level 7
The immunoglobulin bound to the B lymphocytes is the receptor of theB lymphocyte (BCR). It is also an existing form of the antibody, so the BCRhave the light chain and the heavy chain.
What is the machinery of the formation of the antibody? It is due tothe rearrangement of the gene. There are two types of chains in the antibody,and the formation of each other is different. In the formation of the lightchain, V gene segment (variable, V) and J gene segment (Joining, J) first jointogether, becoming the V-J. Then the V-region (V-J) is close to the C-region,but there is also a short intron between the V and C. After transcription,under the RNA splicing, the mRNA get rid of the intron between V and L (leadersegment, which can direct the protein into the cell’s secretory pathways), andthe intron between V and C, and the mRNA can be translated. As for the heavychain, there is a diversity segment (D) between the V and J. The D and J joinstogether first, then it is similar to the formation of the light chain. But,what is different is that the C-region has a lot of exons, between each exonthere is an intron, and one of the exons can become a hinge region after ittranslated, for it can form the disulfide bonds.
How can the DNA rearrangement be regulated? There is a RSS(Recombination signal sequence) adjacent to the functional segment. RSSincludes three parts, a heptamer (conversed,-5’CACAGTG3’-), a spacer (either 12or 23 base pairs) and a nonamer (conversed,-5’ACAAAAACC3’-). With the 12/23rule, the V and J(or V, D and J can join to each other orderly).
When the joining segments are in the same orientation, the signaljoint is in a circular piece of extrachromosomal DNA, which is lost from thegenome when the cell divides. While in the opposite orientation, alignment ofthe RSSs requires the coiled topology, rather than a simple loop like theformer, so that joining the ends of the two heptamer sequences results in theinversion and integration of intervening DNA into a new position on thechromosome. Again, the joining of the V and J segments creates a functionalV-region exon.
Thereare lots of enzymes play roles in the joining, like RAG1/2, Ku70/80 etc. Normal 0 7.8 磅 0 2 false false false EN-US ZH-CN X-NONE /* Style Definitions */ table.MsoNormalTable{mso-style-name:普通表格;mso-tstyle-rowband-size:0;mso-tstyle-colband-size:0;mso-style-noshow:yes;mso-style-priority:99;mso-style-parent:"";mso-padding-alt:0cm 5.4pt 0cm 5.4pt;mso-para-margin:0cm;mso-para-margin-bottom:.0001pt;mso-pagination:widow-orphan;font-size:10.5pt;mso-bidi-font-size:11.0pt;font-family:"Calibri","sans-serif";mso-ascii-font-family:Calibri;mso-ascii-theme-font:minor-latin;mso-hansi-font-family:Calibri;mso-hansi-theme-font:minor-latin;mso-bidi-font-family:"Times New Roman";mso-bidi-theme-font:minor-bidi;mso-font-kerning:1.0pt;}
