mRNA ⇒ 5´-- 5´
Protein ⇒ N - MET PRO GLY LEU ARG - C
Explanation:
The first step before protein arrangement is to synthesize messenger RNA, mRNA. This is the transcription process and occurs in the nucleus. When the DNI molecule separates into two strands to form the transcription bubble, we can identify two separate segments: coding strand and template strand. The coding strand goes in direction 5´ to 3´, while the complementary strand -template strand- grows in direction 3´ to 5. This last segment is the one that is going to be complemented by the mRNA.
Once mRNA synthesis is over, the molecule leaves the nucleus to start the transcription process in the cytoplasm. The ribosome reads mRNA in the 5´ to 3´ direction, and, according to the codons that are being readen, tRNA transfers the correct amino acids to build the polypeptide chain. A codon is a short sequence of three nucleotides that store the genetic information for the aminoacids´ assembly. Each tRNA has two important sites. One of them that couples with the codon of the mRNA molecule, named anticodon. The other site couples with an amino acid. tRNA allows amino acids to align according to the nucleotidic sequence in the mRNA molecule.
The protein is synthesized from the amino terminus to the carboxy terminus, adding amino acids to the chain according to the codon sequence in the mRNA. mARNs also have a start and end codon that are the signals of the synthesis initiation and finish. When the ribosome reaches the end codon, protein synthesis is over.
• The start codon is AUG and places near the 5´extreme of the molecule.
• The end codons are UAA, UAG, UGA.
Protein synthesis initiates in the AUG start codon -Metionin-, and ends when reaching either of the stop codons UAA, UAG, UGA.
When talking about amino and carboxy terminus, the word Terminus refers to the extremes of the polypeptide. The first extreme to be translated carries the amino-terminal group, while the other extreme carries the carboxy-terminus group.
Conventionally, proteins are written from left to right, beginning by the N-terminal extreme carrying the free amine group, and ending by the C-terminal extreme carrying the carboxyl free group. However, we need to know that the free amine group actually places at the end of a protein.
In the exposed example we have the following DNI template strand ⇒5'3'
Transcription:
The template DNI strand is read in direction 3´→ 5´ to build the mRNA molecule in direction 5´→ 3´.
template DNI strand ⇒ 5'--3'
mRNA ⇒ 3´-- 5´
Translation:
rRNA and tRNA read mRNA in the direction 5´→ 3´ to build the protein.
Start codon AUG -Metionin-, near the 5´ extremeEnd UAA, UAG, UGA.
The first portion of mRNA is not read nor translated. This is the untranslated region (UTR), placed before the start codon.
mRNA ⇒ 3´-- 5´
Remember that transcription occurs from 5´ to 3´ extremes, so we need to read the codons in this direction too, beginning on the 5´ extreme.
To make it easier, we can turn the mRNA direction, and write it from 5´to 3´.
mRNA ⇒ 3´-- 5´
mRNA ⇒ 5´-- 5´
Now, we need to find the initiation codon: AUG coding for Metionin.
mRNA ⇒ 5´-CG AUG CCA GGU CUC AGG UGA CGC AAG- 5´
Codons are separated by a space left between them. AUG is the start codon placed near the 5´ extreme.
Now, let us find the end codon, near the 3´extreme.
mRNA ⇒ 5´-CG AUG CCA GGU CUC AGG UGA CGC AAG- 5´
rRNA will read mRNA until it reaches UGA codon, which is the stop signal.tRNA will add amino acids from the start codon, not before.
tRNA anticodons ⇒ UAC GGU CCA GAG UCC
Anticodons are separated by a space left between them.
Protein ⇒ N - MET PRO GLY LEU ARG - C
Each mRNA codon codifies for an amino acid. The start codon codifies for methionine. AUG= Met, CCA= Pro, GGU= Gly, CUC= Leu, AGG= Arg, UGA= Stop codon. The amino terminus is represented as an N and the carboxy terminus is a C. The first extreme to be translated carries the amino-terminal group, while the other extreme carries the carboxy-terminus group.
