Question

1. Radioactive (14C) uridine (lots of it) is added to a growing culture of bacteria for 10 seconds and then the drug rifamycin is added (rifamycin blocks the initiation of transcription, but not continued elongation of RNA polymerases that have already begun transcribing). At two-minute intervals, 1 ml samples are removed and placed on ice. The bacteria are lysed (broken open) and an enzyme is added that will chew up all DNA present (so that we don't have to worry about DNA in the sample). Then, all macromolecules larger than a few nucleotides are collected together and precipitated, after which they are collected on a filter. You measure the radioactivity present on the filters and obtain the following data: Time (minutes 2 . Radioactivity (counts per minute) 50,240 35,050 27,510 23,725 21,850 20,920 20,460 20,230 20,110 20,010 10 12 14 16 18 Explain everything you possibly can about this data. Try to derive every bit of available information out of it. A good place to start might be to imagine why this experiment was done. 2. A strain of yeast translates mRNA into protein with a high level of inaccuracy. Individual molecules of a particular protein isolated from this yeast have the following variations in the first 11 amino acids compared with the sequence of the same protein isolated from normal yeast cells. Assuming that this abnormality results from a single mutation in the cell, what do you think is the most likely cause of this variation in protein sequence? Why? normal sequence Met Thr Ala Ile Val Ser Asn Thr Gin Ile Lys Met variants Met Thr Ala Ala Val Ser Asn Thr Gln Ile Lys Thr Ala Gly Val Ser Asn Lys Met Thr Ala Val Val Ser Asn Thr Gin Ile Lys Met The Ala Ile Val Ser Asn Thr Gin Ala Lys Met Thr Ala Ile Val Ser ASn Thr Gin Gly Lys Met Thr Ala Ile Val Ser Asn Thr Gin Val Lys

PLEASE COMPUTER TYPED* OR VERY CLEAR HANDWRITING with details

Answer 1

1) It can be understood that this experiment was done to analyse the effect of rifamycin on radioactive labelled bacterial colony and its growth. In the given data, addition of radioactive Uridine leads to radioactive labelling of precursor RNA of the cells, there after when the drug Rifamycin is added to the culture, it prevents RNA synthesis and break down of the labelled RNA occurs. This is evident from the decreasing radioactivity per minute in the given table.

2) The most likely cause of this variation can be base substitution. It is a type of mutation in which one base is exchanged for another, this changes a codon to one that encodes a different amino acid. Thus, leading to a different protein produced.

3) RNA is used to produce primers because it contains coding DNA and does not contain introns. Also, there is unavailability of DNA primers. The RNA primers complimentary to cellular DNA are easily synthesized by DNA Primase enzyme. Primase initiates DNA synthesis while RNA polymerase uses the existing RNA or DNA strand as a template, hence, RNA polymerase is not used for primer synthesis during DNA replication.

4) Insertion or deletion of a single nucleotide is more harmful form of mutation as it gives rise to frame-shift that changes the reading of subsequent codons and, therefore, alters the entire amino acid sequence and hence the gene product is altered.

5) The given error rate is : 1/10000 = 0.0001. that is 0.01 %

Hence for 100 amino acid long protein % of proteins produced = (100*0.01 ) -100=99%. Similarly

90 % for 1000 amino acid long protein

100 % for 10,000 amino acid long protein and 100,000 long amino acid

6) No, because each of the 20 amino acids is chemically distinct.

, there are 20 × 20 × 20 × 20 = 160,000 different polypeptide chains possible.

20ⁿ different polypeptide chains n amino acids long are possible.