In order to obtain a better understanding of the function, processes, and reactions of cells, it is imperative that we have a good understanding of the ways in which they reproduce and divide: namely, meiosis and mitosis.
To help you get your head around the concepts, we have put together everything you need to know about the two processes, including the key differences to remember between meiosis and mitosis.
What Is Meiosis?
Meiosis is the process of cell division during which a single cell divides twice.
The result is four new cells, containing half the amount of genetic material – or half the number of chromosomes – as the original cells, and these cells then become our gametes, or sex cells – that is, eggs in females, and sperm in males.
During the process of meiosis, a genetic recombination will occur to introduce new gene combinations into a specific population, and the end result is four cells.
All of these four cells will be genetically different from one another. In the simplest terms, meiosis is responsible for the production of sex cells.
There are nine stages to this process, and these are divided into two sections – the first time the cell divides (Meiosis I), and the second division of the cell (Meiosis II).
Meiosis I can be divided into five stages, and we will explore these in greater detail below:
During the Interphase stage, the DNA of the original cell is copied, and this results in two full sets of chromosomes that are identical, and that are full sets.
To understand this stage more completely, there are two sets of terms that we need to get to grips with: centrosomes and centrioles.
Centrosomes are structures within the cell that are responsible for organizing the spindle fibers, which are the main component of the cell’s cytoskeleton.
They also play an important role in the movement of chromosomes from one part of the cytoplasm to another.
These microtubule-organizing centers are made up of proteins such as γ-tubulin and are located at the base of each centriole.
Centrioles are cylindrical organelles with a diameter of 0.5 μm, and they are surrounded by a membrane.
Centrioles are found in all eukaryotic cells, and they are essential components of the centrosome.
The centrosome has two distinct parts – the distal portion contains the centrioles, while the proximal portion consists of the pericentriolar matrix, which is where the microtubule nucleation takes place.
Just outside the nucleus – the core of the cell – there are two centrosomes.
Each of these contains a pair of centrioles, and these are the key to the process of successful cell division.
During interphase, microtubules will extend from these centrosomes.
The second stage is known as Prophase I. Here, the chromosomes that have been copied condense into clear, X-shaped structures which can be seen using a microscope.
Each of these chromosomes has two sister chromatids, and these share identical genetic information, and these chromosomes pair up, ensuring that both copies of chromosome 1 are paired, as well as both copies of chromosome 2, etc.
Recombination, or crossing over may then occur -this is when the various pairs of chromosomes exchange bits of DNA.
Once Prophase I is complete, the membrane that encloses the nucleus of the cell will dissolve, and the chromosomes will be released.
At this stage, the meiotic spindle will extend across the cell between the centrioles -this contains proteins and microtubules.
Metaphase I begins once the chromosomes have been released, and it is here that the mitotic spindle forms.
This is a structure composed of microtubules, and it helps to align the chromosomes so that they are correctly oriented for the next step.
Essentially, the chromosomes line up beside one another in the center of the cell, with the centrioles at opposite poles, and the meiotic spindles extended – these meiotic spindles attach to one chromosome per pair of chromosomes.
Anaphase I occurs when the chromosomes separate, and this means that the sister chromatids move away from each other towards opposite poles of the cell.
The chromosomes now become individualized, and they begin to migrate towards the two different ends of the cell.
When the chromosomes reach their destination, they are ready to divide.
Telophase I And Cytokinesis
Telophase I occurs once the chromosomes have reached their respective destinations at opposite ends of the cell.
It is during this phase that the nuclear envelope breaks down, and the new daughter cells are formed, with a new membrane formed around each set of chromosomes, creating new nuclei.
The daughter cells are called blastomere, and they are separated by a cleavage furrow, creating two daughter cells with a nucleus and a full set of chromosomes – this is known as cytokinesis.
Meiosis II has four stages, and these are as follows:
Prophase II is similar to prophase I, but it differs because the chromosomes are not condensed. Here, there are two daughter cells with 23 chromosomes each.
