Acrocorinth is the acropolis of the Greek city Corinth.
Neutron
Neutrons, with protons and electrons, make up an atom. Neutrons and protons are found in the nucleus of an atom. Unlike protons, which have a positive charge, or electrons, which have a negative charge, neutrons have zero charge which means they are neutral particles. Neutrons bind with protons with the residual strong force.
Neutrons were predicted by Ernest Rutherford, and discovered by James Chadwick, in 1932. Atoms were fired at a thin pane of beryllium. Particles emerged which had no charge, and he called these 'neutrons'. They were later added to the modern image of the atom.
Neutrons have a mass of 1.675 × 10g, which is a little heavier than the proton. Neutrons are 1839 times heavier than electrons.
Like all hadrons, neutrons are made of quarks. A neutron is made of two down quarks and one up quark. One up quark has a charge of +2/3, and the two down quarks each have a charge of -1/3. The fact that these charges cancel out is why neutrons have a neutral (0) charge. Quarks are held together by gluons.
Neutrons can be found in almost all atoms together with protons and electrons. Hydrogen-1 is the only exception. Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element.
The number of neutrons in an atom does not affect its chemical properties. However it affects its half-life, a measure of its stability. An unstable isotope has a short half-life, in which half of it decays to lighter elements. By contrast, a stable isotope has a long half-life, much longer than that of an unstable isotope. The stability of an isotope is related to radioactivity: an unstable isotope can be highly radioactive.
Neutrons are the key to nuclear chain reactions, nuclear power and nuclear weapons.
LiveJournal
LiveJournal (often called LJ) is a website on which Internet users can keep a blog. It is also the name of the software that was designed to run it. LiveJournal is different from some other blogging sites, because a user can interact with friends in certain ways that are unique to the website (such as instant messaging in LJ Talk). It is based in San Francisco, California.
LiveJournal was started in 1999 by Brad Fitzpatrick as a way of keeping his high school friends updated on his activities. In January 2005, a blogging software company called Six Apart bought Danga Interactive, the company that was owned by Fitzpatrick, which operates LiveJournal.
LiveJournal has a service called a "friends list", which makes the site a way to socialise with friends and other people, even though it is a blog service too.
LiveJournal lets users change their accounts in several ways. A programming language called S2 can help users make their blogs look different. Users may upload small pictures (maximum 100 by 100 pixels) called "userpics", which appear next to the username. People can pay to get more features, like extra userpics, the ability to record their voices into a post ("voice posts"), the ability to make polls, and better layouts.
Each user also has a "User Info" page, which may contain contact information, a biography of the user, and lists of friends, interests, communities, and even schools the user had attended.
Currently LiveJournal has five kinds of accounts. About 95% of all LiveJournal users use a non-paid account ("plus" accounts). People can pay for their accounts ("Paid" accounts) and get more features, or they can get a "sponsored" account, which is free and has more features, but has advertisements. "Early adopters" are the accounts of people who signed up for LiveJournal back in 1999 or 2000. There are also permanent accounts. Most people can not get a permanent account. LiveJournal only sells them sometimes, and they may not sell them again.
RNA
RNA is an acronym for ribonucleic acid, a nucleic acid. Many different kinds are now known.
RNA is physically different from DNA: DNA contains two intercoiled strands, but RNA only contains one single strand. RNA also contains different bases from DNA. These bases are the following:
Adenine forms bonds with uracil, and guanine forms bonds with cytosine. In this way, we say that adenine is complementary to uracil and that guanine is complementary to cytosine. The first three bases are also found in DNA, but uracil replaces thymine as a complement to adenine.
RNA also contains ribose as opposed to deoxyribose found in DNA. These differences result in RNA being chemically more reactive than DNA. This makes it the more suitable molecule to take part in cell reactions.
RNA is the carrier of genetic information in certain viruses, especially the retroviruses like the HIV virus. This is the only exception to the general rule that DNA is the hereditary substance.
The main function of RNA is to carry information of amino acid sequence from the genes to where proteins are assembled on ribosomes in the cytoplasm.
This is done by messenger RNA (mRNA). A single strand of DNA is the blueprint for the mRNA which is transcribed from that DNA strand. The sequence of base pairs is transcribed from DNA by an enzyme called RNA polymerase. Then the mRNA moves from the nucleus to the ribosomes in the cytoplasm to form proteins. The mRNA translates the sequence of base pairs into a sequence of amino acids to form proteins. This process is called translation.
DNA does not leave the nucleus for various reasons. DNA is a "very" long molecule, and is bound in with proteins, called histones, in the chromosomes. mRNA, on the other hand is able to move and to react with various cell enzymes. Once transcribed, the mRNA leaves the nucleus and moves to the ribosomes.
Two kinds of non-coding RNAs help in the process of building proteins in the cell. They are transfer RNA (tRNA) and ribosomal RNA (rRNA).
Transfer RNA (tRNA) is a short molecule of about 80 nucleotides which carries a specific amino acid to the polypeptide chain at a ribosome. There is a different tRNA for each amino acid. Each one has a site for the amino acid to attach, and an anti-codon to match the codon on the mRNA. For example, codons UUU or UUC code for the amino acid phenylalanine.
Ribosomal RNA (rRNA) is the catalytic component of the ribosomes. Eukaryotic ribosomes contain four different rRNA molecules: 18S, 5.8S, 28S and 5S rRNA. Three of the rRNA molecules are synthesized in the nucleolus, and one is synthesized elsewhere. In the cytoplasm, ribosomal RNA and protein combine to form a nucleoprotein called a ribosome. The ribosome binds mRNA and carries out protein synthesis. Several ribosomes may be attached to a single mRNA at any time. rRNA is extremely abundant and makes up 80% of the 10 mg/ml RNA found in a typical eukaryotic cytoplasm.
Small nuclear RNAs (snRNA) join with proteins to form spliceosomes. The spliceosomes govern alternative splicing.
Genes code for proteins in bits called exons. The bits can be joined together in different ways to make different mRNAs. Thus, from one gene many proteins can be made. This is the process of alternative splicing. Any unwanted versions of the protein get chopped up by proteases, and the chemical bits re-used.
There are a number of RNAs which regulate genes, that is, they regulate the rate at which genes are transcribed or translated.
Micro RNAs (miRNA) act by joining an enzyme and blocking mRNA, or speeding its breakdown. This is called RNA interference.
Small interfering RNAs (sometimes called silencing RNAs) interfere with the expression of a specific gene. They are quite small (20/25 nucleotides) double-stranded molecules. Their discovery has caused a surge in biomedical research and drug development.
Transposons are only one of several types of mobile genetic elements.