Chapter 12 DNA & RNA

The genetic code is the way in which the cell stores information that it passes on from generation to generation.

GRIFFITH & TRANSFORMATION

Griffith studied ways in which types of bacteria cause

pneumonia.

His main concern was to make sure that pneumonia causing bacteria did not produce toxins.

He worked with 2 slightly different strains of bacteria.

Both strains of bacteria grew well in his lab but only one of them actually caused the disease.

*The disease causing bacteria grew into SMOOTH colonies on the culture plates…Page 288 Figure 12-2

The harmless strain grew into rough TEXTURED colonies.

GRIFFITH’S EXPERIMENT:

1.He injected mice with the SMOOTH bacteria & they developed pnuemonia and died.

2.He injected mice with the ROUGH non-disease causing bacteria and the mice LIVED.

??Do disease causing bacteria produce a poison???

3. Griffith heat killed the disease causing bacteria and injected them into the mice…the mice survived!

??Maybe Pneumonia is not caused by a toxin released by the disease causing bacteria??

4. In his next experiment Griffith injected a mixture of harmless bacteria & heat-killed bacteria into the mice…the mice developed pneumonia.

When mixed, the heat-killed harmful strain had passed on its disease causing ability to the harmless rough bacteria.

Due to this surprise Griffith recovered the bacterial strains from the animals

that had developed pneumonia & grew them on culture plates…the colonies that grew were SMOOTH!

CONCLUSION: One strain of bacteria had been transformed into another strain. Griffith referred to this as TRANSFORMATION.

Video Classroom Clip: Griffith’s Video Classroom Clip: Griffith’s Experiment Experiment

The Transforming Factor

GRIFFITH’S HYPOTHESIS: When live harmless bacteria & heat-killed bacteria were mixed, a

factor was transformed from the heat-killed cells to the live cells.

In 1944 a group of scientists lead by Oswald Avery (@Rockefeller Institute, NY) repeated Griffith’s experiment & identified DNA as the transforming factor.

Video Segment 5 minutes:Video Segment 5 minutes:

Oswald Avery, Colin MacLeod and Oswald Avery, Colin MacLeod and MaclynMaclyn McCardyMcCardy: Confirming DNA is the Transform: Confirming DNA is the Transform

DNA- deoxyribonucleic acid

The nucleic acid that stores & transmits the genetic information from one generation of an organism to the next…DNA CARRIES THE GENETIC CODE!

Alfred Hershey & Martha Chase studied the viruses that infect bacteria. Viruses are non-living particles smaller than a cell that can infect living organisms.

BACTERIOPHAGE- “bacteria-eater”…a virus that infects bacteria.

Video Segment: Hershey ChaseVideo Segment: Hershey Chase

Bacteriophages are composed of :

1.A DNA Core

2.A Protein Coat (Capsid)

The DNA core & Protein Coat make up the head of the

bacteriophage.

The bacteriophage also has a tail.

How a Bacteriophage infects a bacterial cell:

1-The bacteriophage attaches to the bacterial cell & injects a material into the bacterium.

2. The injected material reproduces using the DNA of the bacterial cell…more bacteriophages are made.

3. The bacterial cell becomes so filled with the bacteriophages that it LYSES (breaks open) & infects other cells.

Hershey & Chase wanted to find out what part of the infecting virus was the material that entered the bacterium…was it the protein coat, the DNA or both?

Hershey-Chase Experiment

Radioactive Tagging

(Page 290)

Two groups of bacteriophages were prepared: one had radioactive sulfur-35 added to it & the other had radioactive phosphorus-32 added.

The radioactive isotopes were used to trace the pathway of certain materials….this was a way of “labeling” the viruses.

The protein coat was labeled with sulfur-35 & the DNA was labeled with Phosphorus-32.

Proteins contain little or no phosphorus & DNA does not contain sulfur…

If sulfur was found in the bacteria this would mean that the viruses protein coat was injected into the cell.

If phosphorus was found, then it was the DNA that had been injected.

RESULTS: Nearly all of the radioactive sulfur remained with the viruses…nearly all of the radioactive phosphorus had entered the bacteria.

CONCLUSION: The virus’s DNA enters a bacteria & the protein coat remains behind on the outside of the bacteria…This was convincing evidence that DNA is the molecule that contains genetic code!

THE STRUCTURE OF DNA

Page 291

How can DNA store information AND duplicate itself?

DNA is a polymer found in units called NUCLEOTIDES.

P.A. P.A. LeveneLevene: DNA Composition of 4 Nitrogen Bases and : DNA Composition of 4 Nitrogen Bases and DeoxyriboseDeoxyribose Sugar Sugar (01:38) (01:38)

NUCLEOTIDE- a molecule made up of three basic parts:

1-A 5 carbon sugar = Deoxyribose

2-A Phosphate Group

3-A nitrogen containing base

NucleotideNucleotide

DNA contains 4 Nitrogen Bases:

1.Adenine & 2.Guanine

*Both belong to a group of compounds known as purines.

