Download
1 / 39
390 likes | 565 Views
CHAPTER 15. THE GENETIC CODE. 生物学基地班 李柯 200431060019. Outlines. THE CODE IS DEGENERATE THREE RULES GOVERN THE GENETIC CODE SUPPRISSOR MUTATIONS CAN RESIDE IN THE SAME OR A DIFFERENT GENE THE CODE IS NEARLY UNIVERSAL. TOPIC 1 THE CODE IS DEGENERATE.
E N D
CHAPTER 15 THE GENETIC CODE 生物学基地班 李柯 200431060019
Outlines • THE CODE IS DEGENERATE • THREE RULES GOVERN THE GENETIC CODE • SUPPRISSOR MUTATIONS CAN RESIDE IN THE SAME OR A DIFFERENT GENE • THE CODE IS NEARLY UNIVERSAL
The phenomenon that many amino acids are specified by more than one codon called degeneracy Codons specifying the same amino acid are synonyms
This table lists all 64 permutations, with the left-hand column indicating the base at the 5’ end of the triplet, the row across the top specifying the middle base and the right-hand column identifying the base in the 3’ position
Code degeneracy 1 Often, when the first two nucleotides are identical, the third nucleotide can be either C or U without changing the code. A and G at the third position are interchangeable as well 2 Transition in the third position of a codon specifies a same amino acid. Transversion in this position changes the amino acid about half the time.
Codon degeneracy, especially the frequent third-place equivalence of cytosine and uracil or guanine and adenine, explains how there can be great variation in the AT/GC ratios in the DNA of various organisms without correspondingly large changes in the relative proportion of amino acids in their prteins.
Critical stem and loop regions of the tRNA structure are labeled. The red hexagons linked to the G denote methylation at the N1 positions of the base. Note that the codon is shown in a 3’ to 5’orientation
Perceiving Order in the Makeup of the Code Inspection of the distribution of codons in the genetic code suggests that the genetic code evolved in such a way as to minimize the deleterious effects of mutations. Code degeneracy may serve as a safety mechanism to minimize errors in the reading of codons.
Wobble in the Anticodon It was first proposed that a specific tRNA anticodon would exist for every codon. But is it true? According to our knowledge, the answer is definitely NO Some tRNA could recognize several different codons
Inosine Cases were discovered in which an anticodon base was not one of the 4 regular ones, but a fifth base, inosine
The wobble conception The base at the 5’ end of the anticodon is not as spatially confined as the other two, allowing it to form hydrogen bonds with more than one bases located at the 3’ end of a codon.
The Wobble Rules The Wobble Rules The pairings permitted are those give ribose-ribose distances close to that of the standard A:U or G:C base pairs. The ribose-ribose distances: Purine-purine: too long Pyrimidine-pyrimidine: too short
The ribose-ribose distances for all the wobble pairs are close to those of the standard A:U or G:C base pairs
In the 3-D structure of tRNA, the three anticodon bases- as well as the two following bases in the anticodon loop-all point in roughly the same direction, with their exact conformations largely determined by stacking interactions between the flat surfaces of the bases. Thus, the first anticodon base is at the end of the stack and is perhaps less restricted in its movements than the other two anticodon bases-hence, wobble in the third position of the codon. By contrast, not only does the third anticodon base appear in the middle of the stack, but the adjacent base is always a bulky modified purine residue. Thus, restriction of its movements may explain why wobble is not seen in the first position of the code,
Three Codons Direct Chain Termination UAA, UAG, and UGA, are read not by special tRNAs but by specific proteins known as release factors. Release factors enter the A site of the ribosome and trigger hydrolysis of the peptidyl-tRNA occupying the P site, resulting in the release of the newly synthesized protein.
How the Code Was Cracked By 1960, the general outline of how messenger RNA participates in protein synthesis had been established. In 1961, just one year after the discovery of mRNA, the use of artificial mRNAs and the availability of cell-free systems for carrying out protein synthesis began to make it possible to crack the code
Stimulation of Amino Acid Incorporation by Synthetic mRNAs The addition of fresh mRNA to extracts caused an immediate resumption of synthesis. [XMP]n + XDP = [XMP]n+1 + P
The figure shows the reversible reactions of synthesis or degradation of polyadenylic acid catalyzed by the enzyme polynucleotide posphorylase
This technique lead the scientist an opportunity to elucidate the nature of the code using synthetic RNA
Poly-U Codes for Polyphenylalanine Under the right conditions in vitro, almost all synthetic polymers will attach to ribosomes and function as templates. Luckily, high concentrations of magnesium were used in the early experiments. Poly-U was the first synthetic polyribonucleotide discovered to have mRNA activity. It selects phenylalanyl tRNA molecules exclusively, thereby forming a polypeptide chain containing only phenylalanine.
Codon Assignments from Repeating Copolymers Organic chemical and enzymatic techniques were used to prepare synthetic polyribonucleotides with known repeating sequences.
Mixed Copolymers Allowed Additional Codon Assignments Poly-AC molecules can contain eight different codons, CCC, CCA, CAC, ACC, CAA, ACA, AAC, and AAA. The proportions of the 8 codons incorporated into polypeptide products depend on the A/C ratio
Transfer RNA Binding to Defined Trinucleotide Codons A direct way of ordering the nucleotides within some of the codons was developed in 1964 Specific amino-acyl -tRNA can bind to ribosome-mRNA complexes The addition of trinucleotide results in corresponding amino-acyl-tRNA attachment, whereas if AAA is added, lysyl -tRNA specifically binds to ribosomes.
Using a combination of organic synthesis and copying by DNA with simple repeating sequences can be generated. RNA polymerase will then synthesize long polyribonucleotides corresponding to one or the other DNA strand, depending on the choice of ribo-nucleoside triphosphate added to the reaction mixture
The three rules: The first rule holds that codons are read in a 5’to 3’direcion. The second rule is that codons are nonoverlapping and the message contains no gaps. The final rule is that the message is translated in a fixed reading frame, which is set by the initiation codon.
Three Kinds of Point Mutations Alter the Genetic Code 1 missense mutation An alteration that changes amino acid is called a missense mutation 2 nonsense or stop mutation A more drastic effect results from an alteration causing a change to a chain-termination codon 3 The third kind of point mutation is a frameshift mutation.
Genetic Proof that the Code Is Read in Units of Three Because the gene could tolerate three insertions but not one or two, the genetic code must be read in units of three
Topic 3: SUPPRESSOR MUTATIONS CAN RESIDE IN THE SAME OR A DIFFERENT GENE
Reverse mutations: The effects of harmful mutations can be reversed by a second genetic change. Some of these subsequent mutations are easy to understand, being simple reverse mutations, which change an altered nucleotide sequence back to its original arrangement, Suppressor mutations: Genes that cause suppression of mutations in other genes are called suppressor genes.
Suppressor genes: genes that cause suppression of mutations in other genes. Suppressor mutations work by producing good (or partially good) copies of the protein that are made inactive by the original harmful mutation.
The figure show how a minor tyrosine tRNA species acts to suppress the nonsense codon in mRNA
The results of large-scale sequencing of genomes have confirmed the universality of the genetic code. It allow us to directly compare the protein coding sequences among all organisms.
