Genes Code for Proteins

1. Genes Code for Proteins

2. Ex Biochem c2-gene code 2.1 Introduction Figure 2.1

3. Ex Biochem c2-gene code Genes and locus • Genes 基因: units of inheritance that are passed down to the next generation • Arranged as DNA segments in a chromosome • Locus 基因座: specific positions of every gene

4. Ex Biochem c2-gene code Allele「對偶基因」或「等位基因」 • The genesfound at the same locus on different homologous chromosomes • Genes that are alleles of one another always control the same characteristic • 2 alleles result in different expression of the same character: heterozygote (same: homozygote)

5. Ex Biochem c2-gene code Allele • 一位於染色體上單一位置的特定基因，其所有的不同形式中之任一形式即謂之 allele(顯性/隱性; 血型A/B/O) • 同一位置的基因控制同一性狀 • 某個基因的基因型是由該基因所擁有的一對等位基因所決定(顯性/隱性)

6. Ex Biochem c2-gene code 2.2 A Gene Codes for a Single Polypeptide • The one gene : one enzyme hypothesis summarizes the basis of modern genetics: • A gene is a stretch of DNA coding for a single polypeptide chain • Sometimes may not be true Figure 2.2

7. Ex Biochem c2-gene code 2.3 Mutations in the Same Gene Cannot Complement • A mutation in a gene affects only the protein coded by the mutant copy of the gene. • It does not affect the protein coded by any other allele.

8. Ex Biochem c2-gene code 2.4 Mutations May Cause Loss-of-Function or Gain-of-Function • Recessive mutations are due to loss-of-function by the protein product. • Dominant mutations result from a gain-of-function. Figure 2.4

9. Ex Biochem c2-gene code 2.8 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 shift in the triplet sets after the site of mutation. • Combinations of mutations that together insert or delete three bases (or multiples of three) insert or delete amino acids; • But they do not change the reading of the triplets beyond the last site of mutation

10. Ex Biochem c2-gene code Figure 2.9

11. Ex Biochem c2-gene code 2.9 Every Sequence Has 3 Possible Reading Frames • Usually only one reading frame is translated and the other two are blocked by frequent termination signals. Figure 2.10

12. Ex Biochem c2-gene code 2.11 Several Processes Required to Express Protein Product of a Gene • A prokaryotic gene is expressed by transcription into mRNA and then by translation of the mRNA into protein. Figure 2.14

13. Ex Biochem c2-gene code • In eukaryotes, a gene may contain internal regions that are not represented in protein. • Exon, intron • Internal regions are removed from the RNA transcript by RNA splicing • This gives an mRNA that is colinear with the protein product Figure 2.15

14. Ex Biochem c2-gene code RNA structure • Each mRNA consists of: • a nontranslated 5′ leader • a coding region • a nontranslated 3′ trailer (poly A tail) Figure 2.13

15. Ex Biochem c2-gene code RNA extraction • More water soluble than DNA • Purification of total RNA • Tissue/cell homogenate/lysis in phenol and guanidine thiocyanate: inhibition of RNase • addition of bromochloropropane or chloroform: separation into aqueous and organic phases. RNA to aqueous phase, DNA to interphase, and proteins to organic phase • RNA precipitated from the aqueous phase with isopropanol. • Purification of mRNA • Oligo-dT column, bind poly A tail of mRNA • Elute with NaCl