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CHAPTER 9 DNA, RNA, Translation and Transcription
The Discovery Of DNA Mendel studied the pea plants in the late 1800’s Scientists wanted to know what it was that contained hereditary factors An epidemic of Pneumonia in London in the 1920’s sparked research which eventually leads to the discovery of DNA.
GRIFFITH’s EXPERIMENTS 1928, London, England: Fredrick Griffith was attempting to discover a vaccine for the virulent strain, or disease causing strain of the Streptococcus pneumoniae bacterium. Streptocuccus pneumoniae was causing pneumonia
GRIFFITH’s EXPERIMENT By studying the bacterium he discovered that: The virulent form was surrounded by a capsule made of polysaccarides that protects it from the body’s defenses. So, when grown in a petri dish, they grow as smooth colonies. (S strain) There was an another strain that did not cause pnuemonia. This harmless strain lacked a capsule and as a result grew into rough colonies in the petri dish. (R strain)
GRIFFITH’s EXPERIMENTS Griffith experimented with mice. He injected a mouse with the S strain and it died He injected another mouse with the R strain, it lived He heated-killed the S strain and injected it into the mouse, it lived He then combined heat-killed S strain and live R strain and injected it into the mouse.
GRIFFITH’s RESULTS When injected with the heat killed S strain combined with the R strain, the mouse died. Griffith concluded that transformation occurred. Transformation is a type of transfer of genetic material from one cell to another OR from one organism to another organism.
Avery’s Experiments 1940’s, United States: Oswald Avery wanted to test whether the transforming agent in Griffith’s experiments was protien, RNA or DNA. The scientists used enzymes to destroy separately each of the 3 molecules in the Heat-Killed S strain.
Avery’s Experiments Experiment #1: Protease was used to destroy the protien in one batch of the heat-killed S strain. Experiment #2: RNase was used to destroy the RNA in another batch of heat-killed S Strain Experiment #3: DNase was used to destroy the DNA in another batch of heat-killed S strain. Each batch was combined with a batch of live harmless R strain and injected into different mice
Avery’s Results The strain missing the protein and the RNA were still able to transform the R strain into the S strain and kill the mice. The strain missing DNA did NOT transform the R strain and the mouse lived. Result: DNA was responsible for transformation in the bacteria
HERSHEY-CHASE EXPERIMENT 1952, United States: Martha Chase & Alfred Hershey tested whether DNA or protein was the hereditary material viruses transfer when viruses enter a bacterium. Bacteriophages – viruses that infect bacteria
HERSHEY-CHASE EXPERIMENT Radioactive isotopes was used to label the protein and DNA in the bacteriophage. Radioactive Sulfur ( 35 S) was used to label protein Radioactive phosphate ( 32 P) was used to label DNA Then, they allowed the protein-labeled and the DNA-labeled bacteriophages to independently infect E-coli bacteria.
HERSHEY-CHASE RESULTS The scientists removed the coats of the viruses by putting the solution in a blender The scientists then separated the bacteriophages from the E coli by centrifuge They discovered that all of the viral DNA and only a little bit of the protein had entered the E coli. Conclusion: DNA is the hereditary molecule in Viruses
SECTION 2: DNA STRUCTURE 1950’s: By now most scientists understood and accepted that DNA was molecule that contained the hereditary information. What they wanted to discover was how this molecule could replicate, store & transmit hereditary information
THE DOUBLE HELIX 1950’s, James Watson, Maurice Wilkens and Francis Crick Won the Nobel Prize in Medicine for their theory about the structure of DNA. Structure was developed with the help of Rosalind Franklin and her x-ray photographs of DNA crystals
DNA NUCLEOTIDES DNA is made up of TWO long chains (or strands) of repeating subunits called nucleotides Nucleotides contain THREE parts: –Deoxyribose: A 5-carbon sugar –A phosphate group –A nitrogen base – contains a Nitrogen, Carbon and accepts hydrogen: Adenine, Guanine, Cytosine, and Thymine
Bonds Hold DNA together The DNA double helix is similar to a spiral ladder. The alternating sugar and phosphate molecules form the sides of the ladder Nucleotides are held together by covalent bonds between the sugar of one nucleotide and the phosphate of the next nucleotide There are 10 nucleotide pairs for each spiral turn of the DNA helix
Bonds Hold DNA together: the rungs The bases (nitrogen bases) face toward the center of the DNA molecule. The bases form hydrogen bonds with bases on the other side and make up “the rungs of the ladder” All pairs are of uniform width: in each pair, one base has a two ring structure and other base has a one ring structure
Nitrogen Bases While the sugar and phosphate groups are identical, the nitrogen bases could be one of four kind (broken down to two groups by structure): Purines: contain a double ring of carbon & nitrogen: Adenine & Guanine Pyrimidines: contain a single ring of carbon & nitrogen: Thymine & Cytosine
COMPLIMENTARY BASES Just before DNA structure was confirmed: it was found that the percentage of adenine is equal to the percentage of thymine It was also found that the percentage of cytosine was equal to the percentage of guanine Upon Observation, Base-Pairing Rules was uncovered – cytosine pairs only with guanine, and adenine pairs only with thymine
COMPLIMENTARY BASES These pairs are known as Complementary base pairs – one double-ringed purine and one single-ringed pyrimidine Because of these base pairing rules, the order if bases on one side of a chain of DNA is complementary to the order of bases on the other side. Base Sequence: the order of nitrogen bases on a chain of DNA
Complementary Bases WHY IS THE BASE PAIRING IMPORTANT? Hydrogen bonds b/t the base pairs help hold the two strands of a DNA molecule together The complementary nature of DNA helps explain how it is that DNA replicates before a cell divides – One strand of a DNA molecule can serve as a template for making a new complimentary strand.
