Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

1. The laws of inheritance and the discovery of chromosomes

18186666: Gregor Mendel publishes his findings on the laws of inheritance : Gregor Mendel publishes his findings on the laws of inheritance based on experiments with pea plants. based on experiments with pea plants. He He develops develops threethree “principles” “principles”: : DominanceDominance,, Segregation Segregation,, and and Independent AssortmentIndependent Assortment

18821882:: ggerman biologist Walter Flemerman biologist Walter Flemmming, by staining cells ing, by staining cells with dyes, discovers rod-shaped bodies he calls with dyes, discovers rod-shaped bodies he calls ""chromosomeschromosomes."."

MendelMendel’s ’s laws of inheritance laws of inheritance are rediscoveredare rediscovered inin 1900, 1900,

byby the the German German botanist Carl botanist Carl CorrensCorrens

the Dutch the Dutch botanist botanist Hugo De Hugo De VriesVries

and the Austrian and the Austrian agronomist agronomist Erich von Erich von TschermakTschermak

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

2. Mendel's principles DominanceDominance

each inherited characteristic is determined by two each inherited characteristic is determined by two alternative alternative heredithereditarary factors, y factors, and one factor is and one factor is dominant odominant over the otherver the other..

SegregationSegregation the sex cell of a plant or animal may contain one factor the sex cell of a plant or animal may contain one factor

(allele) for different traits but not both factors needed to (allele) for different traits but not both factors needed to express the traits. express the traits.

Independent assortmentIndependent assortment Different Different characteristics are inherited independently characteristics are inherited independently

from from each each other. other.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

3. Where do genes reside?In 1902, Walter Sutton (an American who at the time was a graduate student) and Theodor Boveri (a German biologist) recognized independently that the behavior of Mendel's particles during the production of gametes in peas precisely parallels the behavior of chromosomes at meiosis: genes are in pairs (so are chromosomes); the alleles of a gene segregate equally into gametes (so do the members of a pair of homologous chromosomes); different genes act independently (so do different chromosome pairs). After recognizing this parallel behavior, both investigators reached the same conclusion that the parallel behavior of genes and chromosomes suggests that genes are located on chromosomes.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

4. The word GENETICS is formulated

19091909: : Danish botanist Wilhelm Johannsen Danish botanist Wilhelm Johannsen proposes the term "proposes the term "genegene" (from the Greek " (from the Greek word "genos" which means "birth") to refer word "genos" which means "birth") to refer to a Mendelian hereditary factor. Johannsen to a Mendelian hereditary factor. Johannsen also proposes two terms, also proposes two terms, genotypegenotype and and phenotypephenotype, to distinguish between one's , to distinguish between one's genetic make-up and one's outward genetic make-up and one's outward appearance.appearance.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

5. Genes are lined-up on chromosomes 1915: 1915: Thomas Hunt Morgan, an American geneticist, publishes Thomas Hunt Morgan, an American geneticist, publishes

The Mechanism of Mendelian Heredity, in which he presents The Mechanism of Mendelian Heredity, in which he presents results from experiments with fruit flies that prove results from experiments with fruit flies that prove genes are genes are lined up along chromosomeslined up along chromosomes. He also describes the principle . He also describes the principle of "of "linkagelinkage" — that alleles located relatively close to one " — that alleles located relatively close to one another on a chromosome tend to be inherited together. another on a chromosome tend to be inherited together.

By studying the frequency with which traits are inherited together, By studying the frequency with which traits are inherited together, Morgan and co-workers create a "Morgan and co-workers create a "genetic mapgenetic map" of fruit fly " of fruit fly chromosomes showing the relative locations of the genes responsible chromosomes showing the relative locations of the genes responsible for dozens of traits, along with approximate distances between them for dozens of traits, along with approximate distances between them on the chromosome. This work establishes the basis for gene mapping on the chromosome. This work establishes the basis for gene mapping principles still used today.principles still used today.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

6. Genes may cause quantitative variation

19181918:: R.A. Fisher R.A. Fisher sshowhowss that continuous that continuous traits can be explainedtraits can be explained by Mendelian by Mendelian segregationsegregation of genes: of genes: Mendel’s laws give Mendel’s laws give basis for basis for statisticalstatistical relationships between relationships between parents and offspringparents and offspring..

