Chapter 12: DNA

• Slide 1: Title Slide• Slide 2: Table of Contents12.1 Identifying the Substance of Genes• Slide 3: Bacterial Transformation• Slide 4: Griffith’s Experiment• Slide 5: Transformation• Slide 6: The Molecular Cause of Transformation• Slide 7: Bacterial Viruses• Slide 8: Bacteriophages• Slide 9: The Hershey-Chase Experiment• Slide 10: The Role of DNA• Slide 11: Storing Information• Slide 12: Copying Information• Slide 13: Transmitting Information12.2 The Structure of DNA• Slide 6: The Components of DNA• Slide 7: Solving the Structure of DNA• Slide 8: The Double-Helix Model12. 3 DNA Replication• Slide 9: Copying the Code• Slide 10: Replication in Living Cells

TABLE OF CONTENTS

- What clues did bacterial transformation yield about the gene?

To understand genetics, scientists had to discover the chemical nature of the gene. When the gene was identified, then it would be easy to understand how they control the inherited characteristics of living cells. In 1928, the British scientist Frederick Griffith was trying to figure out how bacteria make people sick and this is how it is discovered. He did an experiment with S and R strand to see which one caused pneumonia.

SC.192.N.1.6 Describe how scientific inferences are drawn from scientific observations and provide examples from the Benchmark being studied.

Bacterial Transformation

• In Griffith’s experiment, he had an S strain which was the disease causing bacteria and the R strain which was the harmless bacteria. At the end of the experiment, the S strain grew into smooth colonies of culture plates and the R strain grew into a colony with rough edges. The S strain killed the mice and the R strain didn’t so Griffith went more into it. He then heated the S strain and killed them and then injected them but the mice did not die. But when he did it again except he mixed it with the R strain all the mice died. Why did this happen?

Griffith’s Experiment

• What happened in Griffith’s experiment was that when he mixed the two types of bacteria together, some chemical factor from the heat killed S strain moved into the R strains. Griffith said that this process was known as transformation because one type of bacteria (the harmless) was changed into another type of bacteria (the disease-causing form). This factor was concluded to be a gene.

Transformation

• In 1944, a group of scientists at the Rockefeller Institute in New York decided to repeat Griffith’s experiment. They did the experiment with enzymes that destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA but transformation still occurred. Again, they did the experiment except this time they used enzymes that broke down DNA and transformation did not occur. This concluded that the nucleic acid in DNA stores and transmits genetic information from one bacteria to the next.

The Molecular Cause of Transformation

- What role did bacterial viruses play in identifying genetic material?

The most important experiment made to convince scientists of the chemical makeup of a cell is by the discovery made by Avery’s team. Alfred Hershey and Martha Chase were American scientists that performed the experiment in 1952 that studied viruses which infect living cells.

Bacterial Viruses

• A bacteriophage is a kind of virus that infects bacteria. When a bacteriophage enters a bacterium, it injects its genetic information into in. The viral genes creates a lot of bacteriophages, which destroys the bacterium. And so, hundreds of new viruses are made.

Bacteriophages

• Hershey and Chase wanted to determine which part of the virus, the protein coat or the DNA core entered the bacterial cell to support or disprove Avery’s finding that genes were made of DNA. They grew cultures of radioactive isotopes of phosphorus-32 and sulfur-35 because proteins contain no phosphorus and DNA contains no sulfur. They mixed the viruses with the bacterial cells and waited for the viruses to inject their genetic material, then they separated the viruses form the bacteria and tested the bacteria for radioactivity. The conclusion was that all the radioactivity was from phosphorus which is found in DNA and so this proves that all genetic material in genes are in DNA in all living cells.

The Hershey-Chase Experiment

- What is the role of DNA in heredity?Scientists now wanted to know what else genes were capable of. They believed one crucial assumption. The DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell.

The Role of DNA

• One of the very important roles of DNA is to store DNA. Genes control patterns of development in an organism. This means that the instructions must be written into the DNA of every organism. Like how a flower is purple or someone’s eye color.