3. Cytosine & 4. Thymine

*Belong to pyrimidine group;

Individual nucleotides are joined together to form a long chain.

The sugars & phosphate groups form the BACKBONE of the chain. The nitrogen bases form the MIDDLE PART of the chain.

X-RAY EVIDENCE

Rosalind Franklin, in the early 1900’s

purified a sample of DNA, stretched it

out & made X-ray diffraction

pictures of the

molecule.

See page 292 Time line…

VIDEO CLIP:VIDEO CLIP:RosalindRosalind

Franklin and Maurice Wilkins: X-Ray Crystallography of DNA Franklin and Maurice Wilkins: X-Ray Crystallography of DNA

(It was noted that the fibers that make up the DNA molecule are twisted like strands of a rope…see pg 292…the X near the center)

**It was also noted that large groups of molecules in the fiber are spaced out at regular intervals along the length of the fiber.

*The X-ray did not allow us to determine the structure of DNA…it was more like a fingerprint to give scientists evidence to work with.

BUILDING A MODEL OF DNA

Francis Crick (British physicist) & James Watson (25 year old American biochemist) tried to determine the structure of DNA by building a 3-D models of the atomic groups in DNA.

After viewing Franklin’s x-ray pattern…they realized something important about the pattern…& within weeks Watson & Crick determined the structure of DNA.

THE DOUBLE HELIX

The shape that Watson & Crick used for their model was a helix similar to a spiral but it was twisted.

Franklin’s idea of DNA containing 2 strands was also useful in determining the structure.

Watson & Crick decided that the structure would be a double helix.

Characteristics Described by the Watson & Crick Model:

1. The nitrogen bases on each of the strands of DNA were positioned exactly opposite each other.

2. Nitrogen base positioning allows weak Hydrogen bonds to form between the nitrogen bases ADENINE & THYMINE…and between CYTOSINE & GUANINE.

Erwin Chargaff observed that in any sample of DNA, the # of adenine molecules was equal to the # of thymine molecules.

This was also true for cytosine and guanine.

Video clip:Video clip:ErwinErwin ChargaffChargaff: DNA is Not Equal For All Species and Ratio of Bases Varies : DNA is Not Equal For All Species and Ratio of Bases Varies

AmonAmon (01:24) (01:24)

This observation allowed Watson & Crick to determine that adenine bonds only to thymine and cytosine bonds only to guanine.

BASE PAIRING- the attraction of one base for the other.

Base pairing holds the 2 strands of a DNA double helix together.

Waston & Crick’s model was quickly accepted because it also showed how DNA could replicate.

(The model explained much about the structure of DNA, including placement of the nitrogen bases & the formation of hydrogen bonds!)

In 1962 Watson, Crick, & Maurice Wilkins were awarded the Nobel Prize for medicine/physiology…for their discoveries concerning the DNA molecule.

Rosalind Franklin was not included because she had already died.

12-2 Chromosomes & The Replication of DNA

Before a cell divides, the DNA must be duplicated in a process called REPLICATION that is carried out by a series of enzymes.

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Duplicating the DNA ensures that each resulting cell will have a complete set of DNA

DNA Replication/Synthesis

Defined: the process in which DNA is duplicated before a cell divides.

The 2 strands of a DNA double helix are COMPLEMENTARY…the 2 halves are not identical but one half tells us what the other half looks like.

DNA contains 4 nitrogen bases..and each half of the DNA strand can specifically direct the sequence of the other half by complementary base pairing.

Each strand of DNA serves as a template or pattern from which a new strand is made.

The enzymes that carry out DNA replication separate or unzip the DNA molecule, insert the appropriate bases, & produce sugar phosphate links to extend the growing DNA chains.

The enzymes also proof read the bases to make sure that they have been paired correctly.

Replication begins when the DNA molecule unzips.

The unzipping occurs when the hydrogen bonds between the base pairs are broken and the molecule unwinds.

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(Template paper example)

EX: T-A-C-G-T-T will produce complementary bases

A-T-G-C-A-A

The 2 DNA molecules that are produced are identical to each other…and identical to the original molecule.

12-3

RNA & Protein Synthesis

RNA

The DNA model that was developed by Watson & Crick did not explain how the information contained in the DNA molecule was used by the cell.

This process involves a 2nd nucleic acid called RNA (ribonucleic acid).

TRANSCRIPTION

Defined: the process of copying DNA to form a complementary RNA molecule called :

Messenger RNA (mRNA)….

this is the name given because of how the RNA strand is made from a DNA pattern.

RNA is the nucleic acid that acts as a messenger between the DNA in the nucleus & the ribosomes during the process of making proteins from amino acids in the cell.

Transcription is the 1st step in the synthesis (making) of cell proteins.

In transcription, information is transferred from DNA to RNA.

This transfer of information is important because DNA is found only in the nucleus & a messenger or carrier must bring the genetic information from the nucleus out to the ribosomes in the cytoplasm.

mRNA-Messenger RNA- 1 of the 3 main types of RNA…

(Carries information from the nucleus to the cytoplasm!)

