The flow of information starts with the sequence on DNA, and by a process of transcription and translation, is converted into protein. This one-way flow of information from DNA to protein is called the central dogma of molecular biology.
DNA is long, double-stranded molecule found in the nucleus of all cells (in bacteria, DNA is not enclosed in a nucleus). Inside the nucleus, the DNA is wound into tight coils called chromosomes. Chromosomes are stabilised by positively charged proteins called histones. A genome is the sum total of all the information on the chromosomes.
Each DNA strand is composed of sub-units called nucleotides or, “bases” for short. There are four types of nucleotides – adenine, A; thymine, T; guanine, G and cytosine, C. An A always pairs with T, and G with C. Thus the two strands of DNA are complementary to each other.
Nucleotides are arranged in a specific order on DNA – this is called the sequence of DNA. This sequence is further sectioned into genes – a short segment of DNA that is made into one polypeptide (protein) chain.
Cells convert DNA to protein in a two-step process.
The first step is called transcription, and results is formation of messenger RNA (mRNA). This process takes place inside the nucleus of the cell (or cytoplasm in bacteria).
To transcribe a gene, the DNA is first unwound using the enzyme DNA helicase, which breaks the bonds between the nucleotide pairs. Then an enzyme called DNA polymerase “reads” one of the DNA strands and converts it into a complementary strand called pre-mRNA. Pre-mRNA is also made up of nucleotides, but uses uracil, U instead of T. Thus an A base on the DNA strand will result in a U base in the complementary mRNA strand.
pre-RNA is further modified by a process called “splicing” – this removes the bases called as “introns” from the pre-mRNA, leaving only the “exons” joined together to form mRNA.
Three types of RNA are involved in the translation process – mRNA, ribosomal RNA (rRNA) and transfer RNA (tRNA).
Three bases – called a codon – on an mRNA molecule translate to one amino acid (amino acids are the building blocks of polypeptides). This is called as the genetic code. There are around 20 naturally occuring amino acids in the body, thus all our proteins (e.g. muscles) are made from a combination of these 20 amino acids.
Each tRNA molecule is clover-leaf in shape and carriers one amino acid molecule. The lower half of the structure includes bases called as “anti-codons” – each tRNA with a particular sequence of anti-codons will carry a unique amino acid. Anti-codons on tRNA can bind to their complementary codon on mRNA.
Translation begins when a ribosome attaches to the mRNA molecule. Each ribosome also has a binding site for two tRNA molecules. Two tRNA molecules complementary to the mRNA sequence come in and binds to the mRNA. The amino acids on the two tRNA’s are linked together, via a peptide bond, and one of the tRNA’s is released. The ribosome then shifts three bases along the mRNA, and the whole process is repeated.
The last set of codons (last three bases) of an mRNA molecule are called the “stop” codon – they do not have a corresponding tRNA. When the ribosome reaches this sequence, it detaches from the mRNA. The polypeptide chain (100′s to 1000′s of amino acids long) is released, and migrates to the Golgi appartus where it is modified (if required) and then eventually folds into a 3-D shape called a protein.
The protein is packaged into little vesicles by the Golgi and localised to the appropriate section of the cell.