Chapter 12: DNA
12.1 Identifying the Substance of Genes
Bacterial Transformation
- @@The first scientist to help figure out what genes are made of was Frederick Griffith@@
- Griffith injected mice with four different samples of bacteria
- Disease-causing bacteria that had been heat-killed did not kill the mice
- Harmless bacteria did not kill the mice
- But when the two strains were mixed together, the mice died
Griffith concluded that genetic information could be passed from one bacterial strain to another
- This experiment led Griffith to discover transformation, a process in which one strain of bacteria is changed by a gene or genes from another strain of bacteria
- @@A team led by Oswald Avery tried to find out what molecule causes transformation@@
- Avery and other scientists discovered that DNA stores and passes genetic information from one generation of bacteria to the next
Bacterial Viruses
- Other scientists tried to confirm Avery’s discovery
- Alfred Hershey and Martha Chase used viruses (tiny, nonliving particles that can infect living cells) to study DNA
- A bacteriophage is a kind of virus that infects bacteria by sticking to the surface of the cell and injecting its genetic information into it
- Hershey and Chase used a bacteriophage that had a DNA core and a protein coat to find out which part of the virus—the protein coat or the DNA core—entered bacterial cells
- Hershey and Chase’s experiment with bacteriophages confirmed Avery’s results, convincing many scientists that DNA was the genetic material found in genes
The Role of DNA
- @@The DNA that makes up genes must be capable of storing, copying, and passing on the genetic information in a cell@@
- The genetic material stores information needed by every living cell
- Before a cell divides, its genetic information must be copied
- When a cell divides, each daughter cell must receive a complete copy of the genetic information
12.2 The Structure of DNA
The Components of DNA
- @@DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds@@
- Nucleic acids are long molecules found in cell nuclei
- Nucleotides are the building blocks of nucleic acids and are made up of three basic parts: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base
- Nitrogenous bases are bases that have nitrogen in them
- DNA has four kinds of nitrogenous bases: adenine, guanine, cytosine, and thymine
- The nucleotides in a strand of DNA are joined by covalent bonds formed between the sugar of one nucleotide and the phosphate group of the next
Solving the Structure of DNA
- The next step was to figure out how those long chains of nucleotides are arranged
- Erwin Chargaff and Rosalind Franklin both helped solve the puzzle of the structure of DNA
- @@Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule@@
- Her X-ray pictures showed that the strands in DNA are twisted around each other in a shape known as a helix
- She also showed that DNA is made of two strands
- @@The clues in Franklin’s X-ray pattern allowed Watson and Crick to build a model that explained the specific structure and properties of DNA@@
- Watson and Crick determined that DNA has the structure of a double helix that looks like a twisted ladder
- The double-helix model explains Chargaff ’s rule of base pairing and how the two strands of DNA are held together
- The two strands of DNA run in opposite directions, or, “antiparallel”
- Because of this arrangement, the nitrogenous bases on both strands meet at the center of the molecule, allowing each strand of the double helix to carry a sequence of nucleotides
- Watson and Crick discovered that hydrogen bonds could form between certain nitrogenous bases
- Though hydrogen bonds are fairly weak forces, they have just enough force to hold the two strands of DNA together
- These bonds would form only between certain base pairs: adenine paired with thymine, and guanine paired with cytosine
- This nearly perfect fit between A–T and G–C nucleotides is known as base pairing
12.3 DNA Replication
Copying the Code
- Watson and Crick realized that each strand of the double helix has all the information needed to make the other strand
- Because each strand can be used to make the other strand, the strands are said to be complementary
- @@Replication@@ @@is the process of copying DNA prior to cell division; it makes sure that each daughter cell has the same complete set of DNA molecules@@
- During DNA replication, the DNA molecule makes two new complementary strands, with each strand of the double helix serves as a template for the new strand
- The two strands of the double helix separate, making two replication forks
- As each new strand forms, new bases are added following the rules of base pairing
- The end result is two DNA molecules, each identical to the other and to the original DNA molecule
- DNA replication is carried out by special proteins called enzymes that pull apart a molecule of DNA by breaking the hydrogen bonds between base pairs and then unwinding the two strands
- @@The principal enzyme involved in DNA replication is DNA polymerase@@
- DNA polymerase joins individual nucleotides to make a new strand of DNA
- DNA polymerase produces the sugar-phosphate bonds that join nucleotides together to form the new strands
- DNA polymerase checks each new DNA strand so that each molecule is a close copy of the original
- The telomere is the end part of the chromosome, a region in which DNA is difficult to replicate
- Cells use a special enzyme called telomerase that makes it less likely that genes will be damaged or lost during replication of rapidly dividing cells
Replication in Living Cells
- @@In most prokaryotic cells, replication starts from a single point, and it continues in two directions until the whole chromosome is copied@@
- @@In eukaryotic cells, replication may begin in hundreds of places on the DNA molecule@@
- Replication then occurs in both directions until each chromosome is completely copied