The opening screen of topics in Science Volume 3 provides access to sections describing the structures and functions of a chromosome, hemoglobin, sperm cell, microvilli and a nerve cell. Protein formation shows a short animation of protein formed from amino acids. Membrane diffusion animation clip demonstrates several diffusion processes. The immune system section shows schematically the interaction between the different cells of our immune system

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Chromatin fiber is the basic component of the hromosome. The fiber itself is built from a hellical coil of DNA in form of small pockets called nucleosomes. Nucleosomes consist of a core of histone proteins around which the DNA is wound. The chromatin fiber forms a helical solenoid which itself coils into a hollow tube about 200 nm in length. The genetic material is present in this form in a cell which is in the interphase - in between cell division. As the cell enters mitosis - the hollow tube folds further to form the chromatids.

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Proteins are made of a sequence of specific amino acids. They are characterized by the kind of amino acids they are made of and by the order in which the amino acids are put together. A Ribosome is the cellular machine that assembles proteins.Large ribosomal subunit comes together with small ribosomal subunit and a messenger RNA molecule (mRNA). Messenger RNA provides assembly instructions the two ribosomal subunits need to sequent the correct amino acids into the growing polypeptide. Once the functional ribosomal unit is assembled an initial segment of peptide is formed.

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Membranes regulate the transport of ions and molecules between the cell and it’s environment. This regulatory mechanism is essential for the cell functions. It ensures supply of nutrients, oxygen for respiration and the disposal of waste products.
The animation clip describes the three forms of transport through the membrane: oxygen and water molecules diffuse through the membrane bi-layer. Glucose molecules are transported by a passive facilitated transport process through a transport protein (on the right of the animation window). Sodium and potassium pump (left side of the animation window) pumps both ions simultaneously in opposite directions.

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