Unit 7 • The History, Structure, Function, and Applications of DNA
The History of DNA • It took a lot of different scientists a long time to figure out that DNA is the molecule controlling inheritance of genetic traits
Soon after chromosomes were discovered, scientists were able to grind them up and learn that they were about 50% protein and 50% nucleic acid - which is DNA. % % DNA Protein
Frederick Griffith • 1928 • Experimented with Streptococcus pneumoniae, a bacterium that causes the lungs to fill up with fluid. • identified two strains • Smooth (S) strain Streptococcus • Rough (R) strain Streptococcus
S strain bacteria appear smooth under the microscope because they have a slimy mucus coating outside their cell walls. • This makes them much harder to cough up or for the immune system cells to attack.
R strain bacteria appear rough under the microscope because they don’t have the mucus coating.
Injected S strain into mice • mice died • conclusion: S strain is lethal • Injected R strain into mice • mice survived • conclusion: R strain harmless • Prediction: The bacteria must have the genetic ability to make mucus to be lethal.
Injected mice with boiled, heat-killed S strain • prediction: mice would survive because the bacteria were dead. • observation: mice survived • conclusion: Bacteria must be smooth, alive, and reproducing to cause the mice to die.
Injected mice with a mixture of dead S and living R bacteria • prediction: mice would survive • observation: mice died • conclusion: A new question - what happened?
Examined blood samples from the mice that died after injection with mixture of dead S and living R bacteria • observation: found living S bacteria • conclusion: living R are able to absorb a transforming factor from the dead S and transform themselves into S bacteria • conclusion: this transformation is passed to new bacteria as they divide, so it must be genetic material.
Avery and Macleod • 1944 • continued Griffith’s experiment • concluded that the transforming factor was probably DNA, but their evidence was not widely accepted by other scientists.
Hershey and Chase • 1952 • experimented with bacteria and viruses that infect bacteria called bacteriophages • knew that bacteriophages are made off only two things, DNA and protein
Hershey and Chase experiment • prediction: bacteriophages must inject their genetic material into their host bacteria cells in order to reproduce. • This genetic material must be either the protein or the DNA
Experimental Group 1. • grow bacteriophages and feed them radioactive sulfur • this will produces bacteriophages with radioactive protein only, since DNA contains no sulfur • Allow the bacteriophages with radioactive protein to infect host bacteria • observation: the radioactive protein did not get inside the host bacteria • conclusion: the protein is not the genetic material
Experimental Group 2. • grow bacteriophages and feed them radioactive phosphorus • this will produces bacteriophages with radioactive DNA only, since protein contains no phosphorus • Allow the bacteriophages with radioactive DNA to infect host bacteria • observation: the radioactive DNA was injected inside the host bacteria • conclusion: the DNA is the genetic material
Hershey and Chase proved without question that DNA is the genetic material, not protein.
Edwin Chargaff • 1950’s • analyzed the DNA of different animals to figure out if the proportions of adenine, thymine, guanine, and cytosine in their DNA could be used to tell them apart.
Chargaff’s observations • the amount of adenine in any animal’s DNA is always equal to the amount of thymine • the amount of guanine in any animal’s DNA is always equal to the amount of cytosine • This is called Chargaff’s Rule: • amount of A = amount of T • amount G = amount of C
Rosalind Franklin and Maurice Wilkins • used x-ray crystallography to photograph DNA crystals • produced a diffraction pattern • were able to measure the width and distance between repeats in the DNA molecule • concluded that the DNA molecule was a certain width with regular repeats
James Watson and Francis Crick • Feb 28,1953 • figured out the double helix shape of the DNA molecule
Watson and Crick were the first to put all the pieces together • from Chargaff they learned that DNA was made of two purines (adenine and guanine) and two pyrimidines (thymine and cytosine) • Chargaff’s rules meant that their was a relationship between A and T, C and G
From Franklin and Wilkins they learned that DNA was long, skinny, and the same width all the way down
The Double Helix Model • DNA is made of two parallel strands • The two strands are held together on the inside by hydrogen bonds between A and T and hydrogen bonds between G and C • The two strands twist together in a spiral or helix • The outside of each strand is made up of alternating deoxyribose and phosphate groups.
Beadle and Tatum • 1941 • experimented with mutated molds • discovered that a single mutated gene produces a single mutated enzyme • mutated enzymes don’t work properly and can cause disease
The one gene-one protein idea • Every different protein or enzyme made in the cell must have its own unique gene stored in the genetic material. • defective genes make defective proteins, and this can cause genetic diseases like • cystic fibrosis • sickle cell anemia
The Structure of DNA • DNA is a double helix molecule • Each strand is complementary to the one across from it • A only pairs across from T • G only pairs across from G
DNA Replication • DNA replication copies DNA • occurs during the “S” part of the cell cycle • occurs inside the nucleus • Each strand acts as a template for the newly forming strand • Enzymes work like machines to replicate DNA
DNA replication- how the enzymes do it • 1. DNA helicase unwinds the helix and unzips the two strands by breaking the weak hydrogen bonds • 2. DNA polymerases attach to each side and begin adding complementary nucleotides • DNA ligases seal the phosphate to sugar bonds
The Function of DNA • So far we have covered • the history of scientific research proving DNA is the molecule storing hereditary information • the structure of DNA and how it copies itself • Now we will investigate exactly how DNA codes for proteins.
DNA controls heredity by coding for how proteins are made • enzymes operate cell metabolism • examples: catalase, amylase, sucrase, proteases, polymerases, etc. • structuralproteins build many cell parts • example: keratin builds hair • example: actin and myosin build muscle
DNA is like a set of recipes the cell uses to manufacture just about everything involving proteins • Organisms inherit slightly differentrecipes, therefore their proteins are slightly different • Protein synthesis is the manufacture of proteins according to recipes.
Protein Synthesis • takes place in the cytoplasm • actual work is performed by ribosomes • Ribosomes are made of RNA
RNA • made of 4 RNA nucleotides • adenine, guanine, cytosine, and uracil • single-stranded NOT double stranded • contains the sugar ribose instead of deoxyribose.
DNA double-stranded Nitrogen bases = adenine, thymine, guanine, cytosine stored inside nucleus built with deoxyribose RNA single-stranded Nitrogen bases = adenine, uracil, guanine, cytosine made inside nucleus but used in the cytoplasm built with ribose
Protein Synthesis takes place in two steps • transcription - inside nucleus • translation - in cytoplasm
Transcription • A section of DNA containing a gene (protein recipe) unwinds and unzips • RNA polymerase builds a strand of RNA complementary to the DNA
Transcription produces a strand of RNA complementary to the DNA called mRNA • mRNA is a temporary “copy” of the gene • mRNA can leave the nucleus through pores in the nuclear membrane • mRNA can be “read” by the ribosomes in the cytoplasm to make proteins.
Translation • takes place in the cytoplasm • a ribosome attaches to the mRNA • as the ribosomes slides along, the mRNA strand is read in 3-base long words called codons. • Each codon specifies only one amino acid • Each codon is matched to a complementary anticodon on a tRNA molecule • each tRNA molecule carries the correct amino acid
Building a polypeptide chain • mRNA codons are translated to tRNA anticodons • as the tRNA’s line up side by side on the ribosome, they deliver amino acids in sequence • Each new amino acid attaches to the one before it with a peptide bond • The chain of amino acids is called a polypeptide
Polypeptides are folded into proteins • after completion, most polypeptide chains are moved inside the RER • inside the RER, the polypeptide chain is folded into a three-dimensional shape • The shape of a protein determines its function • enzyme proteins must be folded to produce their active sites
The codon chart • each codon specifies one and only one amino acid • Examples • UUU codes for phenylalanine (phe) • GUG codes for valine (val)
Some amino acids have up to six different codons, while others have only one • Serine(ser) can be specified by the codons UCU, UCA,UCG,UCC, AGU, and AGC • Tryptophan has only one codon - UGG
Signal Codons • RNA polymerase needs a signal to know where to start translating the mRNA strand, and a signal to stop translating: • AUG is the “start” codon that begins the polypeptide chain with Methionine (met) • UAA, AUG, and UGA are “stop” codons that signal the ribosome to let go of the mRNA strand.