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Growth : is an irreversible increase in size and or dry mass. The basis of growth is cell division by mitosis, cell differentiation and cell enlargement.
Growth can be recognized in several ways. In unicellular organism, growth is recognized by an increase in size and mass of the cell. When the unicellular organism reproduces, such as by binary fission, there is an increase in the number of individuals produced, this reproduction is known as the evidence of growth.top↑
In multicellular organisms e.g Man, growth is recognized by an increase in the number of cells. When cells assimilate(absorb) nutrients, there is an increase in size, length and mass.
Note : All the above points are evidence of growth in both multicellular and unicellular organism.
Truly growth is not reversible, when a body enlarges in size or in mass due to the uptake water, such enlarges is not a true growth. A good way to determine the trueness of growth is to compare the dry weight of the representative samples of the organisms over a period of time. If the number of cells making up an organism increases without there being any accompanying increase in size or mass, here can still be classified as true growth.top↑
In the early development stage of fertilized frog egg, numerous cell divisions or cleavages by mitosis result in more and more cells but there is at least initially, no increase in the size or mass of the embryo.
Below Are The Stages In Early Cleavages In Frog Embryo
This type of cell division may occur in connection with growth, or in connection with repair of wear and tear (worn out tissues and cells). In unicellular organisms, usually each cell can be divided but in multicellular organisms, all cells cannot be divided.
In plants, mitosis occurs normally only in cells located at the tips of shoot and roots in the cambium, or in other specific parts.top↑
Cambium : is a layer of cells between the xylem and the phloem of plants which is responsible for the secondary growth of roots and stems.
These cells that can be divided are called meristematic and a group of meristematic cells are called meristem.
Mature plant cells do not normally divide but a wound may cause mature plants cells to become meristematic , such as when a hedge is cut.
In animals, cells which can be divided are not located at the tips of the body but are in various tissues all over the body.top↑
Mitosis consists of a division of the nucleus which is followed by a division of the cytoplasm. The division of the nucleus is a continuous process, but for convenience of description, it is considered to be made up of steps or stages called prophase, metaphase, anaphase and telophase. Now let examine them one by one, this explanation will explain mitosis in animals.top↑
Prophase : which is also known as chromosomes appear in the nucleus. These chromosomes can now be stained and observed under microscope, whereas in an unidividing cell, they cannot be observed. The number of chromosomes is constant in each cell of a species, for example, in maize plant, the number of chromosomes in the nucleus of a body cell is 20, while in the body cell of human being the number of chromosomes is 46.top↑
Early in prophase, the two centrosomes, that is, the two organelle, near the nucleus in the cytoplasm of organisms which controls the organization of microtubules (e.g proteins, I.e cytoskeleton), are separately located outside the nucleus of the cell, each moving towards the two opposite poles.
When the centrosomes arrive at the opposite poles, a fine filament called aster fibres(a star shaped structure) form around the centrosomes. Meanwhile changes continue in the nucleus. The nucleolus disappears, the nuclear membrane disintegrates and the chromosomes lie freely in the cytoplasm. When this happens, filaments are formed which run from the middle of each cell to the poles, then these filaments join with the star shaped structure "aster fibres" to form a structure known as the spindle fibre.top↑
I hope you haven't forget that all we have stated above is in the early prophase, let also check what has happened in the late prophase.
In late prophase, the chromosomes are observed to be composed of two strands each. These two strands are called chromatids and are held together at a point called the centromere.
Metaphase is the next stage after prophase. The chromosomes, each made up of two chromatids, come to lie at the equator of the cell where each chromosome is attached to a spindle fibre at the centromere.
In anaphase, the two chromatids in each chromosome separate at the centromere and begin to move toward opposite poles attached to the spindle fibres.top↑
On the arrival of the chromatids at the poles. At each pole, a new daughter nucleus is reconstituted containing the number of chromatids equal to the number of chromosomes in the parent cell. At this point, a new nucleus is formed and while this is going on, the spindle gradually breaks down. Don't worry, the diagram will be expressed in our next class.
Let call it a day here guys, till next class where will be discussing about cytoplasmic division and growth curves, thank you for joining us.top↑
Next Topic : Cytoplasmic Division
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