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Chapter 8 8-1 DNA How do genes work? What are they made of? How do they determine the characteristics of an organism? Griffith and Transformation A. Griffith finds a ‘transforming principle.’ B. Griffith experimented with the bacteria that cause pneumonia. C. He used two forms: the S form (deadly) and the R form (not deadly). D. A transforming material passed from dead S bacteria to live R bacteria, making them deadly. Avery identified DNA as the transforming principle. A. Avery isolated and purified Griffith’s transforming principle. B. Avery performed three tests on the transforming principle. Hershey and Chase confirm that DNA is the genetic material. A. Hershey and Chase studied viruses that infect bacteria, or bacteriophages. a. Virus – non-living particles smaller than a cell that can infect living organisms. b. Radioactive markers – used in viruses to determine that the genetic material of the c. bacteriophage was DNA, not protein. 8-2 Structure of DNA * genes had to carry information from one generation to the next * genes had to put that information to work by determining heritable characteristics for organisms * genes had to be easily copied because all genetic information is copied before every cell division Scientists who contributed to our knowledge of DNA structure: Chargaff’s Rules: The amount of A = amount of T amount of C = amount of G Franklin – used x-ray diffraction to get information on the structure of DNA A. strands of DNA are twisted around each other like coils of a spring (helix) B. 2 strands C. nitrogenous bases are near the center of the molecule Watson and Crick – working on the structure of DNA, saw Franklin’s x-ray pattern of DNA which led to their breakthrough with DNA structure A. DNA – double helix where 2 strands are wound around each other B. Hydrogen bonds from between certain nitrogenous bases and provide the force to hold the two strands together C. Base pairing rule – hydrogen bonds can only form between a. A-T and C-G DNA is composed of four types of nucleotides. A. DNA is made up of a long chain of nucleotides. B. Each nucleotide has three parts. –a phosphate group –a deoxyribose sugar –a nitrogen-containing base (A, T, C, G) 8-3 DNA Replication Replication copies the genetic information. A. A single strand of DNA serves as a template for a new strand. B. The rules of base pairing direct replication. C. DNA is replicated during the S (synthesis) stage of the cell cycle. D. Each body cell gets a complete set of identical DNA. Proteins carry out the process of replication. A. DNA serves only as a template. B. Enzymes and other proteins do the actual work of replication. C. DNA Polymerase – polymerizes individual nucleotides to produce DNA. Also “proof reads” the new DNA D. Enzymes “unzip” the DNA molecule (hydrogen bonds are broken, 2 strands unwind) E. Free-floating nucleotides form hydrogen bonds with the template strand. Replication is fast and accurate. 8-4 Transcription converts a gene into a singlestranded RNA molecule. RNA and Protein Synthesis DNA → RNA → protein RNA – long, single-strand of nucleotides; carries out DNA’s instructions 5C sugar (Ribose), phosphate group, nitrogenous base · Uracil replaces thymine in RNA · RNA is like a disposable copy of DNA (“working copy” of a single gene) Transcription RNA polymerase binds to DNA and separates the DNA strands. It then uses 1 strand of DNA as a template from which nucleotides are assembled into a strand of RNA. Binds to regions of DNA called promoters (signals in DNA that indicate to the enzyme where to bind to make RNA) Transcription makes three types of RNA. • Messenger RNA (mRNA) – carry copies of instructions • Ribosomal RNA (rRNA) – site of protein assembly at ribosome • Transfer RNA (tRNA) – transfers each amino acid to the ribosome Transcription and replication both involve complex enzymes and complementary base pairing. •The two processes have different end results. –Replication copies all the DNA; transcription copies a gene. –Replication makes one copy; transcription can make many copies. 8-5 Translation converts mRNA messages into polypeptides. The Genetic Code - the “language” of mRNA instructions. •A codon is a sequence of three nucleotides that codes for an amino acid. They are on mRNA. RNA contains A, U, C and G Ex. U C G C A C G G U Is read as UCG – CAC – GGU these represent serine – histidine – glycine 3 amino acids in a protein A. Because of the 4 different bases (a, u, c, g) there are 64 possible 3-base codons on the mRNA. Some amino acids are specified by more than one codon. There are also some codons, like AUG that can code for methionine, or “start” codon for protein synthesis. Some codons are “stop” codons that do not code for any amino acid. B. The sequence of amino acids in an mRNA are the instructions for the order the amino acids should be in for a certain protein. There must be something to read the instructions, and assemble the correct sequence of amino acids. In the cell, this is the ribosome (rRNA + protein). •Amino acids are linked to become a protein. •An anticodon is a set of three nucleotides that is complementary to an mRNA codon. •An anticodon is carried by a tRNA. A. mRNA must be transcribed from DNA in the nucleus and released into the cytoplasm. B. Translation begins when an mRNA molecule in the cytoplasm attaches to a ribosome. a. As each codon is “read” by the ribosome, the proper amino acid is brought to b. the ribosome by the tRNA C. The ribosome forms a peptide bond between the first and second amino acids, breaks the bond that held the first tRNA to its amino acid, and releases the tRNA molecule. The ribosome then moves to the third codon, where a tRNA molecule brings it the amino acid specified by the third codon. D. The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule. a. When a stop codon is reached, the ribosome releases the newly formed b. polypeptide and the mRNA molecule completing the process of translation. 8-6 Prokaryotic cells turn genes on and off by controlling transcription. A. A promotor is a DNA segment that allows a gene to be transcribed. B. An operator is a part of DNA that turns a gene “on” or off.” C. An operon includes a promoter, an operator, and one or more structural genes that code for all the proteins needed to do a job. –Operons are most common in prokaryotes. –The lac operon was one of the first examples of gene regulation to be discovered. –The lac operon has three genes that code for enzymes that break down lactose. •Eukaryotes regulate gene expression at many points. A. Different sets of genes are expressed in different types of cells. B. Transcription is controlled by regulatory DNA sequences and protein transcription factors. •RNA processing is also an important part of gene regulation in eukaryotes. A. Many RNA molecules have sections that are “edited” out before the molecules become functional. B. Introns – intervening sequences of bases that are cut out of RNA in the cell nucleus. C. Exons – expressed sequences of bases that are spliced together to form the final mRNA (after “cap” and “tail” added) D. 3 Steps: 1)Introns are removed and exons are spliced together. 2)A cap is added. 3)A tail is added. 8-7 Some mutations affect a single gene, while others affect an entire chromosome. A. A mutation is a change in an organism’s DNA. B. Many kinds of mutations can occur, especially during replication. C. A point mutation substitutes one nucleotide for another. Mutations may or may not affect phenotype. A. Chromosomal mutations tend to have a big effect. B. Some gene mutations change phenotype. –A mutation may cause a premature stop codon. –A mutation may change protein shape or the active site. –A mutation may change gene regulation. Mutations can be caused by several factors. A. Replication errors can cause mutations. B. Mutagens, such as UV ray and chemicals, can cause mutations. C. Some cancer drugs use mutagenic properties to kill cancer cells. Roles of DNA and RNA DNA – the “master plan” RNA – the “blue print” Genes and Proteins Most genes contain nothing more than instructions for assembling proteins. **Proteins are the keys to almost everything that living cells do.