Mitosis and Meiosis
In this page, we will be discussing sexual and asexual cellular reproduction - or meiosis and mitosis.
Mitosis is an asexual process which begins with a parent cell and ends with two genetically identical daughter cells. Meiosis is a sexual process which begins with one parent cell and ends with four genetically different daughter cells.
I honestly have no idea why their names are so similar, though... bro replaced the "it" with the "ei" and made a whole new word. It's giving copy and paste 💀
But either way, let's get started.
Mitosis and meiosis are part of the cell cycle. The cell cycle is the cycle of life of an average cell, and it has two large phases - interphase and the mitotic phase. I put a picture of it below.
Image credit: Khan Academy
The average cell, as you can see above, spends most of its time in interphase. First, in the G1 phase, it grows. Then, it encounters a checkpoint. Before it can go to the S phase of interphase, it must pass a routine analysis of its function, to see if it is growing correctly and is stable enough to continue. The G1 checkpoint is the most important - if a cell passes this checkpoint, it is very likely the pass all the others. Once this checkpoint is passed, the cell moves into the S phase, where DNA replication begins. This replication is done in preparation for the mitotic phase. After it passes the S checkpoint, the cell moves into the G2 phase where it grows and performs its average functions. Lastly, the it must pass the G2 checkpoint to begin mitosis or if it is a reproductive cell, meiosis. The cell enters the mitotic phase, where it goes through the process of splitting in two, and then two daughter cells are formed, allowing the cell cycle to start again.
Let's learn some vocabulary before we get into mitosis - first, let's see what a chromosome is. A picture of a chromosome is shown to the right. A chromosome consists of two sister chromatids, a centromere, and kinetichore fibers (protein complexes that help the centromere hold the chromosome together). DNA is not always in chromosome form. Most of the time, DNA just floats around, called chromatin. Humans have 22 pairs of autosomes (chromosomes that do not determine sex) and 2 sex chromosomes, which are the X and Y chromosomes. Females have 2 X chromosomes and males have 1 X and 1 Y chromosome. The locus of a chromosome is the location of a gene on the chromosome. And lastly, homologous chromosomes are chromosomes that are similar in length, and size, and the genes are at similar loci. Now that we have the background information we need, we can explore mitosis.
Mitosis
Mitosis has four phases - prophase, metaphase, anaphase, and telophase, which is followed by cytokinesis. However, the lines between telophase and cytokinesis are kind of blurry, so I grouped them into one in the following pictures.
Mitosis is a process of asexual reproduction - it occurs without a partner and produces a clone of the cell. Mitosis is used in the body for growth and maintenance, and it is an extremely important cellular process. It produces two diploid cells - cells with two sets of 23 chromosomes.
Prokaryotes have their own process called binary fission to reproduce asexually. In this process, the chromosomes are duplicated and separated. The cell elongates and divides into two daughter cells.
The amount of chromosomes an organism has actually has nothing to do with how complex they are! A fruit fly has 8 chromosomes. A pea has 14 chromosomes. A human has 46 chromosomes. But a potato has 48 chromosomes and a goldfish has 94!
Prophase
As the first phase of mitosis, in this phase, chromosomes form, centrosomes appear, and mitotic spindles form. The nuclear membrane is taken apart. Both the kinetichore fibers mentioned earlier and the polar fibers (the ones that appear from each end of the cell) are formed.
Metaphase
This phase is fairly simple. The chromosomes line up in the center of the cell, and the mitotic polar fibers latch onto them.
Anaphase
In this phase, the chromatids separate from the centromeres and are taken to each side of the cell. They are pulled apart by the mitotic spindle fibers.
Telophase and Cytokinesis
In telophase, the mitotic spindle fibers disassemble. The nuclear envelope reforms and the DNA unravels into chromatin. In animal cell cytokinesis, a cleavage furrow is formed and the cell is pinched in half. In plant cell cytokinesis, vacuoles containing cell wall material line up in the center of the cell. These vacuoles fuse together to separate the two daughter cells.
Meiosis
Mitosis has eight phases, and they kind of repeat - prophase I, metaphase I, anaphase I, and telophase I and cytokinesis, followed by prophase II, metaphase II, anaphase II, and telophase II and cytokinesis.
Meiosis, unlike mitosis, is a process of sexual reproduction - it produces genetic variability and gametes (egg and sperm cells). Meiosis is used in the body for the creation of gametes. It produces four haploid cells - cells with one set of 23 chromosomes.
Prophase I, however, is actually quite a complicated step. In this step, the chromosomes which have been replicated do something called "crossing over." In this step, homologous chromosomes touch each other and cross over - genes are traded between these chromosomes. The site of this crossover is called a chiasma, and this step is super helpful in genetic variability. Let's explore all the steps of meiosis together.
Prophase I
In this step, first of all, chromosomes form, centrosomes appear, and mitotic spindle fibers form in this step. The nuclear membrane is disassembled. However, like I just said, crossing over occurs, which is a process where homologous chromosomes exchange genes among each other, producing genetic variability. These chromosomes hold onto each other.
Metaphase I
This step is very similar to the previous one in mitosis. The crossed-over chromosomes line up in the center of the cell, and the polar mitotic spindle fibers latch onto them.
Anaphase I
The spindle fibers pull the crossed-over chromosomes apart and towards either side of the cell.
Telophase I and Cytokinesis
The chromosomes gather at the poles of the cell and the cell divides via cytokinesis. The mother cell and split and formed two daughter cells with the same amount of chromosomes - however, these daughter cells are not genetically identical due to the crossing over process.
Prophase II
Now, prophase II must begin. This process is now basically the same as mitosis. A new set of spindle fibers forms and the process begins again.
Metaphase II
In this step, the chromosomes line up in the center of the cell. The spindle fibers grab onto them.
Anaphase II
The sister chromatids are pulled apart by the spindle fibers towards either end of the cell.
Telophase II and Cytokinesis
Lastly, to conclude meiosis, the cell splits in two, producing four total haploid cells that are genetically different.
Cell division is very regulated inside the body. With all the checkpoints and the careful considerations, the body tries very hard to watch over all the multiple trillion cells in your body... yeah, pretty much everyone can see that they're going to miss something, - and they do miss things - something like cancer.
Cancer, quite literally, is uncontrolled cell division. It is when a cell in your body reproduces way too much, forming a disorganized junkyard of cells. Cancer can either form benign or malignant tumors. Benign tumors don't grow or move, but malignant tumors are harmful. They grow, move, and try to get to your bloodstream, where they travel wherever they would like within your body.
But cancer and the cells in your immune system that fight it are always in a constant battle. Watch this video for a much more detailed explanation on cancer, how it forms, and how the immune system fights it:
https://youtu.be/zFhYJRqz_xk?si=1tkNnBiNGPo1ABIM
As for image credit, whichever pictures I did not cite are pictures I drew myself. Unfortunately, that is all the information I have for you today.. but don't fret! I will be making more pages next week :)
As always, thank you for reading and I'll see you in the next one!