Where are we going with this? The information on this page should increase understanding related to this standard: Explore the process of mitosis understanding why mitosis happens, the main goal of mitosis, and the key terms involved in mitosis.
Review of Mitosis Basics
"Mitosis is a process where a single cell divides into two identical daughter cells (cell division). During mitosis one cell? divides once to form two identical cells. The major purpose of mitosis is for growth and to replace worn out cells" (Souce, 2021-11)So, cells split… hmm…
"Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction. ... Meiosis I is a type of cell division unique to germ cells, while meiosis II is similar to mitosis" (Source, 2021-12).
A Closer Look At Meiosis
The Process of Meiosis
The word comes from the Greek words, "homo" and "logos" meaning
to have the same composition, arrangement, order, ratio, or proportion.
So… let's talk about the homologs a little, and use humans as the example. Each human cell has 46 chromosomes. Okay? But!!! It is actually 23 pairs of homologs. One chromosome in each homologous pair comes from the mother and one chromosome in each pair comes from the father.Think of the homolog as a chromosome pair that has genetic information about certain aspects of the organization. For instance hair and eye color are parts of a certain chromosome. Each human cell has one of these chromosomes from the mother and one of these from the father.So there are 23 different types of chromosomes and you get one of each type from the mother and one of each type from the father. A homolog is any two chromosomes of the same type.How about we get a little ridiculous? Say of the 23 types of chromosomes, we have
Milkshake chromosomesHamburger chromosomesFrench fry chromosomesApple pie chromosomesand 19 others
If you take the dad-milkshake and the mom-milkshake, that's a homolog.If you take the dad-hanburger and the mome-hamburger, that's a different homolog.… and so on…
You have 2 complete sets of 23 different chromosomes: a dad set and a mom set.
Let's run with this analogy a little… At the beginning of meiosis, you have one dad-milkshake chromosome and on one mom-milkshake chromosome. One homolog.
After Interphase I, you have two dad-milkshake chromosomes and on two mom-milkshake chromosome. Two homologs. Plopped atop each other. That's a tetrad. You have one cell with 4 milkshake chromosomes: 2 dad-milkshakes, and 2 mom-milkshakes.
The tetrad is a very important structure (as will be discussed as we go). It is recognizable by the two pairs of homologs that are attached to each other. This will frequently be represented in images such that it looks like two X's clost to each other or even overlapping.
After Meiosis I, you still have two dad-milkshake chromosomes and two mom-milkshake chromosome. However, the Tetrad has been pulled apart into different cells.
Now, you have two cells, but still with 4 milkshake chromosomes: 1 dad-milkshake and 1 mom-milkshake chromosome in EACH of the two cells.
NOTE: Something else happens in Prophase I that sort of mixes things up. You can think of it as the milkshakes swapping straws and lids with each other. Sorta. More later on this!
This goes back to the purpose of creating two "half cells" that can be joined to become a "whole cell." More accurately, Meiosis creates two bodies (gamites) that have half of the chromosomes which, combined, create a cell with all of the chromosome pairs.
After Meiosis II, you still have two dad-milkshake chromosomes and two mom-milkshake chromosome. However, they are now in 4 different cells
Now, you have four cells, but still with 4 milkshake chromosomes: 1 dad-milkshake in two of the cells and mom-milkshake chromosome in two of the cells.
NOTE: However, in Prophase I they mixed up a little… swapped lids and straws, sorta More later on this!
This is the little mixing up mentioned in earlier milkshake examples.
|After crossing over, one of the gametes from the father has some of the genetic material from the mother and one of the gametes from the mother has some of the genetic material from the father.|
Stop and look at the image again. Longer…
Crossing over occurs in Prophase I, and greatly increases genetic variability.
Looking Closer at the Cross-over Process
1. Beginning with one diploid cell having pairs of homologs (one mother chromosome and one father chromosome,2. you end up with four haploid cells having one copy of a chromosome from each pair,3. but the copies are partially mixed up.
Summary and Review
- In meiosis, matching chromosomes duplicate and stay together, forming a tetrad.
- The phases of meiosis are very similar to those in mitosis, with a few exceptions.
- Crossing over happens in Prophase I
- There is no Interphase II because Meiosis II does NOT replicate DNA.