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You started off as a fertilized cell inside your mom, called a zygote. Now, you’re a thriving community of hundreds of millions of cells, all working together towards a common purpose: to keep you alive. How did so many cells come from just one?
Generally speaking, the answer is straightforward: many cells come from just one by repeated cell division. Your first form as a zygote split to make two cells. Then those cells split, making four...and so on and so forth, until you became the living, functioning organism you are today.
There are two ways cell division can happen in humans and most other animals, called mitosis and meiosis. When a cell divides by way of mitosis, it produces two clones of itself, each with the same number of chromosomes. When a cell divides by way of meiosis, it produces four cells, called gametes. Gametes are more commonly called sperm in males and eggs in females. Unlike in mitosis, the gametes produced by meiosis are not clones of the original cell, because each gamete has exactly half as many chromosomes as the original cell.

The concept of a chromosome

A chromosome is a thread-like object (scientists literally called them threads or loops when they were first discovered) made of a material called chromatin. Chromatin is made of DNA and special structural proteins called histones. One way to think of a chromosome is as one very long strand of DNA, with a bunch of histone proteins stuck to it like beads on a string.
Figure of a chomosome, chromatin fiber, histones, nucleosome, and DNA
Chromosomes are stored in the nuclei of cells. If you compare the diameter of a cell nucleus (between 2 and 10 microns) to the length of a chromosome (between 1 and 10 centimeters, when fully extended!), you can see that a chromosome must be scrunched up into a very small package in order to fit inside a nucleus. Actually, the average chromosome is about a thousand times longer than a cell nucleus is wide. The situation is a bit like how a very long snake can coil up into a tight ball.
Illustration of an uncoiled and coiled snake
The basic construction of chromosomes (made of chromatin) and structure (long but scrunched up) is the same in all animals. The difference is that each species has its own set number of chromosomes. For instance, all human cells (except gametes) have 46 chromosomes. Cells of nematodes (worms), other than gametes, have 4 chromosomes. The number of total chromosomes in the non-gamete cells of a particular species is called the diploid number for that species. The diploid number of humans is 46, and the diploid number of nematodes is 4.
Figure of human and nematode diploid and haploid counts
The total number of chromosomes in the gametes of a particular species is referred to as the haploid number of that species. This number is always half of the diploid number. For instance, the haploid number in humans is 23, and the haploid number in nematodes is 2.

The concept of mitosis

The purpose of mitosis is to make more diploid cells. It works by copying each chromosome, and then separating the copies to different sides of the cell. That way, when the cell divides down the middle, each new cell gets its own copy of each chromosome.

The phases of mitosis

Diagram of the five phases of mitosis
In the first step, called interphase, the DNA strand of a chromosome is copied (the DNA strand is replicated) and this copied strand is attached to the original strand at a spot called the centromere. This new structure is called a bivalent chromosome. A bivalent chromosome consists of two sister chromatids (DNA strands that are replicas of each other). When a chromosome exists as just one chromatid, just one DNA strand and its associated proteins, it is called a monovalent chromosome. Here is a drawing of what happens in a nematode nucleus (diploid number 4) during interphase, with individual chromatids represented as numbers, sister chromatids as the same number, and the centromere represented as a “-”.
The second and third steps of mitosis organize the newly created bivalent chromosomes so that they they can be split in an orderly fashion. A lot of care has to be taken with this process, because unequal splitting of chromosomes creates malfunctioning cells. Down syndrome is one disease that results from unequal splitting of chromosomes.
In the second step, prophase, the bivalent chromosomes condense into tight packages, the mitotic spindle forms, and the nuclear envelope dissolves. Imagine the difference between a slinky fully stretched out, and a slinky that has been pressed back together. That's what happens to chromosomes during prophase: they get pressed together into tight packages.
In the third step of mitosis, called metaphase, each chromosome lines up in a single file line at the center of the cell. By this point in time, the membrane enclosing the nucleus has dissolved, and mitotic spindles have attached themselves to each chromatid in all the chromosomes. Here is a diagram of what a nematode cell nucleus looks like after prophase and metaphase.
In the fourth step, anaphase, the mitotic spindles pry each chromatid apart from its copy, and drag them to the opposite side of the cell. Four bivalent chromosomes become two groups of 4 monovalent chromosomes.
Diagram of prometaphase
Once anaphase is over, the heavy lifting of mitosis is complete. In the final phase, telophase, membranes form around the two new groups of chromosomes, and the mitotic spindles that provided the power to create these groups are disassembled. Once mitosis is complete, the cell has two groups of 46 chromosomes, each enclosed with their own nuclear membrane. The cell then splits in two by a process called cytokinesis, creating two clones of the original cell, each with 46 monovalent chromosomes.
Diagram of metaphase
Diagram of anaphase
Diagram of telophase and cytokinesis

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  • piceratops ultimate style avatar for user Spope
    What is the difference between a chromosome and a chromatid. We often see pictured the 23 pairs of chromosomes in a human Karyotype. Do BOTH of the copies of each chromosome participate in Mitosis? So in Interphase are their actually TWO pairs of each chromosome, giving a total of 92 Chromosomes ( in a human)? Is this correct? This is the part that has always been the most difficult for me to grasp.
    (15 votes)
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    • duskpin ultimate style avatar for user Tyler Hall
      I think another way to think about it is remembering the difference between "sister chromatids" and "homologous chromosomes". In a regular somatic cell (before DNA is replicated in the S phase), there are 46 chromosomes - 23 of each kind as well as their homologous opposite. I.e. - you have an 'A' chromosome (1 'A' chromatid) and an 'a' chromosome (1 'a' chromatid), 'B' and 'b', 'C' and 'c' and so on, each coming from a different parent. 'A' and 'a' are still 2 different chromosomes, though they are homologous to each other (code for the same genes).