DNA MODELS You will see the structure of DNA simplified from the actual double helix model to a straight ladder model or just the base pairings This is because the only variable is the base pairs, the sugar and phosphate groups are identical in all DNA
Section 3: DNA REPLICATION The discovery of the double helix structure of DNA explains how it can replicate exactly each time a cell divides, the key feature of hereditary material.
How DNA replicates DNA Replication is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis or binary fission. Because the two strands of DNA are complimentary, each serve as a template to make a NEW COMPLIMENTARY STRAND After replication, the 2 identical double- stranded DNA molecules separate & move to new cells formed during cell division.
STEPS OF DNA REPLICATION Step 1 Helicases: Enzymes that separate the DNA strands Helicase move along the strands and breaks the hydrogen bonds between the complimentary nitrogen bases Replication Fork: the Y shaped region that results from the separation of the strands
STEPS.. Step 2 DNA Polymerase: enzymes that ADD complimentary that ADD complimentary nucleotides. nucleotides. Nucleotides are found floating freely inside the nucleus freely inside the nucleus Covalent bonds form between the phosphate group of one the phosphate group of one nucleotide and the deoxyribose nucleotide and the deoxyribose of another of another Hydrogen bonds form between the complimentary nitrogen bases
STEPS.. Step 3 DNA polymerases finish replicating the DNA and fall off. The result is two identical DNA molecules that are ready to move to new cells in cell division. Semi-Conservative Replication: this type of replication where one strand is from the original molecule and the other strand is new
Something about Step 2 In step 2, remember that each strand is making its own new strand. DNA synthesis is occurring in two different directions One strand is being made towards the replication fork and the other is being made away from the fork. The strand being made away from the fork has gaps. Gaps are later joined by another enzyme, DNA ligase
Prokaryotic Replication Prokaryotic Cells have one circular chromosome. Two replication forks are formed in the same area of the chromosome. They proceed in opposite directions – like two zippers Replication continues along each fork until they meet and the entire the entire molecule is copied
DNA ERRORS Usually DNA replication occurs without any errors. Only about one error in about every billion replications occurs. DNA polymerases have a repair function that “proofreads” the DNA. It will replace a wrongly placed nitrogen base Mutation: a change in the nucleotide sequence. A mutation can have serious effects on the function of an important gene and disrupt an important cell function
DNA Errors Chemicals and Ultraviolet radiation from the sun and tanning booths can change DNA Some mutations can lead to cancer. Mutations in the way the cell divides can lead to tumors. An effective mechanism for the repair of damaged DNA is very important to the survival of an organism. Studying DNA replication is important to understanding and treating various types of cancer.
Section 4: Protein Synthesis DNA contains genes that code for a hereditary characteristic, example: hair color. DNA contains genes that code for a hereditary characteristic, example: hair color. The gene that codes for hair color directs the making of a protein called melanin in the hair follicle. The gene that codes for hair color directs the making of a protein called melanin in the hair follicle. The protein is made through an intermediate (middle man) – a nucleic acid called RNA, Ribonucleic Acid The protein is made through an intermediate (middle man) – a nucleic acid called RNA, Ribonucleic Acid
RNA STRUCTURE & FUNCTION DNA and RNA are similar in that they are both made up of nucleotides. DNA and RNA differ in Four Ways: 1. RNA has ribose, DNA has deoxyribose 2. RNA contains a nitrogen base uracil instead of thymine 3. RNA is single stranded* 4. RNA is much shorter than DNA. It contains the information for one gene.
TYPES OF RNA Three Major Types of RNA Three Major Types of RNA 1. Messenger RNA (mRNA) ~ single stranded RNA molecule that carries the instructions from a gene to make a protein. Carries the genetic “message” from the DNA in the nucleus to the ribosomes in the cytosol
TYPES OF RNA Three Major Types of RNA 2. Ribosomal RNA (rRNA) ~ part of the structure of ribosomes, where protein synthesis occurs 3. Transfer RNA (tRNA) ~ transfers amino acids to the ribosome to make a protein.
FLOW OF GENETIC INFORMATION How the information goes from DNA to the Ribosomes and into protein form. 1.Transcription – DNA acts as a template for the synthesis of RNA 2.Translation – RNA directs the assembly of the proteins. 3.Protein Synthesis – proteins are formed based on information in DNA and carried out by RNA in the ribosomes
TRANSCRIPTION Transcription means that the information within DNA is transcribed/“rewritten” as an RNA molecule Occurs in three steps: 1.RNA polymerase, an enzyme that catalyzes (starts) the formation of RNA on a DNA template. a.A promoter is a specific nucleotide sequence of DNA where RNA polymerase binds and initiates transcription. b.After RNA polymerase binds to the promoter, DNA strands unwind and separate
TRANSCRIPTION THREE STEPS OF TRANSCRIPTION 2.RNA polymerase adds free RNA nucleotides that are complementary to the nucleotides on one of the DNA strands. The resulting chain is an RNA molecule. a.Complementary base-pairing determines the nucleotide sequence in the newly made RNA. b.Transcription only occurs in a specific area (one gene) of the DNA. RNA polymerase moves past the area and DNA rewinds
TRANSCRIPTION THREE STEPS OF TRANSCRIPTION 3.RNA polymerase reaches the terminal signal, a specific sequence of nucleotides that marks the end of the gene. a.Upon reaching this mark, RNA polymerase releases both the DNA & the newly formed RNA. b.The newly formed RNA can be any type of RNA, free to perform it’s job within the cell.
The Genetic Code What makes up a protein? –Amino Acids Instructions on which amino acids to assemble are coded within the sequence of nucleotides. Genetic Code – the term for the rules that relate how a sequence of nitrogen bases corresponds to a particular amino acid There are 20 different amino acids found in living things
The Genetic Code Codon – each 3-nucleotide sequence in mRNA that encodes an amino acid or signifies a start or stop sequence
unwindsunzips RNA Polymerase unwinds and unzips DNA (but does not proof-read… why not?) Complementary NTP’s add to template DNA strand from 5’ to 3’ RNA Polymerase begins transcribing the DNA at a specific point RNA strand is identical to the non-coded DNA (and complementary to the template strand) EXCEPT FOR...
Same process Same process as Prokaryotes! After mRNA is transcribed from DNA then the mRNA has a different fate in prokaryotes and eukaryotes translating Prokaryotes immediately begin translating the mRNA. Eukaryotes must process it first.
mRNA Processing: intron/exon methyl cap poly-A tail No mRNA Processing
Viral DNA injected into cells nucleasesCells evolve nucleases in cytoplasm that chomp up any RNA or DNA out there Nucleases can’t get through the nuclear envelope so DNA is safe mRNA sent out into the cytoplasm must be protected –Methyl cap is a block –Poly A tail is a fuse mRNA is still chomped up into NTP’s and recycled, but the Poly A tail gives it some time
“non- coding DNA”Eukaryotic DNA is composed mostly of “non- coding DNA” (or “junk DNA”) –We’re still not entirely sure what it does –Was probably inserted by different viruses over time –The ultimate selfish gene just hitching a ride on a successful group of genes… introns exonsThe introns are the sections of DNA not expressed, the exons are the sections that are expressed (ex-ons are ex-pressed, get it?) SpliceosomeSpliceosome loops out the introns and snips them out So now we’ve got some mRNA that codes for a protein
TRANSLATION the making of a protein The start codon: AUG – a specific sequence (Adenine, Uracil, Guanine) of nucleotides in mRNA that indicates where translation should begin – Methionine UGA – stop codon - doesn’t code for an amino acid but signals for translation to stop
TRANSLATION Every protein is made up of one or more polypeptides. Polypeptides are amino acid chains which are linked by peptide bonds Each polypeptide chain may consist of hundreds of thousands of the 20 different amino acids – arranged in a sequence that is specific to THAT PARTICULAR PROTEIN
TRANSLATION STEP ONE Initiation: tRNA and mRNA join together. The tRNA carries a anticodon – three nucleotides that are complimentary to the sequence of codon on the RNA (the start codon) The start amino acid is methinonine but this would be removed later
TRANSLATION STEP TWO Elongation – the chain is put together Another tRNA carrying the second appropriate amino acid pairs with the mRNA A peptide bond forms between the first amino acid and this one (and so on). The first tRNA detaches
TRANSLATION Step Three The polypeptide chain continues to grow Another tRNA moves in, carrying the amino acid for the next mRNA codon The growing chain moves from one tRNA to the amino acid attached to the next tRNA
TRANSLATION Step Four The ribosome reaches the stop codon The newly made polypeptide falls off. Step Five The ribosome complex falls apart. The newly made polypeptide chain is released The last tRNA leaves the ribosome The ribosome is free to translate the same or another mRNA
THE HUMAN GENOME The Human Genome Genome – the complete genetic content In 1999, scientists have mapped the entire gene sequence of the human genome They now know the order of the 3.2 billion base pairs in the 23 human chromosomes The next challenge is to learn what information the DNA sequences actually code for.