- Demonstrate an understanding of the process and importance of meiosis.
- Investigate the processes of meiosis in Easter lillies.
- Investigate the number of chromosomes in cells before, during, and after meiosis.
- Compare the processes of mitosis and meiosis.
What is the Function of Meiosis?Edit
Meiosis is a special type of cell division that only occurs in reproductive organs. Meiosis produces reproductive cells called gametes (eggs or sperm) which can form a zygote with chromosomes from both parents. The gametes are haploid (n) meaning they only have one copy of each type of chromosome that the diploid (2n) parent cell contains.
The first part of meiosis reduces the chromosome number from diploid to haploid (reduction division). Human gametes contain n = 23 chromosomes compared to the 2n = 46 chromosomes found in somatic cells. Each human sperm or egg cell contains 22 autosomes and one sex chromosome (either X or Y). Autosomes are chromosomes that are not directly involved in determining the sex of an individual. Females have two X chromosomes and males have one X and one Y.
Each diploid cell has two copies of each chromosome, one copy from the female gamete (egg) and one from the male gamete (sperm). During fertilization, the union of haploid gametes forms a diploid zygote, so each of a person's cells has a copy of each of the mother's chromosomes and each of the father's chromosomes.
Phases of MeiosisEdit
Meiosis is similar to mitosis but goes through two sequences of the phases, meiosis I and meiosis II.
During interphase chromosomes are replicated and condensed into the X-shaped pair of chromatids linked by a centromere.
In prophase I similar chromosomes called homologous chromosomes pair to form homologous pairs. The homologous pair, which is made up of four chromatids is called a tetrad from the Greek word "tetra" meaning "four".
Homologous chromosomes are similar to each other but not identical. The chromosomes are homologous because they are made up of the same genes but may have different forms of those genes called alleles. Alleles can be dominant or recessive which determines whether a trait is expressed or not.
During the pairing process, crossing over of chromatids can occur, in which non-sister chromatids exchange genes, which allows for the recombination of genes in each chromosome and contributes greatly to genetic variation. As a result of crossing over, individual chromosomes contain some genes from the mother and some from the father.
After prophase I, a spindle fibre attaches to the centromere of each chromosome. A spindle fibre from one pole attaches to one pair of sister chromatids, and a spindle fibre from the opposite pole attaches to the other pair in the tetrad. The spindle fibres pull each tetrad to the equator of the cell where the chromosomes line up in double-file. Some chromosomes from each parent will be on either side of the cell's equator. This random positioning is called independent assortment.
During anaphase I the homologous chromosomes separate and move to opposite poles of the cell, pulled by the shortening spindle fibres. The centromeres do not split and the sister chromatids are held together so only one chromosome from each pair will move to each pole of the cell.
Telophase I does not occur in all cells, in which case the cells go directly to meiosis II. If telophase I does occur, the homologous chromosomes begin to uncoil and the spindle fibres disappear. The cytoplasm is divided, the nuclear membrane forms around each group of homologous chromosomes, and two cells are formed. Each of these new cells contains one copy of each chromosome. Because each chromosome already consists of two chromatids, a second chromosome replication does not take place between telophase I and prophase II. In females, meiosis II occurs after the egg is fertilized by a sperm cell.
At the end of telophase I, each cell contains some maternal chromosomes and some paternal chromosomes, due to the independent assortment of chromosomes during metaphase I. Each cell also contains chromosomes that are made up of a combination of maternal and paternal alleles as a result of crossing over during prophase I.
The phases of meiosis II are identical to mitosis. Two cells from telophase I go through prophase II, metaphase II, anaphase II, and telophase II. Each cell beginning in meiosis II is haploid but consists of replicated chromosomes (sister chromatids). At the end of meiosis II the four daughter cells are still haploid but each cell contains single unreplicated chromosomes. The daughter cells at the end of meiosis II are called gametes in animals and either gametes or spores in plants.
The end result of meiosis is the production of gametes. This process, called gametogenesis, results in the production of sperm and eggs. In animals, the process of male gamete production is called spermatogenesis and the process of female gamete production is called oogenesis.
Meiosis in males takes place in the testes, the male reproductive organs. The production of the male gametes, sperm, starts with a diploid germ cell called a spermatogonium. This cell enlarges and undergoes meiosis I and meiosis II. The final product is four haploid sperm cells. Following meiosis II, the sperm cells develop into mature sperm. Each cell loses cytoplasm and the nucleus forms into a head. A long, tail-like flagellum is formed for locomotion. Spermatogenesis can occur throughout the year in some organisms, including humans, while in others sperm production occurs only during a certain time of the year called breeding season. For example many species of migratory birds reproduce only during the spring and summer months.
In females, meiosis takes place in the ovaries, the female reproductive organs. The process starts with a diploid germ cell called an oogonium. This cell enlarges and undergoes meiosis I and meiosis II. At the end of meiosis I, the cytoplasm is not equally divided between the two daughter cells. The cell that receives most of the cytoplasm is called the primary oocyte. The other cell is called a polar body and is not a viable sex cell. As the primary oocyte undergoes meiosis II, the cytoplasm is again unequally divided. Only one cell becomes an egg, or ovum, and contains most of the cytoplasm. The other cell, a polar body, is not a viable sex cell. The purpose of the unequal division of the cytoplasm is to provide the ovum with sufficient nutrients to support the zygote in the first few days following fertilization.
Meiosis I and meiosis II are not continuous in many organisms. In humans, for example, meiosis I begins in the ovarian tissue of the embryo before birth and does not continue beyond prophase I. The continuation of meiosis I occurs after the female reaches puberty. Normally only one oogonium undergoes this process each month. Meiosis II takes place after fertilization by a sperm cell. The production of ova (two or more egg cells) in females continues from the start of puberty until menopause, which usually occurs between 40 and 50 years of age.