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Biology: Sexual Reproduction and Role of Meiosis


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About this Lesson

  • Type: Video Tutorial
  • Length: 10:31
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 114 MB
  • Posted: 02/10/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Final Exam Test Prep and Review (42 lessons, $59.40)
Biology Review (19 lessons, $27.72)
Biology: Cell Reproduction - Mitosis and Meiosis (16 lessons, $23.76)
Biology: Meiosis (5 lessons, $8.91)

There are two types of reproduction, asexual reproduction and sexual reproduction. Asexual reproduction creates genetically identical offspring. Several types of asexual reproduction are binary fission, which occurs in bacteria, budding, which occurs in coral, and spores, which some plants produce.

However, sexual reproduction is selected for, genetically. This is because sexual reproduction produces new genetic combinations in offspring that allow for more variety and competition. These genetic combinations are known as recombinant genes. However, sexual reproduction requires a reductive cell division, called meiosis, to produce gametes. Gametes are haploid sex cells, instead of the normal diploid somatic cells and are necessary to keep the correct number of chromosomes in DNA.

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science.

Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel.

His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp.

He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

About this Author

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Cell Reproduction
Sexual Reproduction and the Role of Meiosis

Did you ever why wonder natural selection chose sex? This whole theme of this—everything I say has to do with
natural selection. This is selected for, this is selected against. The natural environment will select the most efficient
way to get something done. And sexual reproduction at first glance seems like it should be an evolutionary loser.
Think about all the problems that go with sexual reproduction and I’m not talking about your social lives here. I’m
talking about biology. Think about this. The first thing is you’ve got a chromosome problem. And that’s what we’re
going to spend a lot of time talking about. But forget about that. Then you got to make a special group of cells. Then
you got to get one cell to the other. All right? In order to get that cell to the other, you have to evolve all these
behaviors constantly in competition with each other and, you know, what dance do I need to do, and then you got to
care for your young. Asexual reproduction seems to be a lot simpler. Think about it. Asexual reproduction,
reproduction without sex. All right. Binary fission and bacteria. How simple is that? One, two. No mating rituals.
You don’t have to go out and buy clothes and impress people and take them out to dinner. It’s very simple. And even
some more advanced organisms do asexual reproduction. You know, corals. Have you ever been on a coral reef?
Now corals reproduce both sexually and asexually, but the bottom line is corals and creatures of their ilk--if this is a
coral polyp, reproduce asexually by a thing called budding. And what they in essence do is they make an asexual
offspring that grows off of their side like so and eventually forms a brand new organism and that’s the way a reef
forms. And they’ll build this calcium carbonate skeleton around their outside and you’ll have a coral reef. Plants.
Plants, okay, yes. Plants do sexual reproduction, but many of them can do asexual reproduction. You’ve heard of
spores. You’ve heard of—many of you have taken cuttings where you cut a piece of plant, you throw it in the ground
and you get a new plant. How simple is that?
So why then has sexual selection or sexual reproduction been selective for? And the answer to that is remember one
thing. Variety is the spice of evolution. You see, the point here is this. Darwin’s theory of natural selection stated
some very elegantly simple context and that was one. There’s a whole lot offspring born. And, two, there are
varieties in these offspring and the varieties compete and competition isn’t necessarily, That’s my hamburger and get
out of the way! Competition can be fleeing from a predator. Speed. Competition can be having a variation that
allows you to hide from a predator. Competition can mean a lot of different things. So the key here is sex has been
selected for and at least we can say this generically, sex has been selected for because variety is selected for. And
sexual reproduction produces new combinations of genes, therefore variety. In other words it comes down to this. It
comes down to the fact that a female, symbol for female, crossed with a male, symbol for male, will share genes. And
you get a mixing of the genes. You get what are called recombinants or recombinations. And in these recombinants
or these recombinations, you’re going to get new variety, and in getting new variety, you’re going to have different
levels of fitness. Not fitness as in physical fitness, but physical as in natural selection. And some will be more fit in
this environment, but guess what? If the environment changes, other ones may be more fit. And if it doesn’t change,
well, they had a good time on the plant. So the bottom line is, it seems that sexual reproduction is probably here to
But there are some problems associated with sexual reproduction and I don’t want to talk about all those social things
like courtship rituals. That’s another day. I want to talk chromosomes. Chromosomes. Back to asexual reproduction.
So reproduction with mitosis was easy. Think about it. Now here’s a typical human cell going through mitosis; 46
chromosomes, doubled, split, 46 daughter cells. Now you’ve got two nice cells. But now all of a sudden, we’re going
to have a problem. And the problem is this. If we want to come up with a new group of cells called gametes, sex
cells, we can’t have those gametes just like the daughter cells produced by mitosis, can we? Now again, if you go
back in the fossil record, mitosis has been around as long as cells have been around. But it doesn’t work for sex.
Why not? Well, let’s see why not.
It’s a simple case of numbers. Give me a cell right here with 46 and let’s turn it into a sperm and an egg that are
going to fertilize and form a zygote. Now this is you, okay? A normal human being with 46 chromosomes, all right?
Now let’s just say this person is going to now, instead of making regular old what we’re going to call somatic or body
cells, is going to make gametes. And this happens to be a male person and he’s going to make 46. Well, are you
starting to see the problem? Let me show you.
If this is Dad and this is Mom and we’re assuming Mom being in the same species is going to have the same number
of chromosomes and she’s going to make eggs with 46 guys, we’ve got a problem here. Because think about it.
Sperm meeting egg, 92? 92? That’s not human. That’s not going to happen. Okay?

