How meiosis reduces chromosome number by half: crossing over, meiosis I, meiosis II, and genetic variation.
Mitosis is used for almost all of your body’s cell division needs. It adds new cells during development and replaces old and worn-out cells throughout your life. The goal of mitosis is to produce daughter cells that are genetically identical to their mothers, with not a single chromosome more or less.
Meiosis, on the other hand, is used for just one purpose in the human body: the production of gametes—sex cells, or sperm and eggs. Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell.
To put that another way, meiosis in humans is a division process that takes us from a diploid cell—one with two sets of chromosomes—to haploid cells—ones with a single set of chromosomes. In humans, the haploid cells made in meiosis are sperm and eggs. When a sperm and an egg join in fertilization, the two haploid sets of chromosomes form a complete diploid set: a new genome.
Phases of meiosis
In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar strategies to organize and separate chromosomes. In meiosis, however, the cell has a more complex task. It still needs to separate sister chromatids (the two halves of a duplicated chromosome), as in mitosis. But it must also separate homologous chromosomes, the similar but nonidentical chromosome pairs an organism receives from its two parents.
These goals are accomplished in meiosis using a two-step division process. Homologue pairs separate during a first round of cell division, called meiosis I. Sister chromatids separate during a second round, called meiosis II.
Since cell division occurs twice during meiosis, one starting cell can produce four gametes (eggs or sperm). In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase.
Before entering meiosis I, a cell must first go through interphase. As in mitosis, the cell grows during G phase, copies all of its chromosomes during S phase, and prepares for division during G phase.
During prophase I, differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome carefully aligns with its homologue partner so that the two match up at corresponding positions along their full length.
For instance, in the image below, the letters A, B, and C represent genes found at particular spots on the chromosome, with capital and lowercase letters for different forms, or alleles, of each gene. The DNA is broken at the same spot on each homologue—here, between genes B and C—and reconnected in a criss-cross pattern so that the homologues exchange part of their DNA.
Image of crossing over. Two homologous chromosomes carry different versions of three genes. One has the A, B, and C versions, while the other has the a, b, and c versions. A crossover event in which two chromatids—one from each homologue—exchange fragments swaps the C and c genes. Now, each homologue has two dissimilar chromatids.
One has A, B, C on one chromatid and A, B, c on the other chromatid.
The other homologue has a, b, c on one chromatid and a, b, C on the other chromatid.
This process, in which homologous chromosomes trade parts, is called crossing over. It's helped along by a protein structure called the synaptonemal complex that holds the homologues together. The chromosomes would actually be positioned one on top of the other—as in the image below—throughout crossing over; they're only shown side-by-side in the image above so that it's easier to see the exchange of genetic material.
Image of two homologous chromosomes, positioned one on top of the other and held together by the synaptonemal complex.
You can see crossovers under a microscope as chiasmata, cross-shaped structures where homologues are linked together. Chiasmata keep the homologues connected to each other after the synaptonemal complex breaks down, so each homologous pair needs at least one. It's common for multiple crossovers (up to !) to take place for each homologue pair .
The spots where crossovers happen are more or less random, leading to the formation of new, "remixed" chromosomes with unique combinations of alleles.
After crossing over, the spindle begins to capture chromosomes and move them towards the center of the cell (metaphase plate). This may seem familiar from mitosis, but there is a twist. Each chromosome attaches to microtubules from just one pole of the spindle, and the two homologues of a pair bind to microtubules from opposite poles. So, during metaphase I, homologue pairs—not individual chromosomes—line up at the metaphase plate for separation.
The phases of meiosis I.
Prophase I: The starting cell is diploid, 2n = 4. Homologous chromosomes pair up and exchange fragments in the process of crossing over.
Metaphase I: Homologue pairs line up at the metaphase plate.
Anaphase I: Homologues separate to opposite ends of the cell. Sister chromatids stay together.
Telophase I: Newly forming cells are haploid, n = 2. Each chromosome still has two sister chromatids, but the chromatids of each chromosome are no longer identical to each other.
When the homologous pairs line up at the metaphase plate, the orientation of each pair is random. For instance, in the diagram above, the pink version of the big chromosome and the purple version of the little chromosome happen to be positioned towards the same pole and go into the same cell. But the orientation could have equally well been flipped, so that both purple chromosomes went into the cell together. This allows for the formation of gametes with different sets of homologues.
