Unit Two: Inheritance

Chapter 9 DNA: 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)

Living R strain Living S strain Heat-killed S strain Living R strain, heat-killed S strain Bacterial strain(s) injected into mouseResultsConclusions R strain does not cause pneumonia. S strain causes pneumonia. Heat-killed S strain does not cause pneumonia. A substance from heat-killed S strain can transform the harmless R strain into a deadly S 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 wall destroyed; phage released. bacterial chromosome phage chromosome bacterium 1 Phage attaches to bacterium. 2 Phage injects its chromosome into bacterium. 3 Phage chromosome is replicated. 4 Phage parts synthesized, using bacterial metabolism. 5 Complete phages 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 be radioactive. 2. If bacteria are infected with bacteriophages containing radioactively labeled protein, the bacteria will not be 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 are radioactive; phage coats are not. 5 Measure radioactivity of phage coats and bacteria. 1 Label phages with P 32 or S Infect bacteria with labeled phages; phages inject genetic material into bacteria. 3 Whirl in blender to break off phage 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 are radioactive; bacteria are not. Radioactive sulfur (S 35 )Radioactive phosphorus (P 32 ) Radioactive DNA (blue) Radioactive protein (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

Figure E9-5 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc. 5 end 3 end

Figure E9-6 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc. 5 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 DNA polymerase #2 DNA polymerase # continuous synthesis discontinuous synthesis

Figure E9-7cd Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc. DNA polymerase #2 DNA polymerase # continuous synthesis discontinuous synthesis continuous synthesis discontinuous synthesis DNA polymerase #2 leaves DNA polymerase #3 DNA polymerase #1 continues along parental DNA strand

Figure E9-7de Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc continuous synthesis discontinuous synthesis DNA polymerase #2 leaves DNA polymerase #3 DNA polymerase #1 continues along parental DNA strand DNA polymerase #3 leaves DNA polymerase #4 3 5 DNA ligase joins daughter DNA strands together.

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 Happen DNA 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