You already know that cells contain many different types of molecules. Three types of biomolecules are DNA, which stores genetic information, there are several types of RNA involved in the copying and implementation of hereditary information, and there are proteins, which carry out most of the processes in living systems.
In this topic, we will look at how these molecules are interconnected and how information from DNA is used to synthesize proteins.
DNA, RNA, proteins
DNA is a long, stable molecule that encodes, stores, and transmits all of the information needed for organisms to live, develop, and reproduce. RNA is less stable, so it is not used for permanent storage of genetic information, but it is used for the synthesis of proteins as well as many regulatory functions that control the cell. Proteins are made up of amino acids and perform many functions like catalyzing reactions, storing energy, transmitting signals, and forming cellular structures.
Central dogma of molecular biology
DNA, RNA, and proteins are interconnected by what we call the central dogma of molecular biology. If we imagine that DNA is a text string of words, then the words in the text will be genes. One gene codes for one protein, and RNA is the intermediary or messenger that helps a cell turn information from DNA into a unique and functional protein.
Before Francis Crick and James Watson discovered the structure of DNA, scientists understood that traits are passed from parents to children, but they did not understand how this happened. For a long time, there was an erroneous assumption that proteins are responsible for hereditary traits, but this turned out not to be the case. Until the 1950s, the theory closest to reality was that DNA had a three-stranded structure, but Crick and Watson proposed the double-stranded structure of DNA based on x-ray photographs taken by Rosalind Franklin and Maurice Wilkins. A simple structure of 2 antiparallel chains perfectly explained previous observations of DNA, such as the presence of adenine and thymine (cytosine with guanine) in equal amounts, as well as the doubling of the molecule by cutting the molecule into 2 equal parts and completing the missing chain.
This discovery was revolutionary! Crick called it the answer to the mystery of life. Crick, Watson, and Wilkins received the Nobel Prize in 1962, and soon after, the central dogma of molecular biology was formulated, explaining the process of realization and transmission of hereditary information in all living organisms.
The dogma is formulated as follows: DNA makes RNA, and RNA makes protein.
In this diagram, we see 3 distinct processes: replication, transcription, and translation. You will notice there is a specific flow of information, and the opposite direction, or protein to DNA is not possible.
Replication
This is the process by which DNA and RNA are copied. DNA duplication is very important, as it is the main way of transferring information between generations of living organisms. RNA replication is carried out by retroviruses. They can replicate single- or double-stranded RNA (picornaviruses, which include foot-and-mouth disease virus and coronaviruses, have single-stranded RNA). Replication takes one nucleic acid strand and produces a second, completely identical strand from that first strand.
Transcription
DNA stores hereditary information, but to implement this information, RNA is needed. Transcription is the process of synthesizing RNA from a DNA template. During transcription, a special enzyme, RNA polymerase, "walks" along a strand of DNA and copies the information into a new strand of messenger RNA. Transcription occurs in all cells of all living organisms.
Translation
Translation is the step in which the genetic information transcribed into RNA in the previous step is used to create a protein.
The DNA sequence, which was copied into messenger RNA, encodes the amino acid sequence of the future protein. In the genetic code, each amino acid is defined by a three nucleotide sequence. The ribosome reads the sequence of nucleotides carried by the RNA, and builds an amino acid chain based on the genetic code. Amino acids are added one by one to the growing polypeptide, which, after folding and post-translational modifications, will become a protein. All cells are continuously producing new proteins via translation.
Exceptions
Everywhere in life there are exceptions. Most of them are retroviruses.
Sometimes the processes of information transfer proceed in the opposite direction from RNA to DNA (but never from protein to RNA). This method of information transfer is called reverse transcription. The process is catalyzed by an enzyme called reverse transcriptase. Most often, reverse transcription occurs in virus-infected cells, allowing the virus to transfer its genome into the cell's DNA for subsequent reproduction. Reverse transcription is performed by HIV, the retrovirus that causes AIDS.
Conclusion
Proteins and nucleic acids are closely related to each other by the processes of realization of hereditary information. These processes that transfer information from DNA to RNA to proteins are governed by a set of rules called the central dogma of molecular biology. Retroviruses are an important exception to these general rules, as their RNA performs the function of DNA.