Week of March 4, 2019

Biology

DNA vs. RNA – A Comparison Chart

Comparison	DNA	RNA
Full Name	Deoxyribonucleic Acid	Ribonucleic Acid
Function	DNA replicates and stores genetic information. It is a blueprint for all genetic information contained within an organism	RNA converts the genetic information contained within DNA to a format used to build proteins, and then moves it to ribosomal protein factories.
Structure	DNA consists of two strands, arranged in a double helix. These strands are made up of subunits called nucleotides. Each nucleotide contains a phosphate, a 5-carbon sugar molecule and a nitrogenous base.	RNA only has one strand, but like DNA, is made up of nucleotides. RNA strands are shorter than DNA strands. RNA sometimes forms a secondary double helix structure, but only intermittently.
Length	DNA is a much longer polymer than RNA. A chromosome, for example, is a single, long DNA molecule, which would be several centimetres in length when unravelled.	RNA molecules are variable in length, but much shorter than long DNA polymers. A large RNA molecule might only be a few thousand base pairs long.
Sugar	The sugar in DNA is deoxyribose, which contains one less hydroxyl group than RNA’s ribose.	RNA contains ribose sugar molecules, without the hydroxyl modifications of deoxyribose.
Bases	The bases in DNA are Adenine (‘A’), Thymine (‘T’), Guanine (‘G’) and Cytosine (‘C’).	RNA shares Adenine (‘A’), Guanine (‘G’) and Cytosine (‘C’) with DNA, but contains Uracil (‘U’) rather than Thymine.
Base Pairs	Adenine and Thymine pair (A-T) Cytosine and Guanine pair (C-G)	Adenine and Uracil pair (A-U) Cytosine and Guanine pair (C-G)
Location	DNA is found in the nucleus, with a small amount of DNA also present in mitochondria.	RNA forms in the nucleolus, and then moves to specialised regions of the cytoplasm depending on the type of RNA formed.
Reactivity	Due to its deoxyribose sugar, which contains one less oxygen-containing hydroxyl group, DNA is a more stable molecule than RNA, which is useful for a molecule which has the task of keeping genetic information safe.	RNA, containing a ribose sugar, is more reactive than DNA and is not stable in alkaline conditions. RNA’s larger helical grooves mean it is more easily subject to attack by enzymes.
Ultraviolet (UV) Sensitivity	DNA is vulnerable to damage by ultraviolet light.	RNA is more resistant to damage from UV light than DNA.

What are the key differences between DNA and RNA?

Function

DNA encodes all genetic information, and is the blueprint from which all biological life is created. And that’s only in the short-term. In the long-term, DNA is a storage device, a biological flash drive that allows the blueprint of life to be passed between generations2. RNA functions as the reader that decodes this flash drive. This reading process is multi-step and there are specialized RNAs for each of these steps. Below, we look in more detail at the three most important types of RNA. 

What are the three types of RNA?

• Messenger RNA (mRNA) copies portions of genetic code, a process called transcription, and transports these copies to ribosomes, which are the cellular factories that facilitate the production of proteins from this code.  
• Transfer RNA (tRNA) is responsible for bringing amino acids, basic protein building blocks, to these protein factories, in response to the coded instructions introduced by the mRNA. This protein-building process is called translation. 
• Finally, Ribosomal RNA (rRNA) is a component of the ribosome factory itself without which protein production would not occur3.

Sugar

Both DNA and RNA are built with a sugar backbone, but whereas the sugar in DNA is called deoxyribose (left in image), the sugar in RNA is called simply ribose (right in image). The ‘deoxy’ prefix denotes that, whilst RNA has two hydroxyl (-OH) groups attached to its carbon backbone, DNA has only one, and has a lone hydrogen atom attached instead. RNA’s extra hydroxyl group proves useful in the process of converting genetic code into mRNAs that can be made into proteins, whilst the deoxyribose sugar gives DNA more stability4.

