The cell is the basic unit of life. The cell theory, set forth in the 1850's says that:

Cells are the fundamental units of life, because a cell is the simplest unit capable of independent existence.  All living things are made of cells.

Cells are small organisms with the central nucleus, or "brain".

Cells are 90% water. The rest of the present molecules are:

50% protein
15% carbohydrate
15% nucleic acid
10% lipid
10% others

By elements, a cell's composition is:

60% Hydrogen
25% Oxygen
10% Carbon
5% Nitrogen

All living organisms contain cells.

There are 2 types of cells, including animal and plant cells.

Both types contain different parts, like the chloroplast and the hard cell wall in plant cells.

Organisms can contain 1 cell and be called Monorans while most other organisms contain hundreds, thousands, or MILLIONS of cells.

The more complex organisms, called Prokaryotes and eukaryotes, have many similarities:

     They both have DNA as their genetic makeup
     They are both contained by a membrane
     They both have ribosomes
     They have a basic metabolism
     They both have many different forms
All living organisms contain cells.


Mitosis is the process by which cells divide. The parent cell has already duplicated its chromosomes, providing both daughter cells with a complete copy of genetic information.

Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material.

One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.

Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.

The Central Dogma of Molecular Biology

Transcription of DNA to RNA to protein: This dogma forms the backbone of molecular biology and is represented by four major stages.

1. The DNA replicates its information in a process that involves many enzymes: replication.

2. The DNA codes for the production of messenger RNA (mRNA) during transcription.

3. In eucaryotic cells, the mRNA is processed (essentially by splicing) and migrates from the nucleus to the cytoplasm.

4. Messenger RNA carries coded information to ribosomes. The ribosomes "read" this information and use it for protein synthesis. This process is called translation.

Proteins do not code for the production of protein, RNA or DNA.
They are involved in almost all biological activities, structural or enzymatic.

The Genetic Code

DNA transfers information to mRNA in the form of a code defined by a sequence of nucleotides bases. During protein synthesis, ribosomes move along the mRNA molecule and "read" its sequence three nucleotides at a time (codon) from the 5' end to the 3' end. Each amino acid is specified by the mRNA's codon, and then pairs with a sequence of three complementary nucleotides carried by a particular tRNA (anticodon).

Since RNA is constructed from four types of nucleotides, there are 64 possible triplet sequences or codons (4x4x4). Three of these possible codons specify the termination of the polypeptide chain. They are called "stop codons". That leaves 61 codons to specify only 20 different amino acids. Therefore, most of the amino acids are represented by more than one codon. The genetic code is said to be degenerate.

Cloning from Adult Vertebrate Cells

The nucleus of a differentiated cell from the skin of an adult frog contains all the instructions, encoded in DNA, to control the formation of an entire tadpole. A similar experiment was recently performed in sheep.

Repressor Protein Switching Genes On and Off


If the level of tryptophan inside the cell is low, the tryptophan repressor protein does not bind tryptophan and thus cannot bind to its control region‹the operator (green)‹within the promoter (yellow). RNA polymerase can thus bind to the promoter and transcribe the five genes of the tryptophan operon (left). If the level of tryptophan is high, however, the repressor protein binds tryptophan, in which state it can bind to the operator, where it blocks the binding of RNA polymerase to the promoter (right). Whenever the level of intracellular tryptophan drops, the repressor releases its tryptophan and is released from the DNA, allowing the polymerase to again transcribe the operon.

Control of Gene Expression

Examples of regulation at each of the steps are known, although for most genes the main site of control is step 1: transcription of a DNA sequence into RNA.

The exact order in which the general transcription factors assemble on the promoter is not known with certainty. One view holds that the general factors assemble off the DNA with the polymerase and that this whole assembly then binds to the DNA in a single step. The general transcription factors have been highly conserved in evolution; some of those from human cells can be replaced in biochemical experiments by the corresponding factors from simple yeasts.




We are not made up, as we had always supposed, of successively enriched packets of our own parts.  We are shared, rented, occupied.  At the interior of our cells, driving them, providing the oxidative energy that send us out for the improvement of each shining day, are the mitochondria, and in a strict sense they are not ours.  They turn out to be little separate creatures....  Ever since, they have maintained themselves and their ways, replicating in their own fashion, privately, with their own DNA and RNA quite different from ours.  They are as much symbionts as the rhizobial bacteria in the roots of beans.  Without them, we would not move a muscle, drum a finger, think a thought.

Mitochondria are stable and responsible lodgers, and I choose to trust them.  But what of the other little animals, similarly established in my cells, sorting and balancing me, clustering me together?  My centrioles, basal bodies, and probably a good many other more obscure tiny beings at work inside my cells, each with its own special genome, are as foreign, and as essential, as aphids in anthills.  My cells are no longer the pure line entities I was raised with; they are ecosystems more complex than Jamaica Bay.

