A biological cell is transparent and so is difficult to see with a microscope. Biologists usually dye cells before they use a microscope to examine them. The dyes they use are called stains and the dyeing process is called staining. A stained animal cell then shows the features in the picture above – it is… Continue reading 20.9 Chromosomes 1 – mitosis

People have been selectively breeding animals and plants for thousands of years – to provide better sources of materials for food and clothing or for decoration. Carassus auratus is a small brown fish related to the carp. But, occasionally, an individual appears with an unusual shape or colour (usually red or yellow). By selectively breeding… Continue reading 21.8 Genes

Before you read this, I suggest you read post 21.4; it would also be useful to read post 21.6. Some protein molecules can form a three-dimensional structure call the α-helix (α is the Greek letter “alpha”), shown in the picture above. In this picture, the coloured balls represent different atoms: black for carbon, white for… Continue reading 21.7 The alpha-helix

Before you read this, I suggest you read post 21.3. A corkscrew is used to remove corks from a bottle – see the picture above. It is made by bending a metal rod, with a circular cross-section into a shape called a helix. The centroids of all the circular cross-sections lie on a helical line… Continue reading 21.6 The helix

Before you read this, I suggest you read post 21.3. In post 21.3 we met the simple spiral whose equation in polar coordinates is r = aθ                    (1) where a is a constant, r is the radius of the spiral and θ is an angle that is positive for anticlockwise rotation. In this post we… Continue reading 21.5 Logarithmic spirals

Proteins are condensation polymers of amino acids. The structural formula of an amino acid is shown at the top left-hand corner of the picture above, where R is any chemical group. (Two amino acids, proline and hydroxyproline, are slightly different). In post 20.27 we met alanine – the amino acid in which -R is the… Continue reading 21.4 Proteins

I have used polar coordinates in a previous post. But that post contained mathematics that was more much difficult than is needed here. So, I am going to begin with a simple description of polar coordinates. Usually we describe the position of a point in space using Cartesian coordinates; in two dimensions the position of… Continue reading 21.3 Polar coordinates, circles and spirals

DNA is an example of a nucleic acid. Its name is an abbreviation for deoxyribonucleic acid. It was discovered in 1869 by a Swiss doctor called Friedrich Miescher (1844-1895). Most people know that DNA is responsible for coding genetic information but may not know much about its chemical composition, how it codes this information or… Continue reading 21.2 Nucleic acids

While I was compiling the index to this blog, I noticed that I had written several posts about mechanical simple harmonic oscillators but hadn’t written anything about electrical simple harmonic oscillators– the subject of this post. A simple harmonic oscillator is a system that oscillates indefinitely by exchanging potential and kinetic energy. In a real… Continue reading 21.1 An electrical simple harmonic oscillator

Before you read this, I suggest you read post 20.38. Cells (post 16.18) in plants are surrounded by a rigid wall made of the polysaccharide (post 20.38) cellulose that consists of repeating β-D-glucose residues (post 20.38) as shown above. In this picture, the second residue is identical to the first but I have shown it… Continue reading 20.40 Polysaccharides