Within each of these daughter cells, the chromosomes will now condense, transforming into X-shaped structures that can be seen using a microscope.
Here, the nucleus in each of the daughter cells will dissolve, and the chromosomes are released, allowing the centrioles to duplicate, and leading to the formation of meiotic spindle forms.
This phase is very similar to metaphase I.
The chromosomes in each daughter cell line up end-to-cent in the center of the cells, meaning that the centrioles are at opposite ends of the cells.
Then, meiotic spindle fingers at the poles attach to the sister chromatids.
This stage is very similar to anaphase I, and it is where the sister chromatids separate and the chromosomes line up alongside one another.
Each separated chromatid is now its own, individual chromosome.
The final stage of meiosis II is telophase II.
Here, the nuclear envelope breaks down, and the chromosomes are pulled apart by the microtubules of the meiotic spindle.
The resulting daughter cells are called blastocysts, which contain 46 chromosomes.
Cytokinesis Or Cell Division
Cytokinesis is the last step in the process of meiosis, and it is also the first step in mitosis.
In this step, the cytoplasm divides, and the two daughter cells are created.
This involves the breakdown of the nuclear envelope and the separation of the chromosomes.
The daughter cells then undergo a series of changes, including the division of the cytoplasm.
Meiosis: An Overview
In short, meiosis is a process in which nuclear cells divide to create daughter cells, and each of these daughter cells contains half the chromosome numbers of the original cells, with one daughter cell containing a completer set of chromosomes – in humans, this takes the chromosome number from 46 to 23.
Germ cells undergo meiosis to create gametes.
There are two main stages to meiosis, and the end result is four cells, each of which has a complete set of chromosomes, as opposed to two separate sets of chromosomes.
What Is Mitosis?
Mitosis is the process of dividing a single cell into two daughter cells.
These two daughter cells are identical to each other, and both have a complete set of chromosomes.
This means that each of the daughter cells contains all the genetic material required for the development of an organism.
There are five main stages to mitosis, and these are:
In the interphase stage, the DNA in a cell is copied to ready the process of cell division, and this creates two full identical sets of chromosomes.
There are two centrosomes outside of the nucleus, and each of these centrosomes contains a pair of centrioles – as we mentioned in meiosis, the centrioles are critical for successful cell division.
Microtubules extend from the centrosomes during the interphase stage.
Prophase begins when the cell enters the M phase of the cell cycle.
During this time, the DNA within the nucleus is replicated, and the chromosomes begin to line up along the equator of the cell.
Chromosomes condense into X-shaped structures, which are visible under a microscope, and each of these contains two sister chromatids.
Each of these chromatids contains identical genetic information, and these lineup so that pairs of chromosomes are together.
Following the end of the prophase stage, the membrane that surrounds the nucleus of the cell will dissolve, and the chromosomes are released.
Mitotic spindles then extend across the cell between the centrioles, and these contain proteins and microtubules.
Prometaphase occurs after the chromosomes have lined up on the equator.
At this point, the centromere (the part of the chromosome that attaches to the microtubule) moves towards the pole of the cell.
Metaphase is the next phase of mitosis. It is characterized by the alignment of the chromosomes along with the metaphase plate, as the chromosomes line up end-to-end along the center of the cell.
The chromosomes are arranged in pairs, and they are attached to the microtubules of a spindle apparatus, and the centrioles are at opposite poles of the cell.
The mitotic spindle fibers attach themselves to each of the sister chromatids.
During anaphase, the centromeres move away from the poles of the cell as the mitotic spindle pulls the chromatids apart.
As the centromeres move further away from the poles, the chromosomes become longer.
After anaphase, the nuclei form at the center of the cell. This is known as telophase.
The nuclei are positioned so that their DNA will be inherited by the daughter cells, and a full set of chromosomes gather together at each pole of the cell.
