Gene Expression

Transcription

“Transcription is the mechanism by which the base sequence of a gene on a DNA strand is converted into the complementary base sequence of mRNA”: first stage of protein synthesis.

  1. RNA polymerase links to DNA at the beginning of the sequence to be transcribed. Only one of the DNA strands is used as a template.
  2. As RNA polymerase moves along the strand it picks up appropriate free RNA nucleotides from the nucleoplasm (that have entered the nucleus from the cytoplasm). It joins guanine to cytosine, but joins uracil to the DNA’s adenine. This happens until it reaches a ‘stop’ signal.
  3. The pre-mRNA detaches from the template strand. It is therefore a copy of the base sequence on the coding strand of the length of DNA. The two DNA strands join together by complementary base pairing. The DNA molecules winds back up into a helix.

The pre-mRNA has to be modified before it leaves the nucleus. Chemicals are added to the ends to ensure stability and introns are removed by splicing. Exons can be spliced together differently, producing different final polypeptides. After splicing, the mature mRNA leaves the nucleus and goes to ribosomes, located in the cytoplasm.

Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.

A. RNA polymerase controls transcription / is the enzyme used in transcription;

B. DNA is unwound by RNA polymerase;

C. DNA is split into two strands;

D. mRNA is made by transcription;

E. promoter region (by start of gene) causes RNA polymerase to bind;

F. anti-sense / template strand of DNA is transcribed;

G. direction of transcription is 5’- 3’;

H. free nucleotide triphosphates used;

I. complementary base pairing between template strand and RNA nucleotides / bases;

J. RNA contains uracil instead of thymine;

K. terminator (sequence) stops RNA polymerase / transcription;

L. mRNA is released / RNA polymerase released