Parthenogenesis – Sci-advent…

 

Parthenogenesis

I know that strictly speaking there should not be an entry for December 25th in the Sci-advent, but to tell you the truth I could not help myself and decided to do one more. This time it is about parthenogenesis: Parthenos  (παρθένος), meaning virgin in Greek and Genesis (γένεσις), meaning birth. The name Parthenos appears for instance in Greek mythology in the story of the daughter of Apollo and Chrysothemis , who died a maiden and was placed among the stars as the constellation of Virgo (fittingly enough…).

Almost all animal species reproduce sexually, by mixing the genes of two different individuals from meiosis. About 1% of animal species reproduce by parthenogenesis, while an even smaller fraction switch between sexual and asexual reproduction (known as cyclical parthenogenesis). One method of parthenogenesis involves sex cell division and recombination, while another just produces an egg with a full complement of DNA. Parthenogenesis is known to happen in some species of fish, amphibians and reptiles… but not in humans…

 

 

Chromosomes – Sci-advent – Day 23

ChromosomeAll known living organisms have their genetic information encoded in a molecule called deoxyribonucleic acid or DNA. Genetic information is encoded as a sequence of four nucleotides: guanine (G), adenine (A), thymine (T), and cytosine (C) recorded using the letters G, A, T, and C.  DNA molecules are double-stranded helices that strands run in opposite directions to each other

If we were to extended DNA molecules, they would be very long, however DNA is instead coiled and packaged in structures called chromosomes, which in turn are contained in the nucleus of the cell. Different species have different numbers of chromosomes (humans have 46 chromosomes, or 23 sets of chromosome pairs; peas have 14 chromosomes or 7 pairs; and tomatoes 24 chromosomes or 12 pairs). In sexual reproduction, one chromosome in each pair is contributed by each parent.
Each chromosome has a narrowing point called centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

 

Element 22: Titanium – Sci-advent – Day 22

Titanium CrystalElement 22 was named after the Titans – sons of the Earth – in Greek mythology. Titanium was discovered by William Gregor in 1791 in Cornwall, England and it is the ninth most abundant element in the Earth’s crust. It is found in minerals such as rutile, ilmenite and sphene. Pure titanium was first produced in 1910 by Matthew A. Hunter.

Titanium is a strong light metal: to get in idea, it is as strong as steel, but 45% lighter. It is resistant to corrosion and does not react with the human body; it is paramagnetic and has a low electrical and thermal conductivity. Due to its characteristics it is used in a number of components that are exposed to sea water. In alloys it is used in airplanes and rockets, and in implants such as artificial hips, pins and other biological implants. Titanium oxide (TiO2) is used as a pigment to create white paint and accounts for the largest use of the element. Titanium tetrachloride (TiCl4), another titanium compound, has been used to make smoke screens. Pure titanium oxide is relatively clear and is used to create titania, an artificial gemstone. Powdered titanium is used in pyrotechnics as a source of bright-burning particles.

Photoelectric Effect – Sci-advent – Day 21

photoelectric effectWe have seen how light could be described in terms of a wave, as demonstrated by the double-slit experiment. Nonetheless, that is not the whole story. For instance, in 1888, Wilhelm Hallwachs describes an experiment using a circular zinc plate mounted on an insulating stand and attached by a wire to a gold leaf electroscope, which was then charged negatively. The electroscope lost its charge very slowly. However, if the zinc plate was exposed to ultraviolet light, charge leaked away quickly. The leakage did not occur if the plate was positively charged.

By 1899, J. J.Thomson established that the ultraviolet light caused electrons to be emitted, the same particles found in cathode rays: atoms in the cathode contained electrons, which were shaken and caused to vibrate by the oscillating electric field of the incident radiation. In 1902, Philipp Lenard described how the energy of the emitted photoelectrons varied with the intensity of the light: doubling the light intensity doubled the number of electrons emitted, but did not affect the energies of the emitted electrons. The more powerful oscillating field ejected more electrons, but the maximum individual energy of the ejected electrons was the same as for the weaker field.

