Question

1. Attenuation is a regulatory mechanism that has been well studied in the E. coli trp operon, involved in tryptophan biosynthesis. Which of the following accurately describes attenuation?
A: The goal of attenuation is to suppress the expression of biosynthetic enzymes when the end product of the pathway is available.
B: The attenuator is a stem loop structure at the 5´ end of the mRNA transcript that resembles the structure of the factor-independent termination sequence.
C: When tryptophan levels are low, the trp operon transcript is attenuated before the operon's structural genes are transcribed.
D: Attenuation allows fine-tuning of the levels of mRNA transcript from the trp operon in response to small changes in the availability of tryptophan.
E: The trpL mRNA contains a "stall" sequence for the ribosome that includes two adjacent trp codons.

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2. Which of the following are true statements concerning the similarities and differences between prokaryotic and eukaryotic DNA replication:
A: Prokaryotes use DNA polymerase III as the primary replicative enzyme, whereas eukaryotes use DNA polymerase I.
B: Most prokaryotes replicate a single chromosome from a single origin of replication, whereas eukaryotes have thousands of origins of replication spread over multiple chromosomes.
C: In eukaryotes, DNA replication is continuous, whereas prokaryotes replicate their DNA discontinuously in a process that generates Okazaki fragments.
D: Eukaryotic DNA replication is more complex than prokaryotic replication, because eukaryotes must dissociate and replicate chromatin as part of the process.
E: A homodimer of DNA polymerase III catalyzes the replication of both leading and lagging strands in prokaryotes, whereas two different DNA polymerases are required to replicate the leading and lagging strands in eukaryotes.

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3. Ammonia is toxic and must be converted to urea in order to be excreted. The following statements describe the process:
A: ammonia is transported into the liver as either alanine or glutamine.
B: transamination is a key step in the release of ammonia from glutamine.
C: urea is formed and excreted primarily by the kidneys.
D: alanine carries ammonia waste from muscle to the liver; once in the liver, alanine is converted via pyruvate to glucose, which may be returned to the muscle.
E: carbamoyl phosphate is a key intermediate in the conversion of ammonia to urea.
F: urea is ultimately formed by hydrolysis from ornithine

Answer 1

1) d) option is correct . when supply of tryptophan is low , the entire operon , including leader sequence is transcribed into mRNA but when when supply of typtophan is more only 140 bses are transcribed .the structural gene are not transcribed. attenuation is the repressible mechanism when tryptophan is present in higher quantity then attenuation process will happen .attenuation process alter the intition process of transcription and also can alter the process when transcription already started. In attenuation process loop is formed in 3' end not in 5 ' prime end and in tryptophan model leader sequence is present between the operator and structural gene . Leader sequence is small nucleotide sequence (140 ) , yes two adjacent codon codes for tryptophan operon . The case is very simple if the tryptophan operon level is high then repressible mechanism will take place because no need of synthesis of biosynthetic enzymes of tryptophan operon and if tryptophan level is less then their is need of it synthesis in bacteria so no attenuation will take place.

2) yes b and d statement show the difference in dna replication , that prokayotes have single origin of replication and eukaryotes have multiple origin of replication in a chromosome . Statement c is wrong because in both okazaki fragment are formed but in eukaryotes length of okazaki fragment are less as compare to prokaryotes..In prokaryotes DNA replication takes place in cytoplasm and in eukaryotes takes place in nucleus and mostly we see in prokaryotes have circular dna and follow circular modes of replication and in eukaryotes dna is linear And times consuming in prokaryotes DNA replication is less as compare to eukaryotes . Yes the eukaryotic replication is more complex because DNA is wrapped around histone protein in eukaryotes and in termination process is also diiference. Statement E is also wrong. In prokaryotes polymerase III and I both are major polymerase in laggimg stand and in eukaryotes and are major polymerase in lagging stand and in leading strand that are similar to DNA pol III and I in prokaryotes.