The apple doesn’t fall far from the tree.
Traditional proverb
In the last section, we talked about cell division that gives us new cells for growing, repairing, and replacing old cells. Most of our cells use this kind of cell division, but we do have in our bodies special types of cells called gametes. These cells, sperm in males and eggs in females, are used for reproduction (or making more members of a species) in many plants and fungi, all animals, and even occasionally in protists. These special types of cells are made in a process known as meiosis. As we are going through this, I encourage you to grab a deck of cards and follow along (if you want to follow the entire way, you would need two decks of cards that have different backs).
Imagine that we have a card game that we want to play that needs a deck of cards that has just two of each of the numbers of cards (2-10, plus the jack, queen, king, and ace), but they can be any color or suit; the more mixed up they are, the better. How could we do this when a regular deck of cards has one heart, diamond, club, and spade for each number? This is like trying to get as many combinations of genes as possible; this genetic diversity helps to make groups of creatures strong.
Before we get to the nuts and bolts of meiosis, we have to understand a little something more about chromosomes. As you know, you have 46 chromosomes (and other creatures have different numbers), but did you know that actually those 46 chromosomes come in pairs? Those 23 homologous pairs of chromosomes are similar in that they have the same genes in the same location along their DNA strands, like how a deck of cards has each number in red and black. The matching genes on the pairs of chromosomes are the two parts that come from each of your parents. But how? Meiosis, of course (what, were you expecting tightrope walking?).
Meiosis goes through the same phases as mitosis (interphase followed by prophase, metaphase, anaphase, and telophase – remember PMAT?), but they are going to go through them twice, called meiosis I and meiosis II. Just like in mitosis, the cell has an s-phase in the interphase when each of the cell’s 46 chromosomes are copied to make sister chromatids. This would be like how in a deck of cards there are two red and two black cards for each number.
Next, prophase I sees the envelope around the nucleus breaking down and the chromosomes condensing; in meiosis, the homologous pairs also move so that they can be near each other. This would be like taking our deck of cards and sorting all of them by number, putting the two red cards for each number in a stack next to the two black. Another quite marvelous thing can happen during prophase I as well: the chromatids of the homologous pairs can cross over one another and the genes can swap! For example, let’s say that one of chromosomes has a freckle gene, while the other one has the no-freckle gene; in meiosis, these genes will sometimes swap places. This would be like taking a red 7 and swapping it with the black 7; this process increases how randomly the cards will be divided or how much genetic diversity a creature has.
Following that is metaphase I, where the chromosomes line up in the middle of the cell. This time, however, they line up with the homologous pairs next to each other. This would be like taking our cards and setting them up in a line with both stacks of 3’s and both stacks of kings and so forth, one after another. The trick is that you would not care which color goes in which column, but rather the right and the left would have both red and black cards mixed together. The cell then goes through the anaphase I and telophase I to separate the homologous pairs to either end of the cell and then split into two new cells (In animal and fungus cells, the envelop around the nucleus sometimes does not reform, but overall the process is really similar to mitosis. In many plant cells, however, the telophase does not happen at all, but the process continues inside the same cell.) With the cards, this would be like separating the pairs of cards into two piles and then gathering those piles together to make two decks that have half as many cards each.
At this point, we have two quite unusual cells; most of our cells have 46 chromosomes (as we have said), but these cells have only 23 – not 23 pairs, just 23 chromosomes. There are still the sister chromatids (just like we still have two of each number card), which still need to separate in the next round of division. Cells with half as many chromosomes as normal are called haploid cells, while those with the normal number are called diploid cells. Notice that even though they have half as many chromosomes as the cells they came from, they still have all the types of chromosomes, just like the deck of cards has all of the numbers but only one of the colors.
The cells take a little break to rearrange (a phase known as interkinesis), and then both of the daughter cells from the last part go back into meiosis II, which is just like mitosis only with fewer chromosomes. In the end, you go from one regular diploid cell with 46 chromosomes (each of them x-shaped sister chromatid copies) to four haploid cells with just 23 chromosomes (those are just single lines). Your deck of cards will go from four of each number to one of each number, which will be randomly red or black. We have now formed our sperm and egg cells.
When these sperm and egg cells come together from a mother and father to form a new creature, you can see the real wonder of meiosis. You get a great variety of genes from the parents, some of which came from each of its four grandparents! This would be like going through the process of making the four decks with two regular decks of cards (with different colored backs – we’ll use blue and yellow decks to illustrate) and then picking two decks at random to put together. If you look through the decks, you might have a red 8 on a blue card and a red 8 on a yellow card, but a black queen on blue card and a red queen on a yellow card. Remember, these chromosomes all carry genes – so where do the two parts of your genes come from? The two homologous chromosomes that come from the egg and sperm each contribute to what genes we have. All of this mixing of genes is why you can have a brother with blonde hair and brown eyes while his sister has black hair and blue eyes.