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Gene Expressionand Protein Synthesis

MECHANISM OF PROTEIN SYNTHESIS

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DNA is transcribed into RNA, which istranslated into protein.

In eukaryotes, the termination sites in DNA are present far away from the corresponding actual 3′ end of mRNA, thus, to produce HnRNA (heterogeneous nuclear RNA). The 3′ end of the mRNA is generated after the processing of HnRNA by snurp (small nuclear RNA-protein complex). In addition to these extra nucleotides at 3′ end, the HnRNA may also contain extra nucleotide sequences at the 5′ end and at the internal positions. These extra nucleotide sequences at internal positions are called introns, whereas, the nucleotide sequences in between the introns

Fig. 8.12 Transcription and processing of mRNA in eukaryotes,

The process of synthesis of proteins from mRNA (translation of language of nucleic acids into the language of proteins) is called translation. There are 20 different types of amino acids, which constitute various proteins, and these amino acids themselves cannot recognize their respective codons in the mRNA. Different amino acids are carried by their specific tRNA molecules at the
site of protein synthesis (mRNA). There are about 55 types of tRNA molecules available in the cytoplasm, so that one amino acid may have more than one tRNAs.

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Fig. 8.16 Translation process of protein synthesis in prokaryotes.

i.ρ-dependent Termination: In some prokaryotes, the termination of transcription is helped by a ρ (rho) protein that gets attached at the 5′ end of the newly synthesizing mRNA. The ρ then moves along the mRNA and induces the formation of hairpin loop near the 3′ end of mRNA due to the presence of inverted repeated sequences. This helps in the detachment of mRNA from the DNA (Fig. 8.9).


i.RNA polymerase I (or A) found in nucleolus for the synthesis of rRNAs (28S and 18S).

ii.RNA polymerase II (or B) found in nucleoplasm for the synthesis of HnRNA (heterogeneous nuclear RNA) which, after processing gives rise to mRNA.

iii.RNA polymerase III (C) found in nucleoplasm for the synthesis of tRNA and 5S rRNA.

iv.Organeller RNA polymerase found in organelles, mitochondria and chloroplasts, (in photosynthetic organisms) and resembles the prokaryotic RNA polymerase.

Drag-and-Drop Protein Synthesis: Transcription - zeroBio

In prokaryotes, the transcription and translation processes occur simultaneously as there is no nuclear membrane to separate the genetic DNA and the cytoplasm.

The activity of those lat genes (yes or not in state of transcription) is often determined by a confluence of external factors and internal cellular signals.

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Protein Synthesis -Translation and Regulation

The process of synthesis of RNAs (mRNA, tRNA and rRNA) from DNA by the enzyme RNA polymerase is known as transcription. At the time of transcription, the RNA polymerase binds with double stranded DNA (gene) at a particular site (in prokaryotes known as promoter site) and after unwinding of the two strands of DNA by the rotation of the DNA, it starts copying one of
the two strands, known as coding strand (sense strand or template strand). The other strand of the DNA, which is not copied for the RNA synthesis, is known as non-coding strand (antisense strand) (Fig. 8.6).

Protein synthesis - Biology-Online Dictionary

that are present in mRNA and contain the information of proteins are known as exons. Thus, HnRNA produced after transcription is quite longer than the mRNA. Most of the extra nucleotide sequences, including introns, are cleaved by snurp. Moreover, after removal of the extra nucleotides from the 3′ end of the HnRNA, poly A tail is added that is required for the stability of the mRNA. Similarly, after removal of extra nucleotides from the 5′ end, a cap of 7-methyl guanosine (7mG) is added that is required for the translation process. The production and processing of HnRNA occurs in the nucleus from where it escapes into the cytoplasm through nuclear pores for the translation process (Fig 8.11 and 8.12).

i will use it to build my knowledge on protein synthesis process.


The protein synthesis occurs by means of transcrition (in the nucleus: production of RNA with nitrogenous bases that are complementary to one of the ; thus DNA -> RNA) and translation (in polyribosomes and the both located in the cytoplasm: RNA codes -> specific chains of aminoacids, i.e.

08/05/2006 · Protein Synthesis: Transcription and ..



Fig 8.14 Initiation of translation in eukaryotes.
4. The ribosomes have two sites, the A-site (amino acyl site) for the location of new amino acyl-tRNA (except for the fmet-tRNAfmet) and a P-site (peptidyl site) for locating the tRNA attached with newly synthesizing amino acid chain (di and polypeptide). The fmet-tRNAfmet after binding withIF2, is located on the initiation codon AUG at the P-site of the 30S subunit of ribosome which requires energy in the form of GTP. The role of IF1 is not
known. (In eukaryotes more initiation factors i.e., eIF1 to e1F6, are involved in nitiation).

Protein Synthesis: Transcription & Translation

5. After initiation of the translation process, elongation process for the formation of dipeptide and ultimately the polypeptide chain is started by the attachment of AA2-tRNAAA2 (amino acyl tRNA or second amino acid attached with its specific tRNA) at its respective codon at the empty A-site with the help of the elongation factors EFTu (temperature unstable elongation factor) and EFTs (temperature stable elongation factor). This process also requires GTP. (In eukaryotes the elongation factors eEF1 and eEF2 are involved in the elongation process).

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