7.3 – Transcription
7.3.1 – State that transcription is carried out in a 5′ → 3′ direction
The 5’ end of the free RNA nucleotide is added to the 3’ end of the RNA molecule that is already synthesised.
7.3.2 – Distinguish between the sense and antisense strands of DNA
Sense strand – The coding strand that carries the promoter sequence of bases to which RNA polymerase binds and begins transcription. It has the same base sequence as mRNA, except with uracil instead of thymine. It also carries the terminator sequence of bases at the end of each gene, causing RNA polymerase to stop transcription
Antisense strand – The template strand for transcription by complementary base pairing. It has the same base sequence as tRNA with uracil instead of thymine.
7.3.3 – Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleotide triphosphates and the terminator
In prokaryotes, transcription and translation occur together because there is no nucleus and the mRNA molecule is in direct contact with the cytoplasm.
Promoter Region
This acts as the start signal for transcription, and is located immediately before the gene
RNA Polymerase
This is the enzyme that makes the single strand of RNA, using the antisense strand of DNA as a template. It recognises start and stop signals to control the length of the RNA molecule.
Polymerisation occurs in a 5’ to 3’ direction, making covalent bonds between nucleotides. Complementary base pairing is used, although uracil replaces thymine in the RNA strand.
Nucleotide Triphosphates
As in replication, free nucleotides are found as nucleotide triphosphates. The condensation reaction causes two phosphates to be released.
Terminator
This is the base sequence that signals the end of the gene and causes transcription to stop. RNA polymerase and the mRNA strand are freed from the site of the gene.
7.3.4 – State that eukaryotic RNA needs the removal of introns to form mature mRNA
Between the exons, there are introns, which are non-coding sequences. The introns are spliced out and broken down in the nucleus. The remaining, mature mRNA is exported from the nucleus for translation. The process is called post-transcriptional modification
