The most fundamental thing in biology is the cell. Nothing
researched in biology makes complete sense without the cell. Yet this basic
unit of life cannot be seen by the human eye and even viewed though the aid of
a microscope, nothing about it at first glance gives much indication of how
significant it is. So exactly did we find the cell and work out its
significance?
Well, it all begins back in the 1600s.
The first time a cell was ever seen was by Robert Hooke.
Robert Hooke, 1680.
Microscopes had been invented in about 1620 and in 1665,
Hooke looked though a compound microscope, a simple act that had huge
ramifications.
A compound microscope uses two lens - an eyepiece and
objective lens. The objective lens forms a magnified image which is then
magnified further by an eyepiece lens. This gives much higher magnification
than a simple microscope, using one lens, can give. Compound microscopes are
still used in optical microscopes today, although with a much higher
magnification than what was possible at the time. Remember, microscopes were
not initially designed for cells- they were designed just for
magnification. No one had any idea how important microscopes would
ultimately be.
How a
compound microscope works.
Hooke initially started with mundane human objects; he
discovered tiny pores in thin slices of bottle cork and named them 'cells'.
This after the latin word 'cella' which means 'small room' - used to describe
the living quarters of monks, and also 'cellulae', meaning the sixth-sided cell
of a honeycomb.
Hooke was
reminded of honeycombs when looking though his microscope.
Hooke then looked at microorganisms - specifically the
micro-fungus Mucor. His microscope - handcrafted and tooled in leather and gold
can still be seen in Maryland. Whilst beautiful, leather and gold are not
practical tools for modern microscopes!
Hooke's
microscope.
Hooke made drawings of what he saw and had them published in
'Micrographia'. This was the first book to have illustrations of insects and
plants seen through microscope. This publication inspired mass interest in the
new field of microscopy. Samuel Pepys called the book 'the most ingenious book
that ever I read in my life'. It is rather strange to think that cells, one of
the most important and essential aspects of biology, were discovered a year
before the Great Fire of London. For context, many people in this time could
still remember the beheading of Charles I!
Hooke was not able to see any internal components of the
cells- like nuclei and had no clue that these' cells' were alive. The
magnification needed for this was not advanced yet and it would take a few more
developments in microscopy before that was possible. In fact, being able to
include that cells were indeed alive happened in Hooke's lifetime.
Robert
Hooke's drawing of 'cells' in cork.
Less than 10 years after the publication of Micrographia in
1665, Anton van Leeuwenhoek, was able to take advantage of the improvements in
microscopes. In 1676, Leeuwenhoek became the first to notice that the
cells were moving- and therefore must be alive. He described many forms of
microorganisms, which he named 'animalcules'. He saw bacteria and protozoa, a
type of single-celled organism that feeds on organic matter. He was also
able to accurately describe red blood cells. Crucially though, he was the first
to see sperm cells of humans and animals and saw that reproduction would require
the sperm cell to enter the egg. This allowed for the prevailing theory of
spontaneous generation to be put to rest. Hooke confirmed his
observations. This was also the first time that cells in animal tissues
could be seen- they are much more fragile than plant cells.
Antonie
van Leeuwenhoek, after 1680
Moving onto 1804, Karl Rudolphi and Johann Heinrich
Friedrich Link were able to prove that cells had independent walls. This
suggested that cells were separate and distinct units; this laid the groundwork
for the idea that organisms are composed of separate structural units.
Karl
Rudolphi
Johann
Heinrich Friedrich Link.
In 1824, Henri Dutrochet was the first to suggest that the
cells were physiological units and that they were a fundamental element to the
organisation of an organism. Henri Dutrochet was also the first to come up with
the process of Osmosis; how water moves across a membrane. This theory is vital
in understanding the physiological processes in cells, giving further
information on how cells actually operate.
Henri
Dutrochet
The work of these scientists all came together and allowed
Theodor Schwann and Matthias Jakob Schleiden, with contributions from Rudolf
Virchow to devise 'cell theory' in 1839. This theory suggested that every
structural aspect of a plant and an animal were either made up of cells or
resulting from cells. This also allowed them to include that the cell was the
most basic unit of life.
Theodor
Schwann
Matthias
Jakob Schleiden
In 1855, Rudolf Virchow was able to add to cell theory that
all cells arise only from cells already existing. This had first been proposed
by Robert Remak in 1852 who also suggested that binary fission was how animal
cells reproduced. This marked the completion of classical cell theory.
