2 12-1 DNAIn 1928, Frederick Griffith, a bacteriologist, was trying to prepare a vaccine against pneumonia
3 Griffith’s Experiments A vaccine is a substance that is prepared from killed or weakened disease-causing agents, including certain bacteriaThe vaccine is introduced into the body to protect the body against future infections by the disease-causing agent
4 A bacteria that is virulent is able to cause disease Griffith discovered that harmless bacteria could turn virulent when mixed with bacteria that cause diseaseA bacteria that is virulent is able to cause diseaseGriffith had discovered what is now called transformation, a change in genotype caused when cells take up foreign genetic material
6 A bacteriophage is a virus Viral Genes and DNAIn 1952, Alfred Hershey and Martha Chase used the bacteriophage T2 to prove that DNA carried genetic materialA bacteriophage is a virusthat infects bacteriaWhen phages infect bacterial cells, they are able to produce more viruses, which are released when the bacterial cells rupture
7 Hershey and Chase carried out the following experiment DNA’s Role RevealedHershey and Chase carried out the following experimentStep 1 T2 phages were labeled with radioactive isotopesStep 2 The phages infect E. coli bacterial cells.Step 3 Bacterial cells were spun to remove the virus's protein coats
8 Hershey and Chase concluded that the DNA of viruses is injected into the bacterial cells, while most of the viral proteins remain outsideThe injected DNA molecules causes the bacterial cells to produce more viral DNA and proteinsThis meant that the DNA, rather than proteins, is the hereditary material, at least in viruses.
10 By the early 1950’s, most scientists were convinced The Structure of DNABy the early 1950’s, most scientists were convincedThat genes were made of DNA.The problem is that no one knew what it looked like.Then along came James Watson & Francis Crick.
12 A Winding StaircaseWatson and Crick determined that a DNA molecule is a double helix—two strands twisted around each other, like a winding staircase.Nucleotides are the subunits that make up DNA. Each nucleotide is made of three parts: a phosphate group, a five-carbon sugar molecule, and a nitrogen-containing base
13 The five-carbon sugar in DNA nucleotides is called deoxyribose Structure of a Nucleotide
15 The nitrogen base in a nucleotide can be either a bulky, double-ring purine, or a smaller, single-ring pyrimidine.
16 Discovering DNA’s Structure Chargaff’s ObservationsIn 1949, Erwin Chargaff observed that for each organism he studied, the amount of adenine always equaled the amount of thymine (A=T)Likewise, the amount of guanine always equaled the amount of cytosine (G=C).However, the amount of adenine and thymine and of guanine and cytosine varied between different organisms
17 Wilkins and Franklin’s Photographs In 1952, Maurice Wilkins and Rosalind Franklin developed high-quality X-ray diffraction photographs of strands of DNAThese photographs suggested that the DNA molecule resembled a tightly coiled helix and was composed of two or three chains of nucleotides
19 Watson and Crick’s DNA Model In 1953, Watson and Crick built a model of DNA with the configuration of a double helix, a “spiral staircase” of two strands of nucleotides twisting around a central axisThe double-helix model of DNA takes into account Chargaff’s observations and the patterns on Franklin’s X-ray diffraction photographs.
20 These base-pairing rules are supported by Chargaff’s observations Pairing Between BasesAn adenine on one strand always pairs with a thymine on the opposite strand, and a guanine on one strand always pairs with a cytosine on the opposite strandThese base-pairing rules are supported by Chargaff’s observationsThe strictness of base-pairing results in two strands that contain complementary base pairs
21 In the diagram of DNA below, the helix makes it easier to visualize the base-pairing that occurs between DNA strands
22 12-2 The Replication of DNA When the double helix was discovered, scientists were very excited about the complimentary relationship between the sequences of nucleotides.Watson and Crick proposed that one DNA strand serves as a template on which the other strand is built.
23 Roles of Enzymes in DNA Replication The complementary structure of DNA is used as a basis to make exact copies of the DNA each time a cell divided.The process of making a copy of DNA is called DNA replicationDNA replication occurs during the synthesis (S) phase of the cell cycle, before a cell dividesDNA replication occurs in three steps
24 Step 1- DNA helicases open the double helix by breaking the hydrogen bonds that link the complementary nitrogen bases between the two strands. The areas where the double helix separates are called replication forks
25 Step 2 - At the replication fork, enzymes known as DNA polymerases move along each of the DNA strands. DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base-pairing rulesStep 3 - Two DNA molecules that form are identical to the original DNA molecule
27 Checking for ErrorsIn the course of DNA replication, errors sometimes occur and the wrong nucleotide is added to the new strand.An important feature of DNA replication is that DNA polymerases have a “proofreading” roleThis proofreading reduces errors in DNA replication to about one error per 1 billion nucleotides
28 The Rate of Replication Replication does not begin at one end of the DNA molecule and end at the otherThe circular DNA molecules found in prokaryotes usually have two replication forks that begin at a single pointThe replication forks move away from each other until they meet on the opposite side of the DNA circle
29 In eukaryotic cells, each chromosome contains a single, long strand of DNA Each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long, each section with its own starting pointWith multiple replication forks working in concert, an entire human chromosome can be replicated in about 8 hours
31 Traits, such as eye color, are determined 12-3 RNA & ProteinTraits, such as eye color, are determinedBy proteins that are built according toThe instructions specified in the DNA.
32 Decoding the Information in DNA Proteins, however, are not built directly from DNA. Ribonucleic acid is also involvedLike DNA, ribonucleic acid (RNA) is a nucleic acid—a molecule made of nucleotides linked together
33 RNA differs from DNA in three ways 1. RNA consists of a single strand of nucleotides instead of the two strands found in DNA2. RNA nucleotides contain the five-carbon sugar ribose rather than the sugar deoxyribose, which is found in DNA nucleotides3. In addition to the A, G, and C nitrogen bases found in DNA, RNA nucleotides can have a nitrogen base called uracil (U)
34 Comparing DNA and RNAThe instructions for making a protein are transferred from a gene to an RNA molecule in a process called transcriptionCells then use two different types of RNA to read the instructions on the RNA molecule and put together the amino acids that make up the protein in a process called translation
35 The entire process by which proteins are made based on the information encoded in DNA is called gene expression, or protein synthesis
37 Transfer of Information from DNA to RNAThe first step in the making of a protein, transcription, takes the information found in a gene in the DNA and transfers it to a molecule of RNARNA polymerase, an enzyme that adds and links complementary RNA nucleotides during transcription, is required
38 The three steps of transcription are Step 1 RNA polymerase binds to the gene’s promoterStep 2 The two DNA strands unwind and separateStep 3 Complementary RNA nucleotides are added
41 Genetic Code: Three-Nucleotide “Words” Different types of RNA are made during transcription, depending on the gene being expressedWhen a cell needs a particular protein, it is messenger RNA that is madeMessenger RNA (mRNA) is a form of RNA that carries the instructions for making a protein from a gene and delivers it to the site of translation
42 The information is translated from the language of RNA—nucleotides—to the language of proteins—amino acidsThe RNA instructions are written as a series of three-nucleotide sequences on the mRNA called codonsThe genetic code of mRNA is the amino acids and “start” and “stop” signals that are coded for by each of the possible 64 mRNA codons
44 RNA’s Roles in Translation Translation takes place in the cytoplasm. Here transfer RNA molecules and ribosomes help in the synthesis of proteinsTransfer RNA (tRNA) molecules are single strands of RNA that temporarily carry a specific amino acid on one endAn anticodon is a three-nucleotide sequence on a tRNA that is complementary to an mRNA codon.
45 Ribosomes are composed of both proteins and ribosomal RNA (rRNA) Ribosomal RNA (rRNA) molecules are RNA molecules that are part of the structure of ribosomesEach ribosome temporarily holds one mRNA and two tRNA molecules
46 The seven steps of translation are: Step 1 The ribosomal subunits, the mRNA, and the tRNA carrying methionine bind togetherStep 2 The tRNA carrying the amino acid specified by the codon in the A site arrivesStep 3 A peptide bond forms between adjacent amino acidsStep 4 The tRNA in the P site detaches and leaves its amino acid behind
47 Step 5 The tRNA in the A site moves to the P site Step 5 The tRNA in the A site moves to the P site. The tRNA carrying the amino acid specified by the codon in the A site arrivesStep 6 A peptide bond is formed. The tRNA in the P site detaches and leaves its amino acid behindStep 7 The process is repeated until a stop codon is reached. The ribosome complex falls apart. The newly made protein is released