Cell Division, Cell Diversity and Cellular Organisation

Cell Division

Eukaryotic cells enter the cell cycle and divide by mitosis or meiosis. In comparisons, prokaryotic cells replicate by binary fission and viruses do not undergo cell division as they are non-living. Viruses replicate inside of the host cells they invade by injecting their nucleic acid in to cell to replicate the virus particles.

Cell Division, Cell Diversity and Cellular Organisation, figure 1

Stem Cells

Multicellular organisms have a diverse range of specialised cells that all originate as undifferentiated stem cells. Stem cells are undifferentiated cells that can continually divide and become specialise.

Differentiation is the process by which stem cells become specialised. They key specialised cells you need to be able to describe the structure of and link to their function are:

  1. Erythrocytes - biconcave shape to increase surface area for diffusion and increase flexibility to fit through narrow capillaries. No nucleus so there is more space to hold haemoglobin to increase oxygen transport.
  2. Neutrophils - Flexible cell to enable them to surround pathogens and engulf them. They contain lysosomes filled with the hydrolytic enzyme, lysozyme.
  3. Squamous epithelial cells - Usually only a single layer of flat cells in contact with the basement membrane of the epithelium. This provides a short diffusion distance.
  4. Ciliated epithelial cells - These cells have hair like projections that sway to move substances, such as mucus in the lungs or an egg in the oviduct.
  5. __Sperm cells __- Flagellum containing many mitochondria to release energy for locomotion and contains digestive enzymes in the tip of the head to digest the wall of the egg cell.
  6. __Palisade cells __- Contain many chloroplasts to absorb maximise light energy for photosynthesis.
  7. __Root hair cells __- Increase surface area for osmosis of water and active transport of minerals due to the projection of the cell. Has a thin cell wall to reduce the diffusion distance.
  8. __Guard cells __- Flexible walls, more so on one side, which results in the cells bending when turgid to open stomata and close when flacid and this helps control water loss.

There are different types of stem cells that have different differentiation abilities. These are totipotent, pluripotent, multipotent and unipotent stem cells.

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Totipotent cells can divide and produce any type of body cell. During development, totipotent cells translate only part of their DNA, resulting in cell specialisation. Totipotent cells occur only for a limited time in early mammalian embryos.

Pluripotent cells are found in embryos and can become almost any type of cell. For this reason, they are used in research with to prospect of using them to treat human disorders. These stem cells could be used to regrow damaged cells in humans, such as replace burnt skin cells, or beta cells for type I diabetics or neurones in Parkinson’s disease sufferers. There are issues with this as sometimes the treatment doesn’t work, or the stem cells continually divide to create tumours. Additionally, ethically there is debate on whether it is right to make a therapeutic clone of yourself to make an embryo to get the stem cells to cure a disease, and then destroy the embryo. Is it safe or even sensible to clone humans and to destroy embryos?

Multipotent and unipotent cells are found in mature mammals and can divide to form a limited number of different cell types. Multipotent cells, such as in bone marrow, can differentiate into a limited number of cells such as erythrocytes and neutrophils, whereas unipotent cells can only differentiate into one type of cell.

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The stems cells in adult plants are found at meristems. This is where the production of tissues, such as xylem and phloem sieve tubes, are formed.

Sources of Stem cells in Mammals:

Embryos, up to 16 days after fertilisation, contain stem cells that are pluripotent and can differentiate into any type of cells.

Umbilical cord blood contains stem cells which are multipotent, like adult stem cells.

The placenta has stem cells that can develop into a limited number of specialised cells.

Adult stem cells, such as in the bone marrow, can produce different cells to repair those within a particular tissue or organ.

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There are pros and cons of using stem cells to treat human disorders. The benefits would include research or even treatment of damaged tissues for neurological conditions like Alzheimer’s and Parkinson’s, and research into developmental biology. Induced pluripotent stem cells (iPS cells) can be produced from adult somatic cells using appropriate protein transcription factors to overcome some of the ethical issues with using embryonic stem cells.

iPS cells are created from adult unipotent cells. These cells, which can be from almost any body cell, are altered in the lab to return them to a state of pluripotency. To do this, the genes that were switched off to make the cell specialised must be switched back on. This is done using transcriptional factors. They are very similar to embryonic pluripotent stem cells, but do not cause the destruction of an embryo and the adult can give permission. Also, the iPS have shown a self-renewal property, in that they can divide indefinitely to give limitless supplies. For these reasons they could be used in medical treatment instead of embryonic stem cells.

