Building a Protein: Transcription

Building proteins is much like building a house:

1. The master blueprint is DNA, which contains all of the information to build the new protein (house).
2. The working copy of the master blueprint is called messenger RNA (mRNA), whic­h is copied from DNA.
3. The construction site is either the cytoplasm in a prokaryote or the endoplasmic reticulum (ER) in a eukaryote.
4. The building materials are amino acids.
5. The construction workers are ribosomes & transfer RNA molecules.

Let's look at each phase of the new construction more closely.
       In a eukaryote, DNA never leaves the nucleus, so its information must be copied. This copying method is called transcription & the copy is mRNA. Transcription takes place in the cytoplasm (prokaryote) or in the nucleus (eukaryote). The transcription is performed by an enzyme called RNA polymerase. To make mRNA, 


RNA polymerase :

               Binds to the DNA strand at a specific sequence of the gene called a promoter Unwinds & unlinks the strands of DNA. Makes use of the DNA strands as a guide or template

               Matches new nucleotides with their complements on the DNA strand (G with C, A with U -- keep in mind that RNA has uracil (U) in lieu of thymine (T)). Binds these new RNA nucleotides together to form a complementary copy of the DNA strand (mRNA). Stops when it encounters a termination sequence of bases (cease codon) mRNA is happy to live in a single-stranded state (as against DNA's desire to form complementary double-stranded helixes). In prokaryotes, all of the nucleotides in the mRNA are part of codons for the new protein. However, in eukaryotes only, there's additional sequences in the DNA & mRNA that don't code for proteins called introns.

This mRNA is then further processed : 

               Introns get cut out

               The coding sequences get spliced together

               A special nucleotide "cap" gets added to finish

               A long tail consisting of 100 to 200 adenine nucleotides is added to the other finish

               No knows why this processing occurs in eukaryotes. Finally, at any moment, plenty of genes are being transcribed simultaneously according to the cell's needs for specific proteins.

The working copy of the blueprint (mRNA) must now go the construction site where the workers will build the new protein. If the cell is a prokaryote such as an E. coli bacterium, then the site is the cytoplasm. If the cell is a eukaryote, such as a human cell, then the mRNA leaves the nucleus through giant holes in the nuclear membrane (nuclear pores) & goes to the endoplasmic reticulum (ER).