Chapter Index
Chapter 8 HOW GENES WORK
- Genes Are Made of DNA (p. 188)
- 8.1 The Griffith Experiment (p. 188; Fig. 8.1)
- A. In 1928, Griffith made a series of unexpected observations while experimenting with Streptococcus pneumoniae.
- B. He found that the virulent strain's polysaccharide coat was necessary for infection.
- C. He experimented further and found that the information specifying the polysaccharide coat could be passed from dead, virulent bacteria to coatless, nonvirulent strains.
- D. Hereditary information could thus be passed from dead cells to live ones, transforming them.
- 8.2 The Avery Experiments (p. 189; Fig. 8.2)
- A. Avery's experiments with the transforming principle from Griffith's experiments demonstrated conclusively that DNA is the hereditary material.
- B. What Avery found was that the purified transforming principle had the same chemistry as DNA, it behaved similarly to DNA, it was not affected by lipid or protein extraction, it was not destroyed by protein- or RNA-digesting enzymes, but it was destroyed by DNA-digesting enzymes.
- 8.3 The Hershey-Chase Experiment (p. 190; Fig. 8.3)
- A. In 1952, the Hershey-Chase experiments used radioactive labels to individually mark the DNA and the protein of viruses.
- B. They labeled the DNA of the viruses with radioactive phosphorus, while they labeled the protein coat with radioactive sulfur.
- C. They infected bacteria using these radioactive viruses, and found the bacteria contained the radioactive phosphorus, but not the sulfur.
- D. This was additional evidence that DNA was the genetic material.
- 8.4 The Fraenkel Conrat Experiment (p. 191; Fig. 8.4)
- A. Conrat, in 1957, used RNA-containing viruses to again show that nucleic acids were the genetic material.
- B. Retroviruses
- 1. Later studies have shown that many other viruses contain RNA rather than DNA.
- 2. To infect cells, these retroviruses make an intermediate double-stranded form of DNA from the RNA, using reverse transcriptase.
- 3. The DNA copy may then insert into the cell's DNA.
- 8.5 Discovering the Structure of DNA (p. 192; Figs. 8.5, 8.6, 8.7)
- A. By the end of the 1950s, then, it was accepted that the genetic material was DNA or RNA, but the structure of these nucleic acids had yet to be determined.
- B. Chargaff found that DNA always had equal amounts of purines (adenine and guanine) and pyrimidines (thymine and cytosine).
- C. He also found that the amount of adenine equaled the amount of thymine and that the amount of cytosine was the same as the amount of guanine, a phenomenon now called "Chargaff's rule."
- D. Chargaff's findings suggested base-pairing that was later found to occur inside the DNA molecule.
- E. Franklin suggested that the DNA molecule was in the form of a helix.
- F. Watson and Crick then connected the ideas of a helix with base-pairing to further elucidate the structure of DNA.
- G. The DNA molecule has a sugar-phosphate backbone with base-pairing on its interior, and is twisted into a double helix.
- H. Watson and Crick also suggested a mechanism by which DNA was able to copy itself.
- 8.6 How the DNA Molecule Replicates (p. 194; Figs. 8.8, 8.9)
- A. Each individual chain of a DNA molecule is complementary to its pair.
- 1. If one chain has the bases ATTGCAT, its partner will have the complementary sequence of TAACGTA.
- B. When DNA copies itself, it is said to have semiconservative replication.
- C. The Meselson-Stahl Experiment
- 1. The hypothesis of semiconservative replication was tested in 1958 by Meselson and Stahl using 14N and 15N.
- 2. They found that after replication, DNA contained an intermediate amount of the two isotopes, suggesting semiconservative replication.
- D. How DNA Copies Itself
- 1. An enzyme called DNA polymerase oversees the operation, and the DNA molecule "unzips" first, revealing an area called the replication fork.
- 2. The polymerase places into position the correct complementary nucleotide until the entire replication fork has been copied.
- 3. A mechanism for DNA repair ensures that very few mistakes are made in the replication process.
- 4. The process is called semiconservative replication because in each new DNA molecule, one strand is "new" DNA, and the complementary strand is the parent DNA molecule.