          The Chemical Structures of Deoxyribose (left) and Ribose (right) Sugars

Bases

The nitrogen bases in DNA are the basic units of genetic code, and their correct ordering and pairing is essential to biological function. The four bases that make up this code are adenine (A), thymine (T), guanine (G) and cytosine (C). Bases pair off together in a double helix structure, these pairs being A and T, and C and G.  RNA doesn’t contain thymine bases, replacing them with uracil bases (U), which pair to adenine1.

Structure

Whilst the ubiquity of Francis Crick and James Watson’s (or should that be Rosalind Franklin’s?) DNA double helix means that the two-stranded structure of DNA structure is common knowledge, RNA’s single stranded format is not as well known. RNA can form into double-stranded structures, such as during translation, when mRNA and tRNA molecules pair. DNA polymers are also much longer than RNA polymers; the 2.3m long human genome consists of 46 chromosomes, each of which is a single, long DNA molecule. RNA molecules, by comparison, are much shorter4.

Location 

Eukaryotic cells, including all animal and plant cells, house the great majority of their DNA in the nucleus, where it exists in a tightly compressed form, called a chromosome5. This squeezed format means the DNA can be easily stored and transferred. In addition to nuclear DNA, some DNA is present in energy-producing mitochondria, small organelles found free-floating in the cytoplasm, the area of the cell outside the nucleus. 

The three types of RNA are found in different locations. mRNA is made in the nucleus, with each mRNA fragment copied from its relative piece of DNA, before leaving the nucleus and entering the cytoplasm. The fragments are then shuttled around the cell as needed, moved along by the cell’s internal transport system, the cytoskeleton. tRNA, like mRNA, is a free-roaming molecule that moves around the cytoplasm. If it receives the correct signal from the ribosome, it will then hunt down amino acid subunits in the cytoplasm and bring them to the ribosome to be built into proteins5. rRNA, as previously mentioned, is found as part of ribosomes. Ribosomes are formed in an area of the nucleus called the nucleolus, before being exported to the cytoplasm, where some ribosomes float freely. Other cytoplasmic ribosomes are bound to the endoplasmic reticulum, a membranous structure that helps process proteins and export them from the cell6. 

References

https://www.technologynetworks.com/genomics/lists/what-are-the-key-differences-between-dna-and-rna-296719

ASTRONOMY

TYPES OF GALAXIES

The most widely used classification scheme for galaxies is based on one devised by Edwin P. Hubble and further refined by astronomer Gerard de Vaucouleurs. It uses the three main types, and then further breaks them down by specific characteristics (openness of spirals, size and extent of bars, size of galactic bulges). In this age of multi-wavelength observing, the sub-classifications also include markers for such characteristics as a galaxy’s star-formation rate and age spectrum of its stars.

Spiral Galaxies

Spiral galaxies are the most common type in the universe. Our Milky Way is a spiral, as is the rather close-by Andromeda Galaxy. Spirals are large rotating disks of stars and nebulae, surrounded by a shell of dark matter. The central bright region at the core of a galaxy is called the “galactic bulge”. Many spirals have a halo of stars and star clusters arrayed above and below the disk.

Spirals that have large, bright bars of stars and material cutting across their central sections are called “barred spirals”. A large majority of galaxies have these bars, and astronomers study them to understand what function they play within the galaxy. In addition to bars, many spirals may also contain supermassive black holes in their cores. Subgroups of spirals are defined by the characteristics of their bulges, spiral arms, and how tightly wound those arms are.

Elliptical Galaxies

Elliptical galaxies are roughly egg-shaped (ellipsoidal or ovoid) found largely in galaxy clusters and smaller compact groups. Most ellipticals contain older, low-mass stars, and because they lack a great deal of star-making gas and dust clouds, there is little new star formation occurring in them. Ellipticals can have as few as a hundred million to perhaps a hundred trillion stars, and they can range in size from a few thousand light-years across to more than a few hundred thousand. Astronomers now suspect that every elliptical has a central supermassive black hole that is related to the mass of the galaxy itself. Messier 87 is an example of an elliptical galaxy. There are some subgroups of ellipticals, including “dwarf ellipticals” with properties that put them somewhere between regular ellipticals and the tightly knit groups of stars called globular clusters.

