- THE FIRST MILLENIUM
1933 – USA
‘‘The Mechanism of Mendelian Heredity’ (1915), ‘The Theory of the Gene’ (1926)’
Starting with Mendel’s laws of segregation and independent assortment, Morgan investigated why there are far fewer chromosomes – the long thread-like structures present in the nucleus of every living cell, which grow and divide during cell splitting, – than there are ‘units of heredity’. Morgan could not see how these few chromosomes could account for all the changes that occur from one generation to the next.
Mendel’s ‘factors of heredity’ had been renamed ‘genes’ in 1909 by the Dane Wilhelm Johannsen.
When the organism forms its reproductive cells (gametes), the genes segregate and pass to different gametes.
Since it had been separately established that chromosomes play an important part in inheritance, then groups of genes had to be present on a single chromosome.
If all the genes were arranged along chromosomes, and all chromosomes were transmitted intact from one generation to the next, then many characteristics would be inherited together. This implicitly invalidates Mendel’s law of independent assortment, which dictated that hereditary traits caused by genes would occur in all possible mathematical combinations in a series of descendants, independent of each other.
Experimental evidence often seemed to back-up the mathematical forecasts for characteristics present in descendants that Mendel had suggested; Morgan felt that the law of independent assortment could not accurately model the process of arriving at the end result.
He began his experiments with the fruit fly, which has just four pairs of chromosomes, in 1908.
He observed a mutant white-eyed male fly, which he extracted for breeding with ordinary red-eyed females. Over subsequent generations of interbred offspring, the white-eyed trait returned in some descendants, all of which turned out to be males. Clearly, certain genetic traits were not occurring independently of each other but were being passed on in groups.
Rather than invalidating Mendel’s law of independent assortment, a simple adjustment was required to unite it with Hunt’s belief in chromosomes to produce his thesis.
He suggested that the law of independent assortment did apply – but only to genes found on different chromosomes. For those on the same chromosome, linked traits would be passed on; usually a sex-related factor with other specific features (such as, the male sex and the white-eyed characteristic).
The results of his work convinced Morgan that genes were arranged on chromosomes in a linear manner and could be mapped. Further testing showed that, as chromosomes actually break apart and re-form during the production of sperm and egg cells, linked traits could occasionally be broken during the exchange of genes (recombination) that occurred between pairs of chromosomes during the process of cell division. He hypothesised that the nearer on the chromosome the genes were located to each other, the less likely the linkages were to be broken. Thus by measuring the occurrence of breakages he could work out the position of the genes along the chromosome.
In 1911 he produced the first chromosome map showing the position of five genes linked to gender characteristics.
In 1933 Hunt Morgan received the Nobel Prize for Physiology.
1905 – Switzerland
- ‘the relativity principle: All laws of science are the same in all frames of reference.
- constancy of the speed of light: The speed of light in a vacuüm is constant and is independent of the speed of the observer’
The laws of physics are identical to different spectators, regardless of their position, as long as they are moving at a constant speed in relation to each other. Above all the speed of light is constant. Classical laws of mechanics seem to be obeyed in our normal lives because the speeds involved are insignificant.
Newton’s recipe for measuring the speed of a body moving through space involved simply timing it as it passed between two fixed points. This is based on the assumptions that time is flowing at the same rate for everyone – that there is such a thing as ‘absolute’ time, and that two observers would always agree on the distance between any two points in space.
The implications of this principle if the observers are moving at different speeds are bizarre and normal indicators of velocity such as distance and time become warped. Absolute space and time do not exist. The faster an object is moving the slower time moves. Objects appear to become shorter in the direction of travel. Mass increases as the speed of an object increases. Ultimately nothing may move faster than or equal to the speed of light because at that point it would have infinite mass, no length and time would stand still.
‘The energy (E) of a body equals its mass (m) times the speed of light (c) squared’
This equation shows that mass and energy are mutually convertible under certain conditions.
The mass-energy equation is a consequence of Einstein’s theory of special relativity and declares that only a small amount of atomic mass could unleash huge amounts of energy.
1915 – Germany
‘Objects do not attract each other by exerting pull, but the presence of matter in space causes space to curve in such a manner that a gravitational field is set up. Gravity is the property of space itself’
From 1907 to 1915 Einstein developed his special theory into a general theory that included equating accelerating forces and gravitational forces. This implies light rays would be bent by gravitational attraction and electromagnetic radiation wavelengths would be increased under gravity. Moreover, mass and the resultant gravity, warps space and time, which would otherwise be ‘flat’, into curved paths that other masses (e.g. the moons of planets) caught within the field of the distortion follow. The predictions from special and general relativity were gradually proven by experimental evidence.
