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Reproduction
Cellular Life
Diploids to Haploids

Budding

Diploid cells of brewer's yeast look, grow, and asexually reproduce by a type of mitosis called budding.

A mature cell that has completed it's cell cycle, replicated its DNA and duplicated its chromosomes enters a traditional mitotic nuclear division almost identical to that seen in all other eukaryotic cells. Each of the duplicated chromosomes are pulled by spindle fibers into genetically equivalent daughter nuclei, which then form new nuclear membranes in a traditional telophase.

However, instead of a traditional cytokinesis, yeast cells "bud". A small swelling develops on the cell wall nearest to one pole of the spindle. This swelling expands outwards and as it enlarges one of the newly forming daughter nuclei begins to reappear within the "bud" or swelling.

Eventually the nucleus completes telophase, and the bud enlarges enough to contain not only this new organelle, but also a selection of mitochondria and other cytoplasmic contents. The bud is cut off from the parental cell, and both new cells begin the cell cycle all over again.

Both haploid and diploid yeast cells grow and reproduce this way. Diploid yeast, however, have another option. They can divide in such a way that the diploid number of chromosomes are reduced down to the haploid number. This kind of division is called meiosis.

diploids to haploids.

A diploid brewer's yeast cell has two sets of biological information. This collection of genes is organized on two more or less identical sets of chromosomes and DNA molecules. The informational content of these genes is normally available to cell all the time. However, the cell must have at least one undamaged, working gene for each function it requires in order to survive.

Maintaining two sets of chromosomes, DNA and genes is relatively "expensive" for the cell. The DNA molecules have to be repaired, organized and controlled on a moment to moment basis. Despite the fact that having two genes coding for each separate protein and function is a good insurance policy, like all insurance, it comes at some cost.

It is not surprising, therefore, that when times become hard (the nutrient media begins to run out of supplies), diploid brewer's yeast cells sacrifice the benefits of being diploid, and undergo as special sort of cell division that results in the halving of their biological information content. This is their form of meiosis.

sorting it out

During meiosis, two things must happen; the biological information must be accurately sorted out so all the daughter cells receive "one of everything", and the original cell must divide it's contents in such as way as to provide for each of the resulting haploid offspring, what ever they may turn out to be.

Sorting out the chromosomes, and the genetic information they carry, is clearly a critical process in which no mistakes can be tolerated. Each of the haploid offspring must get at least one copy of all the genes it needs for survival.

There are, however, two problems;

  • technically, after the S-phase of the cell cycle, where all the DNA molecules are replicated, there are four copies of each gene carried on two sets of more or less identical DNA molecules and chromosomes, and

  • all this information is scattered though out the contents of the nucleus in no particular formal arrangement. The sets of chromosomes are scattered in a similar manner to a dozen pair of different socks all randomly mixed up in a drawer.

How are the chromosome/socks going to be sorted out so that each offspring gets one chromosome/sock of each type?

pairing

The solution to this problem is fairly straight forward. Before the original cell undergoes any kind of nuclear division, the pairs of chromosomes (each itself doubled) find each other in the nucleus and pair together.

This is not a "platonic" friendship, but a physical union in which parts of the paired chromosomes are "spot welded" together at several places along their length. This physical union has two benefits; it keeps the sets of chromosomes together and it allows for the exchange of genes and information between them.

This latter property, i.e. exchanging parts of the chromosome, has important consequences for the resulting haploid cells, and for the yeast species as a whole. It introduces a new idea; the randomization of information and the creation of new possible combinations of genes on a chromosome.

separating


Further randomization of biological information occurs when the spindle fibers get to work and pull apart the chromosome sets. As each partner in a chromosome pair is dragged across the cell and into the location of the new nucleus, it is accompanied by other partners of other chromosomes. These will then form the complete genome and library of the new cell.

The direction in which each partner of each chromosomal set is pulled is totally random. There are no mechanisms in the spindle, or the way in which sides of the partnered chromosomes are attached to the spindle fibers, that ensures that a particular DNA molecule or its genes will end up being preferentially pulled in any one direction.

As a result, each partner of each chromosome set is sorted out into the new daughter nuclei completely independently of the behavior or direction of any other chromosome. This random sorting out of the chromosomes is called:

independent assortment.

As a consequence, the combination and permutation of possible chromosomes, DNA and genes ending up in any one possible haploid daughter cell is very large. Information is randomized and each offspring receives a potentially different library of gene versions to all of its siblings.


BIOdotEDU
© 2002, Professor John Blamire