Mitosis
Mitosis is the process where a mother cell is divided into two genetically identical daughter cells. Mitosis allows an organism’s body to grow by populating the body with cells and replacing old cells with new cells.
Cells go through 4 main phases during mitosis: prophase, metaphase, anaphase, and telophase.
In prophase, the chromosomes start to condense, and the nuclear envelope starts to break down and disappears, thus allowing chromosomes to move freely. Also, in late prophase, microtubules called spindle fibers start to grow.
In metaphase, spindle fibers bind to the chromosomes at the kinetochores, which are proteins found on the centromere, and chromosomes that are bound with spindle fibers line up in the middle as a single file line.
In anaphase, sister chromatids are separated and pulled towards the opposite sides of the cytoplasm by spindle fibers. Meanwhile, spindle fibers that are not attached to chromosomes lengthen and make the cell longer.
In telophase, the cell starts to reestablish its structures. Spindle fibers disappear, chromosomes de-condense, and two new nuclei form.
The additional step that the cell goes through at the end is cytokinesis. During cytokinesis, the cell’s cytoplasm is divided, and two new cells are formed.
Meiosis
Meiosis is a type of cell division that produces gametes, which are sex cells. Diploid cells are cells with two sets of chromosomes (one from each parent), and haploid cells are those with one set of chromosomes. For fertilization to occur, two haploid gamete cells must be united to create one diploid cell. In meiosis, one diploid mother cell produces four unique haploid daughter cells through two rounds of division: Meiosis I and Meiosis II. Meiosis I goes through Prophase I, Metaphase I, Anaphase I, and Telophase I. Meiosis II goes through Prophase II, Metaphase II, Anaphase II, and Telophase II.
Before meiosis occurs, during interphase, DNA in chromosomes are replicated into two copies. These copies are called sister chromatids, and two sister chromatids are held together as one chromosome.
Meiosis I
During Prophase I, chromosomes condense, the nuclear envelope disappears, and spindle fibers form. Homologous chromosomes pair up with each other. Homologous chromosomes are two chromosomes (one from each parent) that carry the same type of genes. These homologous pairs exchange genetic information with one another and create recombinant chromosomes. This process of DNA exchange is called Crossing Over, and it contributes to the genetic variety among offsprings.
During Metaphase I, the homologous chromosomes line up as pairs in the center of the cell. The order in which chromosomes are aligned within a homologous pair is random, which is called Independent Assortment. This also contributes to genetic variety.
During Anaphase I, spindle fibers pull the homologous chromosomes apart, while each chromosome’s sister chromatids remain attached to each other.
During Telophase I, the separated chromosomes arrive at opposite poles. Depending on the species, chromosomes may de-condense and nuclear envelopes may form during this stage.
Meiosis I ends with cytokinesis, which divides the cytoplasm into two haploid cells. Each chromosome in these cells still has two sister chromatids.
Meiosis II
The two cells produced from Meiosis I each go through Meiosis II, which mostly resembles the steps in mitosis.
During Prophase II, chromosomes condense again, nuclear envelopes disappear, and spindle fibers form.
During Metaphase II, chromosomes line up in the center of the cell. Unlike Metaphase I, chromosomes line up in a single line rather than in homologous pairs. Spindle fibers attach to individual kinetochores of each sister chromatid.
During Anaphase II, spindle fibers pull away sister chromatids to opposite poles.
During Telophase II, chromosomes arrive at opposite poles. Chromosomes de-condense and nuclear envelopes form around each set of chromosomes.
Meiosis II ends with cytokinesis dividing each cell’s cytoplasm. The two haploid cells produced from Meiosis I ultimately produce four unique haploid cells.
Onion Root Experiment
We can directly observe mitosis through the onion root experiment. In order to conduct the experiment, we would need to prepare the following materials: a microscope, glass slides, cover slips, onion root, 1M of HCl solution, an alcohol lamp, an acetocarmine stain, razor blades, a pipette, forceps, acetic acid, and ethanol.
First of all, grow onion roots, cut them into small pieces, and place them in aceto-alcohol solution for 24 hours.
Next, put a drop of 1M of HCl on the root tip, expose it for 10 minutes, remove 1M HCl from the slide, and put 2-3 drops of acetocarmine stain.
Warm the slide over the alcohol lamp for 8-10 minutes. Make sure that the slide does not get too hot. Wash off the stain with water, cut the most stained part of the root, and get rid of the remaining portion. And then, place a coverslip on the root tip, not allowing any air bubbles.
Finally, observe the onion root with a compound microscope in 10x objective (switch to 40x objective for better view).
Below are pictures of each phases of mitosis we are expected to see as a result.
Interphase Prophase Metaphase Anaphase Telophase
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