Using Sexual Reproduction in Yeast to Illustrate

the Advantages of Sexual Repoduction

C. Anthony Finney

Flushing International High School, Queens

Summer Research Program for Science Teachers

August 2007

Grade Level: 9th and 10th Grades  

Unit: Lab activity bridging reproduction/genetics and evolution units  

Objectives:

After completing this lab, students will develop:

       SCIENCE SKILLS, including:

       Following good lab practice with microbes (safety, aseptic technique, etc.)

       Identifying components of an experiment (hypothesis, control, etc.)

       SCIENCE CONTENT UNDERSTANDING, including:

       Explain how sexual reproduction increases the genetic diversity of offspring

       Explain how offspring may be genetically similar to, or different from, parents

       Explain how increased genetic diversity contributes to species survival

Materials:

• Schizosaccharomyces pombe mutant strains: URA- and LEU- or any pair of nutrient-dependent phenotypes, one H+ and the other H- (opposite haploid types, capable of sexual reproduction)

• Growth media: Agar + YE5S, minimal medias YE Ura- YE Leu-, and ‘breeding’ media, which contains maltose and induce sexual reproduction

• Transfer loops, orangewood sticks or sterile swabs

•  Hand lenses or dissecting microscope

• Heat source (hotplate)

•  Petri dishes.

• ‘Snail snot’ – a compound to dissolve sporulated yeast cells

• 25°C incubator

Introduction:

Students often discuss the relative advantages of sexual and asexual reproduction in high school biology classes without benefit of lab activities to directly observe the principles at work.  The yeast cell S. pombe is a safe organism that can reproduce quickly in either fashion.  By utilizing two complementary cells which have different nutrient-dependent deletions, a forced cross of the two can create an offspring which can survive an environment neither of the parents could have individually.  This is a powerful demonstration of basic genetics, the adaptive significance of sexual reproduction and an opportunity for hands-on biotechnology in a classroom environment.  Because incubation stages are interspersed in the protocol, the activity can be run concurrent with other classroom instruction.  Given the themes of variation and survival, this also addresses the subject of evolution.

Procedure:

DAY 1 (1 hour)

1. Students learn how to streak colonies of cells onto labeled, prepared trays. (with more time, students can prepare own agar plates) Plates are prepared with variously minimal media, 1 fatal to each strain, the third fatal to both. (leu-, ura- and leu/ura-)
2. Cell plates are incubated overnight.
3. Students predict appearance of following days cells (hypothesis)
4. Students review basic details of S.pombe, or other relevant genetics/cell biology content.

DAY 2 (1/4 to 1/2 hour)

5. Students observe overnight growth and record observations, including looking at cells under light microscope and estimating the number of cells in a colony of a known diameter (assume 1 cell thickness).
6. Students re-streak both cells on common plates of breeding media.
7. Cell plates are incubated overnight.

DAY 3 (1/4 to 1/2 hour)

8. Students dissolve sporulated cells on the breeding media plates with ‘snail snot’ compound to liberate spores, ensure only haploid cells in surviving.
9. Students streak spores from breeding media on plates containing the same media combinations as on Day1.
10. Cell plates are incubated overnight.
11. Students predict appearance of following days cells (hypothesis)

DAY 4 (1/4 to 1/2 hour)

12. Students observe overnight growth and record observations
13.  Students prepare lab report and answer analysis questions

Data Collection:

Data collection includes observation of colonies after each day’s growth

Colony numbers and diameters can be counted for quantitative data (which can be plotted over time) colony shape and color for qualitative data.  But more than numeric data, this lab is like a logic puzzle in which exchange of genetic data does or does not occur.

*Can be extended in an advanced or AP biology course by considering the frequency of crossing-over events which take place to yield the ratio of colonies observed.  Other connections to mathematics are possible by considering exponential growth rates.

Analysis Questions:

After DAY1

1. What are three advantages of asexual reproduction?

2. In sexual reproduction, where do the offspring get their genes from?

After DAY 4

3.  Why are some yeast able to survive on the minimal media that killed their parents.  What does this tell you about the genes inside the surviving yeast?

4. Did all the yeast survive?  Give evidence for your answer.

5. List three ways offspring can have genes different from their parents.

6. In an environment of rapidly changing conditions, which reproductive strategy would contribute more to species survival: asexual or sexual reproduction? Explain.

7. Antibiotic resistance is the ability some pathogens to develop the ability to survive drugs that are designed to kill them.  Using examples from this lab, explain how microbes can develop such so-called ‘resistance.’

Standards:

This LESSON PLAN addresses the following local, state and national standards: