DNA

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.
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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.
Hydrogen bonds
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.
structural proteins 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 different recipes, 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.
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.
Mutations
Mutations occur when the DNA gene’s sequence of base pairs is changed by either a mistake during replication or by a random chemical reaction that damages the DNA
Mutations can be harmful, helpful, or silent
Harmful mutations cause a change in the DNA that produces a defective protein.
Helpful mutations are very rare, and cause a change in the DNA that produces a better protein
Silent mutations cause a change in the DNA that has no effect on the protein
A point mutation occurs when a single base pair is removed and a different base pair is substituted.
Another name for a point mutation is a substitution
Sickle Cell Anemia is caused by a single mutation that changes the codon from GAG to GUG
A frame-shift mutation occurs when a base pair is deleted or an extra base pair is added.
frame-shift mutations are caused by deletions or additions to the gene
after a frame-shift mutation, all the codons downstream from the mutation will be read wrong by the ribosomes during translation.
frame-shift mutations are always harmful.
Sentence analogy to show a frame-shift mutation
THE CAT ATE THE RAT = normal
_HEC ATA TET HER AT = deletion
THE CAT AAT ETH ERA T = addition
Inversion mutations occur when a section of a gene is cut out and re-inserted into the gene backwards in the same location
A transposition mutation occurs when a section of a gene is cut out and re-inserted somewhere else.
Indian corn kernels are different colors because of a transposition mutation
A repetitive sequence mutation produces a stretch of DNA that repeats a series of base pairs over and over.
Huntingtons Disease is caused by the repetitive sequence mutation “CAG”
10-15 repeats = normal
16-35 repeats = mild symptoms
+35 repeats = fatal disease
Mutagens
Mutagens are environmental factors or chemicals that cause mutations in DNA
Mutagens that cause mutations in genes regulating cell division can cause cancer
Mutagens that cause cancer are known as carcinogens
Carcinogens increase the risk of cancer
cigarette smoke
saccharine
asbestos
Ultraviolet (UV) radiation
X-rays
DNA Technology
Scientists have used what they know about DNA to change medicine, industry, and agriculture
Scientists can now change genetic code
Genetic engineering is the manipulation of genetic code to produce combinations of genes
Recombinant DNA is one kind of genetic engineering
DNA from different animals or plants is recombined to produce completely new combinations
bacteria, animals or plants are given metabolic abilities they never had before
This technology is very controversial
Scientists can now grow recombinant tobacco plants that glow in the dark because a gene from fireflies was added to their genome.
Soybeans have been genetically modified so that they are not killed by Roundup, a popular brand of herbicide
Examples of recombinant DNA
bacteria have been engineered to manufacture human insulin for diabetics
How recombinant DNA techniques were used to create insulin-producing bacteria:
1. locate the insulin gene that you want to move to the bacterial cell
2. cut out the insulin gene using restriction enzymes
3. locate a plasmid from a bacterial cell and cut it with the same restriction enzymes
4. mix the insulin gene and the plasmid DNA
5. Add DNA ligase to the mixture to bond the target gene into the bacterial plasmid DNA
6. Insert the recombinant plasmid into living bacteria.
Gel Electrophoresis
Gel electrophoresis is a way to analyze DNA by cutting it up into fragments, then sorting the fragments according to size.
Electricity causes the fragments to sort themselves in a pan of special gel
Restriction enzymes cut up DNA by breaking it at very specific places only.
DNA fingerprinting
get two samples of DNA that you want to compare
cut up each sample using the same restriction enzyme
run each sample through gel electropheresis side by side
compare the banding patterns
Suspect 1
Suspect 2
Crime Scene Specimen