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Meiosis is a type of cell division which results in 4 cells that are genetically non-identical from one parent cell. In order for once cell to divide to result in 4 cells, how many divisions must take place?
Two. 1 cell becomes 2, then 2 become 4:
The first division is called meiosis I, and the second is called meiosis II.
…so far so easy? (it should be!)
Cells resulting from meiosis are gametes such as egg cells and sperm cells, hence meiosis only occurs in sexually reproducing organisms. There are 2 key points about this:
1. Gametes are genetically unlike one another – while cells in other tissues such as muscle or blood must be genetically identical to one another (clones), the very basis of sexual reproduction is genetic diversity. So somewhere in the process of division, something takes place which creates genetic diversity (we’ll come to that shortly).
2. If gametes are to fuse and result in a new organism, it is essential that the number of chromosomes should stay constant. Humans have 46 chromosomes in each cell (of course, apart from cells without DNA in them, and “spoiler alert!”, gametes) – if each gamete had 46 chromosomes, then fusing 2 together would result in a zygote with 92 chromosomes, whose offspring would have 184 chromosomes, and before you know it something terrible would have happened.
The above picture illustrates how the number of chromosomes is halved in the final 4 cells. The terms diploid and haploid refer to the number of sets of chromosomes. In humans, somatic cells (i.e. cells other than gametes) are diploid because there are two sets of chromosomes. Gametes are haploid because they have only one set of chromosomes.
A “set” is made up of all chromosomes which are unique, i.e. are not paired with any homologous chromosomes.
X x X X x X x x <———- haploid = 1 set
XX xx XX XX xx XX xx xx <———- diploid = 2 sets
In the first XX, X and X are homologous chromosomes because they occupy the same space and contain DNA with similar purpose/function. Essentially, they are more or less copies of each other. So when 2 gametes fuse, they form a diploid cell with the complete number of chromosomes.
Wikipedia does us the honour with this epic picture:
On to the very important bit now…
How does meiosis achieve genetic diversity without which you would actually look *just* like your siblings?
10 words: Independent Assortment of Homologous Chromosomes, &
Genetic Recombination by Crossing Over
What an unnecessary mouthful. You still have to learn them though.
Independent assortment of homologous chromosomes means that in meiosis I, when the original diploid line-up a.k.a. XX xx XX XX xx XX xx xx becomes X x X X x X x x in 2 resulting cells, which big X’s and which small x’s end up with each other in each cell is random. Pretty simple concept.
If you split the homologous chromosomes, you get Xx in 2 cells. The idea is that there is no rule saying that black must go with black, and red must go with red. You can end up with Xx and Xx, or Xx and Xx with an equal probability. What can I say, genetics likes being a bit random.
Genetic recombination by crossing over is a lot more interesting. It’s like a bowl of spaghetti. Homologous chromosomes snuggle each other and exchange parts in the process:
Did I mention how important it is to use accurate scientific terminology in the exams? The process is called synapsis, during which mutual exchange of genetic information occurs.
As a finishing touch, I read of this mnemonic to remember the purpose of meiosis.
It is so cringe-worthy, I would rather memorise meiosis off by heart.