[基因Ⅸ(基因9).本杰

[基因Ⅸ(基因9).本杰

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Genes Code for Proteins CHAPTER OUTLINE Introduction - IIBI AGene Codes for a Single Polypeptide The one gene; one enzyme hypothesis summarizes the basis of modern genetics: that a gene is a stretch of DNA coding for a single polypeptide chain. .. • Most mutations damage gene function. Mutations in the Same Gene Cannot Complement A mutation in a gene affects only the protein coded by the mutant copy of the gene and does not affect the protein coded by any other allele. • Failure of two mutations to complement (produce wild phe- notype when they are present in trans configuration in a heterozygote means that they are part of the same gene. Mutations May Cause Loss-of-Function or Gain-of-Function Recessive mutations are due to loss-of-function by the pro- tein 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). • Silent mutations have no effect, either because the base change does not change the sequence or amount of pro- tein, or because the change in protein sequence has no effect. Leaky mutations do affect the function of the gene product, but are not revealed in the phenotype because sufficient activity remains. A Locus May Have Many Different Mutant Alleles The existence of multiple alleles allows heterozygotes to occur representing any pairwise combination of alleles. A Locus May Have More than One Wild-type Allele • Alocus may have a polymorphic distribution of alleles with no individual allele that can be considered to be the sole wild-type. Recombination Occurs by Physical Exchange of DNA Recombination is the result of crossing-over that occurs at chiasmata and involves two of the four chromatids. Recombination occurs by a breakage and reunion that pro- ceeds via an intermediate of hybrid DNA. 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 do not change the reading of the triplets beyond the last site of mutation. Every Sequence Has Three Possible Reading Frames Usually only one reading frame is translated and the other two are blocked by frequent termination signals. Prokaryotic Genes Are Colinear with Their Proteins Aprokaryotic gene consists of a continuous length of 3N nucleotides that codes for N amino acids. The gene, mRNA, and protein are all colinear.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 12/29/2010 for the course LS 112 taught by Professor Hu during the Spring '10 term at Zhejiang University.

Page1 / 14

[基因Ⅸ(基因9).本杰

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online