Irregular Galaxies

Irregular galaxies are as their name suggests: irregular in shape. The best example of an irregular that can be seen from Earth is the Small Magellanic Cloud. Irregulars usually do not have enough structure to characterise them as spirals or ellipticals. They may show some bar structure, they may have active regions of star formation, and some smaller ones are listed as “dwarf irregulars”, very similar to the very earliest galaxies that formed about 13.5 billion years ago. Irregulars are characterised by their structures (or lack of them).

Week of February 25, 2019

Astronomy

Novae and Supernovae

A nova is an explosion from the surface of a white-dwarf star in a binary star system. A nova occurs when the white dwarf, which is the dense core of a once-normal star, “steals” gas from its nearby companion star. When enough gas builts up on the surface of the white dwarf it triggers an explosion. For a brief time, the system can shine up to a million times brighter than normal. As long as it continues to take gas from its companion star, the white dwarf can produce nova outbursts at regular intervals. A supernova is a violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two groups: Type I and Type II. Type I supernovae most likely form as a white dwarf “steals” hot gas from a companion star. If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits, leaving nothing behind. These are the brightest supernovae, and can be used to measure the distances to other galaxies. Type II supernovae are the final stage in the evolution of stars that are at least eight times as massive as the Sun. Such a star reaches a point where it can no longer produce nuclear energy in its core. Without the outward pressure created by this energy, gravity wins out and causes the star’s core to collapse to form a neutron star or black hole. The star’s outer layers “rebound” violently, blasting into space at several percent of the speed of light.

source: https://stardate.org/astro-guide/novae-and-supernovae

Biology

Relationship Between Cell Structure & Function

Nucleus

In the same way that the main building controls a gigantic factory, the nucleus is the control center of the cell. This organelle holds the cell's DNA and the directions for producing proteins and other vital things. The nucleus is bordered by a nuclear envelope that is made up of two membranes. This envelope is dotted with thousands of nuclear pores, which permit material to go through the nucleus. Like messages, instructions and blueprints moving in and out of a main office, a river of proteins go through the nuclear pores to and from the rest of the cell.

Ribosomes

Creating proteins is one of the most crucial jobs in a cell. They are made on ribosomes. Ribosomes are puny pieces of protein found all over the cell. They create proteins through obeying the nucleus’ hinted orders. Each ribosome, in its own way, is like a small machine in a factory, spiraling out proteins on instructions that arrive from its nucleus.

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Endoplasmic Reticulum

Cells also contain the endoplasmic reticulum (en-doh-PLAZ-mik rih-TIK-yuh-lum), or ER. The endoplasmic reticulum is the site where the apparatus of the membrane are assembled, along with proteins and other materials that are exported from the cell. The rough ER is given this name because of the ribosomes found on its surface. Newly made proteins leave these ribosomes and are put in the rough endoplasmic reticulum, where they will be modified chemically.

Golgi Apparatus

Proteins made in the rough ER move next into an organelle called the Golgi apparatus. This organelle was discovered by the Italian scientist Camillo Golgi. The function of this organelle is to deliver proteins and other materials for the cell’s cargo space. In the Golgi apparatus, proteins are put all over the cell

Lysosomes

Even the neatest, cleanest factory needs a cleanup crew, and that's what lysosomes (LY-suh-sohmz) are. Lysosomes are small organelles filled with enzymes. One function of lysosomes is the digestion of lipids into molecules that can be used by the rest of the cell. Lysosomes are also involved in breaking down organelles that have outlived their usefulness. Lysosomes perform the vital function of removing “junk” that might otherwise accumulate and clutter up the cell. A number of serious human diseases can be traced to lysosomes.