A membrane forms around each of these, and two new nuclei are formed.
This is the final stage of mitosis. During cytokinesis, the cytoplasm divides, and the two new daughter cells are formed.
The cytoplasm divides again to produce more daughter cells.
Mitosis: An Overview
To summarize, mitosis is the process where one cell splits into two separate cells.
During mitosis, the chromosomes divide, and the cell goes through four stages, including prophase, prometaphase, metaphase, anaphase, and telophase.
These stages occur sequentially, and there is a gap between each stage. The first step in mitosis is prophase, and it lasts for about 90 minutes.
During this period, the nuclear envelope breaks down, and the chromosomes start to align along the equatorial plane of the cell.
In addition, the centrosome duplicates, and the mitotic spindle starts to assemble.
The second stage is prometaphase, and it lasts for approximately 30 minutes.
During this stage, the chromosomes line up along the equatorial plane, and the centromeres move towards the poles of the cell.
Also during this stage, the microtubules of the mitotic spindle start to grow outwards from the centrosomes.
The third stage is metaphase, which takes place over a period of about 20 minutes.
During this stage, the chromosomes are aligned along the equatorial plane.
Additionally, the chromosomes are pulled towards the poles of the cells by the microtubules of mitotic spindles.
The fourth stage is anaphase, which lasts for about 10 minutes.
During this stage of mitosis, the centromeres of the chromosomes move away from the poles, and the chromatids begin to separate.
The mitotic spindles pull the chromatids apart, and the centromeres move closer together. The fifth stage is telophase, which occurs after anaphase.
During telophase, the nucleus divides into two, and the cytoplasm separates.
The membranes surrounding the nuclei fuse, and the nuclei are pushed back together.
Finally, the two nuclei join together, and the cell becomes two separate cells.
What Are The Differences Between Meiosis And Mitosis?
As you can see, there are a number of similarities between the processes of meiosis and mitosis, but the two also have a number of differences.
During mitosis, the chromosomes are arranged on the surface of the cell. They are not divided, and they do not break apart.
Instead, the mitotic spindles stretch them apart, pulling them apart. In meiosis, however, the chromosomes are divided, and they are broken apart.
Centrioles are found only in eukaryotes. They are cylindrical structures that contain nine triplets of microtubules.
During mitosis, these centrioles duplicate themselves, and then they attach to the plasma membrane.
In meiosis, the centrioles are present at the beginning of prophase I. However, they disappear before the end of prophase I.
The nuclear envelope contains proteins that help hold the chromosomes inside the nucleus. It also helps protect the DNA from damage.
During mitosis, this structure remains intact until late in the cell cycle.
In meiosis, however, the nuclear envelope does not remain intact until the end of the prophase. Instead, it begins to disintegrate early on.
Cytokinesis is the process by which one cell splits into two or more. During cytokinesis, the cleavage furrow forms around the midplane of the cell.
As the furrow progresses, the contents of the cell are expelled through the opening.
In meiosis II, the cell does not divide into two. Instead, the cell undergoes another round of division, forming four daughter cells.
Microtubules are long protein filaments that form the cytoskeleton. During mitosis, the microtubules organize the chromosomes within the cell.
They are made up of tubulin dimers. In meiosis I, the microtubules are organized differently than during mitosis.
The microtubules are shorter, and they are attached to both sides of the chromosome.
This organization allows the chromosomes to be pulled apart during anaphase.
Kinetochores are regions of the chromosome where the microtubules attach.
During mitosis, each kinetochore attaches to a single pole. In meiosis, the kinetochores attach to multiple sites along the chromosome.
Cell division involves the duplication of all of the cellular components. During mitosis, cells grow larger and larger until they reach their final size.
In meiosis, however, cells continue to grow smaller and smaller until they become mature gametes.
During mitosis, the chromosomes have a distinct shape called a metaphase plate.
These plates are formed when sister chromatids come together and align with each other.