I like to think that they work in my interest, each breath they draw for me, but perhaps it is they who walk through the local park in the early morning, sensing my senses, listening to my music, thinking my thoughts.

I am consoled, somewhat, by the thought that the green plants are in the same fix.  They could not be plants, or green, without their chloroplasts, which run the photosynthetic enterprise and generate oxygen for the rest of us.  As it turns out, chloroplasts are also separate creatures with their own genomes, speaking their own language.

We carry stores of DNA in our nuclei that may have come in, at one time or another, from the fusion of ancestral cells and the linking of ancestral organisms in symbiosis.  Our genomes are catalogues of instructions from all kinds of sources in nature, filed for all kinds of contingencies.  As for me, I am grateful for differentiation and speciation, but I cannot feel as separate an entity as I did a few years ago, before I was told these things...

The uniformity of the earth's life, more astonishing than its diversity, is accountable by the high probability that we derived, originally, from some single cell.  It is from the progeny of this parent cell that we take our looks; we still share genes around, and the resemblance of the enzymes of grasses to those of whales is a family resemblance.

Viruses, instead of being single-minded agents of disease and death, now begin to look more like mobile genes.  Evolution is still an infinitely long and tedious biologic game, with only the winners staying at the table, but the rules are beginning to look more flexible.  We live in a dancing matrix of viruses; they dart, rather like bees, from organism to organism, from plant to insect to mammal to me and back again, and into the sea, tugging along pieces of this genome, strings of genes from that, transplanting grafts of DNA in the widest circulation among us.

I have been trying to think of the earth as a kind of organism, but it is no go.  I cannot think of it this way.  It is too big, too complex, with too many working parts lacking visible connections.  The other night, driving though a hilly, wooded part of southern New England, I wondered about this.  If not like an organism, what is it like, what is it most like?  Then, satisfactorily for that moment, it came to me:  it is most like a single cell.

The World's Biggest Membrane

Viewed from the distance of the moon, the astonishing thing about the earth, catching the breath, is that is alive.    The photographs show the dry, pounded surface of the moon in the foreground, dead as an old bone.  Aloft, floating free beneath the moist, gleaming membrane of bright blue sky, is the rising earth, the only exuberant thing in this part of the cosmos.  If you could look long enough, you would see the swirling of the great drifts of white cloud, covering and uncovering the half-hidden masses of land.  It you had been looking for a very long, geologic time, you could have seen the continents themselves in motion, drifting apart on their crustal plates, held afloat by the fire beneath.  It has the organized, self-contained look of a live creature, full of information, marvellously skilled in handling the sun.

It takes a membrane to make sense out of disorder in biology.  You have to be able to catch energy and hold it, storing precisely the needed amount and releasing it in measured shares.  A cell does this, and so do the organelles inside.  Each assemblage is poised in the flow of solar energy, tapping off energy from metabolic surrogates of the sun.  To stay alive, you have to be able to hold out against equilibrium, maintain imbalance, bank against entropy, and you can only transact this business with membranes in our kind of world.

When the earth came alive, its own membrane - [the sky] was constructed, for the general purpose of editing the sun.

Now we are protected against lethal ultraviolet rays by a narrow rim of ozone, thirty miles out.  We are safe, well ventilated, and incubated, provided we can avoid technologies that might fiddle with that ozone, or shift the levels of carbon dioxide.  Oxygen is not a major worry for us, unless we let fly with enough nuclear explosives to kill off the green cells in the sea; if we do that, of course, we are in for strangling.

It is hard to feel affection for something as totally impersonal as the atmosphere, and yet there it is, as much a part and product of life as wine or bread.  Taken all in all, the sky is a miraculous achievement.  It works, and for what it is designed to accomplish it is as infallible as anything in nature.  I doubt whether any of us could think of a way to improve on it, beyond maybe shifting a local cloud from here to there on occasion.  The word "chance" does not serve to account well for structures of such magnificence.    Chance suggests alternatives, other possibilities, different solutions.  This may be true for gills and swim-bladders and forebrains, matters of details, but not for the sky.  There was simply no other way to go.

We should credit it for what it is:  for sheer size and perfection of function, it is far and away the grandest product of collaboration in all of nature.

It breathes for us, and it does another thing for our pleasure.  Each day, millions of meteorites fall against the outer limits of the membrane and are burned to nothing by the friction.  Without this shelter, our surface would long since have become the pounded powder of the moon.  Even though our receptors are not sensitive enough to hear it, there is comfort in knowing that the sound is there overhead, like a random noise of rain on the roof at night.

Lewis Thomas