Shown below is a doublestranded bacterial E coli DNA sequenc

Shown below is a double-stranded bacterial (E. coli) DNA sequence coding for a hypothetical protein. Both strands are shown; the top strand reads 5\' to 3\' left to right, while the bottom strand reads 5\' at 3\' right to left. The nucleotides are numbered 1 to 100. Which strand is used as a template for transcription, the top or the bottom? What is the other strand known as? Where would the promoter be relative to the start of transcription? What are the first 15 nucleotides of the resulting mRNA? Indicate the 5\' and 3\' ends of the mRNA. What are the first 5 amino acids translated from the resulting mRNA? Indicate the amino (NH + 3) and carboxyl -(COO) termini of the protein. Do the underline nucleotides TAA (indicated in blue) encode a stop colon for the protein? Briefly explain your answer. A mutation occurs which results in the insertion of an extra G/C (top strand/bottom strand) base-pair immediately after base pair 11 (shown in bold). What effect will this insertion mutation have on the mRNA transcript and resulting protein? g. A different mutation results in the substitution of the T/A base pair at position 30 (shown in bold and underlined) with a G/C base pair. How would this mutation affect the sequence of the protein that is produced? h. A third mutation occurs which results in the substitution of the C/G base pair at position 42 (shown in bold italics) to a T/A base pair. How would this mutation affect the sequence of the protein that is produced? In class we showed the \"LAC\" operon. In similar way, explain the operation of the trp operon. Use the following sources for studying about the Trp operon.

Solution

a. 3\' to 5\' strand (2nd strand) is the template or the antisense strand. The other strand is the sense strand (5\' to 3\').

b. Promotor are sequences of DNA at the start of genes, that is to the 5\' side (upstream) of the coding region.

c. 5\'- CAUUAUAACACUCUA - 3\' as the transcription starts at and with C

d. CAU UAU AAC ACU CUA - Histidine, Tyrosine, As , Threonine, Leucine

e. Yes, TAA codes for UAA for mRNA. UAA (also known as ochre), UAG (also known as amber) and UGA (also known as opal) are three codons which whenever present in mRNA, would bring about termination of polypeptide chain which would then be released from the ribosome. These codons are called nonsense codons, as they do not code for any amino acids.

f. CUA AUA (Leucine, Isoleucine) ------> CUA (Leucine) GAU (Aspartic acid) A

g. UCC (Serine) -------> GCC (Alanine)

h. CAC (Histidine) ----------> UAC (Tyrosine)

Trp operon

The trp operon encodes five structural genes whose activity is required for tryptophan synthesis. The operon encodes a single transcription unit which produces a 7 kb transcript which is synthesized downstream from the trp promoter and trp operator sites P_trp and O_trp. Like many of the operons involved in amino acid biosynthesis, the trp operon has evolved systems for co-ordinated expression of these genes when the product of the biosynthetic pathway, tryptophan, is in short supply in the cell. As with the lac operon, the RNA product of this transcription unit is very unstable, enabling bacteria to respond rapidly to changing needs for tryptophan.