His work reconciled the Darwinian view of His work reconciled the Darwinian view of evolution with the findings of the geneticists, evolution with the findings of the geneticists, posing the basis of the so-called “Modern posing the basis of the so-called “Modern Synthesis”Synthesis”

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

7. The first mutagenic agent In 1927 In 1927 Herman Herman Muller reported that X Muller reported that X

rays could induce mutations in male fruit rays could induce mutations in male fruit flies. In his investigations Muller found flies. In his investigations Muller found that the mutation rate among the male that the mutation rate among the male fruit flies was linearly related to the fruit flies was linearly related to the radiation dose. His results supplied the radiation dose. His results supplied the first experimental evidence of a mutagenfirst experimental evidence of a mutagen, , in this case, the X rays. Muller's work on in this case, the X rays. Muller's work on mutation induction opened the door to the mutation induction opened the door to the genetic technique of using mutations to genetic technique of using mutations to dissect biological processes, which is still dissect biological processes, which is still used extensively todayused extensively today

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

8. An early hypothesis on the function of genes The first clues about the nature of primary gene function came The first clues about the nature of primary gene function came

from studies of humans. Early in the twentieth century, from studies of humans. Early in the twentieth century, Archibald Garrod, Archibald Garrod, a physician, a physician, made several observations about made several observations about alkaptonuria (a form of arthrytis) and proceeded to propose the alkaptonuria (a form of arthrytis) and proceeded to propose the hypothesis that the information for producing specific enzymes hypothesis that the information for producing specific enzymes in humans is inheritedin humans is inherited. . He observed that inherited diseases He observed that inherited diseases reflect a patient's inability to make a particular enzyme, which reflect a patient's inability to make a particular enzyme, which he referred to as "he referred to as "inborn errors of metabolisminborn errors of metabolism“.“.

Garrod predicted that individuals affected with alkaptonuria would be deficient Garrod predicted that individuals affected with alkaptonuria would be deficient in one of the enzymes in a degradative, biochemical pathway. He had suggested in one of the enzymes in a degradative, biochemical pathway. He had suggested that the specific enzyme was involved in the degradation of homogentisic acid, that the specific enzyme was involved in the degradation of homogentisic acid, an intermediate in the breakdown pathway of phenylalanine and tyrosine. He an intermediate in the breakdown pathway of phenylalanine and tyrosine. He came to this conclusion by feeding homogentisic acid to alkaptonuric patients came to this conclusion by feeding homogentisic acid to alkaptonuric patients and noting that the chemical was excreted in the urine in quantitatively similar and noting that the chemical was excreted in the urine in quantitatively similar amounts to what was administered.amounts to what was administered.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

9. The Beadle and Tatum experiment

Garrod's hypothesis was ahead of its time. Garrod's hypothesis was ahead of its time. Experiments that clarifiedExperiments that clarified the actual function of genes came from research in the 1940s on the actual function of genes came from research in the 1940s on NeurosporaNeurospora by George Beadle and Edward Tatum, who later by George Beadle and Edward Tatum, who later received a Nobel Prize for their work.received a Nobel Prize for their work.

Beadle and Tatum analyzed mutants of Beadle and Tatum analyzed mutants of NeurosporaNeurospora crassa crassa, a fungus , a fungus with a with a haploidhaploid genome. They first irradiated genome. They first irradiated NeurosporaNeurospora cellscells to to produce mutations and then tested cultures from produce mutations and then tested cultures from ascosporesascospores for for interesting mutant phenotypes. They detected numerous interesting mutant phenotypes. They detected numerous auxotrophsauxotrophs strains strains ((that cannot grow on a minimal medium unless the medium that cannot grow on a minimal medium unless the medium is supplemented with one or more specific nutrientsis supplemented with one or more specific nutrients)). In each case, . In each case, the mutation that generated the auxotrophic requirement was the mutation that generated the auxotrophic requirement was inherited as a single-gene mutation: each gave a 1:1 ratio when inherited as a single-gene mutation: each gave a 1:1 ratio when crossed with a wild type.crossed with a wild type.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

