\"Describe how DNA results in a functional protein in the cell. Choose an actual

Question

"Describe how DNA results in a functional protein in the cell. Choose an actual protein and trace it through the process of transcription and translation to its final destination (does it remain within the cell to do work? outside of the cell? A different part of the body?). Be sure to use the following terms: transcription, translation, DNA, mRNA, rRNA, tRNA, codon, reading frame, anticodon, amino acids."

What are some proteins that do this? I know the general process, but I don't know any "actual protiens" to trace.

Of course, any pointers would be extermely appreciated too.

Thanks!

Explanation / Answer

Antibodies are the globular proteins that are synthesized by B- cells in the Bone marrow. During the antigen-independent development of B-cell, B-cell undergoes rearrangement of germ line DNA to generate diversity in the antibody molecule so that it can interact with many different antigens in the antigen binding region of the antibody molecule. Initially, in the pre-Bcell the germ line DNA that is present on different chromosome for V region, for lambda and kappa undergoes DNA rearangement and joining of Variable, Diversity and Joining region takes place to form a rearranged heavy chain having VDJ joined. Similarly in the light chain V and J region combine in the Pro-b cell stage. This heavy chain DNA undergoes transcription to generate a primary transcript.

Transcription is the process of development of RNA or precisely mRNA from the DNA. In case of antibody, the rearranged DNA undergoes transcription in the B-cell nucleus with the help of RNA polymerase enzyme. There are three types of RNA polymerase. RNA polymerase I that produces rRNAs, RNA Polymerase II that produces mRNA or the major product that further undergoes translation to produce protein, and RNA polymerase III that generates t-RNA which are involved in bringing aminoacids for the production of antibody protein.

Initially the DNA unwinds and transcription complex attaches to the DNA and RNA polmerase acts at the promoter region. A lot of transcription factors are involved in the initiation and regulatory machinry of the transcription. These transcription factors are exclusively produced in the pre-Bcell and pro-Bcell stages of B-cell development and are very important for production of antibody molecule. During the process of elongation, a process called splicing takes place which results in removal of introns or the non protein coding regions to generate final mature mRNA. There takes place 5' capping and polyadenylation of the primary RNA transcript containing recombined V-D-J regions and constant mu (IgM) and delta (IgD) that is generated from DNA. These post translational modifications increase the life of mRNA else will get degraded soon. Once the transcription is completed, termination occurs resulting in removal of RNA polymerase from the parent DNa. This mRNA generated has a leader sequence that guides the mRNA to ER for protein synthesis, joined V-D and J regions and constant region for either mu (IgM) or delta (IgD) and polyadenylation in the terminal region. This primary transcript undergoes translation where the mRNA binds to the ribosomes that are present outside the endoplasmic reticulum. There are various factors that are involved in the transcription initiation process.

During protein synthesis, a very important role is played by tRNA that are produced by RNA polymerase only. They are the molecules that tranfer information present in DNA to the protein. this tRNA molecule has different regions, one of the region the anti-codon region directs the amino acids that will bind to the tRNA molecule aminoacid binding region. It is complementary to the codons present in mRNA for the particular aminoacid. protein synthesis is further divided into initiation, elongation and termination steps like transcription only, but the machinery involved i sentirely different. Methionine is the first amino acid in the polypeptide produced. After initiation of protein synthesis the previous tRNA is present on the P site of the ribosome and another site called A-site will recieve the fresh aminoacyl-tRNA. Once next amino acid comes a peptide bond is formed in the amino acid present in the P and A site. After this the mRNA shifts from one codon to another on the ribosome. This is how the polypeptide elongates and a final complete protein chain of antibody molecule is produced and terminal of protein synthesis takes place because of the stop codons or tRNA bearing polypeptide releasing factors.

Antibody heavy and light chains are synthesised, they are 440 and 110 amino acid polypeptides. After theirr production heavy and light chain join each other by disulphide bonds between the cysteine residues.

The final antibody molecule protein is synthesised. I case of mature B-cell that expresses membrane bound IgM and IgD molecule the transcripts will have the membrane domains that will have regions which will code the proteins for the transmembrane region and the cytoplasmic region. For the secretory antibodies like IgA and IgG the transmembrane regions will be absent. Finally the complete antibody molecule is produced in the endoplasmic reticulum, it buds off as a vesicle and fuses with the plasma membrane and gets expressed on the surface of mature B-cell.   

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