Genes Code for Proteins

Ex Biochem c2-gene code 2.1 Introduction Figure 2.1

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

Ex Biochem c2-gene code Allele「對偶基因」或「等位基因」 The genes found 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)

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) 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)

2.2 A Gene Codes for a Single Polypeptide 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

2.3 Mutations in the Same Gene Cannot Complement 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.

2.4 Mutations May Cause Loss-of-Function or Gain-of-Function 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

2.8 The Genetic Code Is Triplet 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

Ex Biochem c2-gene code Figure 2.9

2.9 Every Sequence Has 3 Possible Reading Frames 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

2.11 Several Processes Required to Express Protein Product of a Gene 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

Internal regions are removed from the RNA transcript by RNA splicing 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

RNA structure Each mRNA consists of: a nontranslated 5′ leader 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

RNA extraction More water soluble than DNA Purification of total RNA 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