The Monotremes

The Monotremes

Monotremes are a small but remarkable group of mammals that consists of a single species of duckbill platypus (Ornithorhynchus anatinus) found in Australia and three (perhaps four) species of spiny anteaters (echidnas) found in Australia and New Guinea.

These animals retain several traits of their therapsid ancestors including

• a cloaca — the final segment of the digestive tract into which both the urinary and reproductive tracts empty (monotreme = single hole);

• lay shelled eggs that undergo merobastic cleavage like that of reptiles (and birds) rather than the holoblastic cleavage of all other mammals.

Despite these reptilian features, the monotremes meet all the criteria of true mammals:

• milk secreted from mammary glands (but no nipples);

• hair;

• teeth (only in the young; they are lost in adult montremes).

In the May 8 issue of Nature, a consortium of gene sequencers reported the results of sequencing the complete genome of the platypus.

They identified 18,527 protein-coding genes distributed on 52 chromosomes.

The mix of mammalian and reptilian phenotypic features turns out to be reflected in the genome as well. Examples:

• The platypus has genes for egg yolk proteins that are also found in birds but not in therians.

• The gene content of their X chromosomes resembles that of the Z chromosome in birds, not the X chromosome of other mammals like us.

Other features of their genome reflect their unique biology:

• The platypus produces a venom with genes which in other mammals encode for antimicrobial peptides called defensins.

• The platypus has some 1000 genes for receptors in its vomeronasal organ — far more than found in other mammals. The platypus hunts for food underwater and probably uses these receptors to detect prey (as well as using its electroreceptors for this purpose).

Nucleic Acids (RNA, DNA)

Nucleic Acids (RNA, DNA)

All of the information needed to control and build cells is stored in these molecules.

There are two main types of nucleic acid, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both of these molecules are polymers. They are composed of monomer subunits like the carbohydrates and proteins described previously. The monomers used to build nucleic acids are called nucleotides. The nucleotides are often referred to by the single letter abbreviations A, C, G, T and U. Like all of the monomers described so far, the monomers used to build DNA are similar to each other but are not exactly alike. One of the differences between DNA and RNA is the subset of nucleotides used to build the polymers. DNA contains A, C, G and T while RNA contains A, C, G and U.

Deoxyribonucleic Acid (DNA)

DNA is composed of two long strings (polymers) of nucleotides twisted around each other to form the spiral or helical structure shown below. The twisted molecules are arranged in a particular manner, with specific nucleotides always found across from each other. The nucleotide containing adenine (A) always pairs with the nucleotide containing thymine (T). Likewise, guanine (G) always pairs with cytosine (C). If you look closely at the graphics below you can see the nucleotide pairs interacting in the middle of the helix. The polymers that form DNA can be extremely long, reaching millions of nucleotides per each individual DNA molecule. The following graphic depicts a short strand of double-stranded DNA.

DNA is located in the nucleus cells. All of the nucleated cells in the human body have the same DNA content regardless of their function. The difference is which parts of the DNA are being used in any given cell. For example, the cells that make up the liver contain the same DNA as the cells that make up muscles. The dramatically different activities of these two cell types is dependent on the portions of DNA that are active in the cells. DNA is the storage form of genetic information and acts as a blueprint for cells. As we shall see, changes in the sequence of DNA can lead to alterations in cell behavior. Unregulated growth, as well as many of the other changes seen in cancer, are ultimately the result of mutations, changes in the structure of DNA.

Biology of Cells

Biology of Cells

The cell is the basic unit of life. All organisms are composed of one or more cells. As will be discussed later, humans are made up of many millions of cells. In order to understand what goes wrong in cancer, it is important to understand how normal cells work. The first step is to discuss the structure and basic functions of cells.

First we will introduce the common building blocks of cells. All cells, regardless of their function or location in the body, share common features and processes. Amazingly, cells are comprised almost entirely of just four basic types of molecules. Shown below is a cell surrounded by examples of these building block molecules.

Since they are present in living things these building blocks are called biomolecules. The next sections describe the structures and functions of each of these basic building blocks.

• Carbohydrates
• Proteins
• Lipids
• Nucleic Acids
• Combinations