Section 1: DNA: The Genetic Material
Name:____________________________________ Date:____________ Period:____
Chapter 12 Overview
Section 1: DNA: The Genetic Material
Section 2: Replication of DNA
Section 3: DNA, RNA, and Protein
Section 4: Gene Regulation and Mutation
Note: 1869 - Frederick Miescher discovered DNA, was acidic molecule in nucleus; called it
NUCLEIC ACIDS.
12.1 DNA: The Genetic Material
Fredrick Griffith
▪ Performed the first major experiment that led to the discovery of DNA as the genetic material
– The First demonstration of bacterial transformation.
– Experiments done by Frederick Griffith (in London) in 1928 found there were two different types of the bacterium Streptococcus pneumoniae:
• An "S" or SMOOTH coat strain, which is lethal to mice.
• An "R" or ROUGH strain, which will not hurt the mouse.
– Griffith found that he could heat inactivate the smooth strain.
– However, if he were to take a mixture of the heat-inactivated S strain, mixed with the R strain, the bacteria would die.
– Thus there was some material in the heat-killed S strain that was responsible for "transforming" the R strain into a lethal form.
– Fredrick Griffith (and a lab co-worker) was killed in their laboratory in 1940 from a German bomb.
Griffith’s work continued in U.S.
• in 1944, Oswald Avery, C.M. MacLeod, and M. McCarty carefully demonstrated that the ONLY material that was responsible for the transformation was DNA
• Thus, DNA was the "Genetic material" - however, many scientists were still not sure that it was REALLY DNA (and not proteins) that was the genetic material.
Oswald Avery
▪ Identified the molecule that transformed the R strain of bacteria into the S strain
▪ Concluded that when the S cells were killed, DNA was released
▪ R bacteria incorporated this DNA into their cells and changed into S cells.
Hershey and Chase (1952)
▪ Used radioactive labeling to trace the DNA (P) and protein (S)
▪ Concluded that the viral DNA was injected into the cell and provided the genetic information needed to produce new viruses
12.1 Table Summary of Hershey-Chase Results
|Group 1 Viruses labeled with 32P |Group 2 Viruses labeled with 35S |
|Infected Bacteria |Liquid with Viruses |Infected Bacteria |Liquid with Viruses |
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Chargaff’s rule: C = G and T = A
▪ In 1950, Erwin Chargaff analyzed the base composition of DNA composition in a number of organisms. Purines = Adenine and Guanine; Pyrimidines = Thymine and Cytosine
▪ He reported that DNA composition varies from one species to another.
▪ Such evidence of molecular diversity, which had been presumed absent from DNA, made DNA a more credible candidate for the genetic material than protein.
X-ray Diffraction Structure Analysis (1951-1952)
▪ X-ray diffraction data helped solve the structure of DNA
▪ Indicated that DNA was a double helix
▪ This is the famous Rosalind Franklin - Picture 51 which is believed to have been leaked to James Watson and Francis Crick by Maurice Wilkins.
X-ray Diffraction Rosalind Franklin (1920 - 1958)
▪ The technique with which Maurice Wilkins and Franklin set out to do this is called X-ray crystallography.
▪ With this technique a crystal is exposed to x-rays in order to produce a diffraction pattern.
▪ If the crystal is pure enough and the diffraction pattern is acquired very carefully, it is possible to reconstruct the positions of the atoms in the molecules that comprise the basic unit of the crystal.
▪ Rosalind Franklin died from cancer in April of 1958, at the age of 37.
James Watson and Francis Crick 1953
▪ Built a model of the double helix that conformed to the others’ research
1. two outside strands consist of alternating deoxyribose and phosphate
2. cytosine and guanine bases pair to each other by three hydrogen bonds
3. thymine and adenine bases pair to each other by two hydrogen bonds
Nobel Prize in Medicine/Physiology
• The rules of the Nobel Prize forbid posthumous nominations; because Rosalind Franklin had died in 1958 she was not eligible for nomination to the Nobel Prize subsequently awarded to Crick, Watson, and Wilkins in 1962.
• The award was for their body of work on nucleic acids and not exclusively for the discovery of the structure of DNA.
• By the time of the award Wilkins had been working on the structure of DNA for over 10 years, and had done much to confirm the Crick-Watson model. Crick had been working on the genetic code at Cambridge and Watson had worked on RNA for some years.
DNA Structure – Double Helix
▪ DNA often is compared to a twisted ladder.
▪ Rails of the ladder are represented by the alternating deoxyribose and phosphate.
▪ The pairs of bases (cytosine–guanine or thymine–adenine) form the steps.
Orientation
▪ On the top rail, the strand is said to be oriented 5′ to 3′.
▪ The strand on the bottom runs in the opposite direction and is oriented 3′ to 5′.
Chromosome Structure
▪ DNA coils around histones to form nucleosomes, which coil to form chromatin fibers.
▪ The chromatin fibers supercoil to form chromosomes that are visible in the metaphase stage of mitosis.
12.2 Replication of DNA
Semiconservative Replication
▪ Parental strands of DNA separate, serve as templates, and produce DNA molecules that have one strand of parental DNA and one strand of new DNA.
Unwinding
▪ DNA helicase, an enzyme, is responsible for unwinding and unzipping the double helix.
▪ RNA primase adds a short segment of RNA, called an RNA primer, on each DNA strand. Keeping the DNA strands separate.
Base pairing
▪ DNA polymerase (an enzyme) continues adding appropriate nucleotides to the chain by adding to the 3′ end of the new DNA strand.
▪ One strand is called the leading strand and is elongated as the DNA unwinds.