In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome, however, remain attached to one another and don't come apart.
Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. In some organisms, the nuclear membrane re-forms and the chromosomes decondense, although in others, this step is skipped—since cells will soon go through another round of division, meiosis II. Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells.
Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and simpler process than meiosis I, and you may find it helpful to think of meiosis II as “mitosis for haploid cells."
The cells that enter meiosis II are the ones made in meiosis I. These cells are haploid—have just one chromosome from each homologue pair—but their chromosomes still consist of two sister chromatids. In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes.
Phases of meiosis II
Prophase II: Starting cells are the haploid cells made in meiosis I. Chromosomes condense.
Metaphase II: Chromosomes line up at the metaphase plate.
Anaphase II: Sister chromatids separate to opposite ends of the cell.
Telophase II: Newly forming gametes are haploid, and each chromosome now has just one chromatid.
During prophase II, chromosomes condense and the nuclear envelope breaks down, if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes.
The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate. In anaphase II, the sister chromatids separate and are pulled towards opposite poles of the cell.
In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes decondense. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells.
How meiosis "mixes and matches" genes
The gametes produced in meiosis are all haploid, but they're not genetically identical. For example, take a look the meiosis II diagram above, which shows the products of meiosis for a cell with chromosomes. Each gamete has a unique "sample" of the genetic material present in the starting cell.
As it turns out, there are many more potential gamete types than just the four shown in the diagram, even for a cell with only four chromosomes. The two main reasons we can get many genetically different gametes are:
- Crossing over. The points where homologues cross over and exchange genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis. If meiosis happens many times, as in humans, crossovers will happen at many different points.
- Random orientation of homologue pairs. The random orientation of homologue pairs in metaphase I allows for the production of gametes with many different assortments of homologous chromosomes.
In a human cell, the random orientation of homologue pairs alone allows for over different types of possible gametes.
When we layer crossing over on top of this, the number of genetically different gametes that you—or any other person—can make is effectively infinite.
Check out the video on variation in a species to learn how genetic diversity generated by meiosis (and fertilization) is important in evolution and helps populations survive.
Want to join the conversation?
- In meosis 2 when did the chromosomes duplicate?(10 votes)
- there was no chromosomal duplication in meiosis II only the centrosome duplicated. If there would have been chromosomal duplication cells would never have been able to produce haploid gametes the cell used in meiosis II are the product of meiosis I(46 votes)
- is there random orientation in metaphase 2?(13 votes)
- Good question!
I think that is assumed to be generally true, but it would be very hard to test in most organisms.
The only evidence for this being true that I know of comes from the fungus Neuropsora crassa that makes a linear§ ascus (sac containing the meiotic products).
This allows us to see that in this species independent assortment also occurs in metaphase II.
§Note: The order of the spores within the ascus reflects the meiotic divisions.
- Please specify if the number of chromosomes becomes haploid in meiosis I or meiosis II? And if does in meiosis I then how? In meiosis I chromatids are not separated then how come chromosome number reduces to half??(6 votes)
- The number of chromosomes becomes haploid in meiosis I, because the actual sister chromatids are not pulled apart by spindle fibers. For example, if a cell was undergoing meiosis, and had a total of 4 chromosomes in it, then 2 of them would go to one daughter cell, and 2 of them would go to the other daughter cell. That makes 2 haploid cells.
Then, in meiosis II, each of the 2 sister chromatids in the daughter cells would be split apart by spindle fibers, giving each cell 2 chromosomes. As you said, the fact that in meiosis I chromatids are not separated means that the entire chromosome is moved to one cell; if there were 4, then they would be moved to each daughter cell equally. I hope that helps; if you still have trouble please say so!(23 votes)
- If the starting cell has 46 chromosomes, then how can it produce four cells with 23 chromosomes?(5 votes)
- Remember that when replicating in interphase, the chromosome number DOES NOT CHANGE
in interphase before S (replication phase) we have 46 single stranded chromosomes: 23 are from mom and 23 are from dad (they code for the same things meaning chromosome 1 of mom codes for the same thing as chromosome 1 of dad. Likewise chromosome 5 of dad is similar to chromosome 5 of mom)
after replication how many chromosomes do we have?
answer: still 46, but what's different?
the single strand chromosome (1 chromosome) became two stranded yet attached identical sister chromatids (still 1 chromosome)
it is only when the sister chromatids separate are they each considered separate chromosomes.
