Lesson 3 - Yeast Morphology and Evaluation SAQ
This lesson allows you to test yourself on the material covered in the Yeast Morphology and Evaluation lesson, and determine the level to which you have met the learning outcomes.
Suggested Response - Q1
*Some of the following statements are true; others are not. Indicate which statements are true. Then indicate which statements are false and explain why they are false.
a) Aneuploid yeast cells contain more than one copy of the nucleus.
b) Prokaryotic cells do not have a defined nucleus.
c) The whole of the yeast genome is contained in the 14 nuclear chromosomes.
d) Brewing strains of yeast have been selected because they all have very stable genomes.
e) Lager strains are hybrids of two different yeast strains, one being an ale strain, S. cerevisiae; the other is a wild yeast belonging to the species, S. eubayanus.
f) Brewing yeast cells reproduce by binary fission.
Response:
a) False. All yeast cells contain only a single nucleus. However, the number of copies of each chromosome contained within it may vary. Aneuploid cells contain at least one copy of each chromosome, but not an equal number of each. In other words, some genes may be present several times, others will be present only once.
b) Correct. Prokaryotic cells include bacteria.
c) False. All brewing yeast strains contain 16 chromosomes within the nucleus, not 14. In addition, there are several genes within yeast mitochondria, usually alongside some short, autonomously replicating DNA sequences that are not part of the chromosomal DNA.
d) False. It is true that brewing strains are genetically quite stable as they contain multiple copies of most genes. However, some genes are only present as single copies. Mutations in these can thus lead to the formation of variants.
e) Correct.
f) False. Brewing yeast reproduce by budding. This is an asymmetric process in which the bud should contain an identical genome to the mother but is smaller and a virgin. All brewing yeast cells go through a process of ageing and senescence. They can thus can only bud a certain number of times. This means that a population of brewing yeast cells contains cells of varying ages and metabolic capabilities.