Meiosis and Genes

Brother Gregory speaks to his class,

The subjects for today's lesson are the separation of chromosomes and genes during meiosis and how we can use yeast cells to determine the number and location of genes. You must follow the lesson, answer the questions, then complete the research investigation, if required.

"Let us begin .......

"For thousands of years humans have exploited tiny, single-celled, fungal micro-organisms to make beer, wine, spirits and bread. These are the yeasts, and they have served us well in the winery, brewery and bakery for most of recorded history.

"There are about 350 species of yeast, which grow widely all over the world in an amazing variety of habitats, but humans have mainly taken advantage of those that can bring about the rapid conversion of sugars, found in grapes or grains, into alcohol and carbon dioxide. These little creatures are probably the first organisms of a different species we humans tamed and cultivated. We like what they do!

"But, in recent history, yeast have made another contribution to our understanding of biology and how living organisms work and reproduce. In 1837, Cagniard-Latour showed that beer contained tiny, spherical living cells which could reproduce and grow. These were the "sugar fungi", from which we get the scientific name "Saccharomyces".

"A lot has been learnt about the biological process of fermentation by studying what goes on inside a yeast cell, but our lesson today deals with a different area of biology; meiosis and the process of dividing up biological information during sexual reproduction.

"Yeast cells are special in that they can live and grow in two different genetic states; haploid, in which each individual yeast cell only has a single copy of all the genes, DNA and chromosomes it needs, and diploid, where every cell has two sets of all the genes. [Note; this is the genetic state of all cells in the human body].

As you proceed through this lesson, test yourself as you go by answering some of these "true/false" questions.

Metabolic Pathways - making complex products.

"Yeast cells are capable of a wide variety of metabolic processes. They can convert sugar into alcohol and carbon dioxide, and they can convert simple molecules into very complex molecules such as the amino acid tryptophan. All of these processes involve the use of enzymes which catalyze all the chemical reactions taking place in the yeast cytoplasm.

"Each step in the complex pathway of chemical reactions that produces a product such as tryptophan, is regulated and carried out by its own enzyme, which is coded for by its own gene on the DNA molecules. Each of these enzymes acts like a worker on an assembly line, carrying out its job (chemical conversion) and then passing on the product to the next "worker-enzyme" in the chain.

"If any of the "worker-enzymes" are missing or cannot carry out their job properly, the whole assembly line, or metabolic pathway comes crashing to a halt. The yeast cell can no longer make the final product it needs and so it would starve to death if nothing was done.

"Mutations in genes cause defective enzymes, and the halting of metabolic pathways. In haploid yeast strains, therefore, any mutation in a vital gene will prevent the cell from making what it needs, and will halt it's growth. Since there is only one gene for every enzyme, one mutation is all it takes to halt the whole "assembly line".

Complementation Analysis - bringing different genes together.

"If two different haploid yeast mutants are fused together in a "mating reaction", they form a diploid cell with two genes for every enzyme and function. If the cell receives two defective copies of a particular gene, it is no better off than the original haploid mutants, and it still cannot grow properly. But if it is lucky and receives one mutant gene and one normal gene for a particular enzyme or function, now it can grow!

"This is called complementation, and can be used as a powerful way of determining the number of genes, and hence the number of enzymes, in any particular metabolic pathway.

"But this is not the end of the story. Diploid organisms have a special mode of reproduction in which they produce haploid gametes, and then the gametes from different parents pair up again and reform a new diploid. Yeast diploids can do the same thing.

"Like multicellular organisms, diploid yeast cells can undergo two types of cell division; budding which is their form of mitotic, asexual, cell division, and meiosis, a special kind of division in which the paired, similar chromosomes are separated from one another.

"During the sorting out process of meiosis, homologous pairs of chromosomes are separated from one another, and the genetic content of the resulting cells is now haploid once again.

"In yeast cells, the products of meiosis are single-celled haploids that can once again grow and function independently. After meiosis, an ascus forms that contains four cells, all related to one another, and containing haploid copies of all the genes and chromosomes found in the original diploid.

"Careful manipulation of the "tetrad" of haploid cells (called "spores") under a microscope means that all the products of an individual meiosis can be seen, and studied further. This is called "tetrad analysis", and is yet another powerful genetic technique for teasing out the puzzle of how biological information is inherited.

Meiosis - a complex dance and separation of chromosomes.

"The sub-cellular events of meiosis, and what happens to the genes, DNA molecules and chromosomes can be seen and observed using stains, dyes and powerful microscopes. [Note; this is not normally done using yeast cells, as it is hard to see what is going on inside them].

"Cells that are about to enter meiosis carry out a relatively normal G1, S, and G2, phase of growth, which replicates all the DNA molecules.

"The prophase of a cell entering meiosis (called Prophase I) is a long, very elaborate process that proceeds through many different stages. The outlines of what is going on can be see using powerful microscopes, but the genetic and molecular basis of what is taking place needs other techniques of study.

"Metaphase I and Anaphase I are very similar to those processes seen in cells dividing by mitosis. But it is the homologous chromosome pairs that are separated from one another, not the chromatids. Depending on the particular circumstances, there may or may not be a proper Telophase I.

"In the second round of division, called Meiosis II, the chromatids of each and every chromosome are now separated into individual, haploid cells (often called "gametes" in multicellular organisms). Which can specialize in shape and form to be motile (male gametes), or large and food-filled (female gametes).

"Fusing haploid cells (the gametes) together to form a diploid cell (the zygote), brings together genes from different sources. The diploid cell has "two of everything", and a much better chance of survival. When a diploid cell undergoes meiosis, these genes are randomly separated out into haploids again, creating many different possible combinations of genes. Reproduction that uses this alternation between haploid and diploid genetic states constantly "mixes up" the biological information, creating lots and lots of new possibilities, and therefore many more chances of survival.

Research investigation - #1
check your schedule to see if this is required
How Many Genes?
complementation analysis of yeast mutants
Research investigation - #2
check your schedule to see if this is required
Mapping the Genes
finding the sequence of genes on chromosomes
Concept questions
for the lesson

check your schedule to see if this is required
Properties of Cells - Meiosis
Concept Questions and personal question page
Required Readings
for the lesson
Key Concepts
karyotype -|- homologues -|- types of chromosomes
haploids and diploids -|- life cycles -|- germ cells

Table of Contents
The yeast haploid cell -|- Forming diploids
Sorting things out -|- tetrads and products

before Meiosis starts -|- Prophase I
Meta- and Anaphase I -|- Meiosis II

Independent Assortment

Recommended Reading Meet Brother Gregory
Chapter the First
Science@a Distance
© 2002, Professor John Blamire