10. The life cycle of a fungusThThee fungus fungus NeurosporaNeurospora spends most of its life cycle as a multicellular haploid spends most of its life cycle as a multicellular haploid organismorganism, , in which the cells are joined end to end to form hyphae, or threads of cells. in which the cells are joined end to end to form hyphae, or threads of cells. The hyphae grow through the substrate and send up aerial branches that bud off haploid The hyphae grow through the substrate and send up aerial branches that bud off haploid cells known as conidia (asexual spores). Conidia can detach and disperse to form new cells known as conidia (asexual spores). Conidia can detach and disperse to form new colonies colonies

TThe fungus has alternate mating strains, here he fungus has alternate mating strains, here called type A and type a. Mating can only called type A and type a. Mating can only take place between different mating strains take place between different mating strains and the result is a diploid cell in a long sacand the result is a diploid cell in a long sac (ascus)(ascus). The diploid cell undergoes meiosis . The diploid cell undergoes meiosis producing four haploid cells.producing four haploid cells.

In tIn the ascushe ascus,, the results of segregation during the results of segregation during metaphase 1 are kept in order. These haploid metaphase 1 are kept in order. These haploid cells undergo one cycle of mitosis in the cells undergo one cycle of mitosis in the ascus leading to 8 sporesascus leading to 8 spores (called ascospores) (called ascospores) in order in the ascus.in order in the ascus.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

One set of mutant strains required arginine to grow on a minimal medium. These strains provided the focus for much of Beadle and Tatum's further work.They found that the mutations mapped into three different locations on separate chromosomes, even though the same supplement (arginine) satisfied the growth requirement for each mutant.

11. Auxotroph mutants

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

12. Inferring a metabolic pathwayBeadle and Tatum discovered that the auxotrophs for each of the three loci differed in their response to the chemical compounds ornithine and citrulline, which are related to arginine.

One mutant strain grew when supplied with ornithine, citrulline, or arginine in addition to the minimal medium, another grew on either arginine or citrulline but not on ornithine, and the third grew only when arginine was supplied. On the basis of the properties of the arg mutants, Beadle and Tatum and their colleagues proposed a biochemical model for such conversions in Neurospora

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

13. The one-gene one-enzyme hypothesis Beadle and Tatum concluded thatBeadle and Tatum concluded that a mutation at a particular gene a mutation at a particular gene affectsaffects the the

functioningfunctioning of a single enzyme. The defective enzyme, then, creates a block in of a single enzyme. The defective enzyme, then, creates a block in some biosynthetic pathway. some biosynthetic pathway.

TThis entire model was inferred from the properties of the mutant classes detected his entire model was inferred from the properties of the mutant classes detected through genetic analysis. Only later were the existence of the biosynthetic through genetic analysis. Only later were the existence of the biosynthetic pathway and the presence of defective enzymes demonstrated through pathway and the presence of defective enzymes demonstrated through independent biochemical evidence.independent biochemical evidence.

This model, which has become known as the This model, which has become known as the one-geneone-gene one-enzymeone-enzyme hypothesis, hypothesis, was the source of the first exciting insight into the functions of genes: genes was the source of the first exciting insight into the functions of genes: genes somehow were responsible for the function of enzymes, and each gene apparently somehow were responsible for the function of enzymes, and each gene apparently controlled one specific enzyme.controlled one specific enzyme.

