Hi! Welcome to BIO 301. My name is
Cynthia Anderson Sanchez, and I will be
I will be your instructor for this course.
Today we're going to start by talking a little
bit about genetics and what that has to
do with your gender. So, first let's talk
a little bit about universal questions
that we probably all have. For example,
"Why do children tend to look a little
bit like their mom and a little bit like their
dad?' Also, some deeper questions
like, "What is DNA?". We hear a lot about it
we hear a lot about chromosomes and a lot about
genes. When you to look very closely at
yourself, you'll notice that you are made
up of trillions of cells. The latest
estimate by National Geographic actually
estimates that we have about 40 trillion
cells in our bodies. So, that's pretty
impressive. If you were able to uncoil
all of your DNA and just stretch it out
as far as you could in all of your cells,
it actually is long enough to go all the
way to Pluto and back!
That's 10 billion miles, so that's a lot!
So, let's find out what this DNA is.
DNA stands for deoxyribonucleic acid.
That's why most people just stick with calling it DNA!
Each cell in your body contains
DNA and it lives in the central part of
your cells called the nucleus.
What DNA does, is it's basically an
instruction manual for the cell.
It carries genetic information that's going
to tell the cell information on what
type of cell it is to become, what
functions the cell will have within the
body and also what functions will take
place inside of that cell. In textbooks
sometimes you'll see a picture of DNA
that looks something like this top
picture here. This is the DNA double
helix and this shows how DNA would look
if you stretched it out as much as
possible. In this long, super-thin form, it's
actually not visible even under a
microscope. This form looks somewhat
similar to a telephone cord. If any
of you has ever had the pleasure, or
should I say "torture", of having to use
one of these corded, wired phones,
the phone cord will usually end up tangled
into what is called a supercoil. We see
the super coil formed here. Now this is
not just a fun fact. The reason that I'm
telling you this is the DNA that is
within our bodies will actually supercoil
in on itself during a particular
part of the cell cycle and it's going to
end up forming these nice 'X' shapes that
we see here. Now we always have
chromosomes in our body but the
chromosomes are a lot easier to
visualize when they're in this condensed
to form. This is what your chromosomes
actually look like. Each of your cells
has 46 chromosomes total. You have one
set of 23 chromosomes from your mother
and one set of 23 chromosomes from your
father.
This is an actual photograph of human
chromosomes taken using an electron
microscope. This particular photo was
taken from SCIBLOGS and I
have the link here if you would like to
see this and other pictures as well as
more information on chromosomes. Now
we'll just take a moment to look at a
short youtube video to show you how the
DNA gets packaged into these chromosomes.
Enjoy.
What we're seeing here is an
illustration of a single cell and we're slowly
traveling towards its nucleus; the hub of
genetic material. Inside you can see
chromosomes; the familiar x-shaped blob of DNA.
Chromosomes don't really exist like
this in most cells in fact this is only
a fleeting structure. Most of the time
chromosomes look like this.
These are for typical cell nuclei in
which we've stained all of the DNA with
the blue dye and one of the chromosomes
in green. You can see it looks nothing
like the familiar x-shape cartoon
structure of chromosome. Using a new
technique that we've developed, we've
created a much more accurate picture of
how the DNA folds within the chromosome.
The DNA is represented here by the
single strand that weaves through the 3d
structure. This is how DNA exist within a
chromosome in its usual state; the state in
which all the important functions of the
genome are operating and controlled. 3D
models of chromosomes allow us to map
specific genes and other important
genomic features under the structures.
Putting DNA into its proper context like
this is important because the folding of
DNA and the positions of genes on a
chromosome, contribute to genome control.
Great! So, I hope that you enjoyed that
short clip. Let's talk a little bit
more about our chromosomes and what they
look like when they are in their
condensed state. Here we are looking at a
male human karyotype. Remember that there
are 46 chromosomes total in your cells
but you have 23 from one parent and 23
from the other parent. So, what we do in a
karyotype, is we will pair up the number one
chromosome with mom with the number one
chromosome from dad and then we do the
same thing with a chromosome to
chromosome 3 chromosome for all the way
up to chromosome number 23. Now let's
take a closer look at chromosome number
23. in this particular karyotype, which is
a male karyotype, we see that the number
23 chromosomes look different from one
another
between the male and female versus
everything else. All the other
chromosomes look similar as they're
paired up. Let's look at why that is. The
23rd chromosome has a special function.
It does other things too, but one of the
things that it does is it determines
what gender you are going to be. For this
reason to 23rd chromosomes are also
called sex chromosomes. A female is 'XX'.
What this means is, for the 23rd
chromosome, the female would have
inherited to sex chromosomes that look
like an 'X', as you see here. One X
chromosome must have come from her
mother and the other X chromosome came
from her father. So, females are 'XX'.
A male will have inherited two sex
chromosomes that look different from one
another. One chromosome appears like an 'X'
and the other appears more like a
'Y' shape. For this reason, we consider
males 'XY' for their 23rd chromosomes.
A male inherits the X chromosome from
his mother and the Y chromosome from
his father.
