Concept 11.1 GENES ARE MADE OF DNA
How did biologists discover that DNA was the genetic material? • Understanding structure and function of DNA may be the most significant biological discovery of the 20th century • How did this discovery occur? • Three experiments were key to proving that DNA serves as the genetic material
EARLY EXPERIMENTS • Griffith (1928) experimented with two forms of bacteria (one harmless and the other causing fatal pneumonia in mice) • Proposed the existence of a substance, “transforming factor”, that can be inherited
EARLY EXPERIMENTS • Avery (1944) repeated Griffith’s experiment with bacteria and mice • Focused on DNA and proteins as possible “transforming factor” (both substances found in chromosomes) • Concluded that transforming factor in Griffith’s experiment was DNA and not protein
Hershey & Chase • Hershey & Chase (1952) experiment using radioactive isotopes showed that viruses infect bacteria by injecting their DNA (notprotein) into them • Further proof that DNA is the hereditary material
Hershey & Chase Hershey and Chase offered further evidence that DNA, not proteins, is the genetic material. Only the DNA of the old generation of viruses is incorporated into the new generation.
Concept 11.2 NUCLEIC ACIDS STORE INFORMATION IN THEIR SEQUENCES OF CHEMICAL UNITS
NUCLEIC ACID STRUCTURE • Nucleic acids: organic molecules that store information in cells in the form of a code • Two types of nucleic acids DNA and RNA (function described later) • DNA is a polymer made up of subunits called nucleotides
Nucleic acids chains of monomers called nucleotides Nucleotide: has three components: 1) 5-carbon sugar deoxyribosein DNA 2) phosphate group 3) nitrogen bases four types NUCLEOTIDES
1) purines: double-ring bases 1) adenine (A) 2) guanine (G) 2) pyrimidines: single-ring bases 1) cytosine (C) 2) thymine(T) Types of Nitrogen Bases - DNA
DNA molecule is a double helix--like atwisted ladder Sides composed of alternating sugar and phosphate molecules “Rungs” consist of paired nitrogen bases: adenine, guanine, cytosine, and thymine DNA STRUCTURE
Remember - GCAT ***** Guanine always pairs with cytosine G with C ***** Adenine always pairs with thymine A with T Base Pairing Rules in DNA - GCAT
Watson & Crick - 1953 Described the double helix structure and the pairing of complementary bases Used data from earlier X-ray crystallography experiments by Wilkins & Franklin and other earlier work Few scientific findings in the history of biology have been as important DISCOVERING THE STRUCTURE OF DNA
KNOW THE FOLLOWING STUFF: • EARLY SCIENTISTS: What did Griffith & Avery do? What did Hershey & Chasedo? What did Wilkins & Franklindo? What did Watson & Crick do? • STRUCTURE OF DNA: What is its shape? Where is it found? What are nucleotides? What are they made up of? What is the sugar? What are the 4 bases? What are purines? What are pyrimidines? • BASE PAIRING RULES: GCAT
Concept 11.3 DNA REPLICATION IS THE MOLECULAR MECHANISM OFINHERITANCE
DNA REPLICATION • Before cell division (asexual reproduction) and when organisms reproduce sexually, genetic instructions in DNA must be passed on to offspring • Double stranded DNA and base pairing provides a template mechanism for DNA to make copies of itself • DNA replication occurs before mitosis and before the first division of meiosis
DNA REPLICATION • SEQUENCE OF EVENTS: • Enzyme breaks hydrogen bonds • between nitrogen bases “unzips” the DNA molecule. • 2.Each strand builds its opposite strand by complementary base pairing using free nucleotides in nucleus.
DNA REPLICATION 3. DNA polymerase (enzyme) bonds the new nucleotides into a chain. - process continues until the entire molecule has been replicated Base pairing rules ensure accuracy—Guanine always bonds with Cytosine; Adenine always bonds with Thymine
Each strand is a complement of one of the originals Two DNA molecules identical to the original produced Each DNA molecule has one original and one new strand DNA REPLICATION
KNOW THE FOLLOWING STUFF: • DNA REPLICATION: What is DNA replication? Where does it occur? When does it occur? What are the steps involved? What enzyme is involved? What is the final result?
Concept 11.4 A GENE PROVIDES THE INFORMATION FOR MAKING A SPECIFIC PROTEIN
ONE GENE – ONE POLYPEPTIDE • Beadle & Tatum: demonstrated that each gene (nucleotide sequence) in DNA codes for production of single polypeptide—chain of amino acids that make up a protein • This discovery showed that DNA controls protein synthesis in cells
PROTEIN FUNCTIONS • Proteins form part of every cell structure • Other proteins are enzymes that control most of the chemical reactions in cells • This means that the characteristics of all organisms are determined by proteins • The types of proteins produced are determined by the DNA in the nucleus of the cell
PROBLEM / QUESTION • DNA, located in the nucleus, contains the blueprint for protein synthesis • Protein synthesis occurs in cytoplasm on the ribosomes • DNA does NOT leave the nucleus • So, how do cells build these encoded proteins?
