1. 1 The Interrupted Gene

2. Ex Biochem c3-interrupted gene 2 3.1 Introduction Figure 3.1

3. Ex Biochem c3-interrupted gene 3 3.2 An Interrupted Gene Consists of Exons and Introns Introns are removed by the process of RNA splicing Only mutations in exons can affect protein sequence; however, mutations in introns can affect processing of the RNA, preventing production of protein. Figure 3.2

4. Ex Biochem c3-interrupted gene 4 3.3 Restriction Endonucleases: Key Tool in Mapping DNA Restriction endonucleases can be used to cleave DNA into defined fragments. Only cut DNA with specific sequence Restriction enzyme 限制脢 Figure 3.3

5. Ex Biochem c3-interrupted gene 5

6. 6

7. 7 A map can be generated by using the overlaps between the fragments generated by different restriction enzymes. Figure 3.4 Restriction map

8. Ex Biochem c3-interrupted gene 8 3.4 Organization of Interrupted Genes May Be Conserved Introns can be detected by the presence of additional regions when genes are compared with their RNA products by restriction mapping or electron microscopy. Complimentary DNA (cDNA) Ultimate definition is based on comparison of sequences. Figure 3.5

9. Ex Biochem c3-interrupted gene 9 The positions of introns are usually conserved when homologous genes are compared between different organisms. The lengths of the corresponding introns may vary greatly. Introns usually do not code for proteins. Figure 3.7

10. Ex Biochem c3-interrupted gene 10 3.5 Exon Sequences Are Conserved but Introns Vary Comparisons of related genes in different species show that the sequences of the corresponding exons are usually conserved; but the sequences of the introns are much less well related Figure 3.9

11. Ex Biochem c3-interrupted gene 11 Exons are usually short, typically coding for <100 amino acids. Introns are short in lower eukaryotes, but range up to several 10s of kb in length in higher eukaryotes. Figure 3.12

12. Ex Biochem c3-interrupted gene 12 The overall length of a gene is determined largely by its introns. Figure 3.13

13. Ex Biochem c3-interrupted gene 13 3.7 Some DNA Sequences Code for More Than One Protein The use of alternative initiation or termination codons allows two proteins to be generated where one is equivalent to a fragment of the other. Figure 3.14

14. Ex Biochem c3-interrupted gene 14 when it is read in different reading frames by two (overlapping) genes. Figure 3.15 Nonhomologous protein can be produced from same sequence of DNA

15. Ex Biochem c3-interrupted gene 15 Homologous proteins can differ by the presence or absence of certain regions They are generated by differential (alternative) splicing when certain exons are included or excluded. This may take the form of: including or excluding individual exons choosing between alternative exons Figure 3.17

16. Ex Biochem c3-interrupted gene 16 Figure 3.19

17. Ex Biochem c3-interrupted gene 17 3.9 Some Exons Can Be Equated with Protein Functions exons were the building blocks of evolution and the first genes were interrupted Gene structure is conserved between genes in very distant species Many exons can be equated with coding for protein sequences that have particular functions. Related exons are found in different genes.

18. Ex Biochem c3-interrupted gene 18 Figure 3.20

19. Ex Biochem c3-interrupted gene 19 3.10 Members of a Gene Family Have Common Organization A common feature in a set of genes is assumed to identify a property that preceded their separation in evolution. All globin genes have a common form of organization with three exons and two introns; This suggests that they are descended from a single ancestral gene. Figure 3.21

20. Ex Biochem c3-interrupted gene 20