List the three main stages of the cell cycle:
Your answer should include: Interphase / Nuclear / Division / Cytokinesis
When does DNA replication occur?
Your answer should include: S Phase / S-phase / Interphase
Which stem cells can differentiate into any type of cell?
Totipotent
Which is the longest stage of the cell cycle?
Interphase

The Cell Cycle

The cell cycle comprises three key stages: interphase (G1, S , G2), nuclear division (mitosis or meiosis) and cytokinesis.

Cell Division, Cell Diversity and Cellular Organisation, figure 1

Interphase is the longest stage in the cell cycle. Interphase is when the organelles double, the cell grows and then DNA replicates.

Nuclear division can be either mitosis, creating two identical diploid cells, or meiosis, creating four genetically different haploid cells. Mitosis creates cells with identical DNA for growth and repair, whereas meiosis creates gametes.

Cytokinesis is the final stage. It is the division of the cytoplasm to create the new cells.

Mitosis

Mitosis has four key stages: prophase, metaphase, anaphase and telophase.

Prophase

In this stage the chromosomes condense and become visible. In animal cells, the centrioles separate and move to opposite poles of the cell. The centrioles are responsible for creating spindle fibres which are released from both poles to create a spindle apparatus – these will attach to the centromere and chromatids on the chromosome in later stages. Plants have a spindle apparatus, but lack the centrioles.

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Metaphase

The chromosomes align along the equator of cell. The spindle fibres released from the poles now attach to the centromere and chromatid.

Cell Division, Cell Diversity and Cellular Organisation, figure 3

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Anaphase

The spindle fibres start to retract and pull the centromere and chromatids they are bound to towards the opposite poles. This causes the centromere to divide in two and the individual chromatids and pulled to each opposite pole. These separated chromatids and now referred to as chromosomes.

This stage requires energy in the form of ATP which is provided by respiration in the mitochondria.

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Telophase

The chromosomes are now at each pole of the cell and become longer and thinner again. The spindle fibres disintegrate, and the nucleus starts to reform.

The final stage in the cell cycle is when the cytoplasm splits in two to create the two new genetically identical cells.

The stages of mitosis are visible under a light microscope in onion and garlic root tips. The tips of the roots are constantly dividing for growth and therefore many of the cells are replicating, and the different stages of the cell cycle and mitosis are visible. A small slice of the root tip is placed on a microscope slide and broken down with a needle. A stain is added to make the chromosomes visible and the cover slipped is pushed down. This is to squash the tip to achieve a single layer of cells so it is possible to focus on the cells. The mitotic index can be calculated by counting how many cells are visible in the field of view and the number of cells visible that are in a stage of mitosis.

Then the following formula can be used:

Mitotic index= (the number of cells in mitosis/the total number of cells) x 100

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Genetic Variation

This is introduced in a variety of ways, such as during meiosis cell division, mutations and the random fertilisation of gametes.

Meiosis

This is the type of cell division that creates genetically different gametes. Unlike in mitosis, there are two nuclear divisions in this process which results in four haploid daughter cells.

Haploid (n) = one copy of each chromosome

Diploid (2n) = two copies of each chromosome

The genetic differences are introduction by two key processes in meiosis:

  1. Independent segregation of homologous chromosomes
  2. Crossing over between homologous chromosomes

Cell Division, Cell Diversity and Cellular Organisation, figure 9

The above diagram outlines the stages in meiosis. Before cell division, interphase occurs in which the DNA and organelles double.

As meiosis involves two nuclear divisions the descriptions of the processes can be stated as being in meiosis I or meiosis II, referring to the stage of nuclear division. Both stages include prophase, metaphase, anaphase, telophase and cytokinesis.

Crossing Over

During prophase I (one) the chromosomes condense and become thicker. The homologous chromosomes pair to form bivalents. Crossing over of genetic material can occur between the chromatids of bivalents. Where the chromatids cross over is called the chiasma - there can be more than one chiasma at a time. Breaks can occur in the genetic material and parts of the chromatids are exchanged between the homologous pairs. This results in new combinations of alleles in the resulting gamete.

Cell Division, Cell Diversity and Cellular Organisation, figure 10

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Independent Segregation

During metaphase I, in meiosis I, the homologous pairs of chromosomes line up opposite each other either side of the equator. As there are 23 different homologous pairs, there are 8,388,608 different ways the pairs could assort themselves (2 to the power of 23). It is random each time which side of the equator the paternal and maternal chromosome of each homologous pair align at the equator and as a results each gamete receives different combinations of the maternal and paternal chromosomes.