1. Chapter 2 Genes Code for Proteins

2. 2.1Introduction Early work measuring recombination frequencies between genes led to the establishment of “linkage groups”: –Sets of genes that do not assort independently Figure 2.01: Each chromosome has a DNA containing many genes.

3. 2.2Most Genes Code for Polypeptides The one gene : one polypeptide hypothesis summarizes the basis of modern genetics: –That a typical gene is a stretch of DNA coding for a single polypeptide chain. Genes can also code for RNA products that are not translated into proteins. Mutations can damage gene function.

4. 2.2Most Genes Code for Polypeptides Figure 2.02: Recessive alleles do not produces active protein.

5. 2.3Mutations in the Same Gene Cannot Complement A mutation in a gene affects only the polypeptide encoded by the mutant copy of the gene –It does not affect the polypeptide encoded by any other allele Failure of two mutations to complement (produce wild-type phenotype when they are present in trans configuration in a heterozygote) means that they are part of the same gene.

6. 2.3Mutations in the Same Gene Cannot Complement Figure 2.03: The cis/trans test assigns mutations to genes.

7. 2.4Mutations May Cause Loss-of-Function or Gain-of-Function Recessive mutations are due to loss-of-function by the polypeptide product. Dominant mutations result from a gain-of- function. Testing whether a gene is essential requires a null mutation (one that completely eliminates its function).

8. 2.4Mutations May Cause Loss-of-Function or Gain-of-Function Figure 2.04: Mutations vary from silent to null function.

9. Silent mutations have no effect, either because: –the base change does not change the sequence or amount of polypeptide –the change in polypeptide sequence has no effect “Leaky” mutations do affect the function of the gene product –They are not shown in the phenotype because sufficient activity remains 2.4Mutations May Cause Loss-of-Function or Gain-of-Function

10. 2.5A Locus May Have Many Alleles The existence of multiple alleles allows the possibility of heterozygotes representing any pairwise combination of alleles. A locus may have a polymorphic distribution of alleles with no individual allele that can be considered to be the sole wild-type.

11. 2.5A Locus May Have Many Alleles Figure 2.06: Galactosyltransferase makes A and B antigens.

12. 2.6Recombination Occurs by Physical Exchange of DNA Recombination is the result of crossing-over that occurs at a chiasma and involves two of the four chromatids. Recombination occurs by a breakage and reunion that proceeds via an intermediate of heteroduplex DNA.

13. 2.6Recombination Occurs by Physical Exchange of DNA Figure 2.07: Crossing-over occurs at the 4-strand stage.

14. Linkage between genes is determined by their distance apart. Genes that are close together are tightly linked because recombination between them is rare. Genes that are far apart may not be directly linked –Recombination is so frequent as to produce the same result as for genes on different chromosomes. 2.6Recombination Occurs by Physical Exchange of DNA

15. Figure 2.08: Recombination between genes depends on their distance apart.

16. 2.7The Genetic Code Is Triplet The genetic code is read in nucleotide triplets called codons. The triplets are nonoverlapping and are read from a fixed starting point.

17. 2.7The Genetic Code Is Triplet Mutations that insert or delete individual bases cause a shift in the triplet sets after the site of mutation. –These are frameshift mutations. Combinations of mutations that together insert or delete three bases (or multiples of three) insert or delete amino acids. –They do not change the reading of the triplets beyond the last site of mutation.

18. 2.7The Genetic Code Is Triplet Figure 2.09: The genetic code is triplet.

19. 2.8Every Coding Sequence Has Three Possible Reading Frames Usually only one of the three possible reading frames is translated and the other two are closed by frequent termination signals. Figure 2.10: A DNA sequence usually contains one open reading frame.

20. 2.9Bacterial Genes Are Colinear with Their Products A bacterial gene consists of a continuous length of 3N nucleotides that codes for N amino acids. The gene is colinear with both its mRNA and polypeptide products.

21. 2.9Bacterial Genes Are Colinear with Their Products Figure 2.11: Gene and protein are colinear.

22. 2.10Several Processes Are Required to Express the Product of a Gene A bacterial gene is expressed: –by transcription into mRNA –then by translation of the mRNA into polypeptide

23. 2.10Several Processes Are Required to Express the Product of a Gene Figure 2.14: Bacterial transcription and translation occur simultaneously.

24. 2.10Several Processes Are Required to Express the Product of a Gene In eukaryotes, a gene may contain introns that are not represented in polypeptide. Introns are removed from the RNA transcript by splicing to give an mRNA that is colinear with the polypeptide product. Each mRNA consists of an untranslated 5′ leader, a coding region, and an untranslated 3′ trailer.

25. 2.10Several Processes Are Required to Express the Product of a Gene Figure 2.15: Eukaryotic RNA is processed and exported.

26. 2.11Proteins Are trans-acting, but Sites on DNA Are cis-acting All gene products (RNA or polypeptides) are trans-acting. –They can act on any copy of a gene in the cell. cis-acting mutations identify sequences of DNA that are targets for recognition by trans-acting products. –They are not expressed as RNA or polypeptide and affect only the contiguous stretch of DNA.

27. 2.11Proteins Are trans-acting, but Sites on DNA Are cis-acting Figure 2.17: Mutations in control sites are cis-acting.