In 1905 Einstein gave proposed a way to explain these observations: He assumed that the incoming radiation should be thought of as quanta of frequency hf, with f  the frequency. In photoemission, one such quantum is absorbed by one electron. If the electron is some distance into the material of the cathode, some energy will be lost as it moves towards the surface. There will always be some electrostatic cost as the electron leaves the surface, this is usually called the work function, W. The most energetic electrons emitted will be those very close to the surface, and they will leave the cathode with kinetic energy. This explanation was successful and validates the interpretation of the behaviour of light as particles. In 1921, Einstein was awarded the Nobel Prize in Physics  “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”.

One very prominent application of the photoelectric effect is solar energy produced by photovoltaic cells. These are made of semi-conducting material which produce electricity when exposed to sunlight.

 

 

Mayan Numeral System and Calendar – Sci-advent – Day 20

Numeros Maya

The Mayas are one of greatest human civilisations. Not only did they have excellent agriculture, pottery and hieroglyph writing, but also have some of the most impressive architecture and symbolic art as well as mathematics, astronomy and calendar-making. It is said that they had predicted the “end of the world”, but I would like to think of it as the end and beginning of a calendar cycle. Not so different from the arbitrary December 31st in our calendars…

In order to understand the Mayan calendar cycle, we need to know a bit about their number system, which is a vigesimal system, i.e. base on the number 20. They used three basic number symbols, a shell for zero, a dot for 1 and a line for 5.  Also of note is that they were one of the earliest civilizations anywhere in the world to have the concept of zero. The system is pseudo-positional; in a true positional vigesimal system, the number that appears first would denote the number of units up to 19, the next would denote the number of 20s up to 19, the next the number of 400’s up to 19, etc. In the Mayan system the numbering starts in that way with the units up to 19 and the 20s up to 19, but it changes in the third place and this denotes the number of 360’s up to 19. After this the system reverts to multiples of 20 so the fourth place is the number of 18 × 202, the next the number of 18 × 203 and so on. For example [ 8;14;3;1;12 ] represents

12 + 1 × 20 + 3 × 18 × 20 + 14 × 18 × 202 + 8 × 18 × 203 = 1253912.

As a second example [ 9;8;9;13;0 ] represents

0 + 13 × 20 + 9 × 18 × 20 + 8 × 18 × 202 + 9 × 18 × 203 =1357100.

Now, to the calendar: the calendar was truly behind the number system and vice versa. They had two calendars: Tzolkin with 260 days, with 13 months of 20 days each, and the Haab with 365 days, with 18 months  of 20 days each and a shorter month of 5 days (called Wayeb). The Tzolkin was a ritual calendar, while the Haab was a civil one and the Wayeb was considered “unlucky”. With these two calendars, it is possible to see when they would return to the same cycle:  the least common multiple of 260 and 365 is 18980 days; equivalent to 52 civil years or 73 ritual years. Astronomy also played an important role for instance, Mayan astronomers calculated Venus‘ synodic period (after which it has returned to the same position) to be 584 days. In two 52 year cycles, Venus would have made 65 revolutions and be back to the same position.

A part from those calendars, the Mayas had another way of measuring time using an absolute scale base on a “creation date and time” often taken to be 12 August 3113 BC (but of course that is a matter of debate). This date can be taken as t the zero of the so-called “Long Count”. The Long Count is based on a count of 360 days represented in the Mayan number system. Let us have a look at an example: [9; 8; 9; 13; 0] is the completion date on a building in Palenque in Tabasco, Mexico. It translates to

0 + 13 × 20 + 9 × 18 × 20 + 8 × 18 × 202 + 9 × 18 × 203

which is 1357100 days from the creation date of 12 August 3113 BC so the building was completed in 603 AD.

The Long Count was divided as follows:

  • 1K’in = 1 Day
  • 1 Winal = 20 K’in
  • 1 Tun = 18 Winal = 360 K’in
  • 1 K’atun = 20 Tun = 7200 K’in
  • 1 Baktun = 20 K’atun = 144,000 K’in

On December 21, 2012, the 14th Baktun starts with the representation [13.0.0.0.0] and of course the 13th Baktun finishes… but certainly not the world!

