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Chapter 9 DNA: THE Genetic Material Section1 Identifying the Genetic Material.

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1 Chapter 9 DNA: THE Genetic Material Section1 Identifying the Genetic Material

2 Transformation Frederick Griffith, a bacteriologist, prepared a vaccine against pneumonia Vaccine – a substance that is prepared from killed or weakened disease-causing agents, including certain bacteria To protect the body against future infections by the disease-causing agent

3 Griffith’s Experiments Griffith worked with 2 strains of S. pneumoniae 1 st strain had a smooth capsule that protected the bacterium from body’s defense systems (S) Virulent – (full of poison) able to cause disease 2 nd strain lacked capsule and didn’t cause disease (R) Mice injected with (S) strain died; mice injected with (R) strain lived

4 Griffith injected mice with dead S bacteria – mice lived Griffith injected mice with heat-killed S bacteria-mice still lived Meaning the capsule was not involved with killing the mice He mixed harmless live R bacteria with the harmless heat-killed S bacteria-mice died Transformation- a change in genotype caused when cells take up foreign genetic material

5 Avery’s Experiments Oswald Avery & co-workers demonstrated that DNA is the material responsible for transformation Almost 100 years after Mendel’s experiments

6 Viral Genes and DNA Many scientists remained skeptical Knew little about DNA, so they could not imagine how DNA could carry genetic information

7 DNA’s Role Revealed Alfred Hershey and Martha Chase performed an experiment that settled the controversy Viruses are composed of DNA or RNA surrounded by a protective protein coat Bacteriophage (phage)– a virus that infects bacteria

8 Step 1 – grew E. coli contained radioactive sulfur ( 35 S) protein coat incorporated the sulfur Grew second batch E. coli with radioactive phosphorus ( 32 P) would become part of the phages’ DNA Step 2 Labeled phages used to infect two separate batches of E. coli

9 Step 3 Used centrifuge tubes to separate the bacteria (heavy) from the viral parts (lighter) Concluded that the DNA of viruses is injected into the bacterial cells, while most of the viral proteins remain outside Experiments have shown that DNA is the molecule that stores genetic information in living cells

10 The Structure of DNA Section 2

11 Watson & Crick determined that a DNA molecule is a double helix – two strands twisted around each other Nucleotides – the subunits that make up DNA 3 parts: a phosphate group, a 5-carbon sugar, and a nitrogen- containing base

12 Deoxyribose – sugar molecule from which DNA gets its full name, deoxyribonucleic acid Nitrogen base may be : adenine, guanine, thymine, and cytosine Adenine (A) and guanine (G) are classified as purines –two rings of carbon & nitrogen atoms Thymine (T) and cytosine (C) are classified as pyrimidines –single ring C & N atoms

13 Discovering DNA’s Structure Chargaff’s 1949 observations – the amount of adenine always equaled the amount of thymine; amount of guanine always equaled the amount of cytosine; but amount varied between different organisms

14 Wilkins & Franklin’s Photographs X-ray diffraction to study the structures of molecules 1952 Wilkins & Franklin developed high-quality X-ray diffraction photographs of strands of DNA which suggested that the DNA resembled a tightly coiled helix and was composed of two or three chains of nucleotides

15 Rosalind Franklin was an English scientist who contributed to the discovery of the molecular structure of deoxyribonucleic acid (DNA), 1951

16 Watson & Crick’s DNA Model 1953 Watson & Crick used the information from Chargaff, Wilkins, & Franklin along with their knowledge of chemical bonding, to make the “spiral staircase” configuration of DNA

17 Pairing Between Bases Watson & Crick determined that a purine on one strand of DNA is always paired with a pyrimidine on the opposite strand Base-pairing rule – cytosine pairs with guanine and adenine with thymine Complementary base pairs – sequence of bases on strand determines the sequence of N bases on the other strand of DNA

18 HOMEWORK Section 1 Review p Section 2 Review p

19 THE REPLICATION OF DNA SECTION 3 THE REPLICATION OF DNA SECTION 3

20 Watson & Crick proposed that one DNA strand serves as a template, or pattern, on which the other strand is built DNA replication – the process of making a copy of DNA, which occurs during the (S) phase of the cell cycle

21 Step 1 – The double helix needs to unwind before replication can begin Accomplished by enzymes called DNA helicases which open the double helix by breaking the hydrogen bonds between the two strands

22 Additional proteins prevent the strands from assuming their double- helical shape Replication forks – areas where the double helix separates Enzymes known as DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base- pairing rules – forming two double helixes

