10.1 Meiosis

Meiosis:
     Meiosis includes 3 major steps. It is used during sexual reproduction in order to convert a diploid (46 chromosomes) into a haploid cell (23 chromosomes) or gamete. These sex cells can then be joined with another sex cell to reform a diploid cell with the full number of chromosomes. In total, meiosis can divide a cell into four haploid gametes (haploid = half the chromosomes). The three steps of meiosis include interphase, Meiosis 1 and Meiosis 2.

     Interphase: Cell growth and DNA replication into two sister chromatid chromosomes
     Meiosis 1:

• Prophase I: DNA supercoils, chromosomes condense, nuclear membrane dissolves, 2 sets of homologous pairs join together to allow crossing over of genes. (the four final gametes can have different genetic makeups if crossing over/recombination occurs in this stage)
• Metaphase I: Spindle fibres from the centrioles located at the poles attach to the centromeres of the bivalent chromosomes. The bivalent pairs line up at the cells equator
• Anaphase I: The spindle fibres that are connected to the bivalent contract and split the bivalent into homologus chromosomes. Chromosomes move to opposite poles of the cell
• Telophase I: The chromosomes decondense and a nuclear membrane may reform. Cytokinesis occors to form two haploid daughter cells. 
• Interkinesis: this is a rest period that occasionally occurs between meiosis I and II. 

      Meiosis 2:

• Prophase II: chromosome recondense and if a nuclear membrane was formed it will dissolve at this stage. Once again centrioles will move to opposite poles that are perpendicular to the poles in prophase I
• Metaphase II: The centrioles from the spindle fibres attach to the centromeres of the chromosomes, the chromosomes line up along the cell's equator. 
• Anaphase II: Spindle fibres contract and split the chromosome into sister chromatids (now called chromosomes) which move to the opposite poles
• Telophase II: Chromosome decondense, nuclear membrane reforms, cells divide by cytokinesis to separate into four haploid daughter cells. 

Meiosis is unique because it can create an infinite number of genetic possibilities in gametes. This is due to a couple of reasons. Genetic variety can occur during; 

• the crossing over that occurs in prophase I can form new chromosome combinations in the genetic makeup. Maternal and paternal DNA can be mixed randomly to produce an4 infinite number of possibilities 
•  the random orientation of chromosomes at the cells equator in metaphase I
• separation of homologus chromosomes into two daughter cells. Chromosomes will have originated from either the mother or father.
• there are 23 chromosomes in a haploid cell meaning that there will be 2^23 possible gamete combinations of chromosomes
• Random fertilisations, DNA mutations, chromosome mutations and non-disjunction can also cause genetic variation

A monk by the name of Gregor Mendel discovered two very unique laws about the inheritance of traits

      Law of Segregation : Each characteristic that can be passed from parent to child, is controlled by two alleles. Each parent passes a randomly selected allele to its offspring. The random assortment is caused by the random arrangement of sister chromatids in Metaphase I and crossing over in prophase I

     Law of Independent Assortment: (inheritance law) the passing of a gene for a trait will not influence the passing of another gene. So that different allele combinations will have equal chance of occurring. This law is only true when genes are on different chromosomes and does not apply to linked genes.