Chapter 13 DNA, RNA, and Proteins. Determining the Chemical Composition of DNA As scientists...

Chapter 13Chapter 13

DNA, RNA, and ProteinsDNA, RNA, and Proteins

Determining the Chemical Composition of DNA

As scientists continued their experiments with viruses and bacteria, many results were observed that gave support to the notion that DNA was the hereditary material.

A classic experiment demonstrated the genetic role of DNA.

As scientists continued their experiments with viruses and bacteria, many results were observed that gave support to the notion that DNA was the hereditary material.

A classic experiment demonstrated the genetic role of DNA.

Frederick GriffithFrederick Griffith

Studied the bacterium that caused pneumonia--S. pneumonia.

Worked with two strains: pathogenic and non-pathogenic.

Studied the bacterium that caused pneumonia--S. pneumonia.

Worked with two strains: pathogenic and non-pathogenic.

Frederick GriffithFrederick Griffith

An experimental overview: (S) smooth cells produce mucous

capsules that protect the bacteria from an organism’s immune system--pathogenic.

(R) rough cells have no mucous capsule and are attacked by an organism’s immune system--non pathogenic.

An experimental overview: (S) smooth cells produce mucous

capsules that protect the bacteria from an organism’s immune system--pathogenic.

(R) rough cells have no mucous capsule and are attacked by an organism’s immune system--non pathogenic.

Frederick Griffith, His Experiment

Mixed heat-killed pathogenic (S) bacteria with living non-pathogenic (R) bacteria, the non-pathogenic (R) bacteria began producing the mucous capsule and became pathogenic (S).

The new bacteria that arose from the bacteria were somehow transformed into pathogenic S. pneumonia.

Griffith called this process transformation.

Mixed heat-killed pathogenic (S) bacteria with living non-pathogenic (R) bacteria, the non-pathogenic (R) bacteria began producing the mucous capsule and became pathogenic (S).

The new bacteria that arose from the bacteria were somehow transformed into pathogenic S. pneumonia.

Griffith called this process transformation.

Griffith’s Transformation Experiment

Did not identify DNA as the transforming factor, but it set the stage for other experiments.

Oswaldt AveryOswaldt Avery

Avery worked for a long time trying to identify the transforming factor.

After isolating and purifying numerous macromolecules from the heat killed pathogenic bacteria he and his colleagues could only get DNA to work.

The prevailing beliefs about proteins vs. DNA continued to generate skepticism.

Avery worked for a long time trying to identify the transforming factor.

After isolating and purifying numerous macromolecules from the heat killed pathogenic bacteria he and his colleagues could only get DNA to work.

The prevailing beliefs about proteins vs. DNA continued to generate skepticism.

T2 Phage ReproductionT2 Phage Reproduction

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The Hershey-Chase Experiment

In 1952, Alfred Hershey and Martha Chase performed experiments with viruses showing that DNA is genetic material.

Viruses (aka phages) are DNA or RNA wrapped in a protein.

E. coli is a bacteria that is often used in experiments.

In 1952, Alfred Hershey and Martha Chase performed experiments with viruses showing that DNA is genetic material.

Viruses (aka phages) are DNA or RNA wrapped in a protein.

E. coli is a bacteria that is often used in experiments.

Used the T2 phage because it was generally accepted to be DNA wrapped in protein.

Used E. coli because it was easily obtainable and was readily attacked by T2.

Had to demonstrate whether or it was DNA or protein that was the hereditary factor.

Used the T2 phage because it was generally accepted to be DNA wrapped in protein.

Used E. coli because it was easily obtainable and was readily attacked by T2.

Had to demonstrate whether or it was DNA or protein that was the hereditary factor.

Their experiment demonstrated which part of the T2 entered the E. coli.

They grew T2 in the presence of radioactive sulfur--proteins contain sulfur, DNA does not.

Next, they grew the T2 in a separate batch of radioactive phosphorous. The DNA of T2 contains phosphorous--the proteins do not.

Their experiment demonstrated which part of the T2 entered the E. coli.

They grew T2 in the presence of radioactive sulfur--proteins contain sulfur, DNA does not.

