BY5: Transcription
There are three different types of RNA, messenger RNA, ribosomal RNA and transfer RNA (see year 1 notes). There are two main processes involved in making a protein:
DNA, which does not leave the nucleus of eukaryotic cells, acts as a template for the production of mRNA, which carries the instructions needed for protein synthesis from the nucleus to the ribosomes in the cytoplasm. The function of the ribosomes is to provide a suitable surface for the attachment of mRNA and the assembly of protein. The process of protein synthesis occurs as follows:
Transcription:
The mechanism of transcription:
There are three different types of RNA, messenger RNA, ribosomal RNA and transfer RNA (see year 1 notes). There are two main processes involved in making a protein:
- Transcription = formation of messenger RNA (mRNA)
- Translation = genetic code of mRNA translated into a polypeptide chain
DNA, which does not leave the nucleus of eukaryotic cells, acts as a template for the production of mRNA, which carries the instructions needed for protein synthesis from the nucleus to the ribosomes in the cytoplasm. The function of the ribosomes is to provide a suitable surface for the attachment of mRNA and the assembly of protein. The process of protein synthesis occurs as follows:
Transcription:
- The mRNA is first copied from a specific region of DNA called the cistron
- Often this is equivalent to a gene and codes for one specific polypeptide chain
The mechanism of transcription:
- The enzyme RNA polymerase has two functions in transcription (firstly to unwind and separate the DNA strands and then to synthesise the mRNA molecule against the template DNA strand)
- RNA polymerase first binds to the DNA molecule at the beginning of the DNA sequence to be copied into mRNA and unwinds the double-stranded DNA molecule (RNA polymerase has "DNA helicase activity"). The RNA polymerase then breaks the hydrogen bonds holding complementary bases together resulting in separation of the two DNA strands. Only one of these DNA strands acts as a template against which a matching mRNA can be formed
- The coding strand of DNA is often called the gene strand (this is the base sequence you might see in a text book), however it is actually the opposite template strand that is used for the synthesis of the mRNA
- Transcription occurs when free RNA nucleotides then align themselves opposite the exposed bases of the template DNA strand. Because of the complementary relationship between the exposed bases in DNA and the RNA nucleotides, cytosine in the DNA attracts a guanine, guanine attracts a cytosine, thymine attracts an adenine, and adenine attracts a uracil
- RNA polymerase moves along the DNA template strand forming phosphodiester bonds that link RNA nucleotides together one at a time to form the single stranded mRNA. This results in the synthesis of mRNA alongside the unzipped portion of DNA
- RNA polymerase then rewinds the DNA strands back together behind it to reform the original double helix
- Each amino acid was coded for by a DNA triplet codon. The mRNA molecule carries complementary RNA codons (given in the genetic code table). The mRNA "carries the genetic code, originally contained in the DNA, out of the nucleus" through a nuclear pore to the cytoplasm and attaches to a ribosome consisting of two subunits both made up of ribosomal RNA (rRNA) and protein
- At the ribosome the next stage, translation, occurs whereby the mRNA code is used to build the polypeptide chain
Introductory video (with tutorial) about mRNA transcription and how pre-mRNA is processed to make mature mRNA:
A real-time animated video of mRNA transcription from a template strand of DNA: