Before you read this, I suggest you read post 21.2. Most people who are interested in science know that the DNA molecule is a double helix. But what does this mean? How do we know? And why is it important? By the early 1950s, there was evidence that genes were made of DNA and that… Continue reading 21.12 The DNA double helix

If we believe that chromosomes contain genes (see post 21.10), then genes must be made of either DNA or a type of protein called a histone – because that’s what chromosomes are made of. The question is then – are genes made of DNA or histones? In the early 1940s, most people believed that genes… Continue reading 21.11 Genes are made of DNA

Before you read this, I suggest you read post 21.9. In post 21.9, we saw how cells can divide to produce copies of themselves during growth or to replace old cells in a process called mitosis. During mitosis, chromosomes appeared in the cell nucleus. These chromosomes appeared in pairs, so we can say that each… Continue reading 21.10 Chromosomes 2 – meiosis

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 21.9 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