Learn About Cell Division

2.5.3 State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts (1).

State means to give a specific name, value or other brief answer without explanation or calculation.

• The cell specialises to a particular function in a process called differentiation.

• Through gene expression and protein synthesis there is a specialisation of cell structure and function.

• During this interphase the cell carries out this specialist function.

• The length of the interphase varies from one type of cell to another.

• G1 follows cytokinesis. The cell is involved in the synthesis of various proteins which allow the cell to specialise.

• S-phase involves the replication of DNA molecules which takes place prior to the phases of mitosis.

• G2 preparation for the phases of mitosis which involves the replication of mitochondria and in the case of plants, the chloroplast.

Much of the information and images adapted from Click4Biology.

2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase (2).

Describe means to give a detailed account.

Super coiling: Eukaryotic DNA is combined with histone proteins and non-histone proteins to form chromatin. The method of folding of chromatin is specific to each chromosome leaving genes in predictable positions and a distinctive overall chromosome shape. The human cell has a DNA length of about 1.8 m this has to be packed into a nucleus which has only a 5 um diameter. This packaging process requires up to a X 15,000 reduction. This super coiling makes the structure so dense that it can be see with a light microscope during the phases of mitosis.

In this sequence only one chromosome is illustrated so that we can more clearly follow the process. In a human a complete diagram would have 46 chromosomes each replicating and condensing and separating.

a)The cell membrane is intact during this the interphase. The chromosomes cannot be seen during G1,S and G2.

b) G1,Within the nucleus, genes on the chromosome are being expressed to carry out normal cell function (interphase). Remember you cannot see chromosomes at this stage. The diagram has a 'see's through' the nuclear membrane so you can see inside. In reality it would look just like cell a).

c) S-phase in which DNA replication occurs and the chromosomes are copied. The copies called sister chromatids are held together by a protein to form the centromere. It is still not possible to see this happen with an intact cell.

d) Early Prophase in which the sister chromatids have condensed by super coiling. Note the formation of the spindle microtubules and their attachment to centrioles. The nuclear membrane will now break down to reveal sister chromatids. The internal arrangements of chromosomes can now be seen with a light microscope.

e) Metaphase the chromosomes arranged on the equator of the cell each attached to a spindle microtubule at the centromere

f) Anaphase: The spindle microtubules contract and pull apart the sister chromatids one to each pole of the cell. The centromere splits allowing the sister chromatids to be separate.

g) Telophase: at each pole there are separate groups of the replicated chromosomes the spindles is degenerating

h) Cytokinesis: the cell membrane begins to separate, dividing the cell into two new cells. The nuclear membrane is reforming around each cell.

i) Two daughter cells are formed. They are genetically identical to each other and in effect the basis of a clone. (see 2.5.6)

Notice that cell a) begins with one chromosome and that by step h) there are two cells each with a copy of that chromosome.

As suggested by cell theory, all cells have come from other cells.

Much of the information and images adapted from Click4Biology.

2.5.5 Explain how mitosis produces two genetically identical nuclei (3).

Explain means to give a detailed account of causes, reasons or mechanisms.

• The process of cell division produces genetically identical daughter cells.

• Conservation of chromosome number. The chromosome number in each of the daughter cells is the same as that of the original parental cell

• During the S-phase, each chromosome is copied exactly. The two copies of each chromosome are held together by a protein structure called a centromere.

• Therefore just prior to the beginning of the phases of mitosis there is actually double the number of chromosomes present in a cell.

• Each chromosome in this state is represented by a pair of sister chromatids. These give the now classic cross image of the DNA (see image below)

This pair of sister chromatids image was taken during one of the phases of mitosis.

The two sister chromatids are held together at the centromere

The arms of the chromatids are visible because of a condensation of the molecule called super coiling.

This condenses the molecule some x 15,000 times of its original length The pairs of sister chromatids is a non-random organisation. The position of genes is predicable within the structure seen here. Also there is a unique shape to each of the chromosomes.

Mitosis makes sure that each cell obtains a copy of each of the chromosomes in the parental cell.

However, it is the process of DNA replication during the S-phase that actually copies each DNA molecules to make mitosis possible.

2.5.6 State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis(1).

State means to give a specific name, value or other brief answer without explanation or calculation.

• Growth: multicellular organisms increase their size through growth. This growth involves increasing the number of cells through mitosis. These cells will differentiate and specialise their function.

• Embryonic development is when the fertilised egg cell (zygote) divides to form the multicellular organism. Each cell in the organisms is identical (genetically) to all the other cells. However, each cell will express only a few of its genes to determine its overall specialisms, a process called differentiation. In this way a stem cell may becomes a muscle, or it may become a nerve cell or any one of the many different kinds of cells found in a complex multicellular organism. The best book about this process for the interested reader is

• Tissue Repair: As tissues are damaged they can recover through replacing damaged or dead cells. This is easily observed in a skin wound. More complex organ regeneration can occur in some species of amphibian.

• Asexual Reproduction: This the production of offspring from a single parent using mitosis. The offspring are therefore genetically identical to each other and to their “parent”- in other words they are clones. Asexual reproduction is very common in nature, and in addition we humans have developed some new, artificial methods. Bacteria DO NOT asexually reproduce by mitosis but rather by a process called Binary Fission.

  
  

  Much of the information and images adapted from Click4Biology.