Discovery of DNA Three experiments led to the discovery of DNA as the hereditary factor that Mendel described in his experiments with pea plants. Frederick Griffith’s Experiment (1928)—showed that hereditary material can pass from one bacterial cell to another (transformation) Oswald Avery’s Experiment (1940s)—showed that DNA is the hereditary material that transfers information between bacterial cells. Alfred Hershey and Martha Chase’s Experiment (1952)– confirmed that DNA, and not protein, is the hereditary material in all cells.
Fig Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Living S cells RESULTS EXPERIMENT Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells that allowed the R cells to make capsules. Protective capsule Griffith’s Experiment
Fig EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P)
Fig EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA
Fig EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge Pellet Pellet (bacterial cells and contents) Radioactivity (phage protein) in liquid Radioactivity (phage DNA) in pellet See page 307 Hershey and Chase’s Experiment
Section 2 Vocabulary Pretest 1. Nucleotide 2. Deoxyribose 3. Nitrogenous base 4. Purine 5. Pyrimidine 6. Base-pairing rules 7. Complementary base pair 8. Base sequence A. Sugar found in DNA B. Consists of a sugar, phosphate and nitrogenous base C. Single ring nitrogenous base pair D. Double ring nitrogenous base pair E. Rule stating: A always pairs w/ T and C always pairs w/ G F. Order of bases on an DNA strand G. Contains nitrogen and carbon atoms and is found on the rungs of a DNA ladder H. A and T C and G
DNA Structure By 1950, we knew DNA was the hereditary molecule. How did it work? How did it replicate, store and transmit hereditary information and direct cell function? The answer is found in the unique structure of DNA.
The structure of DNA was discovered in 1953 by James Watson and Francis Crick.
Deoxyribonucleic Acid Described as a double helix (twisted ladder). Formed by two long strands of repeating subunits called nucleotides.
Each nucleotide has three parts: Five-carbon sugar called deoxyribose Phosphate group (phosphorous bonded to 4 oxygens) Nitrogenous base (either adenine, thymine, guanine or cytosine)
The sides of the ladder are formed by covalently bonding the sugar of one nucleotide to the phosphate of another. Sugar Phosphate Covalent bond
The nitrogenous bases form the rungs of the ladder. There are four types of nitrogen bases: Thymine Cytosine Adenine Guanine
Adenine and Guanine have a double ring of carbon and nitrogen atoms and are called purines. Thymine and Cytosine have a single ring of carbon atoms and nitrogen atoms. They are called pyrimidines
The bases pair together to form the rungs of the DNA ladder. Hydrogen bonds hold them together. They always pair according to the following base- pairing rules discovered by Erwin Chargaff in 1949: A – T C – G Note: since this pairing guarantees that a purine always pairs with a pyrimidine, the rungs are always the same length
The base pairs of A/T and C/G are called complementary base pairs. The order of base pairs on a chain of DNA is called its base sequence. Because of its base pairing pattern, one strand of DNA can serve as a template for making a new complementary strand. This is how DNA replicates itself.
A strand of DNA has the following sequence: C T G G A C What is the sequence of the complementary strand? G A C C T G
Section 3 Vocabulary Pretest 1. DNA replication 2. Helicase 3. Replication fork 4. DNA polymerase 5. Semi-conservative replication 6. Mutation A. A change in a nucleotide sequence of DNA B. Enzyme that separates two strands of DNA C. Enzyme that adds nucleotide bases to copying strands of DNA D. Point where two DNA strands separate E. Process of copying DNA F. DNA replication that results in one old and one new strand in each copied molecule
Answer Key 1. DNA replicationE 2. HelicaseB 3. Replication forkD 4. DNA polymeraseC 5. Semi-conservative replicationF 6. MutationA
DNA Replication DNA replication is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission. Steps: Helicases (enzymes) separate the DNA strands by breaking hydrogen bonds between base pairs. This creates an open area of DNA called a replication fork.
DNA polymerases (more enzymes) add complementary nucleotides to each of the original sides. Notice that synthesis on each strand moves in opposite directions.
DNA polymerase enzymes fall off and the two new strands completely separate. An enzyme called DNA ligase must fill in gaps created on the strand being copied in the opposite direction.
The end result is two new identical strands of DNA. This type of replication is called semi-conservative replication because each of the new DNA molecules has kept (or conserved) one of the two (or semi) original DNA strands.
Speed of Replication DNA adds nucleotides at a rate of 50 per second. However, at this rate it would take 53 days to replicate a large human chromosome. Therefore, replication must begin at several, usually thousands, of different points, or origins, at the same time.
