Lezione 9Il codice gene+co -1

L’ evoluzione del concetto di gene

Garrod experiments: the first link between genes and enzymes• Garrod worked with patients who had metabolic diseases

and saw that these diseases often ran in families. He focused on patients with what we today call alkaptonuria. This is a non-fatal disorder where a person’s urine turns black because they cannot break down a molecule called alkapton• By looking at family trees of people with the disorder,

Garrod realized that alkaptonuria followed a recessive pattern of inheritance, like some of the traits Mendel had studied in his pea plants. Garrod came up with the idea that alkaptonuria patients might have a metabolic defect in breaking down alkapton, and that the defect might be caused by the recessive form of one of Mendel's hereditary factors (i.e., a recessive allele of a gene).

Beadle and Tatum experiments: One gene specifies the produc8on of one enzyme

By the early 1950s, researchers had developed two compe7ng theories about the mechanism for protein synthesis.

the pep7de or mul7-enzyme theory, held that proteins were assembled by stepwise coupling of small pep7des, the amino acids, into polypep7de chains, a process guided by enzymes, proteins that catalyze a wide range of reac7ons in the cell. The second was the template theory, which held that proteins were synthesized on templates, one for each protein. The templates were genes.

Scien7sts who followed the discovery of the double helix--many did not--saw that it strengthened the template theory, because it suggested how stretches of DNA might serve as templates for protein synthesis. Among these scien7sts was the physicist and cosmologist George Gamow, who conjectured that DNA provided a direct template for proteins. When looked at in a certain way, Gamow contended, DNA could be seen to have twenty different cavi7es along its length, a number equal to that of the common amino acids.

Source: The Francis Crick PapersDefining the Genetic Coding Problem, 1954-1957https://profiles.nlm.nih.gov/ps/retrieve/Narrative/SC/p-nid/153

The group never met, but Crick used it as a sounding board for some of his most important theories about protein synthesis by circulating his ideas in draft form among its members.

Each member was given a black woolen neck=e with an RNA helix embroidered in green and yellow (photograph, leC to right: Francis Crick (TYR), Alexander Rich (ARG), Leslie Orgel (THR) and James Watson (PRO)).

The physicist and cosmologist George Gamow conjectured that DNA provided a direct template for proteins. To explore further these emerging ideas about genetic control of protein synthesis, Gamow founded the RNA Tie Club, a hand-picked group of twenty molecular biologists, which included Crick, Watson, and Rich. As a badge of membership, each received a tie with the chemical symbol of one of the twenty amino acids embroidered upon it.

1955

Francis Crick – The adaptor hypothesis

Contains the "sequence hypothesis”, which held that gene6c informa6on was encoded in thesequence of the bases in DNA. The base sequence was to be read in linear fashion, from afixed star6ng point and in one direc6on.

The Central Dogma thus implied that acquired changes in a protein could not be inherited, an implication that conformed to Charles Darwin's theory of evolution. The Central Dogma further implied that DNA contained all the information necessary for specifying the sequence of amino acids in a protein, and thus its shape and function; no external information was needed. Finally, Crick asserted that the genetic code was universal to all higher forms of life, as in fact it has proved to be.

• After co-discovering the structure of DNA in 1953, Francis Crick spent more than a decade trying to decipher the code hidden in DNA. The central problem, as Crick saw it, boiled down to “how a sequence of four things (nucleotides) can determine a sequence of twenty things (amino acids)”

• Crick knew that simple pairs of nucleotides couldn’t encode the 20core amino acids because pairs could only encode 16 possibilities (four possible nucleotides in the first position, times four possible nucleotides in the second position). So a sequence of three nucleotides — 4x4x4 or 64 possibilities — was likely to be a minimal word size in DNA.

Benzer. Late 1950s: mapping the rII region of the bacteriophage T4

Crick and Brenner, 1961: Determining the Triplet Nature of the GeneGc Code

Figure 1. Determining the Triplet Nature of the Genetic Code Shown is a suppressor mutant analysis using the rII B region of the genome of phage T4. The FC0 mutant is presumed to have a single nucleotide addition (+ A) to the wild-type sequence. The FC0 suppressed mutant has an additional change, a single nucleotide deletion (− C). The suppressed mutant's phenotype is wild-type (rII+). Combining two single nucleotide additions in rII gives an rII− mutant phenotype. However, combining three nucleotide additions gives an rII+ wild-type phenotype. Given that functional rII proteins are produced in triple addition (+) mutants, and also in triple deletion (−) mutants, the genetic code must be a triplet code with three nucleotides coding for each single amino acid.

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