Vacuoles

Every factory needs a place to store things, and cells contain places for storage as well. Some cells have compositions called vacuoles (VAK-yoo-ohlz) that stock up resources such as proteins and salts. Vacuoles are also found in paramecium. The paramecium contains a vacuole called a contractile vacuole. By contracting rhythmically, this specialized vacuole pumps excess water out of the cell. The control of water content within the cell is just one example of an important process known as homeostasis. Homeostasis is the maintenance of a controlled internal environment.

Mitochondria

All living things require a source of energy. Factories are linked up to the local power system, but what about cells? Most cells get energy in one of two ways—from food molecules or from the sun. Mitochondria are organelles that exchange the energy placed in food into energy that is very convenient for cell usage. Mitochondria are put up with two membranes. The inner membrane is folded up inside the organelle.

Source: https://sciencing.com/relationship-between-cell-structure-function-5154975.html

Week of February 11, 2019

Biology

Eukaryotic Cell vs. Prokaryotic Cell

Diffen › Science › Biology › Cellular Biology

The distinction between prokaryotes and eukaryotes is considered to be the most important distinction among groups of organisms. Eukaryotic cells contain membrane-bound organelles, such as the nucleus, while prokaryotic cells do not. Differences in cellular structure of prokaryotes and eukaryotes include the presence of mitochondria and chloroplasts, the cell wall, and the structure of chromosomal DNA.

Prokaryotes were the only form of life on Earth for millions of years until more complicated eukaryotic cells came into being through the process of evolution.

Comparison chart

 

Eukaryotic Cell versus Prokaryotic Cell comparison chart

	Eukaryotic Cell	Prokaryotic Cell
Nucleus	Present	Absent
Number of chromosomes	More than one	One--but not true chromosome: Plasmids
Cell Type	Usually multicellular	Usually unicellular (some cyanobacteria may be multicellular)
True Membrane bound Nucleus	Present	Absent
Example	Animals and Plants	Bacteria and Archaea
Genetic Recombination	Meiosis and fusion of gametes	Partial, undirectional transfers DNA
Lysosomes and peroxisomes	Present	Absent
Microtubules	Present	Absent or rare
Endoplasmic reticulum	Present	Absent
Mitochondria	Present	Absent
Cytoskeleton	Present	May be absent
DNA wrapping on proteins.	Eukaryotes wrap their DNA around proteins called histones.	Multiple proteins act together to fold and condense prokaryotic DNA. Folded DNA is then organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein.
Ribosomes	larger	smaller
Vesicles	Present	Present
Golgi apparatus	Present	Absent
Chloroplasts	Present (in plants)	Absent; chlorophyll scattered in the cytoplasm
Flagella	Microscopic in size; membrane bound; usually arranged as nine doublets surrounding two singlets	Submicroscopic in size, composed of only one fiber
Permeability of Nuclear Membrane	Selective	not present
Plasma membrane with steroid	Yes	Usually no
Cell wall	Only in plant cells and fungi (chemically simpler)	Usually chemically complexed
Vacuoles	Present	Present
Cell size	10-100um	1-10um Source:https://www.diffen.com/difference/Eukaryotic_Cell_vs_Prokaryotic_Cell

Week of February 4, 2019

The Electromagnetic Spectrum

The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays.

You know more about the electromagnetic spectrum than you may think. The image below shows where you might encounter each portion of the EM spectrum in your day-to-day life.

The electromagnetic spectrum from lowest energy/longest wavelength (at the top) to highest energy/shortest wavelength (at the bottom). (Credit: NASA's Imagine the Universe)

Radio: Your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by stars and gases in space.

Microwave: Microwave radiation will cook your popcorn in just a few minutes, but is also used by astronomers to learn about the structure of nearby galaxies.

