Biology & Life Sciences Chapter 10 You can explain that the researchers collected samples

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CHAPTER 10 MEIOSIS AND SEXUAL LIFE CYCLES

Scientific Skills Exercise

Teaching objective: Students will identify the independent and dependent variables in a table of

data and graph the data to identify phases of meiosis in a yeast culture. They will also get

practice doing calculations with exponents, such as calculating the haploid genome size for this

species of yeast (Saccharomyces cerevisiae, commonly known as budding yeast).

Teaching tips: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

This is a basic graphing exercise with time-related data. Students may have difficulty with the

extremely small units femtograms (fg) used for the DNA. A review of metric units will be

helpful to them. If you want your students to have more practice with calculations, you could

extend question 4 by asking questions such as how many base pairs are in a nanogram (ng) or

how many kilobases (kb) are in a femtogram, etc.

Students may question why the line on the graph is not flat for each stage of meiosis. This is

because the data come from a population of cells, and there is some variation among the yeast

cells in their timing of progression through the process. This also explains why the amount of

DNA does not drop instantly at the end of MI and/or MII phase: Not all of the cells divide at

exactly the same time and so the average includes some pre-division cells, some dividing cells,

and some post-division cells for the relevant time points. This subtlety is addressed in question

Students will not be able to precisely delineate each stage on the graph due to the population

variability. Instruct them to approximate based on what they expect for DNA content at each

stage.

Answers:

1. (a) The independent variable is time, which goes on the x-axis, measured in hours (hr). The

dependent variable is average amount of DNA per cell, which goes on the y-axis and is measured

4. (a) Each haploid cell has 12 fg of DNA. (12 fg of DNA) x (9.78 x 105 base pairs per fg) = 1.2

Suggested Answers for End-of-Chapter Essay Questions

See the general information on grading short-answer essays and a suggested rubric at the

beginning of this document.

7. Scientific Inquiry

The alleles for freckles or no freckles are on all four chromatids at the F locus, and those for

s on the red chromatids are for

freckles and black hair and the alleles on the blue chromatids are for no freckles and blond hair.

By drawing the chromatids, you can follow the alleles through meiosis. The chromatids that did

not participate in crossovers will donate either freckles and black hair alleles or no freckles and

8. Focus on Evolution

When the environment becomes unfavorable, this means it is unfavorable for a particular

genotype or genome that had been successfully adapted to the previous environment. At such

9. Focus on Information

Sample key points:

Genes are stretches of DNA in chromosomes that encode traits and are passed on from parent

to offspring.

Chromosome behavior during sexual reproduction accounts for both similarity and variation

among offspring.

Sample top-scoring answer:

Genes encoding traits exist as stretches of the DNA molecule in a chromosome. Chromosome

behavior during meiosis and fertilization processes that alternate in sexual reproduction

accounts for both the inheritance of traits and the generation of genetic variation. Meiosis in each

10. Synthesize Your Knowledge

With three sets of chromosomes, there is no way for the sets to be separated equally to gametes

during meiosis, and so seeds (which are embryos formed after meiosis and fertilization) cannot

form. Thus, taking cuttings from the plants and rooting them is the only way new plants can be