Next, they grew the T2 in a separate batch of radioactive phosphorous. The DNA of T2 contains phosphorous--the proteins do not.

The scientists now had 2 batches of T2, one labeled with radioactive sulfur and one labeled with radioactive phosphorous.

These 2 batches were separately incubated with non-radioactive samples of E. coli and analyzed shortly after infection.

The scientists now had 2 batches of T2, one labeled with radioactive sulfur and one labeled with radioactive phosphorous.

These 2 batches were separately incubated with non-radioactive samples of E. coli and analyzed shortly after infection.

Shortly after infection, the E. coli samples were spun in a blender to knock off loose parts of T2.

The mixtures were then spun in high speed centrifuges for a long time to separate out various parts of the mixture.

At the bottom of the tube was a pellet of E. coli.

Shortly after infection, the E. coli samples were spun in a blender to knock off loose parts of T2.

The mixtures were then spun in high speed centrifuges for a long time to separate out various parts of the mixture.

At the bottom of the tube was a pellet of E. coli.

The pellet was examined for radioactivity and radioactive phosphorous was found.

The supernatant was analyzed and a lot of radioactive sulfur was found, but no radioactive phosphorous.

This indicates that the DNA got into the E. coli and was in the pellet

The protein did not get into the bacteria and was left in the supernatant.

The pellet was examined for radioactivity and radioactive phosphorous was found.

The supernatant was analyzed and a lot of radioactive sulfur was found, but no radioactive phosphorous.

This indicates that the DNA got into the E. coli and was in the pellet

The protein did not get into the bacteria and was left in the supernatant.

Furthermore, when the bacteria in the pellet were plated on culture medium, they produced more T2 containing radioactive phosphorous.

They concluded: That the virus injects DNA into the E.

coli and it is the genetic material that programs the cells to produce new T2 phages.

The protein stays outside. This experiment provided firm

evidence that DNA was the hereditary material and not protein.

They concluded: That the virus injects DNA into the E.

coli and it is the genetic material that programs the cells to produce new T2 phages.

The protein stays outside. This experiment provided firm

evidence that DNA was the hereditary material and not protein.

• Movie• Movie

Erwin Chargaff’s Experiment

He discovered that the amount of adenine is equal to the amount of thymine and cytosine equaled the amount of guanine.

Chargaff did not know what all of this meant, but after the elucidation of the shape of the DNA molecule, these became known as Chargaff’s Rules.

He discovered that the amount of adenine is equal to the amount of thymine and cytosine equaled the amount of guanine.

Chargaff did not know what all of this meant, but after the elucidation of the shape of the DNA molecule, these became known as Chargaff’s Rules.

Watson and CrickWatson and Crick

In 1953, James Watson and Francis Crick visited a lab of Maurice Wilkins.

Examined lab data (an X-ray diffraction image of DNA) produced by Rosalind Franklin.

In 1953, James Watson and Francis Crick visited a lab of Maurice Wilkins.

Examined lab data (an X-ray diffraction image of DNA) produced by Rosalind Franklin.

DNA StructureDNA Structure

Through trial and error Watson and Crick concluded A paired with T and C with G. This gave them the information they needed to determine the shape and structure of the DNA molecule.

DNA ReplicationDNA Replication

Begins at a site called the origin of replication.

Prokaryotes have one origin of replication.

Eukaryotes have hundreds of thousands of origins of replication.

Begins at a site called the origin of replication.

Prokaryotes have one origin of replication.

Eukaryotes have hundreds of thousands of origins of replication.

Here is an electron micrograph and a schematic representation of bacterial DNA replication.

Primers are the short nucleotide fragments (DNA or RNA) to which DNA polymerase will add nucleotides according to the base paring rules.

Primase is the enzyme that creates a primer that can initiate the synthesis of a new DNA strand.

Primers are the short nucleotide fragments (DNA or RNA) to which DNA polymerase will add nucleotides according to the base paring rules.

Primase is the enzyme that creates a primer that can initiate the synthesis of a new DNA strand.

DNA polymerases are enzymes that catalyze the elongation of DNA at the replication fork.

One by one, nucleotides are added by DNA polymerase to the growing end of the DNA strand.

DNA polymerases are enzymes that catalyze the elongation of DNA at the replication fork.

One by one, nucleotides are added by DNA polymerase to the growing end of the DNA strand.

Helicase is the enzyme responsible for untwisting the double helix at the replication fork.

This separates the parental strands of DNA making them available for use as template strands.

Helicase is the enzyme responsible for untwisting the double helix at the replication fork.

This separates the parental strands of DNA making them available for use as template strands.

Eventually, when replication is complete, two new strands of DNA are formed and the cell is now ready for cell division.

QuickTime™ and a decompressor

are needed to see this picture.

Getting From Gene to Protein

Now that we understand how DNA is replicated and what genes are contained within it, the next question we need to answer is how we get from gene to protein.

Transcription and Translation

These terms describe how we get from gene to protein.

Transcription is the synthesis of mRNA using DNA as the template. Similar to DNA synthesis. mRNA is the message (hence the “m”)

from the gene. Translation is the process that occurs

when the mRNA reaches the ribosome and protein synthesis occurs.

These terms describe how we get from gene to protein.

Transcription is the synthesis of mRNA using DNA as the template. Similar to DNA synthesis. mRNA is the message (hence the “m”)

from the gene. Translation is the process that occurs

when the mRNA reaches the ribosome and protein synthesis occurs.

TranscriptionTranscription

The gene determines the sequence of bases along the length of the mRNA molecule.

One of the two regions of the DNA serves as the template.

RNA polymerase pries apart the DNA and joins RNA nucleotides together.

The gene determines the sequence of bases along the length of the mRNA molecule.

One of the two regions of the DNA serves as the template.

RNA polymerase pries apart the DNA and joins RNA nucleotides together.

The Synthesis of mRNA-Transcription

RNA polymerase encounters a promoter on the DNA near a transcriptional unit and starts synthesizing RNA.

When the RNA polymerase encounters a terminator sequence, transcription stops.

RNA polymerase encounters a promoter on the DNA near a transcriptional unit and starts synthesizing RNA.

When the RNA polymerase encounters a terminator sequence, transcription stops.

TranscriptionTranscription

Movie Movie

TranslationTranslation

mRNA triplets are called codons. Codons are read by the ribosome

along the mRNA and the appropriate amino acid is incorporated into the protein.

When the “start codon” is read by the ribosome, the protein starts growing.

When a “stop codon” is reached, protein synthesis stops.

As this is done, the protein begins to take shape.

mRNA triplets are called codons. Codons are read by the ribosome

along the mRNA and the appropriate amino acid is incorporated into the protein.

When the “start codon” is read by the ribosome, the protein starts growing.

When a “stop codon” is reached, protein synthesis stops.

As this is done, the protein begins to take shape.

Genetic CodeGenetic Code

The genetic code is said to be redundant.

More than one triplet codes for the same amino acid.

One triplet only codes for one amino acid.

The genetic code is said to be redundant.

More than one triplet codes for the same amino acid.

One triplet only codes for one amino acid.

Translation is when the cell interprets the genetic message and builds the polypeptide. tRNA acts as the interpreter.

tRNA transfers aa’s from the cytoplasm to the ribosome where they are added to the growing polypeptide.

Translation is when the cell interprets the genetic message and builds the polypeptide. tRNA acts as the interpreter.

tRNA transfers aa’s from the cytoplasm to the ribosome where they are added to the growing polypeptide.

Movie Movie

RibosomesRibosomes These are the sites of

protein synthesis. These are the sites of

protein synthesis.

The 3 Stages of Protein Building

1. Initiation 2. Elongation 3. Termination

1. Initiation1. Initiation

Initiation brings together mRNA, tRNA and the ribosome.

2. Elongation2. Elongation

The elongation stage is where amino acids are added one by one to the growing polypeptide chain.

3. Termination3. Termination

Termination occurs when the ribosome reads a “stop codon” on the mRNA.

This signals the end of translation.

Termination occurs when the ribosome reads a “stop codon” on the mRNA.

This signals the end of translation.

Chapter 13 DNA, RNA, and Proteins. Determining the Chemical Composition of DNA As scientists...

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