Pyrimidines and purines can exist in alternate forms, or tautomers, shown here. Under common cellular conditions (physiological pH) the most common form of each purine or pyrimidine is shown on the left, and the rare tautomer on the right.
Tautomers differ in the placement of hydrogen atoms and double bonds. Consider cytosine and its tautomer, the rare imino form of cytosine. With which nitrogenous base will the imino form of cytosine
most likely base pair in a double stranded DNA?
A. adenine
B. cytosine
C. guanine
D. thymine
E. uracil
You synthesize polyribonucleotides using a molar ratio of 3 Uracil :
1 Guanine. The resulting polyribonucleotide sequences are random but each polyribonucleotide follows the 3:1 ratio. When this mixture of polyribonucleotides is used as template for translation, proteins with different amino acid sequences are obtained. When you determine the amino acids present in the proteins, you obtain the frequencies for each amino reported in the table below. The frequency of phenylalanine was normalized to 1 and all other amino acids are reported as a frequency of occurrence compared to phenylalanine. As an example of understanding the data, the amino acid glycine frequency is 0.12, meaning that glycine was present in the mixture of polypeptides at a ratio of 12 glycine residues (amino acids) for every 100 phenylalanine residues (amino acids).
Amino acid Frequency
Phenylalanine 1.00
Leucine 0.37
Valine 0.36
Cysteine 0.35
Tryptophan 0.14
Glycine 0.12
Which one of the following statements about this experimentis true?
A. The most likely codon present in the polyribonucleotides synthesized under these conditions is GGG.
B. Codons consisting of one guanine and two uracils, in any order, are likely to encode for either valine, leucine, or cysteine.
C. A codon for tryptophan is as likely to be present as is the codon for phenylalanine.
D. Since only poly(G) and poly(U) RNA are present, the presence of valine shows ribosome slippage during translation.
With regard to mammalian chromosomes, what is a t-loop?
What will be an ideal response?
Each branch point or node of a phylogenetic tree represents:
A. an environmental pressure for evolution.
B. a mutational event that leads to two species.
C. common ancestor to two connected branches.
D. the beginning of a new domain.
E. the point when life started on earth.
An animal virus contains protein, lipid, and DNA. Shown here is the absorption spectra of the three macromolecules purified from the virus. Which of the three samples, A, B, or C, contains DNA?
A. Sample A contains DNA.
B. Sample B contains DNA.
C. Sample C contains DNA.
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