Other researchers obtained similar results for other biosynthetic pathways, and the Other researchers obtained similar results for other biosynthetic pathways, and the hypothesis soon achieved general acceptance.hypothesis soon achieved general acceptance. T The one-genehe one-gene one-enzyme one-enzyme hypothesis became one of the great unifying concepts in biology, because it hypothesis became one of the great unifying concepts in biology, because it provided a bridge that brought together the concepts and research techniques of provided a bridge that brought together the concepts and research techniques of genetics and biochemistry.genetics and biochemistry.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

14. Which macromolecule carry the genes? Chromosomes are composed of both DNA and proteins; Chromosomes are composed of both DNA and proteins;

early observations pointed to DNA as the molecule carring early observations pointed to DNA as the molecule carring the genetic information, the genetic information, but many scientists were very but many scientists were very reluctant to accept reluctant to accept this idea. this idea. DNA was thought to be a DNA was thought to be a simple simple and repetitive and repetitive chemicalchemical. How could all the . How could all the information about an organism's features be stored in such information about an organism's features be stored in such a simple molecule? How could such information be passed a simple molecule? How could such information be passed on from one generation to the next? Clearly, the genetic on from one generation to the next? Clearly, the genetic material must have both the ability to encode specific material must have both the ability to encode specific information and the capacity to duplicate that information information and the capacity to duplicate that information precisely. What kind of structure could allow such precisely. What kind of structure could allow such complex functions in so simple a molecule?complex functions in so simple a molecule?

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

ResolutionResolution was provided in 1952 by was provided in 1952 by Alfred Hershey and Martha Chase Alfred Hershey and Martha Chase with the use of the with the use of the phagephage T2T2 ((a a virusvirus specific to bacteriaspecific to bacteria). The phage is ). The phage is relatively simple in molecular relatively simple in molecular constitution. Most of its structure is constitution. Most of its structure is protein, with DNA contained inside protein, with DNA contained inside the protein sheath of its "head.“the protein sheath of its "head.“

15. An experiment with bacteriophages

The phage attaches to a bacterium's cell wall via The phage attaches to a bacterium's cell wall via its "tails" (shown in green) and injects its genes its "tails" (shown in green) and injects its genes into the bacterium through its syringe-like blue into the bacterium through its syringe-like blue column. It commandeers the bacterium's cellular column. It commandeers the bacterium's cellular machinery to make new phages. The cell machinery to make new phages. The cell eventually becomes so crowded, it bursts, eventually becomes so crowded, it bursts, releasing the new phages that head off to invade releasing the new phages that head off to invade other cells.other cells.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

16. 32P and 35S Phosphorus is not found in proteins but is an integral part of DNA; conversely, Phosphorus is not found in proteins but is an integral part of DNA; conversely,

sulfur is present in proteins but never in DNA. sulfur is present in proteins but never in DNA. Hershey and Chase incorporated the radioisotope of Hershey and Chase incorporated the radioisotope of phosphorus (phosphorus (3232PP) into phage ) into phage

DNADNA and that of and that of sulfur (sulfur (3535SS) into the proteins) into the proteins of a separate phage culture. of a separate phage culture. They then used each phage culture independently to infect E. coli with many They then used each phage culture independently to infect E. coli with many

virus particles per cell. After sufficient time for injection to take place, they used virus particles per cell. After sufficient time for injection to take place, they used centrifugation to separate the bacterial cells from the phage ghosts and then centrifugation to separate the bacterial cells from the phage ghosts and then measured the radioactivity in the two fractions.measured the radioactivity in the two fractions.

When the When the 3232P-labeled phages were used, most of the radioactivity ended up P-labeled phages were used, most of the radioactivity ended up inside inside the bacterial cellsthe bacterial cells, indicating that the phage DNA entered the cells. , indicating that the phage DNA entered the cells. 3232P can also P can also be recovered from phage progeny. be recovered from phage progeny.

When the When the 3535S-labeled phages were used, most of the radioactive material ended up S-labeled phages were used, most of the radioactive material ended up in the phage ghostsin the phage ghosts, indicating that the phage protein never entered the bacterial , indicating that the phage protein never entered the bacterial cell (Figure 8-3).cell (Figure 8-3).

The conclusion is inescapable: DNA is the hereditary material; the phage proteins The conclusion is inescapable: DNA is the hereditary material; the phage proteins are mere structural packaging that is discarded after delivering the viral DNA to are mere structural packaging that is discarded after delivering the viral DNA to the bacterial cell.the bacterial cell.

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

17. DNA is the genetic material

The Hershey-Chase experiment, which demonstrated that the genetic material of phage is DNA, not protein.