Chapter 12- DNA (pages ) a Mr. Bragg presentation

Page  2  Summarize the process of bacterial transformation  Describe the role of bacteriophages in identifying genetic material  Identify the role of DNA in heredity

Page  3 I. 12.1: Identifying the substance of genes A. Bacterial Transformation 1. Background Information- Frederick Griffith (1928) a. Wanted to discover a better vaccine for lobar pneumonia b. Found two strains of the bacteria  Type S- caused pneumonia  Type R – was harmless

Page  4 2. Griffith’s Experiments a.Injected mice with type S- mice died b.Injected mice with type R- mice lived c.He thought that type S made a toxin that killed mice Heat-killed type S and injected into mice – mice lived

Page  5 d. Injected a mixture of type R and heat-killed type S into mice- mice died! e. Cultured the bacteria found in the dead mice and discovered that they were identical to type S!

Page  6 3. Transformation a.Heat-killed bacteria made the harmless bacteria deadly b.Griffith hypothesized that a particular molecule was transferred from type S to type R and made it deadly

Page  7 4. Molecular Cause of Transformation a. Oswald Avery (1944) led a team to discover the molecule responsible for the transformation b. Extracted various molecules one at a time from the heat-killed bacteria and tested them in mice  Transformation still occurred

Page  8 c. In one experiment, they destroyed the DNA  Transformation did not occur d. Conclusion ? DNA was the transforming molecule

Page  9 B. Bacterial Viruses 1. Hershey- Chase experiment (1952) a.Used viruses called bacteriophages to study DNA b.They wanted to find out which part of the virus was entering the bacterial cells c.Used radioactive tags attached to each component

Page  10 d. They used these viruses to infect bacterial cells e. Bacteria received the DNA in their cells but no other component f. Conclusion ??? Confirmed that DNA is the molecule of heredity

Page  11 C. The Role of DNA 1. DNA has 3 major functions a. Storing information  The main job of DNA  The “instructions” that cause a single cell to develop into an oak tree, goldfish, or dog must be written into the DNA

Page  12 b. Copying information  A cell must be able to copy its DNA in order to divide  Cells need to divide to replace damaged cells or for growth of the organism

Page  13 c. Transmitting information  Genes are passed down from parents to offspring  DNA molecules must be carefully stored and passed along  Especially important during meiosis- the loss of any DNA can have dire consequence

Page  14 In class In groups of 2-3 address this question by making trading cards for the following: 1.Frederick Griffith 2.Oswald Avery (and his team) 3.Hershey-Chase 4.DNA – 3 roles (aka: job, function) Each card should have a picture on the front (must relate to the person/persons) plus information on the back that addresses the question. The 4 th card differs from the others, so focus on the roles of DNA on the back.

Page  15  Summarize the process of bacterial transformation  Describe the role of bacteriophages in identifying genetic material  Identify the role of DNA in heredity

Page  16 Do Now  What are the three functions of DNA?  Why is each function important to organisms?

Page  17  Identify the chemical components of DNA  Discuss the experiments leading to the identification of DNA as the molecule that carries the genetic code  Describe the steps leading to the development of the double helix model of DNA

Page  18 II. 12.2: The Structure of DNA A. The Components of DNA 1. Overview a.DNA = Deoxyribonucleic Acid b.Can be copied and passed on from one generation to the next c.Main job: specifies how proteins are made d.Has a special molecular structure

Page  Nucleic acids and nucleotides a. Nucleic acids are long chains of nucleotides b. Found in the nucleus of cells c. Have 3 basic parts  5 carbon sugar (deoxyribose)  phosphate group  Nitrogenous base

Page  Nitrogenous Bases and Covalent Bonds a. DNA has 4 kinds of nitrogenous bases  Adenine (A)  Guanine (G)  Cytosine (C)  Thymine (T)

Page  21 b. The nucleotides in a strand of DNA are joined by covalent bonds between the sugar of one nucleotide and the phosphate of another  This is for the vertical axis, or by analogy the upright sides of a ladder

Page  22 c. They can be joined together in any order d. This structure makes DNA very good at absorbing uv light * Sometimes these mechanisms fail and can lead to mutations

Page  23 B. Solving the Structure of DNA 1. Chargaff’s Rule a. The percentages of A and T bases are almost equal in any sample of DNA b. The percentages of C and G bases are almost equal in any sample of DNA c. A binds to T and C binds to G d. DNA samples of various organisms as different as bacteria and humans obeyed this rule

