 Fig.1 A Typical animal cell. Viruses show some characteristics
of living things but they are not classified among the living as they do not
have cells. They are unable to propagate without enering a host cell.
Fig. 2. A Typical Plant Cell.  Bacteria do not have a proper nucleus, yet they are amongst us, as they have cells enveloped in a cell- membrane. Let us now compare a typical animal cell and a plant cell. There are certain structures covered with membranes inside the cells. These are termed ‘organelles’. They widely differ in size and shape and carry out specific functions. Some of them are listed here. Nucleus - This is the most vital organelle inside a cell. It has two membranes with some pores to communicate with the cytoplasm. It has DNA strands with every aspect of the individual encoded in the form genes. For cell division genes are arranged into chromosomes. Although the cells of an individual may differ the genes are identical in every cell. The nucleolus produces ribosomes. Chromosome - A pair of chromosomes consisting of two chromatids is shown here. They can be seen only during cell division. Reduction of the size of the telomere during each cell division is considered to be the reason for growing old.  Fig. 3. A pair of chromosomes
joined at the centromere. The two chromatids joined at the centromere are from the two parents. Colours are used to show some of the genes.
This is a highly specialized structure that
encloses the cytoplasm and the organelles. It has the ability to select what
comes in and goes out of the cell. While it determines the shape of the cell it
can get attached to other cells of the body. Cytoplasm – This is a jelly like substance that fills the cell. It is mostly water with a few other molecules suspended or dissolved. A network of fibers gives the strength to maintain the shape. Mitochondria - These are the power houses of the cell. This also has some genes that have come from the mother.  Fig 4: Mitochondrion Endoplasmic
reticulum- These are tubes through which synthesized proteins can pass. Proteins necessary for the cell is synthesized from amino acids by ribosomes. There are ribosomes, attached to rough endoplasmic reticulum. Lysosomes- These are vacuoles filled with enzymes. They can digest unwanted organelles or bacteria. Lysosomes are made in the Golgi apparatus. Cyto skeleton -This is a network of fibers in the cytoplasm. This keeps the shape of the cell and holds and moves organelles inside. (These fibers are not shown in the diagram) Cell Division
Binary Fission Fig.5 The stages of reproduction in Amoeba.
An Amoeba is a microscopic protozoan. The whole individual is only one cell, yet it can perform all the living functions. It moves with cytoplasm projecting out wards. These processes are called pseudopodia (false feet) it feed by engulfing food into the cytoplasm and digesting inside a vacuole. As a result of feeding it grows. The stages it undergoes cell division which is called binary fission. The figure shows how this process takes place. After division the two cells separate. Two daughter cells result from one parent. So this amounts to reproduction which may be termed Asexual Reproduction. Stages in Mitosis (Normal cell division) 1 & 2 Interphase - Numbers 1 and 2 illustrate this phase. A DNA strand is formed and it thickens. The nuclear envelop disappear. 3. Prophase - A definite number of paired chromosomes form connected by a part called the centromere. Cytoskeleton forms a mitotic spindle 4. Metaphase - The nuclear envelop disintegrate. Spindle fibers gets attached to chromosomes in the centromeres Spindle fibers arrange the chromosomes in a plane at the centre. 5. Anaphase - Fibers contract pulling chromatids. 6. Telophase - When the daughter chromosomes group together at the two ends of the cell. Cytokinesis - The remaining spindle fibers create a vacuole in the centre cutting of the cytoplasm into two, each getting a set of chromosomes. Stages of Meiosis (Reduction Division)This process is necessary for sexual reproduction. The four daughter cells formed at the end of meiosis are called gametes or sex cells. They have half the number of chromosomes that an adult should have.  |
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