Subtitles are on! Click CC at bottom right to turn off. You can follow the amoebas on Twitter (@AmoebaSisters) and Facebook! I’ve seen too many pictures of chromosomes. Well, diagrams really. Diagrams are awesome, but I’ve learned I
need to be careful about misconceptions that can come with them. See, in diagrams, I’ve seem chromosomes
typically drawn in a X shape so when I was younger, I thought, okay, all chromosomes
are an X shape. Which was really confusing when I heard people
could have 2 X chromosomes or an X and a Y chromosome because then I started to wonder
why are these chromosomes resembling the letters of the alphabet? And then sometimes they looked like single
stick pieces and not X shapes and I wondered what is going on with these things?! I also thought that if humans have 46 chromosomes,
then they’re in there somewhere, deep inside the body. That if you needed to analyze them, you’d
have to find them and make sure not to do anything to them, because you only have 46
of them. Well these are some misconceptions I had. We’re hoping to clear up a little bit about
chromosomes. Now our illustrations are not scientific…obviously…but
they can help with our explanations, which are. While you can find chromosomes in bacteria,
where the chromosomes tend to have circular shapes, we are going to focus on eukaryote
chromosomes. Specifically, we’re going to focus on human
chromosomes. Chromosomes are made of chromatin and chromatin
consists of DNA and protein. A chromosome itself is really intense packaging. It starts with DNA being wound around proteins
called histones---this forms nucleosomes which are often considered to look like “beads.” More and more condensing packaging levels
occur---we highly encourage you to explore this--- until it results into the chromosome
level. But why all this packing? Well, in your cells, DNA has to fit inside
a nucleus. Another reason…it’s also really important
for when you make more cells, like in the process of mitosis. But in mitosis, you have to be able to move
DNA into new daughter cells. You have so much DNA that these highly packed
chromosomal units make it easier to do so. By the way, it’s not always packed up so
tightly. For example, in DNA replication, DNA typically
needs to be unwound. There are a variety of factors that also influence
how tightly packed chromatin is. Let’s get kind of a general idea about how
human chromosomes may be represented. Human chromosomes can be like this as a single
component---or they can be replicated----like this. In our chromosome counting video, we mention
that when chromosomes are duplicated during interphase----the newly made chromosome is
still bound here in this region called the centromere. By the way, this region does not have to be
in the center of the chromosome. Because of it being joined here, we still
count this as one chromosome unit. But that can be a bit confusing. We say that there are 2 sister chromatids. So one chromosome here. It duplicates. And now, while it is still attached, it’s
still one chromosome but it has two chromatids. So in humans, you have 46 chromosomes. In interphase they are duplicated. There are still 46 chromosomes here but that’s
92 chromatids in this picture. During a stage of mitosis, those 92 chromatids
get pulled apart so that each cell will have 46 chromosomes. Now, while most of your body cells have 46
chromosomes, it's important to remember that certain genes on the chromosome may be turned
on or off by a variety of factors. For example, you wouldn’t want your eye
cells actively using a gene to produce stomach enzymes. That would be a problem. So, would you ever get a chance to see all
46 of your chromosomes clearly? Well you could in something called a karyotype. An image of all of your chromosomes. It is possible to have a karyotype made as
part of some types of medical screenings. In a karyotype, the chromosomes are stained
and visible. The chromosomes from a karyotype are typically
from a cell that is in a mitosis stage- typically metaphase- as chromosomes are condensed and
thick; a karyotype during interphase would be a challenge because you don’t have that
condensed DNA packing. Something confusing to clarify: when searching
for karyotypes sometimes it’ll be represented kind of like this diagram. And sometimes you see something like this. While both show 46 chromosomes- the number
of chromatids in the two images here seem different so---what’s going on? Now while the mitosis stages anaphase or telophase
would be a time when each chromosome would have a single chromatid- those phases are
generally not when you do a typical karyotype. For optimal viewing, a typical karyotype is
taken at or right before metaphase- recall from mitosis that in metaphase, the chromosome
would have two joined sister chromatids. It’s just that many times the joined sister
chromatids of the chromosome can be so close together that, to an untrained eye, it might
be difficult to see that each chromosome in the karyotype technically has 2 sister chromatids. When arranging the karyotype, the chromosomes
are arranged in homologous pairs. Homologous chromosomes are about the same
size and contain the same types of genes---and in each homologous pair, you receive one chromosome
from one parent and one chromosome from the other parent. 23 pairs. 22 of the pairs---44 of the chromosomes---are
called autosomes. This means they are not related to your biological
sex. They may have genes related to eye color or
height or hair texture. The last two chromosomes are called sex chromosomes
because they determine biological sex. Females have two X chromosomes. Males have one X and one Y chromosome. Oh just a side note, the name X and the name
Y have nothing to do with the shape of them---the reason is an interesting topic to look up. So 23 of these chromosomes are from the father,
from a sperm cell which is a gamete. Unlike body cells, gametes have half the number
of chromosomes as body cells. Sperms cells are considered haploid because
they only have 1 set of chromosomes. The other 23 of the chromosomes came from
the mother, contained in an egg, which is also a gamete. An egg cell is haploid because like the sperm
cell, it contains only 1 set of chromosomes. When a sperm and egg cell combine, they form
a fertilized egg, known as a zygote. The resulting cell is diploid as it has 2
sets of chromosomes and it will divide to continue to form more diploid cells. You are a diploid organism. Now if you look at the sex chromosomes, you
know that mothers can only give a X chromosome, because that’s the only type of sex chromosome
they have. But fathers can give a X---the baby would
then be XX: female---or a Y---the baby is XY: male. So fathers determine the biological sex. So why do we care about karyotypes or learning
about chromosomes? Well, one reason it can really help when we
are trying to understand genetic disorders. A karyotype can reveal missing chromosomes
or extra chromosomes at specific locations. Do you have an interest in a career that involves
this area of study? The field of genetics continues to expand
and the career of a genetic counselor may be something to read up on. Well that’s it for the Amoeba Sisters, and
we remind you to stay curious!