User:Ilya/Yeast/Ploidy

Why do diploid and haploid versions of yeast exist?

Generally:

• simple organisms are haploid
• multicellular organisms are diploid

Diploid contain alleles (different forms of gene)

• homozygous
• heterozygous

Recessive mutation: both alleles must be mutant to observe mutant phenotype, i.e. homozygous for mutant allele.

Meiosis is fundamentally different from mitosis in the occurrence of high-frequency recombination. Meiotic recombination, which is thought to proceed through a double-strand-break (DSB) repair mechanism (50), follows premeiotic DNA replication. PNAS 98:3249-3253

Diploidy advantages

The genetic consequences of 2n gametes indicate that sexual polyploidization results in greater variability, fitness, and heterozygosity than does somatic doubling. Genetics, v163, p287

Polyploidy can be induced experimentally with an antimitotic drug like colchicine (Its effect is to disrupt the microtubules that are needed to pull the chromatids apart into separate cells). In fact, I think it has been used on some commercial fruits like apples. They are similar apples to the non polyploid root with a larger fruit.

Colchicine - an alkaloid drug extracted from the autumn crocus - inhibits the formation of the mitotic spindle, so cells with two chromosome sets are produced

Freedom to mutate and become better in one environment or able to adapt to other environments. This will allow the plant to live through adverse conditions and work with the ever-changing environment. Combinations of genes can enhance charactoristics such as harvest yield and quality. The larger number of chromosome sets creates a larger cell, thus a larger organism. This can be reproductively advantagous.

Autotetraploid plants are advantageous as commercial crops because, in plants, the larger number of chromosome sets often leads to increased size. Cell size, fruit size, flower size, stomata size, and so forth, can be larger in the polyploid.

Diploidy disadvantages

Too much DNA takes a long time to divide, causing slow growth.

Problems with fertility.

A disadvantage to polyploidy is that it takes a long time to work because there is so much DNA involved. The plant may not evolve fast enough to meet rapidly changing conditions.

Diploid yeast cells undergo meiosis under starvation. Resulting haploid cells require less energy to maintain two sets of genetic material. Mutation rate is higher in haploid? If yes, than better adaptability to environment.