Storing Information

• Most scientists are puzzled at how DNA could replicate and make a copy of everyone of its genes. The only solution was to wait until the DNA structure was known to find out how. Before a cell divides, it must make a complete copy of everyone of its genes to do so.

Copying Information

• Genes are transmitted from one generation to the next as shown in Mendel’s experiment. DNA molecules must be carefully stored and passed along during cell division like meiosis. The loss of any DNA during meiosis could mean a loss of genetic information that could be very important.

Transmitting Information

- What are the chemical components of DNA?

The chemical components of DNA are deoxyribonucleic acid, or DNA, which is a unique molecule. DNA is a nucleic acid made up of nucleotides joined into long strand or chains by covalent bonds.

The Components of DNASC.912.L.16.3 Describe the basic process of DNA replication and how it relates to the transmission and conservation of the genetic information.

• Nuclei acids are long, slightly acidic molecules which are originally identified in cell nuclei. Nucleic acids are made up of smaller subunits which are linked to form long chains. Nucleotides are the basic building blocks of nucleotides. Nucleotides are made up of three basic components: a 5-carbon deoxyribose, a phosphate group, and a nitrogen base.

Nucleic Acids and Nucleotides

• Nitrogenous bases are basically bases that contain nitrogen in them. DNA has four nitrogenous bases: adenine, guanine, cytosine, and thymine. The nucleotides are joined together by covalent bonds formed between the sugar of one nucleotide and the phosphate group of the next. Scientists determine the amount of DNA in a solution by measuring the amount of light it absorbs at a wavelength of 260 nanometers or UV radiation.

Nitrogenous Bases and Covalent Bonds

- What clues helped scientists solve the structure of DNA?

DNA is made up of long strands of nucleotides helps scientists start to understand the structure of this molecule. Now the next step is to understand the way these chains are rearranged in three dimensions.

Solving the Structure of DNA

• Erwin Chargaff, an Austrian-American biocemist, discovered the percentages of adenine, guanine, and thymine, and cytosine. He observed that A=T and G=C and this became known as Chargaff’s rule. But no one could figure out why the DNA samples of different organisms obeyed this rule.

Chargaff’s Rule

• In the early 1950s, the British scientist Rosalind Franklin began to study DNA and used a technique called X-ray diffraction to get more information about it. Her results showed an X-shaped figure showing that the DNA strands are twisted around each other. Also, it shows that there are two strands in the structure. There are also signs that the nitrogenous bases are near the center of the DNA molecule.

Franklin’s X-Rays

• James Watson, an American biologist, and Francis Crick, a British physicist, also tried to determine the structure of DNA. They built a three-dimensional model of DNA which was twisted and stretched. Up until 1953 that Watson was shown a copy of Franklin’s X-ray pattern he knew his model was accurate. The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA.

The Work of Watson and Crick

- What does the double-helix model tell us about DNA?

A double-helix looks like a twisted model. There are two strands of DNA that are twisted around each other in a spiral pattern. The model can tell scientists about how DNA can function as a carrier of genetic information. The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.

The Double-Helix Model

• In the double-helix the two strands of DNA run in opposite directions. In biochemistry, this is known as “antiparallel”. This helps the nitrogenous bases have contact at the center of the molecule. Also, it allows each strand to carry a sequence of nucleotides.

Antiparallel Strands

• Watson and Crick discovered that hydrogen bonds could form between certain nitrogenous bases. This helps the two DNA be able to hold together with this force. Hydrogen bonds are actually a weak chemical force that can separate the two strands of helix to separate. If they do separate, the DNA functions can mess up.

Hydrogen Bonding

• Base pairing is when the two strands of DNA are held together by hydrogen bonds between the nitrogenous bases adenine and thymine, and between guanine and cytosine. For every adenine in a double-helix DNA molecule, there is exactly one thymine and vise versa. Also, for every cytosine in a double-helix DNA molecule, there is exactly one guanine.

Base Pairing