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12.1 Identifying the Substance of Genes. THINK ABOUT IT. How do genes work? To answer that question, the first thing you need to know is what genes are made of. How would you go about figuring out what molecule or molecules go into making a gene?. Griffith’s Experiments.
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THINK ABOUT IT • How do genes work? • To answer that question, the first thing you need to know is what genes are made of. • How would you go about figuring out what molecule or molecules go into making a gene?
Griffith’s Experiments • Griffith isolated two different strains of the same bacterial species. One was harmless, one caused disease.
Griffith’s Experiments • 1.) Griffith took a culture of the S strain, • 2.) Heated the cells to kill them • 3.) Then injected the heat-killed bacteria into laboratory mice. • 4.) The mice survived.
Griffith’s Experiments In Griffith’s next experiment, he mixed the heat-killed, S-strain bacteria with live, harmless bacteria from the R strain and injected the mixture into laboratory mice. The injected mice developed pneumonia, and many died.
Griffith’s Experiments • What happened???
Transformation • DNA transformed the good bacteria into disease-causing bacteria.
Transformation • He called this process transformation, because one type of bacteria had been changed permanently into another.
Oswald Avery Used enzymes to kill off proteins, lipids, carbs and RNA. Transformation still occurred. DNA was the transforming factor!
Bacterial Viruses • Alfred Hershey and Martha Chase • Hershey and Chase studied viruses—nonliving particles that can infect living cells.
Bacteriophages • The kind of virus that infects bacteria is known as a bacteriophage, which means “bacteria eater.”
The Hershey-Chase Experiment Bacteriophage was composed of a DNA core and a protein coat. They wanted to determine which part of the virus—the protein coat or the DNA core—entered the bacterial cell.
The Hershey-Chase Experiment • Hershey and Chase used radioactive markers to see the DNA and protein.
Nucleic Acids and Nucleotides DNA’s nucleotides are made up of three basic components: a 5-carbon sugar called deoxyribose a phosphate group and a nitrogenous base.
Nitrogenous Bases and Covalent Bonds DNA has four kinds of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The nitrogenous bases stick out sideways from the nucleotide chain.
Nitrogenous Bases and Covalent Bonds The nucleotides can be joined together in any order, meaning that any sequence of bases is possible.
Chargaff’s Rules Erwin Chargaff discovered that the percentages of adenine [A] and thymine [T] bases are almost EQUAL in any sample of DNA. The same thing is true for the other two nucleotides, guanine [G] and cytosine [C]. The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules.”
Franklin’s X-Rays In the 1950s, British scientist Rosalind Franklin used X-ray diffraction. X-ray diffraction revealed an X-shaped pattern showing that the strands in DNA are twisted around each other like the coils of a spring.
The Work of Watson and Crick At the same time, James Watson, an American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA. They built three-dimensional models of the molecule.
The Work of Watson and Crick Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern. 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 Double-Helix Model Common names for the structure of DNA: - Double helix - Twisted ladder - Spiral staircase The two strands of DNA are “antiparallel” — they run in opposite directions.
Hydrogen Bonding Hydrogen bonds form between the nitrogenous bases G – C has 3 hydrogen bonds. A – T has 2 hydrogen bonds. Hydrogen bonds are weak. Why is this important to DNA’s function? DNA’s strands have to separate often!
THINK ABOUT IT Before a cell divides, its DNA must first be copied. How might the double-helix structure of DNA make that possible?
The Replication Process Before a cell divides, it duplicates its DNA in a copying process called replication.
DNA Replication The structure of DNA explains how it replicates DNA “unzips” down the middle Free floating nucleotides pair up with both sides of the DNA molecule ATCGA TAGCT ATCGA TAGCT ATCGA TAGCT ATCGA TAGCT Original Exactly Copied DNA Strands
DNA Replication This process produces two exact DNA molecules (Chromosomes) that are the same DNA never leaves the nucleus
The Role of Enzymes How does this happen?? DNA replication is carried out by a series of enzymes. They first “unzip” a molecule of DNA by breaking the hydrogen bonds between base pairs.
The Role of Enzymes -DNA Polymerase - DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA. -DNA polymerase also “proofreads”
Replication in Living Cells How does DNA replication differ in prokaryotic cells and eukaryotic cells? Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied.
Replication in Living Cells How does DNA replication differ in prokaryotic cells and eukaryotic cells? In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.