- From Gene to Protein (p. 196)
- 8.7 Transcription (p. 196; Figs. 8.10, 8.11)
- A. Transcription is the process whereby a messenger RNA (mRNA) molecule is synthesized from a portion of the DNA molecule in the nucleus, and is the first step in gene expression.
- B. The second step, called translation, occurs when the mRNA leaves the nucleus of the cell and directs the production of a protein molecule.
- C. The Transcription Process
- 1. Transcription uses an enzyme called RNA polymerase that binds to the DNA molecule at a specific site called the promoter and then moves along the DNA molecule.
- 2. A strand of mRNA is produced whose nucleotide sequence is complementary to that of the DNA.
- 8.8 The Genetic Code (p. 197; Fig. 8.12)
- A. The genetic code is written such that a three-nucleotide sequence codes for a given amino acid, the building blocks of proteins.
- B. The mRNA sequence that corresponds to the three-nucleotide sequence on DNA is called a codon.
- C. There are 64 different possible codons in the genetic code dictionary, and the same genetic code is employed, for the most part, by every living creature.
- 8.9 Translation (p. 198; Figs. 8.13, 8.14, 8.15, 8.16, 8.17)
- A. In translation, organelles called ribosomes use the mRNA transcript to direct the synthesis of a protein.
- B. The Protein-making Factory
- 1. Translation occurs in the cytoplasm in conjunction with ribosomes, which are made up of proteins and ribosomal RNA (rRNA).
- 2. Ribosomes hold the mRNA in position so translation of the code can occur.
- C. The Key Role of tRNA
- 1. A third type of RNA, called transfer RNA (tRNA) has on one end an anticodon, which is a sequence of three nucleotides complementary to an mRNA codon.
- 2. On the other end of the tRNA molecule, is the amino acid that corresponds with the codon of the mRNA.
- D. Making the Protein
- 1. The role of tRNA is to bring the appropriate amino acid into position along the mRNA molecule held by the ribosome.
- 2. As the ribosome proceeds along the mRNA, the next amino acid is added to the growing peptide chain.
- 3. When the process is finished, the ribosome complex falls apart, and the completed protein is released into the cell.
- 8.10 Architecture of the Gene (p. 201; Fig. 8.18)
- A. Introns
- 1. Prokaryotic DNA is made up of a continuous sequence of genes with no interruptions.
- 2. Eukaryotic DNA is constructed differently because it possesses gene sequences that code for amino acids, called exons, plus intervening, nonusable sequences of nucleotides, called introns.
- 3. Intron sequences must be removed from mRNA before translation can occur.
- B. Gene Families
- 1. A number of other interesting discoveries about the nature of genes have been made in recent years.
- 2. Multigene families arise when genes in cells exist in multiple copies.
- C. Transposons: Jumping Genes
- 1. Transposons are genes that are able to jump from one position to another on a chromosome, perhaps preventing the expression of a portion of a particular gene sequence.
- Regulating Gene Expression (p. 202)
- 8.11 Turning Genes Off and On (p. 202; Figs. 8.19, 8.20, 8.21)
- A. Cells must also have the ability to regulate which genes will be expressed and how often expression occurs.
- B. Repressors
- 1. In some cases, a regulatory protein, called a repressor, is joined to its regulatory site, known as the operator, which prevents the gene from being transcribed.
- 2. When the gene needs to be transcribed, a signal molecule binds to the repressor causing it to change shape so that it can no longer prevent gene expression.
- C. Activators
- 1. In other instances, a regulatory protein known as an activator has to help the DNA unwind prior to transcription.
- 2. No repressor blocks transcription in this case, but the activator must be present for it to proceed.
- D. Enhancers
- 1. A third type of control over gene expression is called an enhancer.
- 2. Enhancers are located on the DNA molecule and help the RNA polymerase locate and bind to the promoter site.
- Altering the Genetic Message (p. 204)
- 8.12 Mutation (p. 204; Fig. 8.22)
- A. In the very large amount of DNA in each cell, mistakes during DNA replication are bound to happen.
- B. Mistakes Happen
- 1. A change in a cell's genetic message is a mutation.