Rudolf
Virchow
Robert
Remak
Of course, further discoveries that altered classical cell
theory would be made later. However, classical cell theory altered biology for
the better. It became clear that to understand life, scientists could no longer
look at whole organisms or tissues and instead look deeper- the cell.
Looking closely at the cell gave rise to cellular physiology and also gave rise
to modern biochemistry.
For example, in 1877, Wilhelm Pfeffer was able to propose
the membrane theory. This was gradually developed over time, with Ernst
Overton suggesting that the cell membrane was made of lipids in 1899. In 1904,
David Nathansohn added to this by suggesting the mosaic theory and Wilhelm
Ruhland refined the mosaic theory to include pores in the cell membrane. Leonor
Michaelis was then able to conclude the role of ions and demonstrated the
membrane potential. All of this work was key in understanding how nerves and
muscles work, as well as how cells transport vital minerals, vitamins and
nutrients. This ultimately led to the first alteration of classical cell theory
to modern; biochemical reactions and metabolic reactions occur within cells. It
also became possible to add a second alteration; the activity of an organism
depends on the total activity of independent cells.
The
mosaic model.
The discoveries that altered classical cell theory and
turned it to 'modern' cell theory also revolved around DNA. In 1869, Friedrich
Miescher first identified what was ultimately DNA. He discovered a microscopic
substance that resided in nuclei - he called it nuclein.
Friedrich
Miescher
In 1878, Albrecht Kossel isolated nucleic acid from nuclein
and isolated its nucleobases- adenine, thymine, cytosine, guanine, and also
uracil. In 1909, Phoebus Levene identified the base, sugar and phosphate
nucleotide unit of 'yeast nuclei acid' -or as we know it; RNA.
Albrecht
Kossel
Phoebus
Levene
In 1927, Nikolai Koltsov suggested a 'giant hereditary
molecule'. At this stage, he wasn't sure what this hereditary molecule actually
was. It was Frederick Griffith in 1928 that came up with the first suggestion
that it could be DNA carrying genetic information.
Nikolai
Koltsov
Frederick
Griffith
In 1929, Levene identified deoxyribose sugar in DNA and
suggested it consisted of four nucleotide units linked together through
phosphate groups.
In 1933, Jean Brachet suggested that DNA was found in the
cell nucleus, with RNA in the cytoplasm. In 1943, Oswald Avery, Colin Macleod
and Maclyn McCarthy devised the transforming principle, suggesting how bacteria
are capable of transferring genetic information.
Jean
Brachet
Oswald
Avery
Colin
MacLeod
Maclyn
McCarty
By 1951, Alec Todd suggested how the backbone of DNA is
structured, which played a crucial role in the work of Franklin, Wilkins,
Watson and Crick. In May 1952, photograph 51 was born and in 1957, Crick
devised the central dogma describing the flow of genetic information. In simple
terms; DNA makes RNA and RNA makes proteins.
Alex
Todd
In 1958, the Meselson-Stahl experiment was done. In the same
year, the discovery of codons allowed for Har Gobind Khorana, Robert W. Holley
and Marshall Warren Nirenburg to decipher the genetic code. Codons are the
triplet DNA bases that give specific amino acids.
Har
Gobind Khorana
Robert W.
Holley
Marshall
Warren Nirenberg, 2002
With this, the molecular age of cell biology had officially
began--and this led to some more adjustments to cell theory. The modern theory,
still in play, now has seven tenants:
1.All known living things are made up of one or more cells.
2.The cell is the basic unit of organisation and function -
all the vital functions of an organism occur within cells; including
metabolism, energy, and DNA synthesis
3.All living cells arise from pre-existing cells; cells have
to divide which requires replication of DNA.
4.Cells contain DNA which is found specifically in the
chromosome and RNA which is found in the cell nucleus and cytoplasm.
5.All cells are similar in chemical composition and
metabolic processes; including proteins, lipids, carbohydrates and biochemical
pathways.
6.Specialised organelles-such as the nucleus, mitochondria,
and ribosomes, perform distinct and essential functions with the cell.
7.Energy flow- metabolism and biochemistry occurs within
cells. Cells are the site of energy transformation essential for life.
This modern theory of cells was almost three hundred years
in the making, starting with Robert Hooke in 1665, and being altered against in
1958. Hooke's simple act of looking though a microscope can be said to
have changed almost everything about biology - and it may still do so.
Discoveries about cells and DNA are still being made on an almost daily basis.
In another three hundred years, could more tenants be added and adjustments be
made to the modern cell theory?
No comments:
Post a Comment