 

 

Double Slit Experiment – Sci-advent – Day 19

Double Slit Experiment

 

The double-slit experiment is one of the most famous experiments in physics and one with great implications in our understanding of Nature. Although the experiment was realised originally with light, it can be done with any other type of wave.

Thomas Young conducted the experiment in the early 1800s. The aim was to allow light to pass through a pair of slits in an opaque screen. Each slit, diffracts the light and thus each acts as an individual light source. When a single slit was open, the light hit a screen with a maximum intensity in the centre and fading away from it. But when there are two slits then the light produces an interference pattern in the screen – a result that would not be expected if light consisted strictly of particles. Although the experiment favours the wave-like description of light, that is not the whole story. This interpretation is at odds with phenomena where light can behave as it is composed of discrete particles, such as the photoelectric effect. Light exhibits properties of both waves and particles, giving rise to the concept of wave-particle duality used in quantum mechanics.

Total Internal Reflection – Sci-advent – Day 18

Total Internal Reflection

We are well acquainted with some optical phenomena such as reflection an refraction; simply take a look at an object half-submerged in a glass of water. But light has other (many other) trick under its sleeve. One very useful trick is total internal reflection. As the name suggests, this phenomenon happens when a ray of light incides in a medium boundary at a very particular angle (known as the critical angle) with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary the light cannot pass through and instead it is all reflected, as if it had hit a perfect mirror.
Total internal reflection is widely used I the operation of optical fibres and devices such as endoscopes and in telecommunications, rain sensors in cars and some multi-touch displays.

Saturn’s Hexagonal Storm – Sci-advent – Day 17

Mini Saturn HexagonSaturn is well-know by its rings and it cannot be denied that they are a feature that makes of this planet an intriguing world. However, in the 1980s NASA’s Voyager 1 and 2 observed a bizarre, but symmetrically interesting feature in the north pole of Saturn: a hexagonal shaped storm. More recently, NASA’s Cassini has been able to image Saturn hexagon in greater detail. The hexagon is 25,000 km (15,000 miles) across. In fact, you could nearly fit four Earth-sized planets there.

The hexagon appears to have remained fixed with Saturn’s rotation rate and axis since first glimpsed by Voyager. The actual reason for the pattern in the storm is still a matter of speculation. Kevin Baines, atmospheric expert and member of Cassini’s visual and infrared mapping spectrometer team at NASA’s Jet Propulsion Laboratory is quoted saying: “Once we understand its dynamical nature, this long-lived, deep-seated polar hexagon may give us a clue to the true rotation rate of the deep atmosphere and perhaps the interior.

Transistor – Sci-advent – Day 16

Pile of TransistorsIf is said that if a cell is the building block of life, then a transistor is the building block of the digital era. Without them a lot of the gadgets, gizmos and technology we use today will simply not be there.

Transistors amplify current, for example they can be used to amplify the small output current from a logic integrated circuit to operate a high current device. A transistor can be thought of as a kind of switch used in a variety of circuits; and this is a function that is very important in computers for instance. The fact that the switch can change between on and off makes it possible to implement binary calculations. In today’s complex computers there are several thousands, even millions of transistors.

 

Laser – Sci-advent – Day 15

Laser Experiment Blue

Lasers have become so common that the number of applications they have do not surprise us. Nonetheless, their characteristics still captivate all of us. Laser is an acronym of Light Amplification by the Stimulated Emission of Radiation and it is indeed a very descriptive name.

A laser consists of three main elements: a gain medium, an energy source and a device to provide feedback to the system. The amplification of the electromagnetic radiation is done by gain medium. This is possible by pumping energy to the system and thus generating stimulated emission. It is very common for typical lasers to use feedback from an optical cavity, such as a pair of mirrors at each end of the gain medium.

Laser light is characterised by properties such as monochromaticity, coherence and power.