Protein Synthesis: Transcription | A-level Biology | OCR, AQA, Edexcel

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1. Sense and Antisense Strands
2. DNA Helicase in Transcription
3. RNA Polymerase in Transcription
4. Splicing

Sense and Antisense Strands

The process of transcription involves forming single-stranded mRNA from double-stranded DNA. Only one of the two strands of DNA contains the codons that are used to code for the amino acids. This strand is known as the sense strand or the coding strand and runs from 5’ to 3’. The complementary strand is known as the antisense strand or the template strand and runs from 3’ to 5’. The antisense strand is used as a template so that the mRNA synthesised has the same sequence as the sense strand.

DNA Helicase in Transcription

The process of synthesising mRNA from DNA is called transcription. In order for transcription to occur, the DNA double helix must be unzipped and the hydrogen bonds broken. This reaction is catalysed by the enzyme DNA helicase which moves along the sugar-phosphate backbone, beginning at the start codon. This exposes a section of both the coding sense strand and the template antisense strand.

RNA Polymerase in Transcription

The DNA antisense strand then act as a template for the mRNA as free RNA nucleotides pair with the exposed complementary bases. The RNA nucleotides are then joined to adjacent nucleotides by phosphodiester bonds to form the mRNA strand. This reaction is catalysed by the enzyme RNA polymerase which travels along the sugar-phosphate backbone in the 3’ to 5’ direction. When transcription ends, the mRNA strand then detaches from the DNA, allowing the double helix to reform. The sequence of bases in the mRNA strand is the same as the DNA coding strand, except the thymine base is replaced by uracil. Just like the coding strand of DNA, mRNA therefore also contains codons.

Splicing

In prokaryotes, the process of transcription results in the direct synthesis of mRNA. In eukaryotes, the process of transcription results in the synthesis of pre-mRNA which must be modified to form mature mRNA. This is because eukaryotic genes contain introns and exons. Exons are sections of DNA that code for proteins. Introns are sections of DNA that do not code for proteins. Before the pre-mRNA can be used to synthesise polypeptides, the introns must be removed. The exons are then joined back together to form mature mRNA - this process is known as splicing. Splicing is the process in which introns are removed from pre-mRNA and the exons joined together to form mature mRNA.

Summary

The sense strand of DNA contains the codons used to code for amino acids - this is the coding strand
The complementary strand of DNA is called the antisense strand and it is used as a template for transcription
In transcription, DNA helicase unzips a section of DNA
Pre-mRNA is then synthesised from the template strand by the enzyme RNA polymerase
The pre-mRNA strand contains the same base sequence as the DNA coding strand
The introns are then removed from the pre-mRNA and the exons joined together to form mature mRNA - this is splicing"

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