Cell Cycle
Animals and plants and the cells within their structures go through cell division. This cycle consists of 4 stages in which the cells first go through growth before division.
G1 Stage
This is the period of cell division which occurs before DNA replication takes place. In this stage the cell grows and absorbs the nutrients and organisms such as the mitochondria and chloroplasts are duplicated.
S Stage
This phase is dedicated towards making sure the daughter cells will have a full set of chromosomes. Each of the 46 chromosomes go through DNA replication to replicate the parent cell’s information.
G2 Stage
The cell makes sure DNA replication has occurred properly and grows some more before becoming ready to do cell division.
Introduction to Chromosomes
Chromosomes help to store the DNA, a molecule that carries all the hereditary information of an organism. During the interphase process, the longest stage of the cell cycle, the DNA is spread out or decondensed, a form known as chromatin. However, when cell division begins the chromatin is condensed by being wrapped around histone proteins to form condensed chromosomes.
Mitosis Approach
Mitosis is the cell division used by somatic cells and many unicellualar organisms for asexual reproduction. This process results in 2 daughter cells that are genetically identical to the parent cell. Before interphase cells start out with 46 chromosomes however after interphase there are still 46 chromosomes but 92 chromatids (copies). Therefore, the resulting daughter cells will end up with a diploid set of chromosomes in other words, 46 chromosomes.
Prophase
The first step of mitosis is known as prophase. At the beginning of prophase, the chromatin starts to condense into chromosomes to make it easier for the DNA to be moved around during division. Furthermore, the centrosomes, an organelle found only in animal cells, forms microtubules. The spindle fibers (microtubules) extend from the centrioles. The nuclear envelope dissolves to allow for the division to occur. Later in prophase, the microtubules bind to the kinetochore, a patch of protein found at the centromere of each chromosome.
Metaphase
In metaphase, all of the chromosomes are lined in the middle of the cell along an invisible plate called the metaphase plate. Before the next step occurs, the cell makes sure that all chromosomes are attached properly to the microtubules and are aligned properly. This is called the spindle checkpoint.
Anaphase
During anaphase, the sister chromatids of each chromosome are pulled apart by the spindle fibers towards the poles of the cell. If the chromosomes fail to separate properly nondisjunction occurs. Each daughter cell receives an uneven amount of chromosomes.
Telophase
At telophase, the cell is pretty much done dividing and the formation of the 2 daughter cells occurs. The microtubules are broken down and the nuclear envelopes start to reappear around each set of chromosomes. Finally, the chromosomes once again decondense back into chromatin.
Cytokinesis
Cytokinesis occurs differently between both plant cells and animal cells. Animal cells form a cleavage furrow and separate into 2 daughter cells. However, plant cells have cell walls and therefore a cell plate forms down the middle of the 2 cells to divide them
Meiosis Approach
Meiosis is the cell division used by germline cells to form sex cells or gametes such as sperm and egg cells. This process is different from Mitosis in that four haploid daughter cells are produced from diploid germ line cells. Before interphase cells start out with 46 chromosomes however after interphase there are still 46 chromosomes but with 92 chromatids. After the end of the first division, Meiosis I, 23 chromosomes are in each cell with 46 chromatids. At the end of the second division, Meiosis II, 23 chromosomes are in each of the four daughter cells but with 23 chromatids. Furthermore, Meiosis allows for genetic variation resulting in unique organisms instead of identical organisms.
Meiosis I
There are many similarities between the mitosis cell division and meiosis cell division. During prophase I, the chromosomes condense and the nuclear envelope dissolves. Each chromosome is an X-shape consisting of two identical chromatids. A significant difference is seen in this phase when the homologous pairs of chromosomes line up together and undergo crossing over. This allows for genetic diversity for the haploid cells produced in the end. Finally, similar to mitosis, the centrosomes and microtubule structures start to form.
Unlike mitosis where the chromatids are split apart, in meiosis the homologous chromosomes are split apart by the spindle fibers. So, in Metaphase I the homologous chromosomes will line up in the middle and form a tetrad. Each chromosome is split apart in Anaphase I and then in Telophase I and cytokinesis the 2 diploid daughter cells with differing DNA form.
Meiosis II
Meiosis II is similar to mitosis except that the parent cells are diploid. Each chromosome will divide in the end at the centromere like in mitosis. The resulting 4 haploid daughter cells will be genetically different due to the crossing over aspect of meiosis.
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