12.9.2 Antibodies and DNA Rearrangement

Question 1

Antibodies and DNA Rearrangement

Answer 1

• The body can produce almost an infinite number of different antibodies, each specific for a particular antigen.
• As B cells mature in the bone marrow, their DNA rearranges, allowing them to produce a particular unique antibody.
• The DNA in each B cell can rearrange in almost an infinite number of ways, allowing the body to produce a comprehensive arsenal of antibodies.

Question 2

note

Answer 2

• Antibodies are proteins consisting of four polypeptide chains, two identical heavy chains and two identical light chains. Each chain is composed of segments encoded for by immunoglobulin genes. Immunoglobulin genes originally have segments for hundreds of different variable (V) regions, many different junction (J) regions, and several different constant (C) regions.
• As a B cell matures, these genes rearrange to produce a unique sequence that will en code a unique antibody.
• The diagram to the left shows a simplified version of DNA rearrangement in immunoglobulin genes. As the B cell differentiates, a stretch of DNA is removed from the original gene. This deletion places the V2
  region next to the J region.
• The DNA is then transcribed, and RNA processing takes place. The protein that will result consists of the V2 region and the J and C regions.
• Keep in mind that this diagram shows only three variable regions and one J and C region. In reality there are hundreds of V regions, several J regions, and several C regions. In addition, there are two different chains (the heavy chain and light chain). Thus, millions of possible antibodies can arise from the rearrangement of antibody genes.

Question 3

The site on an antibody where an antigen binds is referred to as the antibody’s variable region because:

Answer 3

• The region is different on different antibody molecules.

Question 4

True or false?

A human can produce millions of different antibodies because there are millions of antibody genes.

Answer 4

• false

Question 5

Identify the different chains of molecules in the figure below.

Answer 5

1. DNA of undifferentiated lymphocyte;
2. DNA of B cell;
3. pre RNA;
4. mRNA;
5. Polypeptide of antibody.

Question 6

Which statement regarding antibody diversity is not true?

Answer 6

• Antibody diversity is partly due to clonal selection in B cell genes.

Question 7

What is an intron?

Answer 7

• Introns are pieces of DNA that do not code for an amino acid in a polypeptide.