Lecture: Meiosis & Sexual Reproduction
Lecture script · about 15–20 minutes spoken · Week 10
Opening
Picture two siblings at a family dinner. Same mother, same father. One has curly hair and one has straight hair, and the family argues every holiday about who looks like whom. Both children got every one of their chromosomes from the same two people. So how can the same two parents make children who differ? By the end of this lecture, you'll be able to explain it, and even put a number on it.
This matters to a nursing student explaining to a family why two siblings can look so different. It matters to a breeder choosing which plants or animals to cross. And it matters to anyone who has wondered why they resemble one parent more than the other. All of it starts in the two divisions that make every egg and every sperm.
Here's where we are. In Week 1 you met evolution by natural selection, the unifying framework of biology. In Week 3 you met DNA, the cell's instruction molecule. In Week 9 you learned the cell cycle, the chromosome with its two sister chromatids, and mitosis, which makes two identical cells. In this lecture we run a different division on those same chromosomes. I'll take you through the human chromosome numbers, the problem this division solves, its two steps in order, the two shuffles that make every gamete different, and a way to count them. Then we'll set mitosis and meiosis side by side. Next week, in Week 11, Mendel's rules follow single genes through the cells you meet here.
1. Two sets or one: diploid and haploid
Imagine two decks of recipe cards, one from each grandparent. Each deck has a soup card, a bread card, and a salad card, in the same order. The two soup cards cover the same dish, but the recipes on them can differ a little. Hold on to that picture, because it's a good model of the chromosomes in one of your body cells.
A human body cell carries 46 chromosomes, and they come as 23 pairs. One chromosome of each pair came from your mother and one from your father. A cell like that has a name. A diploid cell carries two full sets of chromosomes, one set from each parent, written 2n; a human body cell is diploid, with 46 chromosomes.
An egg or a sperm is different. A gamete is a sex cell, an egg or a sperm, that carries one set of chromosomes and joins with another gamete at fertilization. A haploid cell carries one complete set of chromosomes, written n; a human egg or sperm is haploid, with 23 chromosomes. So for a human, the diploid number is 46 and the haploid number is 23.
Now back to the recipe cards. The two soup cards are a matched pair. Homologous chromosomes are the two chromosomes of a matched pair, one from each parent, that carry the same genes in the same order, possibly as different versions of those genes. Week 11 gives those different versions their name.
Here's the trap I want you to see coming. Homologous chromosomes are not sister chromatids. In Week 9, sister chromatids were the two identical copies of one chromosome, made in the S phase. Sisters are copies. Homologs are a matched pair from two different parents. They carry the same genes, but they can carry different versions.
Pause and try this. Someone says a haploid cell has half a chromosome. How would you correct that, using the human numbers?
Picture a gamete and count what it holds.
Here's the answer. Haploid means one complete set, so a human gamete holds 23 whole chromosomes, one from every pair. Nothing is cut in half; the number of sets is halved, from two to one.
2. The doubling problem
Here's a puzzle. Your body cells carry 46 chromosomes. Suppose an egg and a sperm each carried a full 46 as well. What would the child have?
The child would have 46 plus 46, which is 92. That child's eggs or sperm would then carry 92 each, so the next generation would have 92 plus 92, which is 184. Every generation, the count would double, and it would never settle down. That's the problem. Living things solve it with a division that cuts the count in half before the egg and sperm ever meet.
That division has a name. Meiosis is the two-part cell division that makes gametes, turning one diploid cell into four genetically different haploid cells after a single round of DNA copying. For a human, meiosis takes 46 and halves it to 23.
And the meeting has a name too. Fertilization is the joining of an egg and a sperm, which combines two haploid sets of chromosomes and restores the diploid number. An egg with 23 meets a sperm with 23, and 23 plus 23 gives back 46. So the count stays at 46, generation after generation.
Put those together and you have sexual reproduction: reproduction in which two parents each contribute a haploid gamete, so the offspring carries a new combination of both parents' chromosomes. Compare that with the asexual reproduction from Week 9, where a houseplant cutting grows into a copy of one parent. Sexual reproduction makes something new out of two.
Here's the short line to carry out of this section: meiosis halves; fertilization restores.
Pause and try this. A child has 46 chromosomes. How many came from the mother's egg, and why can't the egg carry all 46?
Think about the sum, and about the next generation.
Here's the answer. The egg carried 23 and the sperm carried 23. If the egg carried 46, the child would have 92, and the count would keep doubling each generation.
