Biology 205 Linkage and Chromosome Mapping

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Chapter 5 - Gene mapping in eukaryotes. See also the linkage worksheet.

I. Discovery of linkage in Drosophila

A. Define linkage and the concept of a linkage map.

• Express genotypes when linkage is present using chromosome notation.
• Distinguish between cis and trans configuration
• Explain the procedure for setting up a two point test cross

B. Test the hypothesis that two loci are unlinked. See p 102 -103.

C. Define r, the rate of recombination and convert r into map units.

D. Explain the concept of a three point test cross.

E. Use a 3 point test cross to determine the relative order and distance between loci

• Find the parental and non parental chromosomes with respect to each pair of loci
• Explain why map distances as calculated are not additive.

II. Mapping in haploid organisms

A. Fungus life cycles.

• Diagram the yeast life cycle as a representative haploid life cycle.

B. Tetrad analysis. See my Neurospora tutorial.

• Define tetrad analysis and ordered tetrad.
• Define ascus and explain why tetrad analysis works in Neurospora.
• Explain how tetrad analysis can be used to calculate gene to centromere distance.

Chapter 6. Mapping prokaryote genomes.

I. Procedures for Genetic analysis of Bacteria.

A. Distinguish between different types of media that might be used for culturing bacteria and other microorganisms.

• Minimal medium - contains only nutrients absolutely required for growth.
• Complete medium supplies everything the microorganisms might possibly need. For instance with yeast we often grow yeast on yeast extract medium made from yeast possible nutrients extracted from yeast.

B. Distinguish between auxotrophs vs prototrophs.

Note that nutritional mutants may involve mutations in either biosynthetic pathways of changes in the ability to use different nutrients.

C. Explain the replica plating procedure used to screen for metabolic mutants.

Auxotrophs are metabolic mutants, unable to synthesize a key molecule for growth and survival using just the substrates available from minimal medium or use a nutrient from the minimal medium. Auxotrophs can grow either on complete medium or on medium containing the substance that the auxotroph cannot synthesize or utilize.

II. Mechanism of recombination in prokaryotes

A. Define conjugation in prokaryotes.

B. Explain Lederberg and Tatum's experiments with E. coli. They mixed two strains: one: met+ bio+ thr leu thi and one with met bio thr+ leu+ thi+. When they mixed these strains they got some offspring that were prototrophs. What did they conclude?

C. Explain the role of the F or sex factor in conjugation.

• Note that the F factor is an example of an episome! An episome is a plasmid that can integrate itself into a bacterial chromosome. In E. coli F+ cells contain the F factor and and are the donor and F- cells lack the F factor. See figure 6.4a.
• Once the F factor is transferred it can then become integrated into the cell's main chromosome as shown in 6.4b. An integrated F factor does not replicate independently. An E. coli cell with an F factor integrated into the main bacterial chromosome is called an Hfr cell. Hfr stands for high frequency of recombination. When these cells encounter an F- cell, congugation can happen and transfer of the the Hfr chromosome with part of the F factor begins. See 4 and 5 in figure 6.4b. As noted in your text the F- cell is rarely converted to an Hfr cell since the entire F factor rarely gets transferred along with the donor cell DNA.
• Hfr cells can become F+ cells when the F+ factor is excised as shown in figure 6.5. The point of this figure is that the resulting episome may pick up genes from the main bacterial chromosome when this excision is not precise. When this episome is transferred to an F- cell the resulting cell is F+ but since the episome is carrying other genes it is called an F' factor. As your text notes the F' factors are named for the gene or genes they pick up.

D. Explain how conjugation can be used to map bacterial chromosomes via interrupted mating experiments. pp 127-128. This technique allowed scientists to infer that the E. coli chromosome was circular. See p 129 and study figure 6.8.

III. Transformation

A. Define transformation and distinguish between natural and engineered transformation.

B. Explain how transformation can be used to map bacterial chromosomes.

IV. Transduction.

A. Define transduction

B. Describe the lytic reproductive cycle of a phage and the lysogenic reproductive cycle of a phage. Figs 6.11, 6.12

C. Explain how transduction can be used to map bacterial chromosomes.

Key points:

Transducing phages can pick up pieces of bacterial DNA and then during the lysogenic phase of the phage reproductive cycle this DNA becomes integrated into the host cell chromosome. Note that double crossovers can lead to integration of a marker gene carried by the phage into the host cell's chromosome. See steps 6 and 7 in figure 6.13

In generalized transduction what piece of DNA the phage picks up is random. Two bacterial genes close together tend to be picked up together.

V. Explain the complementation test and apply this idea to analysis of crosses.

See also the laboratory exercise on linkage.

pgd created: 10/04/03 revised 09/21/04

Be able to do these problems:

Chapter 4.

Questions 4.13, 4.15, 4.26, 4.28

Chapter 5

5.1 ; 5.3; 5.5; 5.11; 5.15; 5.17; 5.18

Chapter 6.

6.3; 6.11. Read about the complementation test. See especially p 141. The idea is that a particular mutant phenotype may arise from mutations in the same gene or different genes.

A rabbit breeder obtains two homozygous recessive albino rabbits from two different sources. Knowing that albinism is inherited as a recessive trait, she expects that when she crosses her albino rabbits she will obtain offspring that are all albino. Instead she finds that all the offspring have pigmented fur. Explain these results.