5. Cell division
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In unicellular organisms, cell division takes place to create new individuals. In multicellular organisms, it also takes place to replace tissues and for growth.
5.1. Mitosis
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All cells (with the exception of gametes) reproduce in sets of two from a single parent cell . These cells are referred to as daughter cells and they have to be identical to each other and their parent. In order for this to happen, the same exact copy of genetic information is transmitted from the parent cell to the daughter cells.
As genetic information is stored in the nucleus, the most important step of cell reproduction is its division through a process called mitosis . In order to provide the same genetic information to both daughter cells, the parent cell must first have two copies. This is why genetic information duplicates before mitosis takes place. Mitosis is then used to provide each daughter cell with a copy.
Cells obtained through the process of mitosis contain the same number of chromosomes as their parent cell (they are diploid cells). The process of mitosis can be divided into four stages: prophase , metaphase , anaphase and telophase .
Prophase:
Chromatin fibres thicken and shorten to form chromosomes.
The nucleolus disappears.
Protein fibres appear between both poles of the cell to form the mitotic spindle .
The nuclear membrane disappears, so chromosomes can move freely around the cytoplasm.
Nucleus during the stage of prophase seen through an optical microscope .
Metaphase:
Chromosomes bond to the mitotic spindle fibres using their centromeres.
This union takes place in the equator of the cell.
Sister chromatids belonging to each chromosome point to the opposite poles of the cell.
Nucleus during the stage of metaphase seen through an optical microscope.
Anaphase:
The mitotic spindle fibres break into two equal halves. This causes the chromosome’s centromere to break. As a result, the two chromatids that made up each chromosome separate into two unconnected half fibres.
The mitotic spindle fibres contract, pulling the chromatids towards the opposite poles. From this point onwards, the chromatids are considered to be independent chromosomes.
Nucleus during the stage of anaphase seen through an optical microscope.
Telophase:
Once the chromatids move to the ends of the poles, the remains of the mitotic spindle fibres disappear.
A new nuclear membrane surrounds each group of chromatids, forming two new nuclei.
Chromatids expand and turn into chromatin.
The nucleolus reappears.
Nucleus during the stage of telophase seen through an optical microscope.
Cytokinesis in animal (A) and plant cells (B)
If all the stages take place correctly, both daughters' nuclei will have exactly the same genetic information and number of chromosomes as their parent cell. This happens because the two chromatids that make up each of the chromosomes of the initial cell are two identical copies and each of the copies is transferred to a daughter cell.
Once mitosis ends, the division of the cytoplasm takes place and cell organelles are shared. This process is referred to as cytokinesis and it can take place in two ways:
In animal cells, the cytoplasm stretches, thins out and eventually separates.
In plant cells, a wall forms which divides the cytoplasm in half.
Weblink 6: Cell division, mitosis and meiosis
5.2. Meiosis
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When cells divide to create gametes , they produce cells that are not identical. Gametes only have a haploid number of chromosomes, meaning they only have half the genetic information.
Gametes join together during fertilisation to form a cell named zygote . This cell has a diploid number of chromosomes, as it has a set of haploid chromosomes from each parent. The zygote will then grow into an organism after completing many cell divisions.
Diagram of fertilisation
In order to form gametes, cells divide through a process called meiosis . Meiosis is a special type of cell division from which four daughter cells are created. Each of these daughter cells carries half the genetic information of the parent cell. The process of meiosis is made up of two mitosis divisions.
Diagram of meiosis
Weblink 7: Meiosis
5.2.1. First mitosis
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This type of mitosis produces daughter cells that have half the chromosomes of the parent cell. It has the same stages as the normal mitosis, but there are some differences in some of them:
In the prophase I , chromosomes appear, but they are in homologous pairs. At this point, a genetic exchange between these pairs occurs, called genetic recombination or crossing over .
In the metaphase I , pairs of homologous chromosomes, not individual chromosomes, bond to the mitotic spindle fibres.
In the anaphase I , a complete chromosome moves to each cell pole, instead of half a chromosome.
There are no differences in the telophase I .
At the end of the first mitosis, there are two haploid daughter nuclei that have n chromosomes.
5.2.2. Second mitosis
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This type of mitosis is the same as the normal mitosis. Chromosomes divide into chromatids and each daughter cell receives one set. This way, meiosis produces four haploid daughter cells, which have half the chromosomes of the original parent diploid cell.
Mitosis II of meiosis seen through an optical microscope.
Video 6: Replication and distribution of DNA during meiosis
5.3. Similarities and differences between meiosis and mitosis
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Both mitosis and meiosis are very similar processes. Both processes start by the condensation of chromatin to form chromosomes. They both create mitotic spindle fibres to which chromosomes attach and are moved to the cell's opposite poles. They also end the same way, chromatids expand to turn into chromatin and the new daughter nuclei are formed.
However, mitosis and meiosis are different processes:
In mitosis, a diploid parent cell creates two diploid daughter cells that have exactly the same genetic information. In meiosis a parent cell creates four haploid daughter cells with different genetic information.
In mitosis, only one process takes place. In meiosis, there are two successive mitosis processes: in the first one genetic recombination takes place and the second one is a normal one.
Even though the stages in mitosis and the first mitosis in meiosis are the same, there are some important differences between them:
Prophase
Prophase I of meiosis takes longer than that of mitosis. Genetic recombination between homologous pairs of chromosomes takes place in prophase I of meiosis. This does not happen in mitosis.
Anaphase
In mitosis, individual chromatids of each chromosome separate. Each chromatid then moves to one of the cell's poles. In anaphase I of meiosis, homologous chromosome pairs separate. Each chromosome then moves to one of the cell's poles.
Telophase
In mitosis, two diploid daughter cells, that are identical to the parent cell, are created. In telophase I of meiosis, two haploid daughter cells, that are not genetically identical to the parent cell, are created.
The two cells obtained during the first mitosis process in meiosis begin the second normal mitosis. At the end of this process, four haploid daughter cells are created.
Weblink 8: Mitosis versus meiosis
5.4. Biological importance of cell division
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Mitosis and meiosis are two sophisticated mechanisms. Their aim is to complete cell reproduction without any mistakes and to make sure cells have the necessary genetic information.
Mitosis creates cells for growth and asexual reproduction. This process makes sure genetic information from the parent cell stays the same in the daughter cells. This is important because any changes in the genetic information would lead to severe consequences.
An example of problems during mitosis is cancer. This illness makes cells of the affected tissues grow in a harmful uncontrolled manner.
Chromosomes in prophase I of meiosis seen through an electronic microscope (genetic recombination is taking place).
Meiosis , on the other hand, is used for sexual reproduction. It makes sure daughter cells get modified genetic information from the parent cell by using the process of genetic recombination.
Diagram showing genetic recombination during prophase I of meiosis.
Genetic recombination in homologous chromosomes is an extremely valuable process, as it provides new genetic combinations to offspring . From an evolutionary point of view, this is very important. These new genetic combinations have led species to evolve to what they are today and they control how they will evolve in the future.
Video 7: How do cancer cells behave differently from healthy ones?
Key concepts
Mitosis is the reproduction process used by somatic cells. This process ensures that exactly the same genetic information is shared with the daughter cells.
Meiosis is a special type of division used for making gametes. Only half the genetic information from the parent cell is passed on to the daughter cells.
