VI. Mutations: changes in the genome

1. There can be an addition or loss of an entire set of chromosomes - CHANGING PLOIDY (THIS IS THE BIGGEST CHANGE POSSIBLE)
2. Within a set, there can be the gain or loss of a chromosome - CHANGING ANEUPLOIDY (trisomies and monosomies)
3. Within a chromosome, there can be a gain or loss of GENES - DELETIONS/DUPLICATIONS
4. Within a gene, there can be the gain, loss, or change of a nucleotide - POINT MUTATION (THIS IS THE SMALLEST CHANGE POSSIBLE), or a changing of exons within a gene.

Mutations only occur in living cells (dead cells don't replicate their DNA or divide, and that is when mutations occur). So, since a living cell already works, it is likely that a mutation (which is usually a random change) will 'hurt' the cell rather than help it. Of course, "it is likely" does NOT mean "always". So, most mutations are 'bad' for the cell and organism, but some mutations can be neutral or beneficial. Now, since the cell already works, a LARGE change is likely to be worse than a small change. Think about a car that runs. If you make a big change to the system, like taking out the engine, or putting one big wheel on, it is likely to have a greater negative effect than a small change (such as changing the direction of the rear-view mirror). So, let's look at these types of changes.

A. A Change in Ploidy

    1. Mechanism #1 - Complete failure of Meiosis

• - There is no reduction; a diploid gamete is produced
• - It is unlikely that this rare event would occur in two different individuals at the same time, so typically this diploid gamete will fertilize a normal haploid gamete, resulting in a TRIPLOID (3n) zygote.
• - Triploidy is usually a reproductive dead-end, because triploids can't produce gametes by meiosis (how do you divide 3n equally?).
• - But, there are triploid species; they have gotten around this by reproducing parthenogetically - without fertilization. They produce gametes by mitosis, without reduction (just like THEY were probably produced...suggesting some possibility of a heritable aspect to this mistake). These 3n eggs develope without fertilization into 3n adults... clones of the original female parent. (All triploid lineages that survive in this way are females).
• - So, when this happens, a new unique lineage is created that does not exchange genes with any other group... it is a new 'instant' species.

b. result

• a diploid gamete is produced.
• usually, this gamete is fertilized by a normal haploid gamete, resulting in a triploid zygote.
• this is almost always BAD - especially in animals - resulting in the inability of the zygote to develop correctly and the spontaneous abortion of the embryo.

Study questions:

- List four broad categories of mutations that change the genome at different scales (or levels).

- Describe how a failure of meiosis can produce polyploidy.

- Why is triploidy rare?