How is mRNA used to make proteins
From DNA to protein
In order to build a protein according to the instructions for a section of DNA, an mRNA transcription is first made. Using these, amino acids are linked together to form chains - and the protein is ready.
The DNA is a secret code for protein production
The DNA corresponds to a book that is in (almost) every cell of a living being and serves as a “building instruction” for the whole organism. In order for our cells to read this DNA book and become a functioning body, they have to translate the DNA language into protein language. To understand this process, you should first read how DNA is made up here.
We remember: DNA consists of two intertwined nucleotide chains and looks like a twisted rope ladder (this structure is called a double helix). There are four different nucleotides in DNA that differ from one another in one building block: the base. These four different bases are called adenine (A), thymine (T), cytosine (C) and guanine (G). Base A is complementary to T, base C is complementary to G. This means that A always pairs with T and C always pairs with G. Three of these successive bases are called “codons” or “base triplets”. Think of a codon like a word in the DNA book, for example the word “AGC”. A gene consists of many of these codons; one could think of a gene as a long paragraph or even several pages in the DNA book.
Transcription: A DNA transcription in RNA language
In order to be able to read the DNA words, i.e. the codons, the twisted rope ladder must first be wound up and the codons translated into a new script. This process is called "transcription". At the beginning of a gene there is a special series of bases - scientists call this place "promoter" - to which an enzyme complex called "RNA polymerase" attaches. You can imagine the RNA polymerase as a cell machine that drives like a locomotive on the "DNA rail". On its way, the RNA polymerase produces a copy of the DNA segment in the form of another nucleic acid that consists of just one strand: the mRNA (messenger ribonucleic acid or messenger RNA).
In the picture below you can see that during the transcription the DNA double helix is untied and the required part of it is split lengthwise. Some of the DNA bases are now unpaired (CATG). The bases of free RNA nucleotides attach to these unpaired DNA bases. A free G pairs with the unpaired C in the DNA, a free U (uracil, which occurs in RNA instead of T and is also complementary to A) pairs with the unpaired A. These previously free RNA nucleotides are chained together. This new chain is the mRNA. It has the same sequence, i.e. base order, as one strand of DNA, with the only difference that U was used instead of T. While the RNA polymerase continues, the DNA strands close behind it like a zipper, and the growing mRNA chain detaches from the DNA.
Translation: Of protein factories and the genetic code
After the required DNA segment has now been transcribed in RNA language, the mRNA is sent from the cell nucleus to the ribosomes in the cytoplasm. This is where the "translation" takes place - the translation of the RNA language into the protein language or the translation of the nucleotide sequence into the amino acid sequence. You can find the code used for this translation here.
Amino acids are linked to form proteins
You can think of ribosomes as protein factories located in the cytoplasm. As you can see in the picture below, the ribosome brings the mRNA and the matching tRNAs together. What are tRNAs? tRNAs consist of the same building blocks as mRNA, but have a different form and function. The tRNAs are translators. They carry a certain amino acid on one side and three bases (the anticodon) on the other side, which perfectly match a codon on the mRNA. If the codon of the mRNA and the anticodon of a tRNA match (i.e. are complementary to each other), the tRNA unloads its amino acid. The amino acids are linked together in the order of the mRNA bases to form a chain that leaves the ribosome when a “stop codon” (the sign to stop) is reached on the mRNA. Then the amino acid chain folds up and the protein is ready. Proteins are building materials and workers in our cells. They have an incredible number of different functions, and for each type of protein there is an instruction manual on a piece of DNA.
Why is the DNA sequence not translated directly into the amino acid sequence, but via the mRNA synthesis step?
- DNA cannot leave the cell nucleus. However, protein production takes place in the cytoplasm.
- A gene consists of exons and introns. Exons are the coding areas, introns are the non-coding areas. Before the mRNA sequence is translated into an amino acid sequence, the introns must be cut out. With some mRNAs, different introns are cut out as required. In the end, different proteins are created from one gene, which differ in certain amino acid segments.
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