We have mentioned two types of nuclear division: mitosis
, where the nucleus divides into two identical nuclei, and meiosis
, which results in the production of four nuclei with half the original number of chromosomes of the parent cell. We will go through all the detailed steps of mitosis and meiosis in the next section, but first let's focus on how the processes have some features in common, yet are still different.
In order to understand these two processes, it is important to become familiar with the terms diploid
. A diploid cell has two of each chromosome, one from each parent. This is in contrast to a haploid cell, which only has one copy of every chromosome. Diploid cells comprise the majority of your body, while examples of haploid cells are eggs and sperm. If a haploid cell has n chromosomes, a diploid cell has 2n (n represents a number, which is different for every species – in humans, for example, n = 23 and 2n = 46).
Both diploid and haploid cells can undergo mitosis. This makes a lot of sense, because mitosis is essentially like making a photocopy: it creates a perfect reproduction of what you started with. Therefore, if a diploid cell undergoes mitosis, the result is two identical diploid cells (2n →2n). Prokaryotic cells, for example bacteria, use this process to reproduce asexually, in a process known as binary fission. If a haploid cell undergoes mitosis, which is something certain types of plant and fungus do as part of their normal life cycles, the end result is two identical haploid cells (n→n). In meiosis, however, you start with a diploid cell that divides twice to produce four haploid cells. In other words a diploid cell that has 2n chromosomes produces four cells, each of which contains n chromosomes.
Now let's step back and talk briefly about chromosomes. Most of the time, each chromosome is a single, long molecule of DNA. But, a chromosome isn't DNA; it also contains proteins, called histones, which the DNA is wrapped around for protection and support. Except in bacteria—their DNA is completely naked. And by naked, we mean doesn't have any histones. Sorry. Together, the DNA and proteins are known as chromatin, which translates roughly as "colored stuff"; in fact, the word chromosome literally means "colored body."
During S phase, that DNA molecule is replicated to form the beginnings of what most people think of as a chromosome – a cross-shaped structure, pinched in at (or near) the middle. Although, it takes a lot more coiling of the DNA and histones to produce this shape, which first appears during prophase.
Every eukaryotic organism has its own particular number of chromosomes of a particular shape and size. This is known as its karyotype. Cytogenetics is the study of karyotypes and is a important tool when studying human chromosomal abnormalities and diagnostically in prenatal screening.
Now we've mentioned that a haploid cell has n chromosomes, while a diploid cell has 2n chromosomes. To recap, a diploid cell has two copies of each type of chromosome. These two copies are not exactly the same, though; one is from Mom and one is from Dad. These two chromosomes, one from each parent, are known as homologous chromosomes
(or a homologous pair). They carry the same type of information (that is, the various genes on that chromosome), but they are not identical because each will have slightly different DNA sequences by which you can tell them apart. Humans have 23 homologous pairs of chromosomes: 22 pairs of autosomes and 1 "pair" of sex chromosomes. Technically, if you are a girl it is an actual pair (XX), but if you are a boy, they are different (XY). We will ignore the sex chromosome differences for now, because we are all about keeping it nice and simple.
Okay, time for a quick analogy: imagine a diploid cell is ordering two hot chocolate drinks, one with whipped cream and one without—both drinks are basically a hot chocolate, but they are not quite the same thing. A haploid cell would only order one type of drink (we'd go for the one with whipped cream, but maybe that is us), because it only has one of each type of chromosome.
You're probably beginning to realize that things become a little more confusing when you add DNA replication into the mix. The important concept here is that replication generates identical
copies of all
of the DNA molecules present in G1
of the cell cycle, but without increasing the total number of chromosomes. Remember: the identical DNA molecules generated by DNA replication are called sister chromatids, and each chromosome now has two chromatids, not one. Therefore, using your cells as an example, all 46 chromosomes will have been copied, but instead of having one chromatid, they now have two, for a grand total of 92. If keeping all these chromosomes and chromatids straight is beginning to make your head spin, just remember that DNA replication generates sisters not homologs
. After DNA replication, each member of a homologous chromosome pair consists of two sister chromatids.
Now comes the tricky part. Once a cell goes through mitosis, the sister chromatids must end up in different cells. This is always true, no matter if a cell is diploid or haploid. In mitosis, this step is absolutely essential to produce two cells that are genetically identical. Now its tricky enough trying to keep track of what's going where and we are using color-coded examples; how does the cell do it?
In mitosis, as we've said, the cell has chromosomes with 2 identical halves, the sister chromatids. The cell must ensure that when it divides into two, each cell gets only one of those sisters. Let's go back to our hot chocolate analogy: two customers, our daughter cells, order two hot chocolates each, they must be thirsty, one with whipped cream, representing Mom's chromosome, and one without, representing Dad's. The barista is efficient and decides to make both cups of the whipped cream hot chocolate at the same time. He puts the two cream ones on the counter next to each other and then does the same with the plain ones. He then checks the orders, putting one of each type of hot chocolate onto two separate trays, ready for each customer to take away. The cell uses this strategy too. Right after DNA replication, the two sister chromatids are held together by a group of proteins that act as a type of glue. When it is time to separate the chromatids the glue is dissolved and voila. Accurate chromosome segregation.
