Neurospora Life Cycle- advantages for genetics.

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The mold Neurospora has a life cycle well suited to its use as a model organism in genetics. First of all, the organism spends most of its life cycle as a haploid organism. This means it is possible to study the expression of genes without worrying about  dominance or recessive alleles.. Any mutations should be easy to detect since mutations will not be masked by another allele.

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

The sac or ascus is the next advantage of  Neurospora because the results of segregation during metaphase 1 are kept in order. For instance notice that the two haploid cells resulting from each mating type are together in the ascus.  These haploid cells undergo one cycle of mitosis in the ascus leading to 8 spores(called ascospores) in order in the ascus.. Scientists have been able to exploit this arrangement to help them screen for mutants and also to do crossover studies with this fungus.

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Another advantage of neurospora is that in addition to ascospores, the fungus also produces asexual spores((5) in sacs called conidia. These spores allow scientists to isolate what amount to clones of any interesting Neurospora genotypes.

Next, the life cycle of Neurospora, is quite rapid requiring about 2 weeks, allowing scientists to rapidly conduct experiments.

Finally,  wild type Neurospora require a very simple chemical diet. Thus, one could screen for mutants by their inability to grow on this so called minimal medium.
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Beadle and Tatum's Experimental Techniques

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This figure outlines the general steps in working with Neurospora and related fungi mutants. Conidia(asexual spores) obtained from wild type Neurospora are irradiated with x rays(1). Then these asexual spores are germinated and cultured on maximal media to produce Neurospora carrying possible mutations resulting from the x rays. These Neurospora are crossed with wild type Neurospora to produce asci containing segregated products of meiosis.

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The ascosposres are isolated(3) and grown on complete media. Many hundreds of tubes are used for this step(4).

Once the cultures are mature, asexual spores are isolated for each tube in step 4. These spores are then grown on minimal medium. Failure of a specific spore to grow on minimum medium indicates the presence of a mutant unable to synthesize a required compound from the raw materials in the minimum medium.

Here,  spores from culture tube 3 are unable to grow in minimal medium except when the amino acid methionine is present. This indicates that these spores are carrying a mutation that prevents the synthesis of methionine.

Analysis of  Methionine Mutants

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Not all  mutants requiring a particular amino acid are going to be the same. For example, different mutants may affect different steps in the methionine synthesis pathway. This table shows the growth of wild type Neurospora in minimal media with at most one compound believed to be part of the metabolic pathway for methionine in Neursospora.

Note that the wild type and all mutants can grow when methionine is present.  If homocysteine is present but not methionine, then strain met 8 cannot grow. Nor can this strain grow in any of the other supplemented minimal media. This suggests that homocysteine is an intermediate toward the end of the pathway for methionine synthesis and that the mutation carried by these Neurospora affects a step in methionine synthesis going from homocycteine to methionine.

Notice that the met 3 strain will not grow unless the medium is supplemented with with any one of cystathionine, homocysteine  or methionine. This suggests that the met 3 strain is carrying a mutation that affects the conversion of some intermediate into cystathione.

Working in this way scientists can infer what compounds are involved in the metabolic pathway for an amino acid and also the order in which biosynthesis takes place. Of course, scientists can also infer metabolic pathways using radioactive tracers, but  it is always best to come at an experimental problem from several ways.

More importantly,  analysis of this type led Beadle and Tatum to the following hypothesis:

The One gene One Enzyme Hypothesis:

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This hypothesis says that each gene codes for one enzyme along the biosynthesis pathway. Later on hypothesis was modified to become the one gene one polypeptide hypothesis since many proteins contain more than one polypeptide chain.

pgd 09/28/02

revised 10/018/03

Linkage analysis in Neurospora

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The fact that the ascus keeps the products of meiosis in order provides a clever way to map the relative distances between a gene locus and its centromere. Suppose we consider a pair of homologous chromosomes with alternate forms of a gene say met 5, and met 5+  in the diploid cell resulting from the fusion of two mating strains and follow what can happen in meiosis to the arrangement of ascospores in the ascus.

If there is no crossing over between the gene and centromere during meiosis then the ascus will have four  met 5 allele bearing ascospores next to each other and then four met 5+ allele ascospores next to each other as shown in the top panel of this figure.

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But suppose there is crossing over between the gene and the centromere. Then other arrangements of spores in the ascus are possible as shown  below. These arise because the met 5+ and met 5 alleles do not segregate until meiosis II when there is crossing over as shown in the figure. This results in patterns of ascospores in groups of two.  One possible pattern is shown here ignoring the mitotic division of each product of meiosis.

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These other arrangements are called second division segregation patterns because as indicated in the figure crossing over has the effect of delaying segregation between the alternate alleles until the second meiotic division.

Map distance between the centromere and the locus is given by:

(1/2)*(asci with second division segregation patterns/total number of asci )*100.

This assumes that the locus is close to the centromere so that multiple cross overs can be ignored..