3. Meiosis I: the pairs split up
Think of a square dance. The dancers start in couples, partners side by side. At one call, the couples line up across the middle of the room. At the next call, every couple splits, and the two partners walk to opposite walls. Meiosis I is that dance, and the dancers are homologous chromosomes.
Before the dance, one thing happens that you already know. In the S phase, the cell copies its DNA, so every chromosome comes in as two sister chromatids joined at the centromere. That copying happens once, before meiosis begins.
Then the first division runs. Meiosis I is the first division of meiosis, called the reductional division, in which homologous chromosomes pair up and then separate, leaving two haploid cells whose chromosomes are still doubled.
In prophase I, each chromosome finds its partner and lines up beside it, along its whole length. Synapsis is the pairing of homologous chromosomes side by side in prophase I of meiosis. Mitosis never does this; in mitosis, homologs never pair. In metaphase I, the pairs line up across the middle of the cell two by two, not single file. Then the partners separate and move to opposite poles, and each one keeps its two sister chromatids together. The cell divides, and you get two cells.
Let's run your lab's paper model. It has three pairs of chromosomes, a long, a medium, and a short, so n is 3 and the cell holds 6 chromosomes. In each pair, call one copy B, from one parent, and the other copy R, from the other parent. After the S phase, the cell holds 12 chromatids. In meiosis I the long B pairs with the long R, and so on, and then each pair splits. You get 2 cells, each with 3 chromosomes, one from every pair, and each still doubled, so 6 chromatids per cell.
Pause and try this. In meiosis I, which separate: the homologs, or the sister chromatids?
Go back to the dance, and ask who walks away from whom.
Here's the answer. The homologs separate in meiosis I. The sister chromatids stay joined, the way each dancer keeps hold of its own copy, so each new cell is haploid but still doubled.
4. Meiosis II: the sisters split up
Now each of the two cells from meiosis I holds 3 doubled chromosomes. Think of each one as a pair of socks still clipped together from the laundry. The second division unclips them.
Meiosis II is the second division of meiosis, called the equational division, in which sister chromatids separate, leaving four haploid cells with single chromosomes. It looks a lot like mitosis. In each cell, the chromosomes line up, the sisters come apart and move to opposite poles, and the cell divides. Two cells each divide once, so you end with 4 cells.
Here's the part that surprises people. There's no S phase between meiosis I and meiosis II, and no second round of copying. The DNA was copied once, and the cell divides twice. That's exactly how the count ends up halved. Copy once, divide twice.
So the order is a fact you can be graded on. Meiosis I comes first, and the homologs separate. Meiosis II comes second, and the sister chromatids separate.
Finish the paper model. You started meiosis II with 2 cells, each holding 3 doubled chromosomes. The sisters separate, each cell divides, and you have 4 haploid cells, each with 3 single chromosomes, one from every pair. One diploid cell with 6 chromosomes became 4 haploid cells with 3 each.
Pause and try this. Someone says the cell copies its DNA again between meiosis I and meiosis II. Is that right?
Count the copies and the divisions.
Here's the answer. No. The DNA is copied once, before meiosis I. Meiosis II only separates the sister chromatids that the single copying already made.
5. Two shuffles: crossing over and independent assortment
Back to the siblings. Each got 23 chromosomes from each parent. So why aren't they alike? Because the gametes that made them were different, and meiosis makes them different in two ways. Genetic variation is the differences in DNA among the members of a family or a species, the raw material that natural selection acts on.
The first shuffle happens while the homologs are paired in prophase I. They trade pieces. Crossing over is the swap of matching segments between nonsister chromatids of paired homologous chromosomes in prophase I, which makes new combinations within a chromosome. And nonsister chromatids are chromatids that belong to different chromosomes of a homologous pair, one from each parent's copy, rather than to the same chromosome. Picture the two soup cards trading their bottom halves. Now each card carries part of each grandparent's recipe.
The second shuffle happens in metaphase I, when the pairs line up. Each pair can face either way: the mother's copy toward one pole and the father's toward the other, or the reverse. Each pair settles that by chance, and no pair affects any other. Independent assortment is the random, independent way each homologous pair lines up in metaphase I, which sends either parent's copy of each pair into a gamete and makes new combinations between chromosomes.
Then comes a multiplier. Random fertilization is the chance meeting of any one egg with any one sperm, which multiplies the variety that crossing over and independent assortment create. Two kittens from one litter have the same parents, yet each came from its own egg and its own sperm, so they can look quite unlike each other.
