SC.912.L.16.17: Compare and contrast mitosis and meiosis and relate to the processes of sexual and asexual reproduction and their consequences for genetic variation.
Purpose of Meiosis
Meiosis is the cellular process used by multicellular organisms to create sex cells known as “Gametes”. Commonly produced examples of gametes are sex cells like sperm and egg cells. Meiosis is designed to allow an organism to transfer copies of some of their genes to the chromosomes that will be contained in their sperm and or egg cells. Only half of the parent’s DNA is placed into the egg or sperm they produce, making the gamete by Haploid meaning it only contains half the normal number of chromosomes. Conversely, the normal somatic (body) cells of an organism are Diploid meaning they have 2 copies of each of their chromosomes (1 copy from mom and 1 copy from dad). Ploidy refers to the number of chromosomes a cell has, it can also be represented with the symbol “n“. If a cell has 2 of each chromosome it can be represented by “2n” and if it has only 1 of each chromosome it is represented as “1n“. It is important that an organism can properly conduct meiosis as any irregularity in the process could potentially cause their gametes to not function properly, pass down genetic diseases, or result in a miscarriage.
Stages of Meiosis
Meiosis is a very similar process to mitosis, but there are a few distinct differences between Mitosis and Meiosis which we will highlight a bit later. Meiosis can be divided into two larger steps Meiosis 1 and Meiosis 2, each containing smaller steps.
Meiosis 1
Prophase 1
During prophase 1 the DNA is condensed to form chromosomes. The DNA has already been duplicated so the chromosomes take on the form of conjoined sister chromatids.
Metaphase 1
Metaphase 1 of meiosis 1 consists of the cell lining up the homologous chromosomes side by side along the equator of the cell. The chromosomes are pushed and pulled by spindle fibers attached to their centromeres.
Anaphase 1
Anaphase 1 consists of the homologous chromosomes being pulled apart, moving towards opposite poles of the cells. It is very important that each homologous chromosome is actually pulled to opposite sides of the cell, if they are not properly separated then future sperm and eggs could have multiple copies of certain chromosomes!
Telophase 1
Homologous chromosomes are pulled to opposite poles of the cell. A cleavage furrow begins to form where the cell is pinching off to form two separate daughter cells.
Cytokinesis
Now that the homologous chromosomes have been placed at very opposite sides of the cell the large cell now divides into two new daughter cells.
Meiosis 2
The role of meiosis 2 is to further divide the genetic material of the 2 daughter cells in half and split the cells again to form 4 daughter cells. Each of the 4 daughter cells will be genetically unique gametes containing a unique combination of chromosomes.
Prophase 2
The nuclear membrane has begun to dissolve, and centrioles will begin to move to the opposite poles of the cell. The chromosomes have condensed to form duplicate sister chromatids in preparation for subsequent steps.
Metaphase 2
The chromosomes are lined up down the equator of the cell. The spindle fibers have pushed and pulled the sister chromatids into their respective positions.
Anaphase 2
The spindle fibers produced by the centrioles are pulling apart the sister chromatids. The spindle fibers are attaching to the centromere of each chromosome.
Telophase 2
The two daughter cells have almost completed meiosis 2. Each daughter cell has migrated the chromosomes and cellular components.
Cytokinesis
Now the process of meiosis is complete and there are 4 genetically unique daughter cells. Each gamete will have its own unique combinations of chromosomes. The nuclear membrane has completely reformed. The daughter cells now have half the original DNA because each gamete only has 1 copy of each chromosome making the cells Haploid or 1n.
Differences between Mitosis and Meiosis
| Mitosis | Meiosis |
| Involved in Growth, Repair/Healing | Involved in Production of Gametes |
| Produces 2 Daughter Cells | Produces 4 Daughter Cells |
| Creates Somatic (Body) Cells | Creates Gamete (Sex) Cells |
| Genetically Identical Daughter Cells | Genetically Unique Daughter Cells |
| Diploid or 2n | Haploid or 1n |
Sources of Genetic Diversity of Gametes
Independent Assortment
Independent assortment is the various examples or ways the chromosomes can be sorted or placed into each respective gamete. Put simply, it is the variety of genetic combinations you can have for each sperm or egg.
This is determined by how the homologous chromosomes are lined up along the equator of the cell during Metaphase 1 of Meiosis 1.
Crossing Over
Crossing over is the exchange of large sections of homologous chromosomes during prophase 1 of meiosis 1. Homologous regions of homologous chromosomes break off and swap places. This process dramatically increases genetic variation among newly produced sperm or egg cells.
Mutations
Any change in the original nucleotide sequences of an organism is considered a mutation. As previously mentioned in other articles, mutations can be caused by a variety of internal or external/environmental factors. If that cell is one in the ovaries or testes of a human and undergoes meiosis to produce gametes, then that mutation will be passed on to every sperm and egg produced.
DNA polymerase is highly accurate but even it can make mistakes. Human DNA polymerase makes an error about 1 in every 100,000 nucleotides. If the other mechanisms used to repair DNA mutations also fail then this mutation will become a permanent part of that cell’s genome.