Question

1. The following is a small stretch of DNA sequence from middle of a bacterial open reading frame that encodes a protein that is 450 amino acids in length :

     5’-GCCTACTCTATGTTTACATCTGTGCTACGC – 3’

     3’-CGGATGAGATACAAATGTAGACACGATGCG – 5’

A) Which of the strands is the template strand for the mRNA that is transcribed from this DNA (“top strand” 5’ to 3’ left to right or “bottom strand” 5’ to 3’ right to left)?

B) What is the basis for your answer to part A?

Answer 1

A) “bottom strand” 5’ to 3’ right to left

B)

For protein synthesis, DNA must first be copied to messenger ribonucleic acid, or mRNA. This process is called transcription. The mRNA holds the coding information to make proteins. Unlike DNA, RNA is single-stranded and not helical in shape. It contains ribose instead of deoxyribose, and its nucleotide bases differ by having uracil (U) instead of thymine (T).

Initially, the enzyme RNA polymerase must assemble the pre-mRNA molecule that complements a section of one DNA’s two strands. Since the goal is not replication but protein synthesis, only one strand of DNA needs copying. The RNA polymerase first attaches to the double helix of DNA and works with proteins called transcription factors to determine what information needs transcribing. The RNA polymerase and transcription factors bind to this DNA strand, called the template strand.

The unit of RNA polymerase and transcription factors moves along the strand in a 3’ to 5’ (3 prime to 5 prime) direction OR (5’ TO 3’ RIGHT TO LEFT)and makes a new strand of mRNA with complementary base pairs. RNA polymerase builds the mRNA with additional nucleotides in elongation. The complementary nucleotides in mRNA, however, differ from DNA in that uracil replaces thymine. The mRNA runs in a 5’ to 3’ (5 prime to 3 prime) direction. After elongation ceases, mRNA separates from the DNA template strand in termination. Then mRNA serves either in a role of messenger in the cell, or it is used in protein formation, or translation