An investigation to find out if there is a relationship between the concentration of phosphate ions in nutrient solutions and the number of cells dividing by mitosis in root tips

Background information:

Mitosis is the nuclear division of a cell producing two genetically identical daughter nuclei, each containing the same number of chromosomes as the parent nucleus.

Mitosis is a continuous cycle but has been divided into four stages; prophase, metaphase, anaphase, telephase. The following are each of the phases described in more detail.

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1. The Prophase

* The two centrosomes of the cell, each with its pair of centrioles, move nnnnn to opposite “poles” of the cell.

* The mitotic spindle forms. This is an array of microtubules, synthesized n from tubulin monomers in the cytoplasm, that develops from each nnnnnncentrosome.

* The chromosomes become shorter and more compact.

* The nuclear envelope disintegrates.

* A protein structure, the kinetochore, appears at the centromere of each nnnnnchromatid.

* With the breakdown of the nuclear envelope, spindle fibers attach to nnnnnnthe kinetochores as well as to the arms of the chromosomes.

The microtubules attached to a kinetochore exert tension on its chromatid. For each dyad, one of the kinetochores is attached to one pole, the second (or sister) chromatid to the opposite pole. Failure of a kinetochore to become attached to a spindle fibers interrupts the process.

2.. Metaphase

The tension is proportional to length; thus if a dyad approaches one pole, the tension in the opposite direction increases and the dyad is pulled back to an equilibrium position midway between the poles. In due course, all the dyads reach this position, the equatorial plane or metaphase plate. The chromosomes are at their most compact at this time.

3. Anaphase

The sister kinetochores suddenly separate and each move to its respective pole dragging its attached chromatid (chromosome) behind it.

Separation of the sister chromatids depends on the breakdown of the cohesins that have been holding them together. It works like this.

* Cohesin breakdown is caused by a protease called separin (also known as separase).

* Separin is kept inactive until late metaphase by another protein called securin.

* Anaphase begins when the anaphase promoting complex (APC) destroys securin (by tagging it for deposit in a proteasome) thus ending its inhibition of separin and allowing separin to break down the cohesins.

4. Telophase

A nuclear envelope reforms around each cluster of chromosomes and these return to their more extended form.

Phosphate is important for all organisms; it is essential for growth and development and has several roles;

* Nucleotides – DNA, RNA & ATP are nucleic acids; phosphate groups are vital to their structure. It combines with the sugar forming a sugar-phosphate backbone, which holds the molecule and covalently bonds different nucleotides to one another. DNA & RNA have one phosphate group and ATP has three. ATP is used by cells to carry out ‘work, therefore is considered as ‘energy currency’ and is needed during the actual process of mitosis.

* DNA replication – two extra phosphate groups are added to nucleotides for activation, allowing them to pair up with complementary bases on the ‘old’ DNA strands (semi-conservative replication).

* Protein synthesis – phosphate groups are required during transcription to produce mRNA otherwise protein synthesis can’t occur; thus no proteins will be made.

* Phospholipids make up the plasma membrane, the phosphate group makes the head hydrophillic whilst the fatty acid tails remain hydrophobic, this is very significant for the membrane’s characteristics. New membranes are synthesised (cytokinesis), therefore phosphate must be available.

( I have obtained some of the factual information above this line from my school notes and my OCR biology 1 textbook)

Experiment Prediction:

High levels of phosphate allows more ATP to be produced so more energy is available. Mitosis is an ‘ active’ process, ATP will be needed by the energy consuming cells. Needed to activate nucleotides in cytoplasm so DNA can be replicated. Insufficient amounts of phosphate means that DNA nucleotides can’t be made or activated, therefore no replication. More phosphate ions allows more nucleotides (DNA & RNA) to be produced, as they are a component of the backbone. Daughter nuclei produced will go through cytokinesis and phosphate ions will be needed to synthesise new membranes. Lack of phosphate means protein synthesis won’t occur or will be inefficient, vital enzymes needed during mitosis can’t be made and mitosis will not take place.

So by taking all of the above into account my overall predication is: that the more phosphate present, the more mitosis taking place.

Preliminary experiment:

In this experiment I prepared and observed of the stages of mitosis in root tips. I did this by preparing a squash slide to view under a microscope. I understood the consequences of adding too much acetic orcein (stained chromosomes too deeply.) I heated the hydrochloric acid by warming over a flame; I developed this for my method, as it was not an accurate technique. The temperature couldn’t be controlled and therefore could disrupt my results so I plan on using a water bath at a set temperature and a thermometer to ensure the temperature was maintained.


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