Structural Biochemistry/Three Domains of Life

Class Project
Work done in this project will enable students to earn extra credit.

Introduction

The three domains of life

The different parts of a Eukaryote and Prokaryote

In the past thirty years, scientists were able to use technological discoveries to redefine the classifications of life on earth. In 1977, American microbiologist, Carl Woese altered the previous two-domain system of Eukaryota (Eukarya) and Prokaryota. The Prokaryota domain was split into the two separate domains of Bacteria and Archaea. Woese was able to look at the similarities and differences of living organisms at the genetic sequencing level. More specifically, Woese analyzed how closely organisms were related based on the 16S ribosomal RNA or rRNA present in all organisms. With the new knowledge from the study of organisms' biochemical differences, scientists were able to classify life on earth into three distinctive groups, or domains: Eukarya, Bacteria, and Archaea.

Classification

Eukarya

All multicellular organisms are made of eukaryotic cells

Most living plants and animals are composed of eukaryotic cells. Eukaryotic cells recieve their name from the Greek eu meaning true and karyon suggesting that they have a true nucleus which contains their DNA. The determining factor for this domain is the presence of a distinct membrane-bound nucleus in each cell. Another distinctive feature of Eukarya is the presence of membrane-bound organelles in many of the cells. This type of cell can divide in two processes mitosis and meiosis. It keeps its DNA in chromosomes. This domain includes all multicellular organisms: plants, animals, fungi and protists. Interestingly, human beings and yeast are both classified under this same domain.

Eukaryotic cells can be categorized in to two types of cells, animal and plant cells.

Prokaryota

Out of the two prokaryotic cells are considerably simpler than eukaryotic cells. Prokaryotic comes from the Greek prefix pro which means before and the Greek root karyon which means kernel. They tend to be smaller in size and are not as complex as eukaryotic cells. As their names suggests, prokaryotic cells lack any organelles enclosed with a membrane. Their DNA is in an area called the nucleoid which also is not enclosed by a membrane. Generally speaking, their chromosomes and plasmids are circular and naked. In addition, prokaryotic cells control genes via operons. But, like eukaryotic cells, they have a plasma membrane surrounded by a cell wall. However, their cell wall contains petidoglycans which can't be found in Eukaryotic cells.

Prokaryotic cells have been split in to two distinct groups, bacteria and archaea.

Bacteria

This domain was also referred to as "Eubacteria". It consists of all the unicellular prokaryotic microorganisms that do not have a cell nucleus or contain any organelles in their cells. Bacteria can be spherical, rod-like, or spiral in shape. . Although bacteria contain ribosomes also found in Eukaryotes and Archaebacteria, bacterial ribosomes are much different in structure. Bacteria are characterized as having a cell wall (except for mycoplasma). Their cell walls are called peptidoglycans, which is composed of peptides and sugars. The cell wall gives the cell structure and shape. It also gives rigidity to prevent water from flowing into the cell membrane of the bacteria and causing the cell to burst. The cell wall usually lies exterior to the cell membrane as in Gram Positive cells. But in the case of Gram Negative bacterial cells, there is a second cell membrane that is exterior to the cell wall. This gives these types of bacteria unique properties to help it survive.

Bacteria also have flagella that help with their motility. The bacteria rotate their flagella in a counterclockwise manner and swim towards an attractant. To move away from a repellent, the flagella rotates clockwise and tumbles to change directions. Bacteria also have pili, which are used mainly for attachment and exchange of genetic information.

Archaea

Archaea, referred to as "archaebacteria" have the appearance of bacteria with their unicellular rod and spiral-like structures. They are prokaryotes and lack a cell nucleus and other organelles. Unlike bacteria, Archaea possess genes and pathways that evolutionarily link them closer to eukaryotes than to bacteria. Archaea contain the enzymes used for transcription and translation which reflects their closer similarity to eukaryotic organisms rather than bacteria. However, they are unlike eukaryotes in that they use a larger range of sources in order to gain energy. As a result of their differences from bacteria and eukaryotes, Archaea was placed into its own category. Archaea can live in extreme and harsh conditions not suited for other living organisms. For example, thermophiles occupy hot springs or volcanoes, while halophiles inhabit salty bodies of water such as the Dead Sea. Archaea were later discovered in a broader range of habitats.

References

In addition, Eubacteria and Archeabacteria are origenated from Prokaryotes.