The situation in meiosis is a wee bit more complicated than mitosis. In meiosis, homologous chromosomes pair (briefly) and each of the four chromatids in that pairing makes it into its own nucleus, giving rise to four haploid nuclei. Unlike mitosis, which occurs in one cell division event, meiosis occurs through two division events. In the first division, the homologous pairs are divided randomly in a process known as independent assortment, meaning that the offspring cell gets either a whole maternal or a whole paternal copy of every chromosome. Since the result of the first division is two nuclei, each of which contains only one homologous chromosome, the nuclei are haploid, even though each chromosome present still has both of its sister chromatids. The sister chromatids only become separated in the second meiotic division. The result is four haploid nuclei containing only one "arm" (or "leg" – you get the point) of each homologous chromosome.
How does the cell orchestrate the separation of chromosomes in this case? Let's return to our hot chocolate (yum). In our mitosis example above, all we cared about was that we separated our drinks, or chromatids, such that each customer, daughter cell, got one drink of each kind. One homologue, or the version of the drink with whipped cream is treated the same as the other, or the version of the drink without cream. But, meiosis is more complicated because we need to be able to keep track of all four chromatids individually. Let's say we have two customers again, the ones who are paying, if you like, but now they have brought their two friends along as well. One pair wants hot chocolate with whipped cream, the other pair is trying to be good, so no cream for them. Our barista is still efficient and makes the two whipped cream versions together, then the two plain ones. This time when he checks the orders, he puts both of creamy ones on one tray for one customer to take away, or the first meiotic division, before she gives one to her friend, or the second meiotic division. The same happens for the plain hot chocolates.
The cell has special linkages called chiasmata
that connect homologous chromosomes together during the earliest stage of meiosis (prophase I
). During the first division, these linkages are dissolved and the two identical chromatids from Mom, or the two hot chocolates with whipped cream, and the two identical chromatids from Dad, or the two hot chocolates without whipped cream, are separated, or put on separate trays. In the next division, one of each of the sister chromatids, or identical drinks, makes it into a separate cell using effectively the same mechanism as in mitosis.
You may also be wondering how the cell only pairs and holds homologous chromosomes together in meiosis and not mitosis. How are chiasmata created? The answer is through recombination
, a process by which breaks in DNA are repaired using another DNA template. In meiosis, the cell actually deliberately generates breaks in its own DNA, in non-sister chromatids
in the homologous pair, where non-sister means that the chromatids are on different chromosomes. It then promotes the repair of this damage using the unbroken chromatids, which are held adjacent to the broken ones with help of the synaptonemal complex
, a special complex of proteins only present in meiosis, as a template.
In actuality, the final haploid cells produced in meiosis don't contain intact copies of either a maternal or paternal version of a chromosome – genetic variation has been introduced through crossing-over. How much recombination you have depends on the size of the chromosome: the bigger it is, the more chance there is for crossing-over to occur. The overall result is kind of like what happens when you shuffle two decks of cards and divide them into two piles. The two new piles are a mix of the two decks. The genetic reshuffling is the reason that each of your gametes, either egg or sperm, depending on whether you are female or male, respectively, contains a combination of DNA from both of your parents.
How does all of this information apply to cells within the human body? You have probably realized by now that the situation of n=3 is a simple scenario. As we mentioned earlier, for a typical diploid cell in the human body, 2n=46; that is, there are 46 chromosomes in total; n (the haploid number) = 23, meaning your lovely diploid cell got 23 chromosomes from Mom and 23 chromosomes from Dad. Or: there are 23 unique types of chromosome in your cells (remember, we are treating the sex chromosomes as the same for simplicity). After replication of the 46 chromosomes, the cell contains 92 chromatids (in 46 pairs). After mitosis two identical cells are created with the same original number of chromosomes, 46. Haploid cells that are generated through meiosis, such as egg and sperm, only have 23 chromosomes, because, remember, meiosis is a "reduction division."
Comparing Mitosis and Meiosis
|Number of cells at completion||2||4|
|Number of chromosomes at completion||Same as parent cell||Half as many as parent cell, keeping one of each type of chromosome|
|Do homologous chromosomes pair?||No||Yes|
|Are sister chromatids held together?||Yes||Yes|
|Number of cell division events||1||2|
|Function||Makes all cells other than gametes. Growth, repair, and asexual reproduction.||Makes gametes for sexual reproduction.|
- The Scientist Walther Fleming named mitosis because the chromosomes in the nucleus looked like long threads. Mito is the Greek root for threads (Harper, 2001).
- Parthenogenesis, or "virgin birth," is a type of reproduction in females where an embryo develops from an unfertilized egg cell. This process occurs in many different types of organisms, such as some bugs, worms, bees, and some reptiles and fish.