This means that in meoisis II when we split the sister chromatids:
the parent cell starts with 23 chromosomes (EACH double stranded=two sister chromatids, so there are 46 chromatids. Anaphase II splits the sister chromatids which now separate (23 chromatids go to one pole and 23 chromatids go to other pole). When the chromatids are separated they are now called chromosomes
so a haploid parent cell of 23 chromosomes (double strand) just created two haploid daughter cells of 23 chromosomes (now single strand).
The above is also how a 46 chromosome (double strand) cell in mitosis can result in 2 daughter cells each with 46 chromosomes (single strand).
Even Sal admits how confusing this is, but he explains all this visually in a separate video differentiating the terms chromatid, chromosome, and chromatin.(5 votes)
- When the new nuclear membrane forms around the chromosomes, how does the cell make sure the centrosomes are outside the nucleus and ALL chromosomes are inside?(8 votes)
- Well, it works based on patterns of nuclear defragmentation. On the places where old fragments of a nucleus are, new form. Also, thanks to cytokinesis, the cell splits exactly half its length.(6 votes)
- will you please explain me all the stages of prophase-1 in meiosis
how can we find the order of stability of covalent compounds by inductive effect(1 vote)
- 1. Chromosomes condense and homologs loosely pair along their lengths, aligned by gene.
2. The paired homologs become physically connected along their lengths through a process called synapsis. This forms a synaptonemal complex.
3.The random rearrangement of corresponding genes occurs between the non sister chromatids (because at this stage each chromosome consists of two sister chromatids).
4. Synapsis ends, and the homologs move slightly apart, no longer bonded along their lengths like in the synaptonemal complex.
5. Some of these homologs have one or more chiasmata, an X shaped region where a genetic rearrangement has occurred. This formation occurs because of sister chromatid cohesion, where a gene that has been given to the homologous pair in synapsis is still bonded to the corresponding part on the sister chromatid of its former chromatid.
6. Centrosomes move to opposite ends of the cell, and the nuclear envelope dissolves.
7. Microtubules from one centrosome attach to the kinetochore (protein structures at the centromeres) of one chromosome from each of the homologous pairs, while the other centrosome connects to the kinetochore of the other chromosome in each homologous pair, and each homologous pair moves towards the metaphase plate (where they line up before anaphase).(14 votes)
- So meiosis is just to make a zygote? What happens after that? Also, why are there different processes of meiosis for sperms and eggs if they only have to join. Someone help, I'm really confused(4 votes)
- Yes, meiosis's goal is to make a zygote. This zygote will (hopefully) turn into an embryo, then a fetus, which eventually becomes a human if everything works out. Meiosis in sperm and eggs is different because, well, sperm and eggs are different. A spermatocyte needs to split into four cells, while an oocyte needs to split into only one because many sperm are needed to fertilize a single egg. Once a sperm reaches the egg, it is only then that they join.(4 votes)
- why is interphase not included as a stage of cell-division in both mitosis & meiosis?(5 votes)
- Interphase is stage of the cell cycle, but not a stage of cell division (meisosis).
Interphase is that gap phase (exactly G0) where cell cycle stops, DNA and organelles grow and synthesize.(2 votes)
- Different between karyogenisis and dikaryogenesis(3 votes)
- Karyogenesis is the formation of a nucleus. Dikaryogenesis is almost non existent on the Internet, but supposedly it has to do with the formation of 2 nucleuses, and there may be a preference in the expression of one of them.(3 votes)
- Is the only point of Meosis 2 to regulate the amount of genetic material within a haploid cell?
What I mean by this is that, in parent diploid cell, each chromosome had 1 chromatid however, at the end of Meosis 1, each chromosome in haploid cell had 2 sister chromatids which renders the amount of genetic material same in daughter and parent cell.(2 votes)
- Correct. Meisosi II is reduction division.
Because, final products of meiosis, gametes are haploid cells.
Just remember that ova and spermatozoids are haploid and than it all makes sense.
Why they are haploid?
within each meiotic cycle number of chromosomes would double, produce polyploidy and polyšloid zygote (incompatible with life). To avoid all of this, Meiosis II is a reduction.(4 votes)