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Translation is the second step in protein synthesis, following transcription. Once mRNA leaves the nucleus, it enters the rough endoplasmic reticulum (rough ER) in the cell's cytoplasm. It's rough in appearance because it has a lot of ribosomes. As soon as mRNA leaves the nucleus for the ER, it's going to encounter a ribosome that can convert its nucleic acid message into functional protein.
Remember that mRNA is a nucleic acid, and protein is a sequence of amino acids. This means that converting the information coded in a gene into a protein means translating that information into a different chemical "language." That translator is rRNA—the RNA enzyme (called a "ribozyme") within ribosomes that reads the information contained within the mRNA's nucleotides to the amino acids of a protein.
To do this, rRNA uses "decoders" called tRNA. These are short pieces of RNA that have a unique codon (a sequence of three RNA nucleotides) on one end and the corresponding amino acid on the other end. Recall that there are four different nucleotides in RNA: A, C, G, and T. This means that there are 4x4x4=64 unique three-nucleotide codon combinations. This more than covers the 20 different amino acids we use to make our proteins. Each tRNA ("transfer RNA") has a unique three-nucleotide codon and a corresponding amino acid attached to it.
If only 20 codons are needed to cover all the amino acids, and there are 64 possible codons, what do we do with the excess? One of the amino acid-coding codons (methionine) is the start codon. The start codon marks the first amino acid of a polypeptide chain.
EXAMPLE
AUG is the start code. This is adenine, uracil, guanine together in this chunk of three nucleotides. It signals the start of a polypeptide chain.Three of the excess codons are stop codons. A stop codon marks the end of a polypeptide chain. Stop codons are UAA, UAG, and UGA. It marks the end of a polypeptide chain and when that polypeptide chain would be finished being made.
The other 41 excess codons are duplicates. This is part of the reason why most gene mutations don't have any effect: Most mutations in any given codon result in another codon for the same amino acid.
In translation, instructions from a single-stranded mRNA, which was formed from transcription, are translated into proteins. This occurs in the rough endoplasmic reticulum of the cell's cytoplasm. There are three stages to translation:
The first step is initiation. During initiation, the small and large ribosomal subunits will join together, and then initiator tRNA will arrive at the start codon (UAG) on mRNA. The start codon marks the beginning of a new polypeptide chain, and codons are groups of three nucleotides that code for amino acids. After this first amino acid (start codon = the amino acid methionine), more amino acids can be added.
The tRNA carries an anticodon, and these will match up with the strand of mRNA using the base pair rules. The tRNA also carries an amino acid.
The same thing will happen with this next codon or group of three nucleotides. The next codon and anticodon will match, which signal for a certain amino acid. These amino acids will start building up in this chain called a polypeptide chain.
The newly formed polypeptide chains compose proteins. These amino acids are held together by a peptide bond. This stage is where those amino acids are being built into a growing polypeptide chain through a process called elongation.
Being the last stage of translation, termination is when the stop codon of mRNA signals the end of translation. With that, the polypeptide chain is complete.
Source: This work is adapted from Sophia Author Amanda Soderlind
