Rabu, 13 November 2013

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Two Stages Synthesis Process Transcription and Translation.

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Protein synthesis occurs in two stages, transcription and translation.

Transcription

Transcription Process

Transcription is a process of RNA synthesis through sense strand in DNA strand. Sense is one of the DNA strands that functions as template. Other DNA strands are called antisense that function in replication process.

Protein synthesis begins with the attachment of RNA polymerase to a site called promoter. Promoter is a starting point of transcription. It also determines which DNA strand used as template. While moving along DNA strands, RNA polymerase separates the two DNA strands and does the addition of mRNA nucleotides in a rate about 60 nucleotide per second. After RNA chain is formed, the unwound DNA strand is wound again. The process continues until RNA polymerase comes to the end of DNA strand called terminator. After it reaches terminator, RNA polymerase and the new mRNA strand detach from DNA. The RNA, then, leaves nucleus toward ribosome to start the second stage.

The component of nitrogen base in mRNA is similar With the one in antisense DNA, except for thymine which is replaced by uracil. For example, if sense DNA has nitrogen bases of AGC TTC AAC CAG and CCG, then the nitrogen bases in RNA are UCG AAG IIUG GUC and GGC.

Messenger RNA molecule which is formed has two different ends, the 5'end and the 3' end. The 5' end is necessary to prevent mRNA destruction by hydrolytic enzyme and give signal to ribosome to attach on mRNA. The 3' end is necessary to restrain mRNA degradation and help the attachment of ribosome on mRNA.



Translation

Translation is the second stage of protein synthesis. In this stage, there Occurs translation of codon sequence in mRNA to become amino acid sequence in ribosome. Translation of one codon produces one amino acid. These are three stages the translation process.



Initiations is a process of small unit of ribosome at the 5'end of mRNA. It is then continued by the attachment of the first tRNA (initiator) that carries amino acid metionin at anticodon UAC on mRNA at start codon, AIJG. Start codon is a triplet indicating the initiation of protein synthesis. Furthermore, the attachment process of large ribosomal subunit to small ribosomal subunit occurs.                                

Initiation is signed by the attachment ofsmall unit ribosome at the 5' end of mRNA

Large ribosomal subunit has 3 special positions of tRNA attachment, which are A, R and E. A (right) is the entering site of tRNA that carries amino acid. P (center) is a site of tRNA to release the amino acid. E (left) is the exit site of tRNA from ribosome. 

Elongation is a composition process of polypeptide carried by tRNA. The process occurs when tRNA comes into ribosome at A position then slides to P position and removes the amino acid. tRNA slides again to E position as an exit site from ribosome. After one IRNA leaves ribosome, the ribosome slides by one codon toward the 3' end in mRNA, so that the other tRNA will take the A position in the empty ribosome. The process keeps continuing until ribosome slide comes to stop codon, which is 7 UGA or UAA or UAG. Slop codon is a triplet signing that the composition process of polypeptide chain has been over.
    Elongation is signed by the arrangement process of polypeptide
Termination is a removal stage of polypeptide chain from ribosome. A protein named removal factor will bind itself to stop codon and add water to polypeptide chain. It causes polypeptide chain to detach from ribosome. 



Source: Life: The Science of Biology, 2003,

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