Pause and try this. Which shuffle makes new combinations within one chromosome, and when does it happen?
Think about which shuffle needs the partners lying side by side.
Here's the answer. Crossing over, in prophase I. It needs the homologs paired, which is why it never happens in mitosis, where homologs never pair.
6. Counting the gametes
Now let's put a number on independent assortment. Think about getting dressed from a closet in the dark. Every shirt can go with every jacket, so the choices multiply. Independent assortment works the same way, except that every chromosome pair offers exactly 2 choices: its B copy or its R copy.
Take the paper model with 3 pairs. The long pair gives 2 choices, the medium pair 2, and the short pair 2. Multiply: 2 times 2 times 2 is 8. That's 2 to the power 3. Here's the whole list, read long, medium, short: BBB, BBR, BRB, BRR, RBB, RBR, RRB, and RRR. Count them. That's 8 different gametes with no repeats, matching 2 to the power 3.
The general rule is short. From independent assortment alone, a cell with n chromosome pairs makes 2 to the power n genetically different gametes. That means 2 multiplied by itself n times. Watch what each added pair does. Two pairs give 4. Three give 8. Four give 16. Five give 32. Each added pair doubles the count.
Now scale it to you. A human has 23 pairs, so independent assortment alone gives 2 to the power 23, which is 8,388,608 genetically different gametes. And that's only a floor, because crossing over raises it without limit. Then fertilization multiplies again. Two model parents, each able to make 8 kinds of gametes, can pair them 64 ways. That's why two siblings almost never draw the same hand.
In your lab, you'll check the doubling in a spreadsheet, where a power-of-2 formula does the multiplying for you. Your job is to say what the number means.
Pause and try this. Someone says a human can make 46 kinds of gametes, since a body cell has 46 chromosomes. What went wrong?
Ask what each chromosome pair does to the count.
Here's the answer. Each pair doubles the count, so the number is 2 multiplied by itself 23 times, 8,388,608. Neither 23 nor 46 counts combinations; they count pairs and chromosomes.
7. Mitosis and meiosis, side by side
You now know both kinds of nuclear division. They share their parts: chromosomes, sister chromatids, and phases with the same names. That's exactly why people blur them. So let me walk you through six differences, one at a time.
First, divisions: mitosis divides once, and meiosis divides twice. Second, cells made: mitosis makes 2, and meiosis makes 4. Third, chromosome sets: mitosis keeps the cells diploid, like the parent, while meiosis makes them haploid. Fourth, how the new cells compare: mitosis makes cells identical to each other and to the parent, while meiosis makes cells that all differ. Fifth, pairing: in mitosis, homologs never pair, and in meiosis they pair in prophase I, where they can cross over. Sixth, purpose: mitosis is for growth, repair, replacement, and asexual reproduction, and meiosis is for making gametes.
Both start the same way, with one round of DNA copying in the S phase. After that they go separate ways. Mitosis keeps every pair together in each new cell. Meiosis splits the pairs between cells, then splits the sisters.
Now the long view. In Week 1 you learned that natural selection favors individuals whose traits suit their surroundings. But selection can only choose among differences that already exist. If every offspring were a copy of its parent, there would be nothing to choose between. Meiosis and fertilization supply new combinations every generation. Variation is the raw material natural selection acts on, and that's the reason sexual reproduction is worth what it costs a living thing.
Pause and try this. Which division would your skin use to close a scraped knee, and which does a body use to make sperm?
Think about whether the new cells should match the old ones.
Here's the answer. The scraped knee heals by mitosis, because repair needs identical diploid cells. Sperm come from meiosis, because a gamete must be haploid and unique.
Closing
Let me pull it together. A human body cell is diploid, with 46 chromosomes in 23 pairs, and a gamete is haploid, with 23. Homologs are a matched pair from two parents; sisters are copies. Meiosis halves the count, and fertilization restores it.
The DNA is copied once. Meiosis I splits the homologs, and meiosis II splits the sisters, leaving four haploid cells. Crossing over in prophase I mixes within a chromosome, independent assortment in metaphase I mixes between chromosomes, and random fertilization multiplies both.
From independent assortment alone, n pairs give 2 to the power n kinds of gametes: 8 for the paper model, 8,388,608 for you. Mitosis makes 2 identical cells; meiosis makes 4 unique ones. And that variety is what natural selection works with.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz, and then the Modeling Meiosis lab, where you build the paper cell yourself. Next week, in Week 11, we start Mendelian genetics, following one gene at a time through the gametes you met here.
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