And so we come to a new process. So sexual reproduction needs a new process and I know you know about mitosis
and please don’t forget your mitosis lessons. Because a second ago I said to you mitosis came first, well, guess
what? This next process is a wrinkle on mitosis. It’s a new wrinkle on mitosis. So what we need is we need not to do
a duplicative division as mitosis did. We need a different kind of division. We need a reductive division. When we
start to make gametes, we need to reduce. So we need to take a body cell, a somatic cell. Soma means body. A
somatic cell and we need to take that somatic cell and turn it into a gamete. And in doing this, we need to reduce the
number of chromosomes. During this process we need to drop, we need to half the number of chromosomes
because that’s what it’s going to come down to. Forget all the stuff about genes. Forget everything, the DNA. The
bottom line is it’s all wrapped up in chromosomes anyway so when we’re talking about chromosomes, we’re talking
about DNA and genes.
So we literally have to go from 46 to cells with 23 each so that when Dad makes those sperm and Mom makes that
egg, we can now reunite these or recombine these into a zygote with 46. That’s what we have to do.
I’m going to lay some vocabulary on you here. So let’s look at this in summary and then we’ll take a look at the body
plan or the plan for meiosis because that’s what we’re going to be talking about. Here’s what we’re talking about. We
need to go from a case where they’re somatic and it’s sometimes referred to if talk about the chromosome number,
we’re going to say that that is diploid. You see the word “di” there. And you’ll see why we say that in a second. To a
gamete. A gamete we refer to as haploid. Now that’s nice but aren’t those just more words, and the answer is yes.
Okay? But one more thing. Let’s take a look at the chromosome number and we’ll make it make perfect sense. If I
refer to the haploid number of chromosomes as N, what would the diploid number be? Well, that’s N and remember
we went from full to half, and this must be 2N. So whether you are a human with your 2N equaling 46 or a potato—
potatoes are creatures, too. You know what? Potatoes have more chromosomes than you guys. How do you like
that? Okay? You have to go from the diploid to the haploid in order to do sex.
Well, how’s this all going to work? Well, let’s take a look at a quick overview and we’re going to call it meiosis where
we are literally going to take a cell, double those chromosomes and end up going from a cell with the diploid number
to cells with the haploid number. In this case, a cell with 2 to 4 cells with 1. Does that sound complicated? If you
have mitosis, meiosis is going to be nothing.

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