Page  Franklin’s X-rays (early 1950’s) a. Used X-ray diffraction to get information on the structure of DNA b. Her X-ray pictures showed that strands of DNA are twisted around each other  This shape is known as a double helix c. She also showed that DNA is made up of two strands ( 2 upright supports)

Page  Watson and Crick a.Built theoretical models of DNA using cardboard and wire b.In 1953, Watson saw a copy of Franklin’s x-rays c.Along with Crick, they were able to build a workable model

Page  26 C. The Double-Helix Model 1. DNA is a double helix  Looks like a twisted ladder 2. The shape explained many of the properties of DNA, such as Chargaff’s rule and how DNA held together

Page  Anti-parallel strands a.The two strands of DNA run opposite to each other b.The nitrogenous bases meet at the center of the strand c.This structure allows each strand of bases to carry a sequence of nucleotides Analogy: like letters in a 4-letter alphabet

Page  Hydrogen bonding a. Hydrogen bonds hold the nitrogen bases together in the horizontal plain  Analogy: like rungs on a ladder b. Hydrogen bonds are weak bonds, but are capable of holding the two sides together

Page  Base Pairing a. Defined- the principle that bonds in DNA can form only between adenine and thymine and between guanine and cytosine  A binds to T  C binds to G b. The base pairing explains Chargaff’s rule

Page  30  Identify the chemical components of DNA  Discuss the experiments leading to the identification of DNA as the molecule that carries the genetic code  Describe the steps leading to the development of the double helix model of DNA

Page  31 Do Now 1. Draw and label the three parts of a nucleotide 2. The percentage of Thymine (T) in a DNA sample is 30%. What are the percentages of Adenine (A), Cytosine (C), and Guanine (G)? Hint: recall how the bases bond to one another. 3. What type of bonds are found on the upright sides of the DNA molecule? How about in the center (between bases)?

Page  Draw and label the three parts of a nucleotide 1 2 3

Page  33  2. The percentage of Thymine (T) in a DNA sample is 30%. What are the percentages of Adenine (A), Cytosine (C), and Guanine (G)? Hint: recall how the bases bond to one another. Adenine? Thymine30% Guanine? Cytosine? 30% 20% Remember: A bonds to T C bonds to G Samples add up to 100%!!!

Page  What type of bonds are found on the upright sides of the DNA molecule? How about in the center (between bases)? = Covalent Bonds

Page  35  Summarize the events of DNA replication  Compare DNA replication of prokaryotes with that of eukaryotes

Page  36 III. 12.3: DNA Replication A. Copying the Code 1. The Replication Process a. Replication – the process of copying DNA prior to cell division b. The DNA molecule first separates into 2 strands  Analogy: unzips like a zipper  Makes two replication “forks”

Page  37 c. Next, the new bases are added to these strands  Each single strand is a template for making its complementary strand  Follows base pairing rules, A-T and G-C  For example, one strand has a sequence of TACGTT and its complementary strand would be ATGCAA

Page  38 d. The end result is two DNA molecules that are identical to the original molecule DNA replication animation

Page  The Role of Enzymes a. DNA replication is carried out by enzymes b. There are enzymes that “unzip” the DNA molecule

Page  40 c. DNA polymerase- is the principal enzyme involved in DNA replication  Job: produces the sugar-phosphate bonds that join nucleotides together  Secondary job: to check each new DNA strand, so that each molecule is a close copy of the original

Page  Telomeres a.Defined: the repetitive DNA at the end of a chromosome b.DNA in this region is hard to replicate c.Special enzymes called telomerase fix the problem Telomeres shown in red

Page  42  Telomerase makes it less likely that genes will be damaged or lost during replication

Page  43  Often switched “off” in normal adult cells  In cancer cells, it may be switched “on”  Shoelace analogy Telomerase (continued) Got aglets?

Page  44 B. Replication in Living Cells 1. Prokaryotic DNA Replication a. In bacteria, DNA replication starts when special proteins bind to one starting point on a chromosome b. Replication goes in both directions until the whole chromosome is copied

Page  45 c. The two chromosomes are attached to different points on the cell membrane d. They divide when the cell divides

Page  Eukaryotic DNA Replication a. Have larger chromosomes than prokaryotes b. Replication can begin in multiple locations on the DNA molecule c. Moves in both directions

Page  47 d. Proteins make sure that the copies are accurate e. Copying errors can occur, which changes the order of the nitrogen bases f. The chromosomes stay together until anaphase of mitosis Copied chromosomes (sister chromatids) separating during anaphase

Page  48  Summarize the events of DNA replication  Compare DNA replication of prokaryotes with that of eukaryotes