Any change in the nucleotide sequence of a DNA molecule is called a mutation. DNA polymerase can check and correct mistakes made during replication. However, mistakes do happen. Mistakes can be spontaneous or caused by environmental factors (radiation, chemicals, etc.) Mutations can be helpful, harmful or harmless. Mistakes made in genes that control cell division can lead to tumors. Mutations
Section 4 Vocabulary Pretest 1. Ribonucleic acid 2. Transcription 3. Translation 4. Protein synthesis 5. Ribose 6. Messenger RNA 7. Transfer RNA A. Nucleic acid important in protein synthesis B. Sugar found in RNA C. RNA that carries instructions from the nucleus to ribosomes D. Process of making an RNA molecule from a DNA template E. RNA that assembles an amino acid chain F. Process of assembling a protein from a coded RNA message G. DNA RNA protein
8. RNA polymerase 9. Promoter 10. Termination signal 11. Genetic code 12. Codon 13. Anticodon 14. Genome H. An organism’s entire gene sequence I. Sequence of nucleotides at the end of a gene J. Sequence of nucleotides that start transcription K. 3-nucleotide sequence on mRNA that encodes an amino acid L. 3-nucleotide sequence on tRNA that complements a codon M. Specifies the amino acid sequence of a protein N. Enzyme that catalyzes the formation of RNA Pretest continued
Answer Key 1. Ribonucleic acidA 2. TranscriptionD 3. TranslationF 4. Protein synthesisG 5. RiboseB 6. Messenger RNAC 7. Transfer RNAE 8. RNA polymerase N 9. Promoter J 10. Termination signal I 11. Genetic code M 12. Codon K 13. Anticodon L 14. Genome H
Protein Synthesis (Big Picture) Cells make proteins. The instructions to make a protein are on the DNA in the nucleus. Ribosomes in the cytoplasm make the proteins Cells MUST be able to get the instructions from the DNA inside the nucleus out to the ribosomes. RNA is the messenger !!!
RNA Structure and Function RNA is different from DNA in 4 ways RNA sugar is ribose Uracil replaces thymine as a base RNA is single stranded RNA is shorter than DNA
DNA vs. RNA DNARNA DoubleSingle DeoxyriboseRibose ThymineUracil LongerShorter
Types of RNA Three major types of RNA Messenger RNA (mRNA) —carries instructions for making a protein from a gene in the nucleus to a ribosome in the cytoplasm Ribosomal RNA (rRNA) —part of a ribosome Transfer RNA (tRNA) —transfers amino acids to the ribosome to make a protein.
mRNA is made from DNA in the nucleus. It carries the message for making a protein out of the nucleus to a ribosome in the cytoplasm rRNA is part of the ribosome. tRNA is folded with many nucleotide bases. However, we emphasize the three at the bottom.
Protein Synthesis Forming proteins based on information in DNA and carried out by RNA is called protein synthesis. DNA RNA protein It involves two processes: Transcription Translation
Transcription Transcription —the genetic code is copied or “transcribed” onto a mRNA in the cell nucleus. Three steps: RNA polymerase (enzyme) binds to a specific site on a DNA molecule called a promoter. This causes DNA to unwind. RNA polymerase uses the base-pairing rules to add the RNA nucleotides that match the DNA code (A/U; C/G) RNA polymerase stops at a termination signal that marks the end of a gene.
Reading the Code The code on the mRNA must next be “read” during the process of translation. This genetic code tells us how a sequence of bases on a DNA molecule (or its RNA messenger) corresponds to a particular amino acid. The code is read three bases at a time. Each 3 base sequence that codes for an amino acid is called a codon.
Codons in mRNA Notice AUG is the start codon UAA, UAG, and UGA are the stop codons.
Amino Acids The genetic code rules are the same for nearly all living things. The same codons always code for the same amino acids. There are 20 different amino acids. A chain of amino acids makes up a polypeptide. Polypeptides join and twist to make up proteins. It is tRNA and the ribosomes that assemble the proteins during translation.
Three bases at one end of a tRNA are complementary to a codon on the mRNA. They are called an anticodon. The specific amino acid that the codon codes for is attached to the top of the tRNA A U G G G A C C U
Translation Translation —the making of a protein Steps: Initiation —ribosomal subunits, mRNA and the tRNA carrying methionine (amino acid of the start signal AUG) bind together. Elongation —the tRNA carrying the amino acid specified by the next codon binds. Peptide bonds form between the amino acids beginning the chain. This continues until a termination signal is reached. Termination —stop codon is reached Disassembly – the ribosome complex falls apart and the peptide is released.
Recap: Protein Synthesis: Big Picture
The Human Genome Genome —the complete genetic material contained in an individual. The entire Human Genome consists of 3.2 billion base pairs. We now know the order of these base pairs and have discovered that humans have approximately 30,000 genes. We now need to learn where and when human cells use each of the proteins coded for in the genome. This can help diagnose, treat, and prevent many genetic disorders.