▪ Leading Strand is synthesized ________________
▪ The other strand of DNA, called the lagging strand, elongates away from the replication fork.
▪ The lagging strand is synthesized discontinuously into small segments, called Okazaki fragments
Joining
▪ DNA polymerase removes the RNA primer and fills in the place with DNA nucleotides.
▪ DNA ligase links the two sections.
Comparing DNA Replication in Eukaryotes and Prokaryotes
▪ Eukaryotic DNA unwinds in multiple areas as DNA is replicated.
▪ In prokaryotes, the circular DNA strand is opened at one origin of replication.
12.3 DNA, RNA, and Protein
Central Dogma: DNA to RNA to Protein
RNA
▪ Contains the sugar ribose (instead of deoxyribose) and the base uracil (instead of thymine)
▪ Usually is single stranded
Messenger RNA (mRNA)
▪ Long strands of RNA nucleotides that are formed complementary to one strand of DNA
▪ Caries genetic information from DNA in the nucleus to direct protein synthesis in the cytoplasm
Ribosomal RNA (rRNA)
▪ Associates with proteins to form ribosomes in the cytoplasm
Transfer RNA (tRNA)
▪ Smaller segments of RNA nucleotides that transport amino acids to the ribosome where proteins are made by adding 1 a.a. at a time
Transcription
▪ Through transcription, the DNA code is transferred to mRNA in the nucleus.
▪ DNA is unzipped in the nucleus and RNA polymerase binds to a specific section where an mRNA will be synthesized
RNA Processing
The code on the DNA is interrupted periodically by sequences that are not in the final mRNA.
• Intervening sequences are called introns – these are ____________.
▪ Remaining pieces of DNA that serve as the coding sequences are called exons.
The Code
Experiments during the 1960s demonstrated that the DNA code was a three-base code.
The three-base code in DNA or mRNA is called a codon.
Translation
• In translation, tRNA molecules act as the interpreters of the mRNA codon sequence.
• At the middle of the folded strand, there is a three-base coding sequence called the anticodon.
• Each anticodon is complementary to a codon on the mRNA.
The 1968 Nobel Prize in Medicine was awarded to Robert W. Holley Har Gobind Khorana Marshall W. Nirenberg for deciphering the Genetic Code. The code is used by cells to translate messenger RNA sequences into protein. The mRNA sequence is read three bases at a time in codons. Each codon identifies a specific amino acid or a stop codon.
One Gene—One Enzyme
The Beadle and Tatum experiment showed that one gene codes for one enzyme. We now know that one gene codes for one polypeptide.
12.4 Gene Regulation and Mutation
Prokaryote Gene Regulation
▪ Ability of an organism to control which genes are transcribed in response to the environment
▪ An operon is a section of DNA that contains the genes for the proteins needed for a specific metabolic pathway
1. Operator
2. Promoter
3. Regulatory gene
4. Genes coding for proteins
The Trp Operon
The Lac Operon
[pic]
Eukaryote Gene Regulation
▪ Controlling transcription
▪ Transcription factors ensure that a gene is used at the right time and that proteins are made in the right amounts
▪ The complex structure of eukaryotic DNA also regulates transcription.
Hoax Genes
Hoax genes are responsible for the general body pattern of most animals.
RNA Interference
RNA interference can stop the mRNA from translating its message.
Mutations
A permanent change that occurs in a cell’s DNA is called a mutation.
▪ Types of mutations
▪ Point mutation
▪ Insertion
▪ Deletion
Protein Folding and Stability
Substitutions also can lead to genetic disorders.
▪ Can change both the folding and stability of the protein
Causes of Mutation
▪ Can occur spontaneously
▪ Chemicals and radiation also can damage DNA.
▪ High-energy forms of radiation, such as X rays and gamma rays, are highly mutagenic.
Body-cell v. Sex-cell Mutation
Somatic cell mutations are not passed on to the next generation.
Mutations that occur in sex cells are passed on to the organism’s offspring and will be present in every cell of the offspring.
Review Questions
1. Which scientist(s) definitively proved that DNA transfers genetic material?
2. Name the small segments of the lagging DNA strand.
3. Which is not true of RNA?
Which is true:
4. The experiments of Avery, Hershey and Chase provided evidence that the carrier of genetic information is _______.
5. What is the base-pairing rule for purines and pyrimidines in the DNA molecule?
What are the purine bases ______________________________________
What are the pyrimidine bases ___________________________________
6. What are chromosomes composed of?
7. The work of Watson and Crick solved the mystery of how DNA works as a genetic code. True or False
8. Which is not an enzyme involved in DNA replication?
9. During DNA replication, what nucleotide base sequence is synthesized along an original strand that has the sequence TCAAGC?
10. Which shows the basic chain of events in all organisms for reading and expressing genes?
11. In the RNA molecule, uracil replaces ___________________.
12. What does each diagram shows?
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13. What characteristic of the mRNA molecule do scientists not yet understand?
14. Why do eukaryotic cells need a complex control system to regulate the expression of genes?
15. Which type of gene causes cells to become specialized in structure in function?
16. What is an immediate result of a mutation in a gene?
17. Which is the most highly mutagenic?
18. Look at the following figure. Identify the proteins that DNA first coils around.
19. Explain how Hox genes affect an organism.
20. Explain the difference between body-cell and sex-cell mutation.
21. What does this diagram show about the replication of DNA in eukaryotic cells?
22. What is this process called?
[pic]
23. What type of mutation results in this change in the DNA sequence? TTCAGG (TTCTGG
24. How could RNA interference be used to treat diseases such as cancer and diabetes?
25. The structure of a protein can be altered dramatically by the exchange of a single amino acid for another. True or False
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