Mitosis vs. Meiosis
Article Nov 20, 2018 | by Charlotte Capitanchik
In order for organisms to grow, cells have two options: they must either replicate themselves to create more cells, or the cells themselves must expand in volume. In humans, tissues such as the skin and blood contain cells that are actively dividing, whilst other tissues such as fat contain cells that expand (good if you need energy for winter, bad if you are trying to fit into some expensive jeans). Other cells, such as neurons, will never divide again once they are terminally differentiated; they are post-mitotic.
In the process of replicating themselves, cells have another choice: do they want to make an identical copy and be left with two cells? Or do they want to make four “half-copies”, in preparation for sexual reproduction, where their genetic content will be made whole again by the process of fertilisation? This choice is the choice between mitosis and meiosis.
Difference Between Mitosis and Meiosis
This article will explore the characteristics of both kinds of cell division, shining a light on how they are similar and in which aspects they are crucially distinct. We will also explore the research into these processes and how cell division might go awry to cause disease states such as cancer and Down’s Syndrome.
Mitosis vs. Meiosis: Overview and commonly asked questions
|What is the purpose of this process?||To create gametes with only one copy of the organism’s genetic information, in preparation for sexual reproduction. Various steps in meiosis create opportunity for genetic diversity in the daughter cells. This is the raw substrate for evolution. ||In a unicellular organism, the purpose of mitosis is to proliferate as a species. In a multicellular organism, the purpose can be to grow during development, or to repair or regenerate a damaged tissue, for example.|
|What is the outcome of this process?||Four haploid cells with different genetic information. ||Two diploid cells with identical genetic information.|
|Which organisms perform this process?||Only organisms which perform sexual reproduction. Archaea and bacteria do not do this, so it might be tempting to think that unicellular organisms do not sexually reproduce. However, there are exceptions; budding yeast will form haploid spores under nutritional deprivation. ||Mitosis is performed by unicellular and multicellular eukaryotes. Bacteria have their own version of mitosis called “binary fission”. This is distinct from meiosis as bacteria typically have one circular chromosome, which is not contained within a nucleus, like eukaryotic chromosomes.|
|How long does this process take?||Meiosis has various timescales in different organisms, which can be affected by several factors including temperature and environment of the organism, and the amount of nuclear DNA. The process lasts 6 hours in yeast but can last more than 40 years in human females, due to a developmental hold at prophase I, until ovulation. Other examples are 1-2 days in male fruit flies and ~ 24 days in human males. ||Mitosis is usually shorter than meiosis. The process can take over 10 hours for mammalian cells in culture , budding yeast can take ~80 minutes to complete a cell cycle , whilst bacteria can divide every 20 minutes.|
|What is an example of a disease caused by an error in this process?||Errors in meiosis can lead to the wrong number of chromosomes ending up in germ cells, this is called aneuploidy. This can trigger miscarriage, but is occasionally tolerated. One example is Down’s syndrome, caused by trisomy 21. Another example is Klinefelter syndrome, where XY males have an additional X chromosome.||Uncontrolled mitosis occurs in cancer, where either genes that stop cell division (tumour suppressors) are switched off, or genes that encourage cell division (oncogenes) are overactive.|
|Etymology?||Meiosis means a “lessening” in Greek. This refers to the outcome of meiosis, where the genetic information in each new cell is halved.||Mitosis is the Greek word for thread, after the thread-like chromosomes that can be seen under the microscope in dye-stained cells during cell division.|
|First described by?||Oskar Hertwig described the fusion of egg and sperm in the transparent sea urchin egg in 1876. ||Walther Flemming in his 1882 work “Cell substance, nucleus and cell division.” |
Setting the scene for mitosis vs. meiosis
The stages of mitosis vs. meiosis
Meiosis and mitosis both have a prophase, metaphase, anaphase, telophase and cytokinesis.
In meiosis, prophase, metaphase, anaphase and telophase occur twice. The first round of division is special, but the second round is more like mitosis.
