Our objective will be to try and achieve an understanding of the molecular mechanisms underlying biological complexity. You will be learning about biology not at the population level or the organisimal level. We will be examining biology at the cellular and molecular level. An organism does not pass on a copy of itself to the next generation, instead it provides it with genetic material containing the information needed to construct a progeny organism. "A molecular blueprint". The aim of molecular biology is to define the complexity of living organisms in terms of the properties of their constituent molecules.
The Cell is the basic living unit of organization for all organisms. The simplest forms of life are solitary cells, and in more complex organisms groups of cells perform specialized functions and are linked by intricate systems of communication.
The cell theory, established in the middle of the nineteenth century proposed that:
Cells vary in size, shape, and complexity Highly varied forms in different plants, animals, and microorganisms. May exist singly - bacteria, protozoa, yeast, algae or packed together by the billions to form complex organisms. All cells fall into one of two categories or major divisions:
Both types of cells are surrounded by a plasma membrane
Prokaryotic (bacteria, rickettsiae (Rocky Mt Spotted Fever), chlamydiae, cyanobacteria)
Present-day prokaryotes, which include all of the various types of bacteria, are divided into two groups- archae and eubacteria which diverged early in evolution.
Some archae live in extreme environments, which are unusual today but may have been prevalent in primitive Earth. Thermoacidophiles live in hot sulfur springs with temperatures as high as 80oC and pH values as low as 2.
The eubacteria include the common forms of present-day bacteria- a large group of organisms that live in a wide range of environments.
Prokaryotic cells exists as single cell compartments, bounded by a plasma membrane that protects the cell from the outside environment (lipid bilayer).
Small (smallest 0.3um in diameter - mycoplasma) (Usually not more than a few micrometers)
First prokaryotic cells may have originated 3.5 to 3.8 billion years ago.
The largest and most complex prokaryotes are the cyanobacteria, bacteria in which the process of photosynthesis evolved.
Prokaryotic Characteristics:
Eukaryotic (fungi [yeasts and molds], protozoa, plant cells, animal cells)
First eukaryotic cells originated approximately 2 billion years ago, following 1.5 billion years of prokaryotic evolution.
Eukaryotic Characteristics:
DIAGRAM THESE CELL TYPES AND DISCUSS IMPORTANCE OF THE NUCLEAR MEMBRANE IN EUKARYOTIC CELLS
Studies of the DNA sequences of archae and eubacteria have suggested that they are as different from each other as present-day eukaryotes. Therefore, a very early event in evolution appears to have been the divergence of three lines of descent from a common ancestor, giving rise to present-day archae, eubacteria, and eukaryotes.
The organelles of eukaryotic cells are thought to have evolved from a symbiotic association witb prokaryotes.
Endosymbiosis
This theory is particularly well supported by studies of mitochondria and chloroplasts, which are thought to have evolved from bacteria living within large cells. They are similar to bacteria in size and they reproduce by dividing in two. In addition, they both contain their own DNA and their own ribosomes. The ribosomes and ribosomal RNAs of these organelles are more closely related to those of bacteria than to those encoded by the nuclear genome of eukaryotes. In addition, both organelles are surrounded by a double membrane. Interestingly, the inner membrane of these organelles has properties which are very "prokaryotic-like"
The endosymbiotic theory is now widely accepted with mitochondria thought to have evolved from aerobic bacteria and chloroplasts from photosynthetic bacteria, such as cyanobacteria.