Transcription and Translation
During cellular division, identical copies of DNA need to be synthesized for each cell. If these copies are not accurate, genetic mutations and abnormal cells may develop.
DNA consists of a structure known as a double helix. Each strand in the double helix consists of nucleotides. In order to ensure the accuracy of DNA replication, theses nucleotides can only be paired up in front of other specific types of nucleotides. During the replication process, the DNA strands separate with the help of certain proteins known as enzymes. Now the genetic information on each strand can be read and utilized. The first step in the process of reading a particular sequence of genetic information on a DNA strand and utilizing it is known as transcription.
During the processes of transcription, the enzyme RNA polymerase moves along the separated DNA strand to look for a particular genetic sequence so that it can make a copy. Initially, the copy is an RNA copy, where RNA is a single strand of nucleotides carrying genetic information, rather than a double-stranded helix like DNA
The resulting copy is known as a messenger RNA (mRNA) molecule. mRNA carries genetic information that acts as a set of instructions for the synthesis of a particular protein. The mRNA molecule exits the nucleus of the cell to the cytoplasm where ribosomes are located, to complete the process of protein synthesis.
The sites where the genetic information is translated into the form of an actual protein are known as ribosomes. Different types of cells generally vary in the number of ribosomes they contain, as this depends on how active they are in protein synthesis. Sometimes, they may be present throughout the cytoplasm in the form of large clusters of multiple ribosomes known as polyribosomes.
Ribosomes are formed of a type of RNA known as ribosomal RNA (rRNA) combined with proteins, which are about 60 in number in single-celled organisms known as prokaryotic cells and about 80 or more in multi-cellular organisms known as eukaryotic cells.
The rRNA acts as a sort of manager, directing and catalyzing the steps of the synthesis of a new protein. This process involves bringing amino acids which are the building block of protein in a particular order and linking them together, guided by the genetic information carried by the mRNA.
When the mRNA travels to the ribosomes in the cytoplasm with the recipe for a new protein, the process of reading this recipe and translating it begins. The order of nucleotides on the mRNA determines the type and order of amino acids that need to be added to the growing protein, where every three nucleotides are referred to as ‘codons’ and each codon refers to a particular amino acid.
A third type of RNA, known as transfer RNA (tRNA), is responsible for matching the correct amino acids with their codons as the mRNA moves along the ribosome while its information is being deciphered. It carries this out by carrying the amino acids to the ribosome, one by one, as their codons appear. These amino acids are linked together in that particular order to create a protein chain.
This process continues until a specific codon known as a ‘stop codon’ is reached, which is a signal that indicates that the desired protein synthesis is now complete. The newly formed protein is then released from the ribosome to carry out its intended function.