CIE A Level Biology复习笔记6.2.4 Transcription
Template & Non-Template Strands
- In the transcription stage of protein synthesis, the section of the DNA molecule where the gene is located (the gene coding for a particular polypeptide) unwinds – the hydrogen bonds between the complementary base pairs break, causing the two DNA strands to ‘unzip’
- Free activated RNA nucleotides then pair up with the exposed bases on the DNA molecule but only with those bases on one strand of the DNA molecule
- This strand of the DNA molecule is called the template strand or the transcribed strand
- This is the strand that is transcribed to form the mRNA molecule (RNA polymerase bonds the RNA nucleotides together to create the sugar-phosphate backbone of the mRNA molecule)
- This mRNA molecule will then be translated into an amino acid chain
- The strand of the DNA molecule that is not transcribed is called the non-template strand or the non-transcribed strand
The template strand of the DNA molecule is the one that is transcribed
Exam Tip
Note the use of sense and anti-sense strands in transcription has been replaced with non-transcribed and transcribed (or template) strands respectively.The mRNA codons have the same base sequence as the non-transcribed strand, and the tRNA anticodons have the same base sequence as the transcribed strand except RNA, which has the base Uracil, replacing Thymine.
Post-Transcriptional Modification
- Within eukaryotic genes, there are both coding and non-coding sequences of DNA
- The coding sequences are called exons and these are the sequences that will eventually be translated into the amino acids that will form the final polypeptide
- The non-coding sequences are called introns and are not translated (they do not code for any amino acids)
- When transcription of a gene occurs, both the exons and introns are transcribed
- This means the RNA molecule formed (known as the primary transcript) also contains exons and introns
- As the introns are not to be translated, they must be removed from the RNA molecule
- The exons are then all fused together to form a continuous RNA molecule called mature mRNA that is ready to be translated
- This process is sometimes called ‘splicing’ and is part of the process of post-transcriptional modification (referring to the modification of the RNA molecule after transcription but before translation occurs)
The RNA molecule produced from transcription of a gene contains introns that must be removed before translation can occur
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