© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 1

The Search for Genetic Material Early geneticists didn’t know what the

genetic material was, what carried genes The genetic material needed to:

Contain information Be able to be copied easily Be variable to account for diversity

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 2

Was it DNA or Protein?

Geneticists thought that Chromosomes might carry the genes (from mitosis)Composed of DNA and protein

Which is the genetic material? Protein is large, complex and stores info DNA seemed too small and simple

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 3

Griffith’s Experiment Two strains of bacteria

R – harmless (1) S – deadly (2)

Heat killed Strain S is also harmless (3)

Something from heat killed Strain S makes Strain R deadly (4)

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 4

Avery, MacLeod, and McCarty

Isolated compounds from Strain S Added these to Strain R DNA transformed Strain R into Strain S Proved DNA was the genetic material

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 5

DNA, Genes, & Chromosomes

Inside a cell, we have chromosomes Each chromosome is made up of one

molecule of DNA and many proteins Along the DNA are regions that code for

information (genes) Animation of this relationship

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 6

Watson & Crick

Determined the 3-D structure of DNA Structure revealed its function

X-ray crystallographic studies showed thatDouble helixLadder twisted into a spiral coil

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 7

The Double Helix Two long strands of

nucleotides Sugar-phosphate

backbone Base-pairing holds the

strands together

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 8

Base-Pairing Rules Strands held together by base-pairing

Hydrogen bonds between bases Strict base-pairing rules:

A binds to T G binds to C

Makes copying the sequence possible

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 9

Fulfills Three Requirements Contains info of the genetic material

Sequence of bases stores this info Easily copied

Each strand is a template for the other Accounts for diversity

Alleles have different DNA sequences

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 10

Individuals have slight differences in sequence

Different species have greater differences

Variation & Diversity

Person 1

Person 2

Chicken

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 11

DNA Replication DNA Synthesis during S-

phase of Interphase H-bonds between 2

strands are broken Two Strands separate Each strand is a template

for the other

DNA Synthesis & Replication:

• DNA “unzips”

• Two opposite strands separate

• Matching bases are added to each side

• Result: Two copies of the original DNA

molecule

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 13

Many enzymes and proteins are involved Initiate replication Unwind the DNA -helicase Stabilize the open strands Add & connect bases to form backbone (DNA

polymerases) Enzymes also needed to correct mistakes Mistakes result in mutation

DNA Replication

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 14

Two Types of Mutations

Base ChangesA change in the sequence of nucleotide basesMost are silent (do not change a protein)

Chromosomal Abnormalities (macro-mutations)Large changes in whole regions of DNAMost are lethal

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 15

Base Mutations Incorrect base-pairings that are not fixed

Mutagens increase the frequency

Next replication includes these changes Source of new alleles (new variation)

A few are beneficial (make a better protein)Most are silent (make no change in a

protein)Some are harmful (most that change a

protein)

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 16

Mutations

Ask for examples of mutant human genes Bad mutations include ->

sickle cell anemia, hemophilia, color blindness.

Can you find a mutation in this picture?

Mutations

There at least two mutations -> • blue eyes, white skin •(occurred long ago).

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 18

DNA Repair

DNA polymerases often

insert incorrect bases DNA mismatch

Proofreading enzymes

correct mistakes

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DNA Repair

Enzymes recognize damaged DNA

Cut the backbone on either side

Damaged DNA is removed

DNA is corrected

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 20

Inherited Repair Disorders

Xeroderma pigmentosum

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 21

The Griffith experiment showed that

A. Something heritable was being passed between the bacterial strains.

B. DNA was the genetic material.

C. Protein was the genetic material.

D. Strain S was lethal to mice.

Concept Quiz

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 22

Watson and Crick showed that

A. A binds to T and G binds to C.

B. One strand is a template for the other strand.

C. The DNA molecule can be easily replicated.

D. All of the above.

Concept Quiz

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 23

DNA repair isA. Only important during replication.

B. Found in some species.

C. Vital to maintaining DNA’s integrity.

D. An inherited disorder.

Concept Quiz

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 24

Process Animation 12.1aDNA Repair: Mutations from DNA Replication

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Process Animation 12.1bDNA Repair: Mutations from Sunlight

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