Hi. It’s Paul Andersen and this
is Life Science Core Idea 1B (LS1B). It’s on growth and development of organisms. By
the time students graduate high school they should have an understanding of growth and
development. In other words, how we go, not only from a kitten to a cat, but how do we
go from a cat zygote to a cat. And so the progression at which we teach this at the
elementary level, at the lower elementary level, we should talk about growth and change.
In other words we go from an acorn to an oak tree but an oak tree doesn’t just look like
a giant acorn. It is somehow changed throughout that time. Or how you go from an egg to a
chicken. It’s now just a giant egg, but it’s changed over time. And so this idea of getting
bigger or growth and then also changing. And so how do we go from a baby to mom to grandma
to great grandma? That’s through growing and changing over our lifetime. And parents are
important in that. So we should teach the importance of parenting, making sure that
organisms survive that whole way. So in humans parental care is incredibly important. But
also in important in birds as they both care for the organism or even in spiders. This
mother is taking care of little spiderlings. And so we should emphasis the importance of
parenting. As we move higher through elementary we should bring this idea of a life cycle
forward. And so in plants we go from an acorn to a germinated seed to a sapling to a tree
and then that tree again is going to produce more acorns and so this life cycle can continue.
In organisms, in animals, excuse me, we go from an egg to a hatched egg to a chick and
then to an adult chicken. And then they lay eggs and this process continues over and over
again. And so now we should bring out these ideas of growth and development. And so if
you grow it means that you get bigger over time. But if you develop that means that you’re
becoming different over time. Or you’re changing. And so there are a lot of steps that are going
on from the egg to the chicken that are making it look different then it did when it was
just an egg. And so plants and animals have evolved great ability to grow at the correct
rate for their environment. And so plants grow really really slow, but they can grow
really really big. And they continue to grow their whole life. Each of the animals in this
community here have evolved to grow and different rates and that’s played against their environment
or their success in their environment. But the way they reproduce at the end of that
growth can either be asexual or sexual. And we should talk about this in middle school.
That there are a number of organisms reproduced by budding off of themselves. And so these
hydra or these fresh water hydra will just make a copy of themselves and that forms a
new hydra. Or these yeast, the way yeast grow they simply make a copy of themselves that
buds of and that forms a brand new organism. There are advantages to that. It’s really,
really easy. But there are disadvantages, because they’re going to look exactly like
the parent. And so a lot of organisms, especially when we move to the higher level plants and
animals, reproduce sexually. And so the function of flowers is simply to transfer pollen from
one plant to another. Or to transfer sperm from one plant to another. And so asexual
reproduction has some disadvantages and so these dragonflies are reproducing sexually.
And that gives us variation. And so you should emphasis in middle school the importance of
plant reproduction and not get so much into animal reproduction. The bee is playing a
role here. It’s getting honey from the flower, but what it’s really doing is it’s transferring
pollen from one flower to another. And what’s inside the pollen, they’re simply sperm inside
there. And so we’re getting variation and we’re going to create seeds that have variation
compared to that adult plant. As we move into high school then we can start talking about
the specifics of sexual reproduction. How does this actually work. And that’s where
we’ll talk about mitosis and meiosis. And so if we have these two birds right, how do
they sexually reproduce? Well the mother is going to produce an egg and the father is
going to produce sperm. And they’re going to transfer that sperm from the male to the
female. What is really inside those egg and inside those sperm are chromosomes. And so
in a typical human for example, we’ll have 46 chromosomes. What is that? It’s our DNA.
It’s our genetic material. We only give half of our chromosomes to our offspring. And so
I’ve really simplified it in this animation, but basically it’s going to have chromosomes
and those chromosomes are sections of DNA. And on the DNA we’re going to have these different
genes. And so when you create sperm or when you create egg you’re really only giving half
of your DNA. So what happens when the sperm fertilizes the egg? We make a zygote. And
that zygote is going to be a combination of the chromosomes from the mother and the chromosomes
from the father. And you can see that the chromosomes will have different lengths. And
we give them numbers. This would be chromosome one and this is two. It’s really oversimplified.
Most organisms are going to have dozens and dozens of chromosomes. But we eventually have
a fertilized egg. And so it’s a union of the chromosomes from the male and the female.
And so that will make an exact duplicate of itself. And that’s called mitosis. And those
two cells are now called daughter cells. And so mitosis is the process of going from that
original cell to two copies of the cell. And then we make more of that cell. And then we
make more of that cell. And so each of these are getting bigger and then they’re dividing
and so we call that growth. But if we were to let that keep going and going and going
and going, what would you get? You would get a bird that just looks like that original
cell and we know that doesn’t happen. And so each of these cells have to differentiate.
What does that mean? They have to become different cells. Some of these cells are going to become
nerve cells and muscle cells and bone cells. And the way they do that is through differentiation.
And so basically what that means is inside the chromosomes they’re going to turn on specific
genes and turn off other genes. And so even as it’s in this early stages, each of these
cells are starting to become different cells or to differentiate. Now we’re talking about
development or change in that organism over time. Eventually these become adults. And
adults are still going to be made up of cells, but each of those cells are going to be different.
So if we were to take these two cells, or two organisms, what happens next? Well they
give off their sperm and egg again. So create the next generation. The creation of sperm
and egg is called meiosis. And meiosis gives us variation. So each of the different sperm
are going to be different. And you can see here that it’s going to be a combination of
chromosomes from the chromosomes that they got from their mother and from their father.
And so there’s a lot of variation within the creation of these sex cells, both egg and
sperm. And eventually there’s going to be union between the egg and the sperm and that’s
going to create the next generation. So that’s development. That’s growth. Growth means gets
bigger. Development means gets different and I hope that was helpful.}