It’s Professor Dave, let’s check out more chromosomes.
We’ve learned a lot about DNA, chromosomes, and heredity.
We’ve also learned about mitosis, the reason why all your cells have the same genetic material,
and meiosis, the reason that gamete production and subsequent fertilization produce such
genetic diversity from generation to generation.
But sometimes there are errors that occur throughout this process.
We have discussed mutation in the genome, due to both endogenous and exogenous mutagens,
as well as random error by replicative machinery.
But there are large-scale alterations to the genome that can occur by completely different
mechanisms, which can have enormous effects on the resulting organism.
Let’s take a look at some of these now.
The first thing we want to examine is alteration in chromosome number.
We take it for granted that all of our somatic cells have 46 chromosomes, which through meiosis
produce gametes with 23 chromosomes, so that fertilization produces a zygote with 46 chromosomes
once again, which grows into a new person.
But there are instances where these numbers can change.
Let’s take another look at meiosis.
We know that in meiosis one, homologous chromosomes line up along the metaphase plate, and then
get pulled apart, resulting in haploid daughter cells with duplicated chromosomes, that then
undergo meiosis two, which is quite similar to mitosis, to give haploid gametes with unduplicated
chromosomes.
But in either of the two processes, something called nondisjunction can occur.
Say that in meiosis one, a particular pair of chromosomes both go to one daughter cell
and not the other, instead of one going to each.
After meiosis two, four gametes will result, two of which will have an additional chromosome,
or N + 1, and two of which will be missing a chromosome, or N – 1.
Alternately, say that in meiosis two, a pair of sister chromatids do not get properly pulled
apart, and they both go to one daughter cell and not the other.
Assuming everything went properly with the other daughter cell from meiosis one, the
end product will be two normal gametes, and then one N + 1 and one N – 1 gamete, due
to the nondisjunction.
If any of these abnormal gametes undergo fertilization with a normal gamete, it will produce a zygote
that exhibits aneuploidy, which means an abnormal number of a particular chromosome.
Typically, this will be either monosomy, meaning only one version of a chromosome, or trisomy,
meaning three versions of a chromosome.
This is because the abnormal gamete had either zero versions or two versions of a particular
chromosome, plus the one from the normal gamete, to give either one or three.
As the zygote divides and develops into a fetus, all of the resulting cells will have
the same abnormality, which will result in huge problems.
In fact, embryos with such abnormalities often result in miscarriage, meaning spontaneous
termination of pregnancy.
In other cases, a baby can be born but with genetic disorders such as Down syndrome, or
trisomy 21, Klinefelter syndrome, involving an extra X chromosome in a male, or Turner
syndrome, which is monosomy X in females.
More on these in the upcoming pathology series.
Now, unlike aneuploidy, which involves an irregular number of a particular chromosome,
organisms can also exhibit polyploidy, which means more than two complete sets of chromosomes.
Normally, our cells are diploid, meaning two sets of all 23 chromosomes, or 46 total.
But organisms can also exhibit triploidy, or 3n, meaning three sets, or even tetraploidy,
or 4n, meaning four sets.
These situations can arise if nondisjunction occurs for all chromosomes to produce a diploid
egg, which is subsequently fertilized, or if a diploid zygote replicates all its chromosomes
and then fails to divide, which after another round of replication and mitosis will yield
a tetraploid embryo.
Polyploid animals are quite rare, although a handful of polyploid fishes and amphibians
do exist.
However polyploid plants are quite common, particularly ones we eat.
Bananas are triploid, wheat is hexaploid, or 6n, and strawberries are octoploid, or
8n, meaning eight complete sets of chromosomes in each cell.
Beyond these examples of alteration in chromosome number, we should also discuss some ways that
the structure of an individual chromosome can change drastically.
We have already learned about point mutations and frameshift mutations, which involve the
alteration, deletion, or insertion of a single base pair.
But chromosomes can also break apart, due to radiation or other damaging agents, which
can result in a few different things.
First, we can observe deletion.
This is when an entire fragment of a chromosome is lost, which may result in missing genes.
This fragment may also become attached to the sister chromatid, which would produce
a duplication, or two identical sections on the same chromosome.
Attachment can also occur on a homologous chromosome, whereby the sections will not
be precisely identical.
This fragment may reattach to the original chromosome but have flipped the other way
first, resulting in an inversion.
And lastly, the fragment can attach to a completely different nonhomologous chromosome, resulting
in translocation.
Deletion and duplication is relatively common during crossing over in meiosis.
This can happen when the segments of DNA that are exchanged are not of equal length, such
that one loses information and the other gains it, hence one deletion and one duplication.
If two gametes with significant deletion in the same location are involved in fertilization,
it can result in a zygote that is missing a number of genes, which can very easily lead
to miscarriage, or death in early childhood.
Much progress has been made in linking certain conditions to specific chromosomal deletions
or translocations, which represents a huge step forward for science.
And with that, we have a better understanding of chromosomes as well as fascinating alterations
in their number and structure.