In meiosis, however, the chromosomes are much longer and thinner than those seen in mitosis.
They are also very closely packed together.
In mitosis, the centromere is a region of the chromosome that holds the microtubule attachment site for the spindle.
In meiosis though, the centromere has no specific function. Instead, it is simply a place on the chromosome where the micro spindles attach.
Nucleoli are small bodies located near the center of the nucleus. Their primary role is to produce ribosomal RNA.
During mitosis, nucleoli are visible because they contain large amounts of rRNA. However, they are absent during meiosis.
The Golgi apparatus is a network of membranes that is responsible for producing glycoproteins.
During mitosis, these structures are present throughout the cytoplasm.
In meiosis, they are only found in the vicinity of the nucleus, while in mitosis, they can be found anywhere in the cell.
DNA replication occurs at different times during mitosis and meiosis.
During mitosis, DNA replication begins before the onset of mitosis. It continues as the cell enters mitosis.
In meiosis, the process starts after the chromosomes have aligned with each other.
Once this happens, the rest of the cell cycle proceeds normally.
Number Of Cells Divided
One of the main differences between meiosis and mitosis is the number of cells that are divided during the process; in mitosis, there is one cell division resulting in two daughter cells, while in meiosis, there are four divisions resulting in eight daughter cells.
This difference is due to the fact that meiotic cells undergo many more divisions than do mitotic cells.
In addition, the cells in mitosis are genetically identical, while those produced during the course of meiosis are not.
Genetic Information Transfer
During the cell cycle, genetic information is transferred from DNA to RNA.
This occurs at different times depending on whether the cell is undergoing mitosis or meiosis.
For example, during mitosis, the transfer of genetic material takes place during the S phase.
During meiosis, however, the transfer of genetic information occurs during the M phase.
Are There Any Similarities Between Meiosis And Mitosis?
Despite clear differences, there are also a number of similarities between mitosis and meiosis, and some of the major similarities between the two processes include:
Spindle Fiber Formation
In both mitosis and meiosis, the formation of the spindle fibers (microtubules) is similar.
This similarity is most apparent during prophase I when the chromosomes begin to line up.
At this time, the chromosomes appear to be surrounded by a ring-shaped structure known as the nuclear envelope.
As the chromosomes move closer together, they eventually become attached to each other via their kinetochores.
At this point, the spindle fibers begin to form around the chromosomes.
Chromosome segregation is another area where the two processes are quite similar.
In both cases, the chromosomes must separate from each other so that they can migrate towards opposite poles of the cell.
To accomplish this, the chromatids break apart into individual segments.
These segments then travel along the microtubules until they reach the opposite ends of the cell.
Once all of the chromosomes have reached their destination, the cell divides.
Cytokinesis is the final stage of mitosis and meiosis where the two daughter cells are formed.
During cytokinesis, the plasma membrane surrounding the dividing cell breaks down, forming two distinct layers.
One layer contains the new cell wall, which will ultimately become the outer surface of the new cell.
The second layer consists of an internal layer of cytoplasm, which becomes the inner surface of the new cell and separates it from the mother cell.
Cell Cycle Progression
The progression through the cell cycle is very similar in both mitosis and meiosis. Both processes start with a period of growth called interphase.
Interphase is followed by prophase, metaphase, anaphase, telophase, and finally cytokinesis.
Mitosis and meiosis share certain features regarding cellular organization. First, both processes occur within a single nucleus.
Second, the chromosomes of mitotic cells are arranged in a linear fashion, while those of meiotic cells are organized in pairs.
Third, the proteins involved in mitosis and meiosis are located in specific areas of the cell.
Finally, the mechanisms used for chromosome separation differ slightly between the two processes.
Meiosis and mitosis both play a key role in cell division, and are important biological processes.
By understanding the similarities and differences, you can gain a more comprehensive understanding of microbiology, and the very structure of our world.
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