Unit Two: Inheritance

Chapter 9DNA: The Molecule of Heredity

• 9.1 How did scientists discover that genes are made of DNA?

• 9.2 What is the structure of DNA?

• 9.3 How does DNA encode information?

• 9.3 How does DNA replication ensure genetic constancy during cell division?

9.1 How Did Scientists Discover That Genes Are Made of DNA?

• Transformed Bacteria Revealed the Link Between Genes and DNA

• In the late 1920’s, a British researcher name Frederick Griffith was trying to make a pneumonia vaccine

• (Bacteria vaccines are a bit tricky)

LivingR strain

LivingS strain

Heat-killedS strain

Living R strain,heat-killedS strain

Bacterial strain(s) injected into mouse Results Conclusions

R strain doesnot cause pneumonia.

S strain causes pneumonia.

Heat-killed Sstrain does not cause pneumonia.

A substance from heat-killed S straincan transform the harmless R straininto a deadlyS strain.

Genes Are Made of DNA

• Later findings by Avery, MacLeod, and McCarty (1940s)– The transforming molecule from the S strain was

DNA

Figure E9-1a Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

DNA

tail

head

Figure E9-1b Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

6 Bacterial walldestroyed; phagereleased.

bacterialchromosome

phage phagechromosome

bacterium

1 Phage attachesto bacterium.

2 Phage injects itschromosome intobacterium.

3 Phage chromosomeis replicated.

4 Phage partssynthesized, usingbacterial metabolism.

5 Completephages assembled.

Figure E9-2 (part 1) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Observations:1. Bacteriophage viruses consist of only DNA and protein.2. Bacteriophage inject their genetic material into bacteria, forcing the bacteria to synthesize more phages.3. The outer coat of bacteriophages stays outside of the bacteria.4. DNA contains phosphorus but not sulfur. a. DNA can be “labeled” with radioactive phosphorus.5. Protein contains sulfur but not phosphorus. a. Protein can be “labeled” with radioactive sulfur.

Hypothesis: Radioactive phosphorus (P32)

Question: Is DNA or protein the genetic material of bacteriophages?

Prediction:1. If bacteria are infected with bacteriophages containing radioactively labeled DNA, the bacteria will beradioactive.2. If bacteria are infected with bacteriophages containing radioactively labeled protein, the bacteria will notbe radioactive.

Figure E9-2 (part 2) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Conclusion: Infected bacteria are labeled with radioactive phosphorus but not with radioactive sulfur,supporting the hypothesis that the genetic material of bacteriophages is DNA, not protein.

Results: Bacteria areradioactive; phage coats are not.

5 Measure radioactivity of phagecoats and bacteria.

1 Label phages with P32 or S35.

2 Infect bacteria withlabeled phages; phages injectgenetic material into bacteria.

3 Whirl in blender to break offphage coats from bacteria.

4 Centrifuge to separate phage coats(low density: stay in liquid)from bacteria (high density:sink to bottom as a “pellet”)

Results: Phage coats areradioactive; bacteria are not.

Radioactive sulfur (S35)Radioactive phosphorus (P32)

RadioactiveDNA (blue)

Radioactiveprotein(yellow)

sugar

phosphate

base = cytosine

sugar

phosphate

base = thymine

sugar

phosphate

base = adenine

sugar

phosphate

base = guanine

9.2 What Is the Structure of DNA?

• DNA Is Composed of Four Nucleotides

• DNA Is a Double Helix of Two Nucleotide Strands

• Many people contributed to the discovery, but Francis Crick and James Watson (and Maurice Wilkins) got the Noble prizes.

Hydrogen Bonds

• Hydrogen bonds hold certain nitrogenous base pairs together– A bonds with T, G bonds with C– Bonding bases called complementary base

pairs

• Ladder-like structure of the two DNA strands are twisted into a double helix

9.2 What Is the Structure of DNA?

• Hydrogen Bonds Between Complementary Bases Hold the Two DNA Strands Together

• The Order of Nucleotides in DNA Can Encode Vast Amounts of Information

How Does DNA Encode Information?

