1. Genetic Code and Interrupted Gene Chapter 4. Genetic Code and Interrupted Gene Aala A. Abulfaraj

2. The Genetic Code Is Triplet The genetic code is read in triplet nucleotides called codons. The triplets are nonoverlapping and are read from a fixed starting point. Mutations that insert or delete individual bases cause a frameshift in the triplet sets after the site of mutation.

3. FIGURE : Frameshift mutations show that the genetic code is read in triplets from a fixed starting point

4. Every Sequence Has Three Possible Reading Frames In general, only one reading frame is translated, and the other two are blocked open reading frame (ORF) – A sequence of DNA consisting of triplets that can be translated into amino acids starting with an initiation codon and ending with a termination codon. FIGURE : An open reading frame starts with AUG and continues in triplets to a termination codon

5. Express the Protein Product of a Gene Each mRNA consists of a untranslated 5′ region (5′ UTR or leader), a coding region, and an untranslated 3′ UTR or trailer. FIGURE 14: The gene may be longer than the sequence coding for protein

6. Express the Protein Product of a Gene Ribosome: A large assembly of RNA and proteins that synthesizes proteins under direction from an mRNA template. Ribosomal RNAs (rRNAs) – A major component of the ribosome. Transfer RNA (tRNA) – The intermediate in protein synthesis that translate the genetic code. –Each tRNA molecule can be linked to an amino acid. –A tRNA has an anticodon sequence that is complementary to a triplet codon representing the amino acid.

7. Several Processes Are Required to Express the Protein Product of a Gene Several Processes Are Required to Express the Protein Product of a Gene In eukaryotes, a gene may contain internal regions, or introns, that are not represented in protein. Introns are removed from the mRNA transcript by RNA splicing to give an mRNA that is translated to a protein product. FIGURE: Gene expression is a multistage process

8. An Interrupted Gene Interrupted gene: is a gene that has not continuous coding sequence due to the presence of introns. Primary (RNA) transcript : The original unmodified RNA produced by transcription RNA splicing: The process of removing introns from primary transcribed RNA and connecting the exons into a continuous mRNA

9. An Interrupted Gene Intron: A segment of DNA that is transcribed, but later removed by splicing Introns usually do not encode proteins Exon – Any segment of an interrupted gene that is represented in the mature RNA product.

10. An Interrupted Gene Consists of Exons and Introns

11. Organization of Interrupted Genes May Be Conserved Comparisons of related genes in different species show that the sequences of exons are usually conserved, but the sequences of the introns much less

12. Gene Family Gene family: is a set of genes within a genome that codes for related or identical proteins or RNAs. Superfamily: is a set of genes that all related by assuming their production from a common ancestor, but now showing considerable variation. Homologous genes (homologs): is a set of genes that share common features.