DNA Recombination Lecture 5

DNA Recombination Lecture 5 7526468 WE DNA a Td At the end of this lecture, students should be able to: • Understand the process of DNA recombination and its types. • Recognize the role of gene rearrangement in immunoglobulin synthesis. Genetic recombination events fall into at least thr...

DNA Recombination Lecture 5 7526468 WE DNA a Td At the end of this lecture, students should be able to: • Understand the process of DNA recombination and its types. • Recognize the role of gene rearrangement in immunoglobulin synthesis. Genetic recombination events fall into at least three general classes. Homologous genetic recombination involves genetic exchanges between any two DNA molecules that share an extended region of nearly identical sequence. The actual sequence of bases is irrelevant, as long as it is simitar in the two DNAs. Site -specific recombination the exchanges occur only at a particular DNA sequence. DNA transposition usually involves a short segment of DNA with the remarkable capacity to move from one location in a chromosome to another “Jumping genes“. to _ mn _ ji s1 jyp s i It serves an important role in DNA repair in bacteria and eukaryotic cells. Recombination occurs with the highest frequency during meiosis. After the DNA is replicated during prophase of the first meiotic division, Genetic information is exchanged between the closely associated homologous chromatids by homologous genetic recombination in a process called crossing over. Homologous genetic recombination 7 o o É III e o exchangin 11 in both one strand Strands DNA Synthesizing enzymes will complete these d will complete the Strand and link it recombination Diversity C Prophase Site -specific recombination It is limited to specific DNA sequences. Recombination reactions of this type occur in virtually every cell, filling specialized roles that vary greatly from one species to another. Examples include regulation of the expression of certain genes and promotion of programmed DNA rearrangements in embryonic development or in the replication cycles of some viral and plasmid DNAs. Each site -specific recombination system consists of an enzyme called a recombinase and a short 20 to 200 bp of unique DNA sequence where the recombinase acts. One or more auxiliary proteins may regulate the timing or outcome of the reaction. I virus use this method to inject DNA into our cells Jumping genes (Transposons) These segments of DNA, found in virtually all cells, move from one place on a chromosome (the donor site) to another on the same or a different chromosome (the target site). Transposition; the new location is determined randomly. Bacteria have two classes of transposons. Insertion sequences (simple transposons) contain only the sequences required for transposition and the genes for the proteins (transposases) that promote the process. Complex transposons contain one or more genes in addition to those needed for transposition. These extra genes might, for example, confer resistance to antibiotics and thus enhance the survival chances of the host cell. Eukaryotes also have transposons, structurally similar to bacterial transposons, and some use similar transposition mechanisms. abstain 55 Donor M a im pyo W's se est Ta r g e t s 95 É idea I Seiichi's Inanna Fog ones frug age transposons to inject their DNA in our cells Basia.im Simple way target a war Replicat MY sequence of transposition an gene of transposes troposon g85IIjwIEwIfoso.s 2Ty p e s of transposition Directed DNA removed from one place and inserted into Replicative DNA copied is a I al I I n u d Some DNA rearrangements are a programmed part of development in eukaryotic organisms. An important example is the generation of complete immunoglobulin genes from separate gene segments in vertebrate genomes. A human (like other mammals) can produce millions of different immunoglobulins (antibodies) with distinct binding specificities. Recombination allows an organism to produce an extraordinary diversity of antibodies from a limited DNA -coding capacity. The genes for these polypeptides are divided into segments, and the genome contains clusters with multiple versions of each segment. The joining of one version of each gene segment creates a complete gene. Immunoglobulin Genes Assemble by Recombination Loniginal keep its DNA intact antibodies MÉ sy i 7 j ELITE kid a chains Recombination of the V and J gene segments of the human lgG kappa light chain. This process is designed to generate antibody diversity. At the top is shown the arrangement of lgG -coding sequences in a stem cell of the bone marrow. Recombination deletes the DNA between a particular V segment and a J segment. After transcription, the transcript is processed by RNA splicing, translation produces the light - chain polypeptide. The light chain can combine with any of 5,000 possible heavy chains to produce an antibody molecule 300 5 lighten It 1500 I ÉÉ The heavy -chain genes contain D regions in addition to V, J, and C regions. First the D and J segments join. Then a V segment is joined to the rearranged DJ region. The introns between J and C regions are removed by splicing to yield heavy -chain mRNA 120 12 go 4 heavy DOG 5 Chain V D u segments T.T T 120 12 y d mis further antibody diversity is generated after the formation of rearranged immunoglobulin genes by two processes that occur only in B lymphocytes: class switch recombination and somatic hypermutation . Class switch recombination results in the association of rearranged V(D)J regions with different heavy -chain constant regions, leading to the production of antibodies with distinct functional roles in the immune response; IgG, IgM, IgA, IgE . Somatic hypermutation increases the diversity of immunoglobulins by producing multiple mutations within rearranged variable regions of both heavy and light chains. These mutations, principally single -base substitutions, occur with frequencies as high as 10 – 3, approximately a million times higher than normal rates of spontaneous mutation. They lead to the production of immunoglobulins with a substantially increased affinity for antigen, and therefore are an important contributor to an effective immune response. Ind Ift 981 AB EEE most common recombination in humans Immunoglobulin gene assembly by recombination constant hag vanint Synth Ig 1 light 2 heavy 9 Dg