ANSWER • The answer lies in another type of nucleic acid, ribonucleic acid or RNA • RNA acts as messenger between DNA and the ribosomes in the cytoplasm to carry out protein synthesis • RNA is a smaller molecule than DNA and is capable of passing through pores in nuclear membrane
3 important differences: RNA contains the sugar ribose - DNA contains the sugar deoxyribose RNA is single-stranded - DNA is double-stranded RNA substitutes the nitrogen base uracil for the nitrogen base thymine found in DNA DIFFERENCES BETWEEN DNA & RNA
FROM RNA TO PROTEIN • Once RNA reaches the cytoplasm it directs the making of a protein • How is the information to build the correct protein carried in RNA? • The answer lies in the genetic or triplet code.
TRIPLET or GENETIC CODE • Genetic information for making a protein is encoded in a sequence of 3 nucleotides in DNA—a codon • In transcription, the information in DNA is encoded in the sequence of nucleotides in RNA • RNA is a code language that tells the ribosomes which amino acids to add first, second, third etc.
Sequence of three nucleotides in DNA codes for one amino acid = a codon Code is transcribed from DNA to RNA Triplet code contains the information the cell requires to make a protein THE TRIPLET (GENETIC) CODE
THE TRIPLET CODE • The triplet code is degenerate • Arranging the four nitrogen bases found in RNA into groups of three results in 64 different codons • Only 20 different amino acids • Result: most amino acids have more than one codon
THE TRIPLET CODE • Triplet code is universal same genetic coding system applies to ALMOST all organisms • One codon (START CODON) initiates the synthesis of a protein • Three codons (STOP CODONS) do not code for any amino acid signal the end of protein synthesis • A stop codon comes at the end of each gene sequence
KNOW THE FOLLOWING STUFF: • DNA TO RNA TO PROTEIN: What is RNA? Name the 3 ways in which RNA is different from DNA? What is the name of the process used to make RNA? Where does this process occur? What is the transcribed RNA used to make? What are the biochemical subunits that make up proteins? What is a codon? How many triplet codes are there? How many amino acids are there? How many triplet codes do not code for any amino acid? What are these triplets called?
Concept 11.5 THERE ARE TWO MAIN STEPS FROM GENE TO PROTEIN
From DNA to RNA to Protein • Involves a series of steps: • Transcription: a gene (one shortsequence of nucleotides in DNA) uses the enzyme RNA polymerase to make messengerRNA or mRNA • mRNA leaves the nucleus and binds to a ribosome in cytoplasm • Translation: a ribosome translates the information found in mRNA into a protein
DNA makes mRNA in process of transcription mRNA leaves nucleus and binds to ribosome Translation: along with a ribosome, the mRNA message is translated to build a protein From DNA to RNA to Protein
TYPES OF RNA • Three different types of RNA work together in protein synthesis: • Messenger RNA (mRNA): brings information from DNA in nucleus to ribosomes in cytoplasm • Transfer RNA (tRNA): the supplier; transports amino acids to the ribosome to assemble a protein • Ribosomal RNA (rRNA): part of the ribosomes; clamps onto mRNA and uses its information to assemble the amino acids in the correct order
TYPES OF RNA • Different types of RNA are produced by separate genes in DNA
TRANSCRIPTION: DNA to RNA • Occurs in nucleus using RNA polymerase • 3 mainsteps: 1. Segment of DNA separates exposing DNA nitrogen bases - only one strand is used as template to make mRNA active strand 2. Free RNA nucleotides base pair with complementary nucleotides on exposed active DNA strand 3. Newly formed mRNA strand separates from DNA strand and is edited before moving to the cytoplasm
DNA strands separate Active DNA strand serves as template to make mRNA Free RNA nucleotides base pair with exposed nucleotides on the active strand Newly formed mRNA molecule separates from DNA molecule TRANSCRIPTION SUMMARY
PROCESSING mRNA • The mRNA initially transcribed in the nucleus is NOT functional • It contains stretches of non-coding nucleotides (introns) that interrupt coding regions (exons) • Before the mRNA leaves the nucleus, enzymes remove the introns and join the exons together • Process is called RNA splicing • End result continuous coding sequence that leaves the nucleus, and moves to a ribosome where the mRNA “message” is translated to a sequence of amino acids
KNOW THE FOLLOWING STUFF: • TRANSCRIPTION - DNA TO RNA: What is messenger RNA (mRNA)? What molecule is used to make mRNA? How many strands of theDNA molecule are involved in making mRNA? What enzyme is involved in the process? How is basepairing different in making mRNA? What is the message in mRNA used for? • EDITING THE RNA MESSAGE: Why can’t the initial RNA molecules produced in transcription be used right away? What is an intron? What is an exon? What is the process of editing RNA called?
TRANSLATION INVOLVES THREE MAIN PLAYERS • Ribosomes: present in cytoplasm • Messenger RNA (mRNA): carries information from DNA in the nucleus to assemble amino acids in the correct sequence to make a protein • Transfer RNA (tRNA): in cytoplasm; translates the message in the three-letter codons in mRNA; • Transfer RNA brings specific amino acids that correspond to information in the mRNA codons so they are assembled in correct order