Chapter 16 The Molecular Basis of Inheritance DNA: The Genetic Material
Intro to DNA Video http://www.youtube.com/watch?v=bVk0twJYL6Y&feature=youtube_gdata_player
Just a thought… Make a stack of books totaling about 10,000 pages. That stack of books represents only about one-fiftieth of the information contained in the DNA of every human cell. Correlate this with the amount of information required to code for a human being.
Quick Review:1. What is the structure of a chromosome?2. Define the term gene.3. Identify the stage in the cell cycle in which DNA is copied.4. What are mutations?5. Summarize Mendel’s theory of heredity.
Answers 1. A chromosome consists of two replicated strands of DNA tightly coiled around proteins. The two strands, called chromatids, are attached at a point called a centromere. 2. A gene is a segment of DNA that codes for a protein or RNA molecule. 3. A cell’s DNA is copied during the synthesis (S) phase. 4. When chromosomes break, the broken pieces can detach completely or can reattach in various ways. Therefore, the chromosome is changed, or mutated.
5. (a) For each inherited trait, an individual has two copies of the gene, one from each parent. (b) There may be alternative versions of genes. (c) When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. (d) When gametes are formed, the alleles for each gene in an individual separate independently of one another, and when gametes unite during fertilization, each gamete contributes one allele.
Vocabulary: 1. DNA /double helix, 2. Nucleosome 3. Semi conservative replication, 4. DNA polymerase, 5. Okazaki fragment
Vocabulary • vaccine • virulent • transformation • bacteriophage • double helix • nucleotide • deoxyribose • base-pairing rules • complementary base pair • DNA replication • DNA helicase • replication fork • DNA polymerase
Chapter: DNA: The Genetic Material 1. Identifying the Genetic Material • A. Transformation • B. Viral Genes and DNA
2. Why did researchers originally think that protein was the genetic material?
Until the 1940s, the case for proteins seemed stronger, especially since biochemists had identified them as a class of macromolecules with great heterogeneity (uniformity) and specificity of function, essential requirements for the hereditary material. Also, little was known about nucleic acids, whose physical and chemical properties seemed far too uniform to account for the multitude of specific inherited traits exhibited by every organism.
Identifying the Genetic Material The experiments of Griffith and of Avery yielded results that suggested DNA was the genetic material.
The capsule killed, non capsule did not! Heat-killed capsule, did not KILL! BUT A MIX OF THE 2 DID!
3. Distinguish between the virulent and non-virulent strains of Streptococcus pneumoniae studiedby Frederick Griffith.
The virulent strains ( with capsules) are pathogenic (disease-causing), whereas the nonvirulent strains ( no capsule) are nonpathogenic (harmless).
What happened?5. Use this figure to summarize the experiment in which Griffith became aware that hereditaryinformation could be transmitted between two organisms in an unusual manner.
information could be transmitted between two organisms in an unusual manner. Griffith studied two strains of the bacterium Streptococcus pneumoniae. Bacteria of the S (smooth) strain can cause pneumonia in mice; they are pathogenic because they have an outer capsule that protects them from an animal’s defense system. Bacteria of the R (rough) strain lack a capsule and are nonpathogenic. To test for the trait of pathogenicity, Griffith injected mice with the two strains. Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells that allowed the R cells to make capsules.
Transformation • Something in the heat-killed bacteria that once were able to produce a capsule that caused them to kill the mice was able to “transform” the once non-virulent bacteria into capsule producing, virulent ones! • But what was that something????
6. Define transformation. A change in genotype andphenotype due to the assimilation of external DNA by a cell. When the external DNA is from a member of a different species, transformation results in horizontal gene transfer.
Griffith 1928 • 1. What happened to the normally non-virulent rough bacteria when mixed with the virulent smooth heat killed ones? • 2. Based on what happened to the bacteria, what was Griffith’s experiment called? • 3. At the end of Griffith’s experiment, did scientists know if DNA or proteins caused the changes?
Griffith 1928 • 1. What happened to the normally non-virulent rough bacteria when mixed with the virulent smooth heat killed ones? they became virulent • 2. Based on what happened to the bacteria, what was Griffith’s experiment called? the transforming experiment • 3. At the end of Griffith’s experiment, did scientists know if DNA or proteins caused the changes? _no
Avery’s experiment • DNA destroying enzymes helped support the fact that it was the DNA not proteins that were the transforming factors!
7. What did Oswald Avery determine to be the transforming factor? Explain his experimental approach.
DNA ! Avery broke open the heat-killed pathogenic bacteria and extracted the cellular contents. He treated each of three samples with an agent that inactivated one type of molecule, and then tested the sample for its ability to transform live nonpathogenic bacteria. Only when DNA was allowed to remain active did transformation occur.
Avery 1944 • 4. Avery attempted to do what? • 5. Did people accept Avery’s results?
Avery 1944 • 4. Avery attempted to do what? identify if the substance was protein or DNA • 5. Did people accept Avery’s results? NO
Hershey and Chase used the bacteriophage T2 and radioactive labels to show that viral genes are made of DNA, not protein.
Alfred Hershey and Martha Chase--Acceptance within scientific community of DNA as genetic material (Video Clip)
bacteriophage • 8. Sketch a T2 bacteriophage and label its head, tail sheath, tail fiber, and DNA.
The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. .
The cell’s DNA is hydrolyzed. The phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell.
Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. The phage directs production of an enzyme that damages the bacteria cell wall, allowing fluid to enter. The cell swells, and finally bursts, releasing 100 to 200 phage particles
Hershey and Chase • More evidence it is DNA!
Radioactive (heavy forms) were provide for the phages to incorporate into their structures • Sulfur in protein 35 S • Phosphorus in DNA 32 P
The radioactive sulfur remained outside with the protein coat of the virus. • The radioactive Phosphorus went inside the bacteria and showed that the DNA was the component that went inside and produced more viral proteins.
10. How did Hershey and Chase “label” viral DNA and viral protein so that they could be distinguished? Explain why they chose each radioactive tag in light of the chemical composition of DNA and protein.
Hershey and Chase used radioactive isotopes of sulfur to tag protein in one batch of T2 and a radioactive isotope of phosphorus to tag DNA in a second batch. Because proteins, but not DNA, contain sulfur, radioactive sulfur atoms were incorporated only into the protein of the phage.
DNA stores the information that tells cells which proteins to make and when to make them.
11. Describe the means by which Hershey and Chase established that only the DNA of a phage enters an E. coli cell. What conclusions did these scientists draw based on these observations?
Separate samples of the non-radioactive E. coli cells were allowed to be infected by the protein labeled and DNA-labeled batches of T2. The researchers then tested the two samples shortly after the onset of infection to see which type of molecule—protein or DNA—had entered the bacterial cells and would therefore be capable of reprogramming them. Hershey and Chase found that the phage of DNA entered the host cells but the phage protein did not. Hershey and Chase concluded that the DNA injected by the phage must be the molecule carrying the genetic information that makes the cells produce new viral DNA and proteins.