Infrared: Night vision goggles pick up the infrared light emitted by our skin and objects with heat. In space, infrared light helps us map the dust between stars.

Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit visible light.

Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason skin tans and burns. "Hot" objects in space emit UV radiation as well.

X-ray: A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag. Hot gases in the Universe also emit X-rays.

Gamma ray: Doctors use gamma-ray imaging to see inside your body. The biggest gamma-ray generator of all is the Universe.

Is a radio wave the same as a gamma ray?

Are radio waves completely different physical objects than gamma-rays? They are produced in different processes and are detected in different ways, but they are not fundamentally different. Radio waves, gamma-rays, visible light, and all the other parts of the electromagnetic spectrum are electromagnetic radiation.

Electromagnetic radiation can be described in terms of a stream of mass-less particles, called photons, each traveling in a wave-like pattern at the speed of light. Each photon contains a certain amount of energy. The different types of radiation are defined by the the amount of energy found in the photons. Radio waves have photons with low energies, microwave photons have a little more energy than radio waves, infrared photons have still more, then visible, ultraviolet, X-rays, and, the most energetic of all, gamma-rays.

Measuring electromagnetic radiation

Electromagnetic radiation can be expressed in terms of energy, wavelength, or frequency. Frequency is measured in cycles per second, or Hertz. Wavelength is measured in meters. Energy is measured in electron volts. Each of these three quantities for describing EM radiation are related to each other in a precise mathematical way. But why have three ways of describing things, each with a different set of physical units?

Comparison of wavelength, frequency and energy for the electromagnetic spectrum. (Credit: NASA's Imagine the Universe)

The short answer is that scientists don't like to use numbers any bigger or smaller than they have to. It is much easier to say or write "two kilometers" than "two thousand meters." Generally, scientists use whatever units are easiest for the type of EM radiation they work with.

Astronomers who study radio waves tend to use wavelengths or frequencies. Most of the radio part of the EM spectrum falls in the range from about 1 cm to 1 km, which is 30 gigahertz (GHz) to 300 kilohertz (kHz) in frequencies. The radio is a very broad part of the EM spectrum.

Infrared and optical astronomers generally use wavelength. Infrared astronomers use microns (millionths of a meter) for wavelengths, so their part of the EM spectrum falls in the range of 1 to 100 microns. Optical astronomers use both angstroms (0.00000001 cm, or 10^-8 cm) and nanometers (0.0000001 cm, or 10^-7 cm). Using nanometers, violet, blue, green, yellow, orange, and red light have wavelengths between 400 and 700 nanometers. (This range is just a tiny part of the entire EM spectrum, so the light our eyes can see is just a little fraction of all the EM radiation around us.)

The wavelengths of ultraviolet, X-ray, and gamma-ray regions of the EM spectrum are very small. Instead of using wavelengths, astronomers that study these portions of the EM spectrum usually refer to these photons by their energies, measured in electron volts (eV). Ultraviolet radiation falls in the range from a few electron volts to about 100 eV. X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then are all the photons with energies greater than 100 keV.

 Show me a chart of the wavelength, frequency, and energy regimes of the spectrum

Why do we put telescopes in orbit?

The Earth's atmosphere stops most types of electromagnetic radiation from space from reaching Earth's surface. This illustration shows how far into the atmosphere different parts of the EM spectrum can go before being absorbed. Only portions of radio and visible light reach the surface. (Credit: STScI/JHU/NASA)

Most electromagnetic radiation from space is unable to reach the surface of the Earth. Radio frequencies, visible light and some ultraviolet light makes it to sea level. Astronomers can observe some infrared wavelengths by putting telescopes on mountain tops. Balloon experiments can reach 35 km above the surface and can operate for months. Rocket flights can take instruments all the way above the Earth's atmosphere, but only for a few minutes before they fall back to Earth.

For long-term observations, however, it is best to have your detector on an orbiting satellite and get above it all!

Updated: March 2013

https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html