DNA Transcription

Transcription is the process through which a DNA strand is copied to mRNA (Messenger RNA), where it will be used to synthesize proteins (long strands of polypeptides). The mRNA is sent out into the cytoplasm of the cell, where it will be translated by Ribosomes.

Stage 1: Initiation

• Polymerase II starts reading the template DNA strand 3' to 5'. The template DNA strand is also called the 'antisense' strand. The opposite strand is called the 'coding' or 'sense' strand because it has (almost) the same nucleotides as the RNA sequence will have.
• The DNA is unwound as the Polymerase II is reading it.
• Once the 'TATA' box is found, proteins mark its location as the start position. This sequence (TATA) is also referred to as the 'promoter' sequence

Stage 2: Elongation

• Polymerase II codes the RNA sequence complimentary to the template strand in the 5' to 3' direction. The sequence is almost identical to the coding strand, which is alos complimentary to the template strand.
• RNA doesn't have a T (Thymine) nucleotide, it uses U (Uracil) instead.
• The RNA Polymerase copies the coding strand and replaces the T nucleotides with U.
• The 5' end of the mRNA strand is called 'upstream' and the 3' downstream, so the RNA is replicated upstream to downstream.

Stage 3: Termination

• When the termination sequence is encountered, the RNA is completed. (Terminator sequence is "AAAUAAA")
• Introns are sections of the DNA that don't have a purpose in coding for genes, and these are removed from the mRNA by proteins called spliceosomes, in a process called 'RNA splicing'. The parts that remain, which code for proteins, are called exons.
• To protect the mRNA from the harsh conditions outside the nucleus, a modified 'G' nucleotide is added to the 5' end and a 'Poly-A Tail' is added to the 3' end. The Poly-A Tail consists of a long strand of A nucleotides.