      When DNA is replicated, you now have 2 copies of the 'A' chromosome (or 2 'A' chromatids) and 2 copies of the 'a' chromosome (2 'a' chromatids), 2 'B' and 2 'b', and so on. However, the 2 'A' chromatids are still linked together by the hip, and thus are considered to still be only one chromosome. In this way, you do have 92 chromatids, but still only 46 chromosomes. Or, you could say you have 46 pairs of sister chromatids, but still 23 pairs of homologous chromosomes.

      This might help a little more:

      Somatic cell (before S phase): 46 chromosomes, 46 chromatids, 23 pairs of homologous chromosomes, 0 pairs of sister chromatids

      For a male, this would look like: A a, B b, C c, D d, E e, F f, G g, H h, I i, J j, K k, L l, M m, N n, O o, P p, Q q, R r, S s, T t, U u, V v, W w, X Y

      Somatic cell (after S phase, before mitosis/meiosis): 46 chromosomes, 92 chromatids, 23 pairs of homologous chromosomes, 46 pairs of sister chromatids

      In a male this would look like: AA aa, BB bb, CC cc, DD dd, EE ee, FF ff, GG gg, HH hh, II ii, JJ jj, KK kk, LL ll, MM mm, NN nn, OO oo, PP pp, QQ qq, RR rr, SS ss, TT tt, UU uu, VV vv, WW ww, XX YY

      Germ cell (after meiosis II): 23 chromosomes, 23 chromatids, 0 pairs of homologous chromosomes, 0 pairs of sister chromatids.

      This could look like: A, b, C, D, e, f, g, H, i, j, K, L, M, n, o, p, q, R, s, T, U, v, w, X

      The most important thing to remember that will clear up the confusion, I think, is that after DNA is replicated in S phase, the 2 sister chromatids (or 2 copies of each chromosome) are still linked together and still considered to be just 1 chromosome. But hopefully the rest helps clear up some things as well.
      (49 votes)
  • blobby green style avatar for user stpeterlauren
    The misconception in many of the comments below is that the article, and its diagrams, are depicting meiosis, when they are actually describing MITOSIS. The whole point of mitosis is to make exact copies of the parent’s cells, so that each cell produced has the same genetic information as its parent cell. In humans, the case is that the each of the 46 chromosomes will be present in the daughter cells after mitosis. For our nematodes example, each cell after MITOSIS should contain four chromosomes given that its diploid number in four. One of the most confusing aspects of the cell cycle is terminology, and you have to be very careful when using it. The words chromosome and chromatid can be referring to several different stages of DNA strands throughout the cell cycle. So, make sure to know the exact state of the DNA strand you are describing.

    During MITOSIS, the parent, diploid (2n), cell is divided to create two identical, diploid (2n), daughter cells. This occurs by undergoing DNA replication (in S phase during interphase) where the monovalent chromosome is duplicated so that it will have two DNA strands that are replicas of each other. These two strands are each now called a sister chromatid, and the two sister chromatids make up a divalent chromosome. This parent cell has a diploid number of 4 because there are four chromosomes present in an autosomal cell.
    Skipping to anaphase, the four sets of divalent chromosomes, which were lined up during metaphase, are each split at the centromere as the spindle fibers pull the now four monovalent chromosomes to each end of the cells. Before cytokinesis, there is a total of eight monovalent chromosomes in one cell, with four chromosomes on each end of the cell. After cytokinesis, the ploidy of the daughter cells remains the same because each daughter cell contains 4 chromatids, as the parent cell did. These daughter cells contain single stranded chromatid, but that does not affect the ploidy (as I commonly confused) because eventually, these cells will undergo DNA replication and have double stranded chromosomes- which are sister chromatids.

