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As a eukaryotic organism grows, its cells are constantly dividing and creating new cells according to the “genetic blueprint” of its DNA. The processes by which these new cells are developed are known as mitosis and meiosis. Mitosis is the method by which somatic (or non-reproductive) are created, while meiosisis the method that creates gametes (reproductive cells like sperm and eggs).
Keep in mind: prokaryotic cells do not have membrane-bound organelles like nuclei, and therefore do not undergo mitosis and meiosis as eukaryotic cells do (instead, they undergo binary fission). Throughout our discussion of mitosis and meiosis, we will be talking only about eukaryotes.
Before we get into the specifics of each process, let’s go over some AP Biology background information that will help us understand the differences between them.
Chromatin, Chromatids and Chromosomes
These are essentially the three forms of a cell’s genetic material. Chromatin is its loosest, least-organized form, which usually floats freely around inside the defined envelope of the nucleus. Chromatids are formed from condensed chromatin and serve as one-half of each chromosome. In its most complete form, two identical “sister chromatids” are joined together by a centromere to form a full chromosome.
Diploid vs. Haploid Cells
Cells come in essentially two “flavors”: diploid and haploid. As the names imply, a diploid cell contains two sets of genetic information in homologous chromosome pairs, while a haploid cell contains only one set of genetic information in single copies of each chromosome.
Non-reproductive somatic cells are diploid cells, containing two sets of chromosomes. Human cells, for example, have 23 chromosome pairs (46 total chromosomes), with one set of genetic information inherited from each of that human’s parents.
Reproductive gametes, on the other hand, are haploid cells, containing only one set of chromosomes. In humans, egg and sperm cells contain only 23 chromosomes. When gametes combine during sexual reproduction, the sets of chromosomes from both parents provide the chromosome pairs for future diploid cells.
Now that we’ve reviewed the necessary AP Bio background, let’s get to the meat of this section: the actual processes of mitosis and meiosis.
The process of cellular mitosis occurs in four primary phases: prophase, metaphase, anaphase and telophase. A fifth “phase,” known as interphase, is the state in which a somatic cell spends most of its lifespan.
Note: you will not need to know the names of these phases for the AP Biology exam, but you will still be required to describe the steps.
Take a look at how each of these phases breaks down.
Not necessarily a true “phase” of mitosis, interphase is the normal, non-division state of somatic cells. If you throw a prepared slide of cells under a microscope, chances are the majority of them will be sitting in interphase, looking relatively inactive and uninteresting. If a cell is not in interphase, it is undergoing mitosis (which is sometimes referred to as “M phase”).
Interphase itself is split into three stages, as follows:
• G1: cell simply grows
• S phase: cell continues growing, starts duplicating DNA
• G2: growth continues while cell prepares for mitotic division
This is where the action begins. As a cell prepares to divide, it enters prophase, in which the nucleoli—spherical structures inside the nucleus that contain RNA and protein—disappear and the chromatin of the nucleus condenses into tightly-packaged chromosomes. Note that because the DNA was duplicated in S-Interphase, each chromosome now contains two copies of the cell’s DNA.
The membrane that surrounds the genetic material of the cell (known as the nuclear envelope) then disappears, and a mitotic spindle is created as the microtubule organization centers (MTOCs) move toward opposite ends of the nucleus. These MTOCs are specialized structures that control the arrangement of a protein called tubulin into long microtubules that can manipulate the positioning of the cell’s genetic material. The mitotic spindle is simply the term for the overall structure of microtubules that guide this material.
As the MTOCs move apart, the microtubules they’ve built increase in length and connect to the centromeres of the chromosomes via a region called the kinetochore. The MTOCs are then capable of moving the chromosomes toward or away from the poles of the cell by shortening or lengthening the microtubules.
During metaphase, the fully-formed chromosomes are aligned by the microtubules at the center of the cell in a plane known as the metaphase plate. Then, the attached microtubules retract, splitting each chromosome into its individual sister chromatids. These resulting chromatids still have a centromere each, however, and therefore are referred to as individual chromosomes from this point forward.
Metaphase ends as soon as the original chromosomes are split.
