Your text mentions that many scientists were reluctant to accept the results of
ID: 2111588 • Letter: Y
Question
Your text mentions that many scientists were reluctant to accept the results of Avery, MacLeod, and McCarty's experiments showing that DNA (rather than protein) was the transforming principle. Why? Consider any experimental weaknesses in their approach as well as any conceptual problems with DNA as a molecule for information transmission.
Your text describes semi-conservative synthesis as well as two alternatives: conservative and dispersive synthesis.
How many rounds of replication does it take to distinguish one form of DNA synthesis from the others in the Meselson-Stahl experiment?
Propose an experiment to distinguish between semi-conservative, conservative, and dispersive synthesis after a single round of replication.
Suggest the possible cause of each of the following aberrations during replication in E. coli:
Accumulation of Okazaki fragments.
The presence of RNA in newly synthesized DNA strand.
Many mismatched base pairs.
DNA replication does not initiate.
DNA (3.5 x 108 base pairs) in diploid cells of Drosophila melanogaster is replicated in about 3 minutes. Assuming that all replication forks move at a constant rate of 104 base pairs per minute, what is the minimum number of replication units present in the Drosophila genome?
Ten ionizing radiation sensitive yeast mutants were tested for complementation:
Identify the number of genes causing the phenotype and the mutants that belong to each complementation group.
Mutational changes were induced in a single codon in the gene of T4 phage. The following amino acid substitutions were observed. By following the base changes in the codons, determine the codon for each mutant. Assume each amino acid substitution resulted from a single base substitution. Codons for several amino acids are provided. UAG, UAA, and UGA are stop codons. No additional information is required to solve this problem (from Fristrom and Clegg, Principle of Genetics. 1988)
Normal :
Mutant A: UGG
Mutant B:
Mutant C:
Mutant D:
Mutant E:
A red kerneled wheat plant is crosssed with a white-kerneled plant, yielding an F1 plant with all red kernels. A self-cross yields a 15:1 red:white ratio. A testcross yields a 3:1 red:white ratio. Explain the results. Draw Punnett squares for both the self- and test-cross.
Plant breeders developed two different pure-breeding plant lines that gave no leaves. Each plant was crossed with normal (leafy) pure-bred plants to generate the following phenotypic ratios:
What's going on here? Why do two apparently similar plant lines produce such different results? For each cross depicted in the table, show the genotypes of all parental, F1, and F2 plants. You will show the results of a total of three crosses.
Explanation / Answer
DNA had been known to be present for a long time in cells, but people refused to believe that it was the substance of hereditary as it was just to simple and boring to code for all the many things going on inside living organisms. You can understand that they thought this if you consider that DNA is made up of only four nucleotidee bases repeated over and over, adenine, thymine, guanine and cytosine - I think this covers the conceptual problem part.
Another reason for peoples scepticism is like you said, the experimental weaknesses. It was a very specific system and the E.coli finding could not be replicated in other organisms. Alfred Mirksy argued that protein contaminants (0.02%) could have effected the results. There was also thepossibilityy that DNA might not be directly responsible for hereditary, but it might act as a chaperone for a protein that was responsible.
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