    During meiosis I, however, the parent, diploid (2n), germ cells are divided to create two haploid (n) daughter cells. This means that in nematodes, the parent cells will contain 4 total chromosomes, but the daughter cells will only have 2. Important terminology here is homologous pairs chromosomes, or homologues. Homologues consist of two sets of chromosomes, one from the mother and the other from the father. These homologues are similar in shape, size and type of genetic information they contain, but are not identical in the alleles they carry. The nematodes have 2 sets of homologous chromosomes (for a total of 4 chromosomes), whereas humans have 23 homologues (for a total of 46 chromosomes).
    The process by which meiosis I occurs is different than mitosis because homologous pairs of chromosomes (called tetrads) are lined up during metaphase I, rather than single divalent chromosomes. These homologous pairs are split apart, and the maternal homologue goes to one pole, while the paternal homologue goes to the other. Therefore, after anaphase I, the daughter cells will contain only one of the two homologous chromosomes, ultimately reducing the overall number of chromosomes present in the daughter cells. It is then during meiosis II, where the newly formed haploid (n) daughter cells are equationally divided by separating the sister chromatids of the homologue, similar to mitosis. The end result is four haploid daughter cells, called gametes.
    (8 votes)
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  • leafers ultimate style avatar for user ff142
    In the meiosis diagrams, two groups of two tetravalent chromosomes are shown, not two groups of two bivalent chromosomes. The diagram should show two sets of homologous chromosomes, not four, and homologous chromosomes should not be connected to each other by a centromere. Each cell after meiosis I should have two bivalent chromosomes with the chromosome numbers 1 & 2, not two tetravalent chromosomes with different chromosome numbers for the different cells (1 and 2 for one cell and 3 and 4 for the other cell), whatever organism it is wouldn't be able to survive in that case.
    (6 votes)
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  • old spice man green style avatar for user rhyshannaford93
    I understand this, but if someone could explain this conceptual problem it would be very much appreciated.
    If the division of chromosomes in meiosis allows some chromosomes to be in some gametes cells and others in other gamete cells (as shown in the first couple diagrams of the meiosis section), then how can gametes posses the correct type and number of 26 chromosomes. Basing off the diagram, it seems that 2 and 4 chromosomes are in one gamete whilst lacking 1 and 3.
    (3 votes)
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    • blobby green style avatar for user Autumn Nelson
      So in meiosis there are two divisions. The first division there are still 2 copies of each chromosome. During the second division, they split so there is only one copy of each chromosome, each one not identical to the other. A normally body cell (humans is 46) contains 2 copies of each chromosome, gametes contain 1 copy of each, therefore has half the chromosomes. Because multiplication happens in both meiosis and mitosis, in meiosis you end up with 4 cells, each containing different genetic information but one of each chromosome and in mitosis you get 2 cells containing identical information with pairs of chromosomes. Does that help?
      (3 votes)
  • leaf green style avatar for user Tadeniji.durls
    What is interphase part in the replication and division of the cell
    (3 votes)
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    • leafers ultimate style avatar for user aditi
      Interphase, in very simple terms, is cell growth. during interphase, the cell prepares for cell division by producing new organelles, replicating the DNA, and preparing for mitosis/meiosis. Interphase doesn't have a part in the division of the cell. Replication is one part of interphase.
      (1 vote)
  • male robot hal style avatar for user shreypatel0101
    Can anyone explain me the last part of the article i.e down syndrome?
    It says that bivalent chromosomes during meiosis II seprate, but there are no bivalent chromosomes (a bivalent is also called tetrad, that is a homologous chromosome is called is called bivalent)
    Please help..!
    (1 vote)
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    • starky ultimate style avatar for user ♪♫  Viola  ♫♪
      So, see how the product of meiosis is 4 gametes which have one copy of each chromosome (monovalent)? In other words, gametes are not supposed to have two sisters chromatids for each chromosome. You're supposed to end up with four little eggs with one copy of chromosome 21, so that the sperm can complete it with its one copy of chromosome 21. In trisomy 21, one gamete contributes 2 copies of ch.21 while the other contributes 1, so you get a zygote with 3 copies.

      In down syndrome, during the step of meiosis where sister chromatids in one cell are pulled apart to form two cells with one chromatid each [the last step in this image: https://biologypost.files.wordpress.com/2013/01/meiosis.jpg], the two sisters do not separate! So one of the cells will get no copy of chromosome 21 while one cell gets 2 copies of chromosome 21 (bivalent). The one with no chromosome 21 is not viable at all. But if a sperm comes along and fertilizes the one with the 2 copies of chromosome 21, then it adds its own copy to the egg, thus the zygote now has 3 copies of chromosome 21, hence the name "trisomy 21".

      Honestly, forget about the monovalnt and bivalent wording. I've never seen it be used in my textbooks, so it's probably not common terminology.

      Here's what I just explained in a simple picture: http://4.bp.blogspot.com/_JI1iL2842nw/SR2FnYjLKYI/AAAAAAAAAEQ/fJSQc5bUfpE/s320/nondis_m22964.gif
      (4 votes)
  • blobby green style avatar for user jsanc153
    I'm still confused about Mitosis. You start with 46 chromosomes (92 chromatids) and then the chromatids replicate and make 46 pairs of chromosomes which will eventually divide through the rest of mitosis making 2 daughter cells, each with 46 chromosomes (23 pairs )? Am I understanding this correctly?
    (2 votes)
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  • starky sapling style avatar for user purpleowl123
    What exactly does the terminology tetrad mean?
    (1 vote)
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  • blobby green style avatar for user RY4N
    Why is this so interesting?
    (1 vote)
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  • blobby green style avatar for user rosmin
    a cell has 8 chromosomes.after mitotic cell division the number of chromosomes in the daughter cells will be
    (1 vote)
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