- 2. Mutations are the raw material for evolution.
- C. The Importance of Genetic Change
- 1. Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives.
- 2. The rate of evolution is limited by the rate of mutation.
- 3. Genetic change through mutation and recombination provides the raw material for evolution.
- 8.13 Kinds of Mutation (p. 205; Table 8.1)
- A. Most mutations are detrimental and their effects may be minor or catastrophic.
- B. Mutations in Germ-line Tissues
- 1. Only when a mutation occurs within a germ-line cell is it passed to subsequent generations.
- C. Mutations in Somatic Tissues
- 1. Changes in somatic cells are not passed on from generation to generation.
- 2. A somatic mutation may have drastic effects on the individual in which it occurs.
- D. Point Mutations
- 1. Point mutations are changes in the hereditary message of an organism that involve only one or a few base pairs of the coding sequence.
- 2. Sometimes the changes involve a base substitution, an insertion or deletion, or a frame-shift mutation.
- 3. Some mutations may arise spontaneously, while others are the result of exposure to mutagens.
- E. Changes in Gene Position
- 1. Individual genes may move from one place to another by transposition or there may be chromosomal rearrangements.
- 8.14 Cancer and Mutation (p. 206; Table 8.2)
- A. Agents thought to cause cancer are carcinogens.
- B. The suspicion that chemicals contribute to the incidence of cancer is called the chemical carcinogenesis theory.
- C. Early Ideas
- 1. The chemical carcinogenesis theory was first suggested in 1761 by an English physician named John Hill.
- 2. Observations in 1775 by Sir Pott, a London surgeon, suggested a relationship between soot and tar and scrotal cancer in chimney sweeps.
- D. Demonstrating That Chemicals Can Cause Cancer
- 1. In 1915, Japanese doctor Katsusaburo Tamagiwa found tar applied to rabbits caused cancer.
- 2. Since that time, it has been shown repeatedly that smoking cigarettes introduces tar into the lungs, leading to an increased rate of lung cancer.
- E. Carcinogens Are Common
- 1. Numerous chemicals have been found to be carcinogenic.
- KEY TERMS
- transformation (p. 188) The Griffith experiment showed how DNA can be passed from dead strains of a pathogenic bacterium to transform a non-virulent strain into one that is pathogenic, thus proving that DNA is the genetic material.
- retroviruses (p. 191) RNA viruses
- nucleotides (p. 192) the building blocks of the nucleic acids
- double helix (p. 192)
- base pairs (p. 192) Hydrogen bonds form between the pairing bases inside the DNA molecule, keeping the molecule at a constant thickness.
- complementarity (p. 194) Each strand of DNA is a complementary, mirror image of the other.
- semiconservative replication (p. 194) When DNA replicates, each existing strand serves as a template for a new complementary strand.
- transcription (p. 196) The process of "reading" the DNA molecule and assembling a complementary strand of mRNA.
- mRNA (p. 196) messenger RNA
- RNA polymerase (p. 196) A sophisticated enzyme that transcribes DNA into RNA.
- genetic code (p. 197) Each three-nucleotide block in a gene corresponds to a specific amino acid.
- codon (p. 197) the three-nucleotide sequence on mRNA that corresponds to an amino acid
- translation (p. 198) Ribosomes use mRNA to direct the synthesis of a protein.
- ribosome (p. 198)
- tRNA (p. 198) transfer RNA
- anticodon (p. 198) The three-nucleotide sequence on the tRNA molecule that is complementary to the mRNA codon.
- intron (p. 201) extra nucleotide sequences in DNA that code for nothing
- multigene family (p. 201) Genes exist in multiple copies, called multigene families.
- transposons (p. 201) jumping genes
- promoter (p. 202) the site on the DNA to which the RNA polymerase binds
- repressor (p. 202) the regulatory protein
- operon (p. 202) a cluster of genes that is transcribed as a unit
- enhancer (p. 203) enhancers help RNA polymerase find its binding site
- mutation (p. 204)
- mutagen (p. 205) an agent (usually radiation or chemical) that causes damage to DNA
- carcinogen (p. 206; Table 8.2) agents thought to cause cancer