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.