Teacher's Study Guide for Lesson Five Chromosomes in Meiosis, Medicine and Evolution by Dr Jamie Love 2002 - 2010

The independent direction in which the tetrad breaks up (in meiosis I) leads to a wide variety of possible gametes.
That variety increases as the number of chromosomes increases.

1. An organism with only one homologous pair of chromosomes can only produce two types of gametes -
one carrying the #1P and the other carrying the #1M.

2. An organism with two homologous pairs (of chromosomes) can produce four types of gametes. These would be

1. #1P and #2P
2. #1P and #2M
3. #1M and #2P
4. #1M and #2M

Each gamete must have one of each chromosome number.

3. You have 23 pairs of homologous chromosomes so you have a haploid number of 23 and a diploid number of 46 because your n = 23.
We could have had any combination as long as we ended up with one chromosome from each homologous pair (one #1, one #2, one #3, etc.) There are millions of possible combinations.

By the time chromosomes become visible in prophase I they are already paired as tetrads and tangled together, entwined in a process called synapsis.
Synapsis is a highly specific and organized alignment of the homologous chromosomes causing their genes (the specific DNA segments) to line up. Their DNA stands actually break and they swap bits of DNA between homologs. This exchange of genetic material is called crossing over and it produces new combinations of genetic material along both chromosomes. The chromatids involved in crossing over will have the same order of genetic materials but the swapping may have introduced some new types of genetic materials.

Crossing over is inferred from the patterns of crossed over chromatids. These regions are called chiasmata (meaning "cross" in Greek). Chiasma is the singular form. There are many chiasmata along the length of a pair and many crossing over events occur among all four chromatids in the pair of homologous chromosomes. Each chiasmata represents a crossing over event and causes recombination along those parts of the chromatids undergoing the exchange.

(How would the location of different genes along the chromosome affect the frequency of crossing over?)
[Genes far away from each other would be more likely to cross over than genes positioned close to teach other.]

(What's the difference between centromeres entering mitosis and centromeres entering meiosis I?)
[In mitosis each centromere has two kinetochores but it meiosis I there is only one kinetochore per centromere.]

Chiasmata help to place the tetrad pairs along the metaphase I plate. When a pair of chromatids become tangled in chiasmata they effectively lock the two chromosomes (dyads) together into a tetrad.

If something goes wrong in anaphase I or anaphase II of meiosis the resulting gametes might not have the correct number of chromosomes. One gamete may get 22 chromosomes and the other would get 24 chromosomes.

(What would be the result, chromosome count, of the fusion of either of those "bad" gametes with a healthy gamete?)
[A zygote with either 45 or 47 chromosomes.]

When the number of chromosomes in a cell is "off" by one or a few, we call that condition aneuploidy.
Down syndrome is also called "trisomy 21" because people with Down syndrome have an extra copy of chromosome #21.

Sometimes a gamete has the correct number of chromosomes but one of them is damaged, perhaps missing a piece of genetic material. There are several human genetic diseases caused by "broken chromosomes" but fortunately they are rare. The chromosome is usually broken at some stage before meiosis and the damaged chromosome passes through meiosis as normal. Zygotes produced from such chromosomes are deficient in genetic materials and that usually causes some severe problems.

Chromosomes are arranged from the largest (number one, or #1) to the smallest (number twenty-two, or #22, in the case of humans) - not counting the two sex chromosomes.

The "short" portion is given the designation "p" and the "long" portion is labeled "q". Special staining procedures produce bands spread unevenly but consistently along the length of each chromosome. A position on a chromosome is called a locus. Genetic information is contained at each locus. Each set of chromosomes has one and only one #5p15 and we always refer to that as a single position (locus) even though there may be several copies or variations of it. At a different locus in the set, say on a different chromosome (#4) or a different position (#5p13), there will be different genes. Two genes cannot share the same locus (but variations of the same gene can). Different genes reside at different loci (the plural of locus).

Polyploidy is when the organism has extra sets of chromosomes so its cells have 3n, or 4n, or 5n, etc. In polyploidy the chromosomes are increased in exact units of "n".

You can sometimes "force" a hybrid plant through its sterility by adding colchicine. Colchicine disrupts nuclear division by inhibiting the formation of spindle fibers.

(How might this be used in a lab to help you see and count chromosomes?)
[Colchicine is used to stop nuclear division so all the condensed chromosomes are halted before metaphase. This allows cytogeneticists to "collect" chromosomes for study.]

Many families of wild flowers are composed of species that consist of simple multiples of a basic number of chromosomes - the basic number being "n". Polyploidy is common among plants, rare among animals and fatal for humans.

Broken and rearranged chromosomes can be involved in evolution.

Chimpanzees and gorillas have 48 chromosomes because they have two extra pairs of chromosomes (so they should have 2n = 50) but they do not have human chromosome pair #2 (so they really have 2n = 48).
Human gametes have 23 chromosomes but chimp and gorilla gametes have 24 chromosomes.
Careful analysis of the chromosomes shows that the human #2 was actually produced by the fusion of the two sets of ape chromosomes!
These chromosomal rearrangements are markers of chromosome evolution and they may also act as a reproductive barrier that made it impossible for our ancestors to mate with the ancestors of chimps and gorillas.