Meiosis is an incredibly important process which allows for gametes to be produced, these gametes are a necessity for sexual reproduction. Two gametes join together and fertilise to form a zygote, which develops into a new organism. Meiosis occurs in the reproductive organs to produce gametes. Meiosis involves a reduction division. Now this reduction division process, means that cells that divide by meiosis have the full number of chromosomes to start with, but the cells that are formed from meiosis have half the number. These cells are called haploid cells, and they have half the normal number of chromosomes (hence the term haploid). The unique feature of the cells formed by meiosis, are that they are all genetically different because each new cell ends up with a different combination of chromosomes.
Meiosis itself is characterized by two divisions, meiosis I and meiosis II. Meiosis I is the reduction division due to the fact it halves the chromosome number. Meiosis I and II are still split into the four phases; PMAT. Prophase, metaphase, anaphase and telophase. To start with, meiosis begins with interphase, in which the DNA unravels and replicates to produced double armed chromosomes called sister chromatids.
In the first Meiosis I division
Prophase I; The chromosomes condense, and coil up, reducing their size and arrange themselves in homologous pairs (this means one chromosome from the father and one from the mother pair up) and crossing over occurs, this is the source of genetic variation. Centrioles begin to move to opposite ends of the cell forming the spindle fibres. The nuclear envelope breaks down.
Metaphase I; The homologous pairs line up at the metaphase plate at the centre of the cells, and spindle fibres attach to the kinetochore.
Anaphase I; The spindles shorten and separate the homologous pairs- one chromosome goes to each end of the cell.
Telophase I; A nuclear envelope forms around each group of chromosomes and cytokinesis occurs to split the cytoplasm and two haploid daughter cells are produced. The chromosome number is halved.
Then the second Meiosis II division follows
The two daughter cells undergo prophase II, metaphase II, telophase II and cytokinesis – in a fashion similar to the stages in mitosis.
In anaphase II, the pairs of sister chromatids are separated- each new daughter cell inherits one chromatid from each chromosome. Four (genetically identical) haploid daughter cells are produced – and these are the gametes.
The production of genetically different cells, is a reason as to why meiosis is an important
process. There are two main events in meiosis that lead to genetic variation.
The crossing over of chromatids in prophase I, means that each of the four daughter cells formed from meiosis contains chromatids with different alleles.
The independent assortment of chromosomes is the other event that leads to genetic variation.
Each homologous pair of chromosomes in your cells is made up of one chromosome from the maternal and one from the paternal. In metaphase I homologous pairs line up and in anaphase I they split up, and as to where which chromosome ends up in which daughter cell, is arbitrary. So the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes. This is called independent assortment, and the shuffling of chromosomes leads to genetic variation in any potential offspring.