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