Although there is no universal agreement as to a definition of life, its biological manifestations are generally considered to be organization, metabolism, growth, irritability, adaptation, and reproduction.

–  The Columbia Encyclopaedia, First Edition, first sentence of the article on “life”, 1935

A cell is the smallest structure capable of basic life processes, such as absorbing nutrients, expelling waste, and reproducing.

Examining a thin section of cork through a homemade microscope Robert Hooke, in 1665, became the first to observe a cell. And the first thing that stuck him was how the rows of tiny boxes making up the dead wood’s tissue looked the rows of “cells” occupied by monks in a monastery; hence the name!

Some microscopic organisms, such as bacteria, consist of a single cell.

Multicellular organisms, such as ourselves, consist of many different varieties of cells working in concert.

The human body is made up of around 20-30 trillion cells of over 200 different varieties. One of the largest cells is the female egg cell which is about one millimetre in diameter; one of the smallest human cells is the male sperm.

There is great variation in cell shapes. One of the longest cells are the nerve cells connecting the end of our toes to the spinal cord and so spanning half the human body. A much quoted contender for the longest cell in the animal kingdom are the nerve cells running down a giraffe’s neck which can be as long as 3 meters!

Cells that make up multicellular organisms are called eukaryotic cells. These have a nucleus containing DNA. This is in contrast to the simpler prokaryotic cells, found only in bacteria, where the DNA mingles freely inside the cell’s interior. Eukaryotic cells also contain other organelles such as mitochondria (in animal cells) and chloroplasts (in plant cells).

It is believed that eukaryotic cells evolved from the more primitive prokaryotic cells around 2 billion years ago when animal mitochondria and plant chloroplasts, that were once free-living bacteria, were engulfed and maintained inside other cells for their ability to produce energy.

This idea of, once free, living cells living symbiotically inside our cells was used in the Star Wars movies and given the name of midi-chlorians (a derivative of mitochondria and chloroplasts) – Jedi warriors were said to have unusually high numbers of these!


We are machines built by DNA whose purpose is to make more copies of the same DNA. … This is exactly what we are for. We are machines for propagating DNA, and the propagation of DNAis a self-sustaining process. It is every living object’s sole reason for living.

– Richard Dawkins

DNA is the hereditary material in all living organisms and is found in every single living cell (except some viruses that contain the related RNA).

This DNA is usually packaged into structures called chromosomes, which were first observed using a special dye; hence the name Gr. chroma, colour; soma, body).

DNA can be a very long molecule made up of a variable sequence of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). These pair up with each other (A with T and C with G) to form units called base pairs which together are arranged in two long strands forming a spiral called a double helix.

DNA can replicate – each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. Therefore, when cells divide each new cell has an exact copy of the DNA.

Human cells contain around 3 billion of the four different bases forming DNA. These are arranged in a specific, yet unique, order to each of us. E. coli cells contains 4,6 million base pairs, the fruit fly has 160 million base pairs and the dog 2.4 billion. However, the size of the genome often bears little relation to the complexity of the organism – cells of the onion contain five times as much DNA as a human cell!

Human DNA contains an estimated 22,000 genes. A gene is the region of DNA sequence that can code for a protein; this information stored is transferred to a similar molecule called RNA which carries the information out of the nucleus and into the cell where it is used to assemble amino acids to form a protein

How do many organisms, seemingly less “advanced” than ourselves, have more genes than we do? While sharing over 99.9% identical gene sequences among ourselves as a species, we also share about 96% of our genes with chimpanzees, 80% with mice, 75% with dogs, 50% with the fruit fly (Drosophila) and 30% with a simple yeast!

One explanation for how this might be is that many genes are capable of making more than one protein, allowing human cells to make perhaps 80,000-100,000 proteins from our genes.

But the important take-home message is that what actually defines us genetically is not how much DNA an organism contains, or even how many genes are present; it is the complexity of how our genes are used and how these genes are controlled to interact with one another to carry out the various functions that give us our unique characteristics as humans.


 The basic structure of proteins is quite simple: they are formed by hooking together in a chain discrete subunits called amino acids.

– Michael Behe

Firstly a protein is not a carbohydrate and not a fat.

Proteins are molecules composed of one or more chains of amino acids in a specific order. This order is determined by the base sequence of nucleotides in the DNA coding for the protein, i.e. a gene.

Proteins are required for the structure, function, and regulation of the body’s cells, tissues, and organs. To do this, each protein has unique functions. Immunohistochemistry, using fluorescently-labelled antibodies against a specific protein, can be used to visualize proteins in a cell, such as an enzyme (green) in a neuron.

Examples of proteins include whole classes of important molecules, among them enzymes, hormones, and antibodies.

The word “protein” was introduced into science by the Swedish chemist Jons Jacob Berzelius (1779-1848) Greek; prota; of primary importance.

In 1955 Sir Frederick Sanger (the only living person to be awarded 2 Nobel prizes) sequenced or determined the complete amino acid sequence of the first protein – insulin.


For each chromosome contributed by the sperm there is a corresponding chromosome contributed by the egg; there are two of each kind, which together constitutes a pair. 

-Thomas Hunt Morgan

A chromosome is a strand of DNA and associated proteins that is found in the nucleus of eukaryotic cells. All the 3 billions base pairs of DNA in a human cell would stretch over 2 meters in length if unravelled out! To allow this huge mass of DNA to fit into a cell, it is packaged – by twisting, folding and wrapping around proteins – into a much more compact structure, known as a chromosome. At 5 µm long these chromosomes allow a 400,000-fold reduction in length!

Different species have characteristic numbers of chromosome pairs, known as a karyotype (Gr. Karyon; nut, used to donate the nucleus). Mice have 20 pairs of chromosomes, dogs 39?, fruit flies 8?, some ant species only 2 and some crayfish species 200 chromosomes! Needless to say, the number of chromosomes in an organism bares little relation to the complexity of the organism.

Humans have 23 pairs of chromosomes. It was a laboratory accident with hypotonic saline that swelled cells allowing the chromosomes to separate sufficiently to get an accurate count in 1956, over a century after chromosomes were first observed.

These 23 pairs are found in each cell of our body. The only exception are gametes (i.e. eggs and sperm) that carry only one copy of each of the 23 chromosomes.  These single copies then combine at fertilisation to give a total of 23 pairs again.

In 1971, at a conference in Paris, these chromosomes were numerically named from 1 to 22 (the 23rd pair were called ‘X’ and ‘Y’) according to their size. This scheme of coding chromosomes is still in use today, though in making the original assessments of size a mistake was made – chromosome 22 is actually longer than chromosome 21 and so the numbers should have been reversed.

The first 22 chromosome pairs are referred to as autosomes whilst the other pair, known as the sex chromosomes, determines the sex of the individual; males have an X and a Y chromosome, while females have two X chromosomes.