Mitosis and meiosis are the two forms of cell reproduction that occur in living organisms. They are the only two ways that new cells can be created. They work through the process of cell division – one cell splits into two (in mitosis) or four (in meiosis) identical cells, known as daughter cells. Mitosis and meiosis have two different purposes. The purpose of mitosis is to produce new cells for the body, and it is required for growth and repair of the body. The purpose of meiosis is to produce sex cells, which are known as gametes. These are sperm and egg cells, that are involved only in reproduction.
The end products of mitosis and meiosis are very different to each other. The end product of mitosis is two daughter cells, which are identical to the original cell that divided to produce them. They are also identical to all other cells in the body, barring any mutations in the cells’ DNA. The end product of meiosis, however, is two cells which each have half the number of chromosomes that a normal cell has (gametes), only having one copy of each chromosome. These are known as haploid cells, as opposed to diploid cells, which have two copies of each chromosome. This happens because, during fertilization, two gametes join together and the resulting cell has two copies of each chromosome and is diploid.
Mitosis and meiosis occur in different places to each other. Mitosis occurs in all living cells in every organism because it is how these cells reproduce. Every cell in the body is produced by mitosis and will undergo mitosis itself. Meiosis, however, does not occur in most cells. It occurs only in the testes and ovaries in animals, as it produces only gametes – sperm and egg cells. Meiosis is not necessary for any other cells, whereas mitosis is essential for all cells in the body.
Errors can occur in both mitosis and meiosis. Errors in mitosis include nondisjunction, where chromosomes don’t separate properly, and endoreplication, where the chromosomes are copied but the rest of mitosis does not occur and the cell doesn’t split. Nondisjunction can also occur in meiosis. However, these errors have very different consequences to each other.
Errors in mitosis affect only the daughter cells, and will not affect any other cells in the body. This makes the consequences of errors in mitosis far less serious than those of errors in meiosis. Errors in mitosis have a much larger effect. This is because the gametes produced by meiosis will then undergo mitosis to create a new individual after fertilization has occurred.
This means that the gamete, which has an error, will be replicated, and the error will be replicated in every cell in the body of the new individual. However, usually the error will prevent the gamete from being able to undergo fertilization, but in some cases, fertilization can still occur.
There is a process which is known as crossing over. This is when genes are exchanged between paired chromosomes. This process does not occur at all in mitosis, the chromosomes of the daughter cells are identical to the chromosomes of the cell undergoing mitosis.
Crossing over does occur, however, in meiosis. As part of both mitosis and meiosis, the chromosome pairs (the two copies of each chromosome) pair up. When this occurs in meiosis, the two chromosomes in each pair exchange genes with each other.
This results in a pair of chromosomes that overall have the same genes, but some of the genes are now located on the other chromosome. This ensures that the chromosomes that are inherited by the gametes, and therefore the chromosomes inherited by offspring, are different to the chromosomes that occur in the cells of the parent. This ensured genetic diversity in offspring.
In mitosis cell division only occurs once, when the cell undergoing mitosis splits into two daughter cells. However, in meiosis, cell division occurs twice, to result in four daughter cells. In the first round of cell division, the cell undergoing meiosis splits into two haploid cells. In the second round of cell division, each of these haploid cells splits into two identical cells.
Both mitosis and meiosis can actually be used in reproduction, despite meiosis being used more commonly. Meiosis is used more commonly because it is used in sexual reproduction, which involves two parents, which is the main mode of reproduction amongst animals.
However, some organisms, such as bacteria, some plants, and even a few animals, can reproduce by asexual reproduction. Asexual reproduction is where an organism clones itself and produces offspring entirely by itself. The offspring produced by asexual reproduction have exactly the same genes as the parent. The mechanism of asexual reproduction is mitosis, as no fertilization occurs, so cells need to have the normal number of chromosomes.
Both mitosis and meiosis are divided into a number of stages, which are called phases. However, these phases differ between the two. Essentially, there are five different phases, which are called interphase, prophase, metaphase, anaphase, and telophase.
Different processes occur in each of these phases, all of which are essential to both mitosis and meiosis. In mitosis, these five phases occur once, and then mitosis is complete. However, meiosis is split into two rounds, called meiosis I and meiosis II. Meiosis I consists of interphase, prophase I, metaphase I, anaphase I, and telophase I. Meiosis II then follows, which consists of prophase II, metaphase II, anaphase II, and telophase II, with interphase not occurring in the second round. The phases are the same process whether they occur in mitosis or meiosis, it is just that meiosis requires them to occur twice.