Steps of Transcription

Translation is very important in the process of making proteins. Without transcription and translation, your body would have no possible way to make proteins, or function. Proteins allow your body to do everything. Muscle proteins allow your muscles to strengthen and grow.

The process of translation can be seen as the decoding of instructions for making proteins, involving mRNA in transcription as well as tRNA. The genes in DNA encode protein molecules, which are the “workhorses” of the cell, carrying out all the functions necessary for life.

1. Step 1: Initiation. Initiation is the beginning of transcription.
2. Step 2: Elongation. Elongation is the addition of nucleotides to the mRNA strand.
3. Step 3: Termination. Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop (termination) sequence in the gene.

Transcription takes place in three steps: initiation, elongation, and termination. The steps are illustrated in Figure 2.

Transcription involves four steps:

• Initiation. The DNA molecule unwinds and separates to form a small open complex.
• Elongation. RNA polymerase moves along the template strand, synthesising an mRNA molecule.
• Termination. In prokaryotes there are two ways in which transcription is terminated.
• Processing.

Translation: Beginning, middle, and end Initiation (“beginning”): in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin. Elongation (“middle”): in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain.

nucleus

The codon AUG is called the START codon as it the first codon in the transcribed mRNA that undergoes translation. AUG is the most common START codon and it codes for the amino acid methionine (Met) in eukaryotes and formyl methionine (fMet) in prokaryotes.

In cases of start codon mutation, as usual, the mutated mRNA would be shunted to the ribosomes, but the translation would not take place. Hence, it cannot necessarily produce proteins, as this codon lacks a proper nucleotide sequence that can act as a reading frame.

The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The most common start codon is AUG (i.e., ATG in the corresponding DNA sequence).

Each three-letter sequence of mRNA nucleotides corresponds to a specific amino acid, or to a stop codon. UGA, UAA, and UAG are stop codons. AUG is the codon for methionine, and is also the start codon.

AUG, as the start codon, is in green and codes for methionine. The three stop codons are UAA, UAG, and UGA. Stop codons encode a release factor, rather than an amino acid, that causes translation to cease.

mRNA

The stop codon only serves as the signal for the end of transcription. So, if you had two start codons in an mRNA sequence, the first start codon when initiate transcription and the ribosome would add the N-terminal formyl-Methionine. The second start codon would cause the ribosome to add an internal Methionine.

The codon for Methionine; the translation initiation codon. Usually, protein translation can only start at a Methionine codon (although this codon may be found elsewhere within the protein sequence as well). In eukaryotic DNA, the sequence is ATG; in RNA it is AUG.

Stop codons are essential for the termination of the translation process. If there is no stop codon in the mRNA, then there is the possibility that the Ribosome would synthesise mRNA until the 3′ end of the mRNA is not encountered. At the 3′ end, there is no codon and thus, the ribosome cannot proceed further.

The findings, to be published on February 21, 2017, in the journal Nucleic Acids Research by scientists in a research collaboration between NIST and Stanford University, demonstrate that there are at least 47 possible start codons, each of which can instruct a cell to begin protein synthesis.

DNA is comprised of 4 different nucleotides (A, C, T, and G), whereas proteins are made of 20 amino acids. Codons are nucleotide triplets that encode for amino acids. Thus, in order for the 4 nucleotides to account for all 20 amino acids, a minimum of 3 base pairs are required.

Codon-Amino Acid Abbreviations