A Science Odyssey: DNA Workshop: Protein Synthesis

Protein Synthesis
In prokaryotes mRNA is translated as it is being transcribed from DNA
In eukaryotes it has to be transported to the cytoplasm before translation
In prokaryotes the mRNA is used immediately but in eukaryotesit must be modified before it leaves the nucleus
Protein synthesis is also much faster in prokaryotes.

Transcription is the first of overall two protein synthesis steps

For a gene to be expressed, i.e., translated into , that portion of the DNA has to be uncoiled and freed of the protective proteins. An enzyme, called , "reads" the DNA (the sequence of bases on one of the two strands of the DNA molecule) and builds a single-stranded chain of the RNA molecule as a complementary, mirror-image sequence. Again, where there is a G in DNA, there will be C in the RNA and vice versa. Instead of thymine, RNA has (U). Wherever in the DNA strand there is an A, there will be a U in the RNA, and wherever there is a T on the DNA molecule, there will be an A in the RNA.

DNA and Protein Synthesis - BioTopics

Cell uses the genes to synthesize proteins. This is a two-step process. The first step is transcription in which the sequence of one gene is replicated in an RNA molecule. The second step is translation in which the RNA molecule serves as a code for the formation of an amino-acid chain (a polypeptide).


9. As the ribosome moves by two codons, next round of protein synthesis is initiated by the attachment of a new ribosome. Thus, at a time, a single mRNA is found to be attached with many ribosomes with their polypeptides of different length, (shortest polypeptide at the 5′ end of the mRNA and longest at the 3′ end), called polysomes.

Fig. 8.15 Peptide bond formation in growing polypeptide.
10. Ultimately, the A-site of ribosome is occupied by the termination codon (UAA,UAG or UGA) at the 3′ end of mRNA, which is not recognized by any tRNA. Thus, the termination of the protein synthesis is helped by the release factors RFl, RF2 and RF3 (in eukaryotes eRF1), which release the newly synthesized polypeptide chain from the P-site (Fig. 8.16).


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).

Steps involved in protein synthesis are ..

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.

how are DNA and RNA involved in protein synthesis? | …

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