Einstein spent much of the rest of his life trying to devise a unified theory of electromagnetic, gravitational and nuclear fields.
1923 – Toronto, Canada
Early research had shown that there was almost certainly a link between the pancreas and diabetes, but at the time it was not understood what it was.
We now know a hormone from the pancreas controls the flow of sugar into the blood stream. Diabetics lack this function and are gradually killed by uncontrolled glucose input into the body’s systems.
Banting believed that the islets of Langerhans might be the most likely site for the production of this hormone and began a series of tests using laboratory animals.
After successfully treating dogs – showing signs of diabetes after the pancreas had been removed – with a solution prepared from an extract from the islets of Langerhans, Banting’s team (Best, MacLeod and Collip) purified their extract and named it insulin.
Human trials successfully took place in 1923 and dying patients were restored to health. The same year, industrial production of insulin from pigs’ pancreas began.
In the Second World War Banting undertook dangerous research into poisonous gas and was killed in an air crash while flying from Canada to the United Kingdom.</p
1927 – Germany
‘It is impossible to determine exactly both the position and momentum of a particle (such as an electron) simultaneously’
The principle excludes the existence of a particle that is stationary.
To measure both the position and momentum ( momentum = mass × velocity ) of a particle simultaneously requires two measurements: the act of performing the first measurement will disturb a particle and so create uncertainty in the second measurement.
Thus the more accurately a position is known; the less accurately can the momentum be determined.
The disturbance is so small it can be ignored in the macroscopic world, but is quite dramatic for particles in the microscopic world.
MAX BORN’S ‘probabilistic’ interpretation, expressed at about the same time, concerning the likelihood of finding a particle at any point through probability defined by the amplitude of its associated wave, led to similar conclusions.
The uncertainty principle also applies to energy and time. A particle’s kinetic energy cannot be measured with complete precision either.
Heisenberg suggested the model of the proton and neutron being held together in the nucleus of the atom after the work of JAMES CHADWICK who discovered the neutron in 1932.
Heisenberg decided to try to develop a new model of the atom, more fundamentally based on quantum theory that worked for all atoms. He believed the approach of trying to visualise a physical model of the atom was destined to fail because of the paradoxical wave-particle nature of electrons.
Every particle has an associated wave. The position of a particle can be precisely located where the wave’s undulations are most intense. But where the wave’s undulations are most intense, the wavelength is also at its most ill-defined, and the velocity of the associated particle is impossible to determine. Similarly, a particle with a well-defined wavelength has a precise velocity but a very ill-defined position.
Since the orbits of electrons could not be observed, he decided to ignore them and focus instead on what could be observed and measured; namely the energy they emitted and absorbed, as shown in the spectral lines. He tried to devise a mathematical way of representing the orbits of electrons, and to use this as a way of predicting the atomic features shown up in the spectral lines.
He showed that matrix mechanics could account for many of the properties of atoms, including those with more than one electron.
Together with PAUL DIRAC, Pascual Jordan created a new set of equations based on the rival theories of Schrödinger and Heisenberg, which they called ‘transformation theory’. Whilst studying these equations, Heisenberg noticed the paradox that measurements of position and velocity (speed and direction) of particles taken at the same time gave imprecise results. He believed that this uncertainty was a part of the nature of the sub-atomic world. The act of measuring the velocity of a subatomic particle will change it, making the simultaneous measurement of its position invalid.
An unobserved object is both a particle and a wave. If an experimenter chooses to measure the object’s velocity, the object will transform itself into a wave. If an experimenter chooses to measure its position, it will become a particle. By choosing to observe either one thing or the other, the observer is actually affecting the form the object takes.
The practical implication of this is that one can never predict where an electron will be at a precise moment, one can only predict the probability of its being there.
1928 – UK
‘Every fundamental particle has an antiparticle – a mirror twin with the same mass but opposite charge’
‘It appears that the simplest Hamiltonian for a point-charge electron satisfying the requirements of both relativity and the general transformation theory leads to an explanation of all duplexity phenomena without further assumption’
1931 – UK
‘A magnetic monopole is analogous to electric charge’
A magnetic monopole is a hypothetical particle that carries a basic magnetic charge – in effect, a single north or south magnetic pole acting as a free particle.
1912 – England
X-rays scattered from a crystal will show constructive interference provided their wavelength ( λ ) fits the equation
2d sin θ = n λ
where d is the spacing between atoms of the crystal, θ the angle through which the rays have scattered and n is any whole number
This is the cornerstone of the science of X-ray crystallography.