Humans

Meiosis 

 

Humans go through the process of meiosis in order to reproduce. This is the process in which haploid gametes are formed. They go through two stages of division, meiosis I and meiosis II. These stages are similar to the process of mitosis, however in mitosis the chromosome number is the same in both parent and daughter cell (46) after division, where as in the process of meiosis the gametes produced will have half the number of chromosomes as the parent cell (23). This means that the gametes produced through meiosis only have 1 set of chromosomes indicating that they are haploid (n). Because human offspring receive 2 sets of chromosomes (one from their mother and one from their father) they are haploid organisms. However, this does not mean that every human is half of their mother’s genetics and half of their father’s. Each parent contributes 23 chromosomes to the offspring all of which are homologous.

                  The process of meiosis involves 2 stages the first is called Meiosis I. Meiosis I begins with Prophase. In this stage the cell is diploid, the chromosomes condense, the nuclear envelope of the cell dissolves and spindle fibers begin to form. The chromosomes begin to line up in pairs, one fraternal and one maternal, this is called synapse. Each homologous pair crosses over and shares genetic information that will determine certain traits the offspring will poses such as eye colour, however the sex chromosomes (X and Y) is the one set of chromosomes that virtually do not cross over but by the end of Prophase I do slightly. The second stage is Metaphase I, in this stage the spindle fiber attach to the chromosomes pulling them to the center of the cell where they line up in a random pair with another chromosome opposite to them. During anaphase I the chromosomes are pulled to opposite sides of the cell by centrioles. The next phase is Telophase I, in this phase a cleavage furrow is formed and the cells nuclear envelope begins to reform. At this point Cytokinesis is preformed, which is the physical splitting of the cell. Also after this stage the DNA becomes decondensed. From here the second stage of Meiosis beings called Meiosis II. The first stage of Meiosis II is Prophase II, here the chromosomes condense during Metaphase II the spindle fibers attach and the chromosomes line up in the middle of the cell. From here is Anaphase II where the centrioles pull the chromosomes to opposite poles of the cell. Next is Telophase II in this phase a cleavage furrow is formed, the cells nuclear envelope reforms and chromosomes decondense. The cells then undergo Cytokinesis where 4 genetically different haploid cells are created. Note that if a persons sex chromosomes are XX they are a female and if the chromosomes are XY they are male (Biology 11, 2011).