Let's look at this in a little bit more
detail . Let's think about which parent
then is the one that is dictating what
sex (gender) the offspring will be. We know that
you get half from each, but your mom was
female
so since she was female she had XX for
her 23rd chromosomes. So that means that
you MUST have received one X from your
mother. Your father, on the other hand,
you could have received his X chromosome or
his Y chromosome, depending on which
spermatozoon fertilized the egg that
made you. An egg must always carry an
X chromosome for the sex chromosome. The
male would have had a 50/50 chance.
every baby has a 50/50 chance of
becoming male or female.
Keep in mind that your chromosomes do a
whole lot more besides just determining
your gender. Today we are just going
to focus on that alone. When we talk
about DNA too, a lot of times we talk
about genetics and genetic traits that
are inherited from your parents.
Chromosomes carry genes. What we mean
by a 'gene' is a gene is a segment of DNA
within the chromosome that carries out a
very specific function. So we have
different genes for different traits. For
example you may have inherited the genes
for red hair freckles and green eyes
like this lovely stick figure here. Now
when the egg and sperm join together to
create the fertilized egg that we call
a ZYGOTE, the result is that you end up
with 46 chromosomes; 23 from each parent.
But, in order for that to happen,
that means that the egg and the sperm
have to have only HALF the number of
chromosomes as all of your other cells
in your body. Let's take a moment to
see why that is. When we talk about our
sex cells,
they have a specific name so that we
don't always have to say "sex cells" or
"eggs and sperm". We can also say GAMETES.
So when we say "gametes", that means all of
your sex cells; the sperm and the eggs.
Everything (all cells) that is not a gamete is
considered a 'body' cell, otherwise known
as a SOMATIC CELL. The word 'somatic'
actually comes from the greek word for
"body", so that makes sense as to why it would be
called the somatic cells. "Gametes", on the
other hand, comes from the greek word for
"to marry", so that kind of makes sense as
well, because when you are joining the
egg and sperm together you are able to get
a whole new organism. All of your 'body' (somatic)
cells divide, no matter what type of cell
they are. So your somatic cells or your
'body' cells are always dividing.
You have to do this in order to grow, to
repair wounds, to replace old worn-out
cells... remember you started as just a
zygote and somehow you went from thatsi
zygote to 40 trillion cells!
That's pretty impressive. Now somatic
cells or body cells divide and when they
divide they do it by a process that we
call mitosis. And you can see here that
one parent cell will end up having two
daughter cells. When we talk about
mitosis we start off with a parent cell
that just like all your other cells has
two sets of 23 chromosomes, 46 total. And
what happens here is the genetic
material is going to double. When it
doubles, it's then going to line up
line up at the center of the
cell and then these spindles are going
to pull the genetic information apart.
When they are pulled apart like that
they're called sister chromatids. So you
have one sister chromatid going to one
side and one sister chromatid going to
the other side. Eventually, that cell is
going to undergo cytokinesis where it's
going to completely separate into two
individual daughter cells. The two
daughter cells are genetically identical
to the parent and they're genetically
identical to one another.
Now your sex cells, or gametes, undergo a
different process. This process is called
meiosis. You can think of it loosely as
two consecutive rounds of mitosis.
So let's learn a little bit about this
process. You start off with a normal
cell that has 46 chromosomes; 23 from mom
and 23 from dad. We call this the parent
cell or sometimes you'll hear it called
the 'progenitor' cell. What happens here is
you also get this doubling of genetic
information. You end up with new with
a chromosome number that iss going to be the
same. You'll still have the 46 but they
are going to double so that they create
this
'X' shape. Now once they've created this X
shape with double the amount of genetic
material, they're then going to "pair up",
kind of like we saw what we saw with the
karyotype. So you're going to have
chromosome number one from mom and
chromosome number one from dad join
together and they're going to basically
lay on top of each other like this (showing one hand overlaying the other hand).
I'm showing them side-by-side here to try to
get you not to be too confused. But they
lay on top of each other like this (showing one hand overlaying the other)
and then they're going to shuffle their
genetic material. You can think of it as
shuffling a card deck.
so at this point the individual
chromosomes are no longer going to look
the same. Chromosome one from mom and
chromosome one from dad are now all
mixed up where each of those chromosomes
are going to have different genes from
different parents. At this point, the
homologous chromosomes (that's what we
call the number one for mom and number
one from dad so they're similar)
now they're going to line up down the
middle of the cell. This time you're
going to have each side of the cell only
have a spindle that's attached to 23
individual chromosomes so when it is
pulled apart you're going to have 23
chromosomes
that looks like an X on in one cell and
23 chromosomes that look like an X in
the other cell. We see here then that that cell
splits into two separate cells. You'll
notice here, as well, that they're still
in that x-shape, so they still have
double the amount of genetic material.
What they're going to do again is
they're going to take those 23
chromosomes which are now a mixture
between each of the parents and they're
going to line those up down the middle
and then the sister chromatids of the
chromosome are going to be separated
using the spindle; one chromatid going to
one side and one chromatid going to the
other side, to eventually pinch off and
create a total of four (daughter) cells at the end
of this process. So, what you need to know
for this process is one parent cell or
one progenitor cell is going to end up
creating four daughter cells. Each of
these daughter cells are genetically
different from one another and they are
genetically different from the
progenitor cell. This is because each of
those chromosomes have been mixed in
that crossing-over event where the DNA
is then shuffled. So the end result is
you get a gamete (an egg or sperm cell)
that has 23 chromosomes. But, those 23
chromosomes are a mixture from the mom
and dad from the owner of the egg and
the host of the spermatozoa.