23 Step 3 The process continues until all of the DNA has been copied & the polymerases are signaled to detach Nucleotide sequences are identical in the two DNA molecules Checking for errors – DNA polymerases are important in “proofreading” the nucleotides – can backtrack Errors in DNA replication about one error per 1 billion nucleotides

24 Rate of Replication Replication does not begin at one end & end at the other Prokaryotes usually have two replication forks Eukaryotic cells – length a problem – 33 days if done with a single point Each human chromosome is replicated in about 100 sections – replicated in about 8 hours

25 HOMEWORK Section 3 Review p Performance Zone p , 6-12 STP p

26 How Proteins Are Made Chap. 10 Section 1

27 Decoding the Information in DNA Traits are determined by proteins that are built according to instruction coded in DNA Ribonucleic acid is also involved RNA differs from DNA 3 ways a single strand five-C sugar, ribose Uracil (U) instead of thymine (T)

28 A gene’s instructions for making a protein are coded in the sequence of nucleotides in the gene Transcription – a process were the instructions for making a protein are transferred from a gene to an RNA molecule

29 Translation – the protein synthesis that takes place at ribosomes & that uses the codons in mRNA molecules to specify the sequence of amino acids to make protein Gene expression (protein synthesis) – the process by which proteins are made based on the information encoded in DNA

30 Transfer of information from DNA to RNA RNA polymerase, an enzyme that adds and links complementary RNA nucleotides during transcription, is required Step 1 RNA polymerase binds to the gene’s promoter-a specific sequence of DNA that acts as a “start” signal for transcription

31 Step 2 - RNA polymerase unwinds and separates the 2 strands of the double helix, exposing the DNA nucleotides Step 3 – RNA polymerase adds & then links complementary RNA nucleotides as it “reads” the gene – transcription follows the base-pairing rules for DNA except that uracil pairs with adenine

32 The RNA polymerase eventually reaches a “stop” signal in the DNA RNA nucleotides are linked together with covalent bonds during transcription Behind the RNA polymerase, the DNA closes up reforming the double helix Behind the RNA polymerase, the DNA closes up reforming the double helix In transcription, new molecule is RNA and only part of one of DNA strands serves as a template

33 Transcription in prokaryotic cells occurs in the cytoplasm; in eukaryotic cells, in the nucleus Many identical RNA are made simultaneously from a single gene Look at Figure 3 page 210

34 The Genetic Code: Three-Nucleotide “Words” Different types of RNA are made during transcription Messenger RNA (mRNA) carries the instructions for making a protein from a gene and delivers it to the site of translation Translated from the language of RNA (nucleotide) to language of proteins (amino acid)

35 Codons - a series of three-nucleotide sequences on the mRNA Marshall Nirenberg, American, deciphered the first codon by making artificial mRNA that contained only the base uracil (U) mRNA was translated into a protein phenylalanine amino- acid subuntis

36 Genetic code – the amino acids and “start” and “stop” signals that coded for by each of the possible 64 mRNA codons

37 RNA’S Roles in Translation Translation takes place in the cytoplasm Transfer RNA molecules and ribosomes help in the synthesis of proteins. Transfer RNA (tRNA) are single strands of RNA that temporarily carry a specific amino acid on one end & an anticodon at the other Anticodon – a three- nucleotide sequence on a tRNA that is complementary to an mRNA codon

38 Ribosomes are composed of both proteins & ribosomal RNA (rRNA) Ribosomal RNA molecules are part of the structure of ribosomes Each ribosome temporarily holds one mRNA and 2 tRNA molecules

39 Step 1 The mRNA and the tRNA carrying methionine bind together “start” codon AUG, signals the beginning of a protein chain Step 2 – The tRNA carrying the amino acid specified by the codon in the A site arrives Step 3 – A peptide bond forms between adjacent amino acids S 4 – The tRNA in the P site detaches and leaves its amino acid behind

40 S 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 arrives. S 6 – A peptide bond is formed. The tRNA in the P site detaches and leaves its amino acid behind. S 7 – The process is repeated until a stop codon is reached. The ribosome complex falls apart. The newly made protein is released.

41 Another ribosome can find the AUG codon on the same mRNA and begin making a second copy of the same protein The genetic code is the same in all organisms, but for a few exceptions

42 mutations

43 duplicating DNA at rates as high as 1000 nucleotides per seconBecause each of the two daughters of a dividing cell inherits a new DNA double helix containing one old and one new strand (Figure 5-5), the DNA double helix is said to be replicated “semiconservatively” by DNA polymerase.d. nucleotidesdouble helixFigure 5-5double helixDNA polymerasenucleotidesdouble helixFigure 5-5double helixDNA polymerase

44 Homework Section 1 review p Chapter review p ,2,6,7,8,12 P. 223 STP 1-3


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