RNA is similar to DNA in structure because it too is a long chain made up of nucleotides.

mRNA is the blueprint of the genetic code.

3 Major Differences Between RNA & DNA

1.The sugar in RNA is ribose…in DNA it is deoxyribose.

2.RNA consists of a single strand of nucleotides although it can form double stranded sections by folding back on itself in loops…DNA is double stranded.

3. RNA contains all of the same nitrogen bases as DNA except for one…RNA contains Uracil in the place of thymine.

There are 3 types of RNA molecules in cells: Messenger RNA (mRNA), Transfer RNA (tRNA), & Ribosomal RNA (rRNA).

PROCESS OF TRANSCRIPTION

1-Portions of the DNA molecule unwind & the 2 strands separate.

2-RNA nucleotides pair with complementary bases on a DNA strand forming a strand of mRNA(complementary strand).

*The single strand of DNA serves as a template/pattern for synthesizing mRNA.

The hereditary information in the sequence of DNA is copied in complementary form into the nucleotide sequence of mRNA.

The enzyme RNA polymerase is very important in transcription.

An enzyme is very specific as to what substance it works on…this particular enzyme works on the polymers DNA & RNA.

(Polymer- a long strand of molecules)

In transcription the RNA polymerase attaches to a specific place on the DNA molecule & separates the two strands of the double helix…a mRNA strand is also made.

Amino acids are the building blocks of proteins.

The genetic code for each amino acid is a specific sequence of 3 nucleotides.

CODON- 3 nucleotide sequence in mRNA that specifies a certain/particular amino acid.

Certain DNA sequences serves as START (GO) signals & are recognized by the RNA polymerase enzyme.

Other areas are recognized as TERMINATION SITES (STOP) where the RNA polymerase releases the newly made mRNA molecule.

PROTEIN SYNTHESIS

Genes contain instructions for assembling proteins.

The sequence of nucleotides in mRNA contain the genetic code, which determines the amino acid sequence of proteins.

The information that DNA transfers to mRNA is in the form of a code.

The code is determined by the way in which the 4 nitrogen bases are arranged in DNA.

DNA & RNA are responsible for making proteins…proteins are responsible for cell structure & movement;Most enzymes are proteins; Proteins direct the making of carbohydrates, lipids, & nucleotides.

The Nature of the Genetic Code

Proteins are made by stringing amino acids together to form long chains called POLYPEPTIDES.

Each polypeptide contains a combination of the many different amino acids.

DNA & RNA contain 4 nitrogen bases, so the genetic code has a 4 letter alphabet.

In order to code for the different amino acids, more than one nucleotide has to make up the code for the amino acid.

The smallest size for a code word in DNA is 3 nucleotides.

CODON- a 3 nucleotide sequence on mRNA that codes for an amino acid (3 letter code word!)

Each codon specifies a particular amino acid that is to be placed in the polypeptide chain.

The genetic code consists of 64 codons along with their corresponding amino acids. These codons are found on mRNA.

There are 20 amino acids. This is why it is possible for an amino acid to be specified by more than 1 kind of codon.

Of the 64 codons, 61 specify a particular amino acid & 3 of them are STOP CODONS which signify the end of a polypeptide chain.

See page 303 Figure 12-17

TRANSLATION

Defined: the process in which a message carried out by mRNA is decoded into a polypeptide chain known as a PROTEIN.

The message is being translated from the language of the nucleic acid to a polypeptide.

During translation, the type of amino acid that is added to the growing polypeptide depends on the codon on the mRNA & the anticodon on the tRNA to which the amino acid is attached.

The cell uses information from mRNA to produce proteins!

Transfer RNA

Defined: the type of RNA that carries amino acids to the ribosomes where amino acids are joined together to form polypeptides. It is a single strand of RNA that loops back onto itself.

tRNA molecules are found in the cytoplasm.

Their function is to carry amino acid molecules to the ribosomes, the sites of protein synthesis.

There are 20 different amino acids in cells, & there is a different form of tRNA for each amino acid.

Each type of tRNA has a 3 nucleotide sequence called an ANTICODON, that is complementary to mRNA.(Homemade transparency…PROTEIN SYNTHESIS…)

The amino acids are joined together in the ribosome to form a polypeptide.

The Role of tRNA

Each tRNA has 2 important sites of attachment:

1.One site called the anticodon binds to the codon on the mRNA molecule.

ANTICODON-ANTICODON-3 nucleotide sequence in tRNA that base pairs with a complementary sequence in mRNA during protein synthesis.

2. The other site attaches to a particular amino acid.

During protein synthesis the anticodon of tRNA molecule base pairs with the appropriate mRNA codon.

ROLE OF THE RIBOSOME

rRNA- the type of RNA that makes up the major part of the ribosomes.

The ribosomes consisting of rRNA & protein, release the new protein.

The ribosomes are essential to making the genetic code work.

Proteins are made up of smaller molecules of amino acids strung together into long chains called polypeptides.

Individual amino acids are attached to the individual codon.

TEST CH 12-1 THRU 12-3