In mitosis, prophase, metaphase, anaphase and telophase occur once.
Chromosomes condense and the centrosomes begin to form an early spindle.
| || |
In metaphase II of meiosis, and metaphase of mitosis, chromosomes line up along the metaphase plate due to the action of microtubule spindle fibres emanating from the centrosomes located at opposite cell poles. These fibres are attached to the chromosomes by kinetochores at the centromeres of the chromosomes.
| || |
In anaphase, chromosomes are split to opposite poles of the cell.
| || |
A nuclear membrane reforms around the newly separated chromosomes, which begin to uncoil, becoming less condense. The spindle microtubules disassociate. Each daughter cell will inherit one centrosome.
The cell plasma membrane pinches, to leave two daughter cells with separate plasma membranes.
| || |
Mnemonics are also helpful, for example a useful mnemonic to remember the order of the steps in mitosis is “I Prefer Mating At Teatime” – Chamillionaire.
|Mitosis Stage||Chromosomes||Centrosomes||Nuclear Membrane||Plasma Membrane|
|Interphase||Are uncondensed but are still organised. The entire genome is replicated to create two identical semi-conserved copies of each chromosome.||The centrosome is duplicated.||Intact.||Intact.|
|Prophase||Condense. Duplicated chromosomes are called sister chromatids.||Microtubules begin to form an early mitotic spindle between the duplicated centrosomes.||Intact.||Intact.|
|Metaphase||Align along the metaphase plate, the midpoint between the two centrosomes. Sister chromatids are joined at the centromere by proteins that form a structure called a kinetochore.||The two centrosomes are now located at opposite poles of the cell.||In higher eukaryotes like vertebrates, by the time metaphase occurs the nuclear envelope has broken down. This is caused by phosphorylation of nuclear lamin proteins.||Intact.|
|Anaphase||Cohesin is cleaved at the centromere of chromosomes, resulting in sister chromatids being pulled to opposite poles of the cell.||Microtubules emanating from the centrosomes shrink as the tension holding the chromosomes at the metaphase plate is broken by cohesin cleavage. ||Broken down.||Intact.|
|Telophase||Chromosomes begin to uncoil, becoming less condensed.||The centrosomes remain segregated to opposite sides of the cell. Each daughter cell will receive one centrosome comprised of two centrioles.||A nuclear envelope reforms around the chromosomes in each daughter cell.||Intact.|
|Cytokinesis||Chromosomes have returned to their interphase structure. This is a topic of much research, but it seems as though each chromosome occupies its own territory within the nucleus.||Centrosomes signal to the cell that it is okay to proceed with cytokinesis. Research shows that cells where centrosomes are destroyed with a laser beam cannot undergo cytokinesis.||Intact.||Pinches to form two separate membranes around the two daughter cells.|
Active research questions
How do cells promote faithful chromosome segregation in mitosis?
How are homologous chromosomes held together, and then separated in meiosis I?
High-throughput single-cell analysis has been regularly acclaimed as a transformative technology for sequencing and a huge step towards the goal of personalized medicine. Will Tapestri, a sequencing platform developed by Mission Bio, be the game-changer in DNA analysis we have been waiting for?READ MORE
A Chinese scientist, He Jiankui, has claimed that he helped make the world-first gene-edited babies: two twin baby girls. Reportedly, he used CRISPR-Cas9 to edit embryos for seven couples during IVF fertility treatment, and engineered the embryos to possess resistance to HIV infection. He Jiankui, from the Southern University of Science and Technology, is now facing investigation over whether the experiment was illegal.READ MORE
CRISPR-based gene editing has been implemented across the scientific community in the past 5 years. Although CRISPR is relatively easy and efficient compared to previous gene editing methods, researchers are continuing to improve the technology. New advancements are designed to make different types of genetic changes, improve efficiency, and improve delivery of CRISPR for different therapeutic approaches.