The Four Stages Of Meiosis

Meiosis is specialized cellular division of sex cells. This type of cellular division occurs in single and multi-cellular organisms that undergo sexual reproduction. This process is split into two cycles: Meiosis I and Meiosis II. Prior to the start of meiosis, interphase occurs. Although interphase is not a stage of meiosis it is a vital preparatory step. It allows cellular growth, DNA replication and prepares for cellular division. Each cycle of meiosis is broken down into four stages for a total of eight stages. Meiosis I is composed of prophase I, metaphase I, anaphase I, and telophase I. Meiosis II is a repeat of each phase of meiosis I. Once meiosis is complete, the result will be four haploid daughter cells meaning that each daughter

cell will only have half the amount of chromosomes as the diploid parent cell. These haploid cells will mature and turn into gametes. Each phase occurs in succession but is a clearly defined process.
During prophase I, chromosomes coil or condense.

One chromosome has been donated from each parent cell in order to create a homologous chromosome pair. These chromosomes have identical lengths and gene placement but can contain different alleles. When homologous chromosomes attach at the centromere they create a tetrad, which is defined as a pair of sister chromatids. Once the sister chromatids are attached, the non-sister chromatids participate in crossing over. Crossing over is the transfer of genetic information in order to create greater genetic variability. In metaphase I, the centromere of each tetrad attaches to spindle fibers. These spindle fibers slowly shift the tetrads position to the center of the cell until they are side by side. Immediately after they line up, homologous chromosomes are separated by microtubules called kinetochore fibers that are used to pull sister chromatids to opposite poles of the cell. Once the sister chromatids are on opposite poles of the cell, anaphase I is complete and the cytoplasm of the cell begins to separate. This is known as cytokinesis and occurs during telophase. Once meiosis I is complete, meiosis II begins and repeats each step, however, instead of two haploid cells there will be a total of

four.
Mendel’s Laws of inheritance are concepts that explain how variability occurs within gametes. Mendel’s First law, the principal of segregation, states that diploid organisms separate during the formation of sex cells in order to create genetic variability. For example a person that is heterozygous for a trait has two alleles that are responsible for eye color. The alleles will separate into two different cells during meiosis in order to increase the chances of genetic variability. Independent assortment, Mendel’s second law, states that gametes can end up with any combination of parental chromosomes. Simply put, traits act independently of one another, meaning that the gene that determines height has no influence the gene for eye color.
During cellular division homologues chromosomes may lack a piece of information from a gene.

This can result in single gene disorders that may or may not be life threatening depending on the mutation. For example, the Maple Syrup Urine Disorder, or MSUD, is a potentially fatal disease that disables the body from breaking down valine, leucine and isoleucine. These three amino acids are used to build proteins and are eventually broken down by branched –chain alpha ketoacid dehydrogenase (BCKD). Individuals who are affected by MSUD have a mutation that lack one of the six proteins that assist in the breakdown of the three amino acids. As a result, increased levels of valine, leucine and isoleucine end up in the blood stream and cause degradation of brain cells. In order for the disease to be inherited the child must obtain an altered gene from each parent, which makes MSUD an autosomal recessive

disorder