properlyEthical issues