Top tip: to determine the number of chromosomes at any time during the process, simply count the number of centromeres.
After the initial separation of the chromosomes, the new chromosomes (the split chromatids) are pulled to the poles of the cell via the shortening of the microtubules. At the end of this phase, each pole contains a complete set of identical chromosomes.
Since the DNA copies made during the S phase of interphase have now split, the chromosomes at the poles consist of single chromatids with only a single copy of the parent cell’s DNA.
To wrap-up the division process, normal cell organelles start to re-build and the newly-formed daughter cells begin to take shape for their own interphase. Nuclear envelopes develop around the genetic material at each pole, the chromosomes unwind and return to loosely-floating chromatin, and the nucleoli appear once more.
While the nucleus reforms, the dividing cell undergoes cytokinesis, which refers to the splitting of the unit and the division of cytoplasm across the two new cells. A cleavage furrow develops at the center of the dividing unit and cinches closed like a drawstring, leaving two separate cells with enclosed cell membranes.
Final result: two diploid daughter cells containing identical genetic material to the parent cell.
Because meiosis has the special task of creating new sex cells for reproduction, its process is unique, though similar to mitosis in many ways. Meiosis essentially goes through the stages of mitosis twice, with some key variations.
Perhaps the most important thing about meiosis is that it enables the independent assortment of genetic material. The determination of which chromosomes end up in which gametes is random, allowing for natural variation in the gene pool. It is this variation and biological diversity that keeps species naturally resilient.
Now that you’re inspired by the beauty of natural genetic diversity, let’s discuss how it happens.
This phase begins similarly to prophase in mitosis, with the nuclear envelope breaking down and the chromatin condensing into chromosomes. In meiosis, however, homologous chromosomes pair up into groups of four chromatids (known as tetrads or bivalents) in a process called synapsis.
During synapsis, genetic material may cross over between non-sister homologous chromatids (chromatids that are not connected by a centromere and are therefore not part of the same chromosome).
Next, homologous chromosome pairs are arranged at the metaphase plate. Instead of a line of single chromosomes, as in mitosis, meiosis sees a line of pairs. Microtubules from each pole then attach to the kinetochore of one chromosome from each pair.
This next phase starts as soon as the tetrads begin to separate. Like in mitosis, the separate chromosomes are pulled by the microtubules to opposite ends of the cell. Unlike mitosis, however, these chromosomes still comprise two sister chromatids.
The first half of the process completes with the formation of nuclear membranes around the chromosomes at the poles. Unlike in mitosis, the cleavage furrow does not yet develop. Note that once this process repeats to form the final four daughter cells, the resulting cells will be haploid.
The fun starts again with prophase II, in which the two newly-formed nuclear envelopes break down again and the mitotic spindle forms. This time, there is no crossing over.
Metaphase II is nearly identical to metaphase in mitosis, with single chromosomes aligning at the metaphase plate. In this case, however, there is half the number of chromosomes present as in mitosis.
Just like metaphase II, anaphase II mirrors the happenings of anaphase in mitosis, but with half as many chromosomes. Each single chromosome is pulled apart by microtubules and the new chromosomes (formerly sister chromatids) are pulled to opposite poles.
The entire process wraps up in telophase II. Four new nuclei form and cytokinesis occurs to form the four final cells. Note that the resulting cells’ chromosomes comprise only one chromatid each, and even when these are replicated during the S phase of interphase the haploid cell will still only contain half the number of chromosomes of the parent cell.
Final result: four haploid daughter cells, each containing copies of half the genetic material of the parent cell.
• Mitosis creates two diploid somatic daughter cells that are clones of the parent cell.
• A somatic cell spends most of its time in interphase, growing and replicating DNA in preparation for mitosis.
• The four phases of actual mitotic division are prophase, metaphase, anaphase and telophase. These names will not need to be memorized for the AP Biology exam.
• Meiosis creates four haploid gamete daughter cells, each containing half of the original cell’s genetic material.
• The phases of meiosis vary in a few key ways from those of mitosis, but follow the same general phase order twice. Again, the names of the phases will not need to be memorized.
That’s all there is to it! Can you describe each of the stages and key structures of mitosis and meiosis for the AP Bio exam?
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