On a linear template, ATP hydrolysis, TFIIE, and the helicase activity of TFIIH (provided by the XPB and XPD subunits) are required for polymerase movement. This requirement is bypassed with a supercoiled template. This suggests that TFIIE and TFIIH are required to melt DNA to allow polymerase movement to begin. To extend elongation into the transcription unit, a kinase complex called P-TEFb is required. P-TEFb contains the CDK9

kinase, which is a member of the kinase family that controls the cell cycle. P-TEFb acts on the CTD to phosphorylate it further on serine 2 of the heptapeptide repeat.

TLS Online TPP Program

#Id: 4622

#Unit 13. Methods in Biology

 A precipitation curve for a system of one antigen and its antibodies. This plot of the amount of antibody precipitated versus increasing antigen concentrations (at constant total antibody) reveals three zones: 

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#Id: 4623

#Unit 13. Methods in Biology

A related quantitative technique, rocket electrophoresis, does permit measurement of antigen levels. In rocket electrophoresis, a negatively charged antigen is electrophoresed in a gel containing antibody. The precipitate formed between antigen and antibody has the shape of a rocket, the height of which is proportional to the concentration of antigen in the well.

TLS Online TPP Program

#Id: 4624

#Unit 13. Methods in Biology

One limitation of rocket electrophoresis is the need for the antigen to be negatively charged for electrophoretic movement within the agar matrix

TLS Online TPP Program

#Id: 4625

#Unit 13. Methods in Biology

The interaction between antibody and a particulate antigen results in visible clumping called agglutination. Antibodies that produce such reactions are called agglutinins.

TLS Online TPP Program

#Id: 4626

#Unit 13. Methods in Biology

Agglutination reactions are similar in principle to precipitation reactions they depend on the crosslinking of polyvalent antigens. Just as an excess of antibody inhibits precipitation reactions, such excess can also inhibit agglutination reactions; this inhibition is called the prozone effect.

TLS Online TPP Program

#Id: 4627

#Unit 13. Methods in Biology

A modification of the agglutination reaction, called agglutination inhibition, provides a highly sensitive assay for small quantities of an antigen. For example, one of the early types of home pregnancy test kits included latex particles coated with human chorionic gonadotropin (HCG) and antibody to HCG.