• How can a molecule with only 4 simple parts be the carrier of genetic information?

• The key lies in the sequence, not number, of subunits

• Within a DNA strand, the four types of bases can be arranged in any linear order, and this sequence is what encodes genetic information

How Does DNA Encode Information?

• The genetic code is analogous to languages, where small sets of letters combine in various ways to make up many different words

– English has 26 letters– Hawaiian has 2 letters– The binary language of computers uses only two

“letters” (0 and 1, or “on” and “off”)

How Does DNA Encode Information?

• The sequence of only four nucleotides can produce many different combinations

– A 10 nucleotide sequence can code for greater than 1 million different combinations

9.4 How Does DNA Replication Ensure Genetic Constancy During Cell

Division?

• The Replication of DNA Is a Critical Event in a Cell’s Life

• DNA Replication Produces Two DNA Double Helices, Each with One Old Strand and One New Strand

free nucleotides

DNA Replication

• Base pairing is the foundation of DNA replication

– An adenine on one strand pairs with a thymine on the other strand; a cytosine pairs with guanine

– If one strand reads ATG, the other reads TAC

DNA Replication

• The two resulting DNA molecules have one old parental strand and one new strand (semiconservative replication)

Figure E9-4 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

5 end

3 end

1

23

4

51 2

345

6

Figure E9-5 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

5 end

3 end

1

23

4

5

1 23

456

789

1

23

4

5 1 23

456

Figure E9-6 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

5 end

3 end

3 end

5 end

Figure E9-7ab Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

replication forks

replication bubbles

DNA

DNA helicase

Figure E9-7bc Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

replication forks

DNA helicase

DNApolymerase #2

DNApolymerase #1

5

3

5

3

continuous

synthesis

discontinuoussynthesis

Figure E9-7cd Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

DNApolymerase #2

DNApolymerase #1

5

35

3

continuous

synthesis

discontinuoussynthesis

53

53

3

5continuous synthesis

discontinuous synthesis

DNApolymerase #2leaves

DNApolymerase #3

DNA polymerase #1continues alongparental DNA strand

Figure E9-7de Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

53

53

3

5continuous synthesis

discontinuous synthesis

DNApolymerase #2leaves

DNApolymerase #3

DNA polymerase #1continues alongparental DNA strand

53

53

3

5 DNApolymerase #3leaves

DNApolymerase #4

3

5

DNA ligase joinsdaughter DNA strandstogether.

Section 9.5 Outline

• 9.5 How Do Mutations Occur?– Accurate Replication and Proofreading Produce

Almost Error-Free DNA– Mistakes Do Happen– Mutations Range from Changes in Single

Nucleotides to Movements of Large Pieces of Chromosomes

– Mutations May Have Varying Effects on Function

Replication And Proofreading

• During replication, DNA polymerase mismatches nucleotides once every 10,000 base pairs

• DNA repair enzymes “proofread” each new daughter strand, replacing mismatched nucleotides

• However…

Mistakes Do HappenDNA is damaged in a number of ways

• Spontaneous chemical breakdown at body temperature

• Certain chemicals (some components of cigarette smoke)

• Alcohol

Mistakes Do Happen• UV light from the sun causes DNA

damage– DNA damage leads to uncontrollable

cell division and skin cancer

Types of Mutations• Point mutation - individual nucleotide in

the DNA sequence is changed

• Insertion mutation - one or more nucleotide pairs are inserted into the DNA double helix

• Deletion mutation - one or more nucleotide pairs are removed from the double helix

Types of Mutations• Inversion - piece of DNA is cut out of a

chromosome, turned around, and re-inserted into the gap

• Translocation - chunk of DNA (often very large) is removed from one chromosome and attached to another