2.3.7 Transcription Flashcards
Transcription
A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)
This process of protein synthesis occurs in two stages:
- transcription - DNA is transcribed and an mRNA molecule is produced
- translation - mRNA (messenger RNA) is translated and an amino acid sequence is produced
Transcription Process
This stage of protein synthesis occur in the nucleus of the cell
Part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break)
The exposed gene can then be transcribed (the gene from which a particular polypeptide will be produced)
A complementary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA
Free RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on one stand (the template strand) of the ‘unzipped’ DNA molecule
The sugar-phosphate groups of these RNA nucleotides are then bonded together (by phosphodiester bonds) to create the enzyme RNA polymerase to form the sugar-phosphate backbone of the mRNA molecule
When the gene has been transcribes (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA stands break and double-stranded DNA molecules re-form
The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope
The Coding Strand and the Template Strand
In the transcription stage of protein synthesis, free RNA nucleotides pair up with the exposed bases on the DNA molecule
RNA nucleotides only pair with bases on 1 strand of the DNA molecule
- this strand of the DNA molecule is known as the template strand (or the transcribed strand) and it is used to produce the mRNA molecule
RNA polymerase moves along the template strand in the 3’ t o 5’ direction
- this means that the mRNA molecule grows in the 5’ to 3’ direction
Because the mRNA is formed by complementary pairing with the DNA template strand, the mRNA contains the exact same sequence of nucleotides as the DNA coding strand (although the mRNA will contain uracil instead of thymine)
