DNA
DNA, short for deoxyribonucleic acid, is a nucleic acid - an acidic molecule that is in the nucleus of cells. The nucleus is a part of the cell separated from the rest of the cell by a membrane or wall. A lot of DNA can be found in the cells of all living things, also known as organisms. This includes animals, plants, germs and viruses. DNA contains the genetic code of the living thing - the instuctions that tell all the cells in the organism proteins to make. The proteins that a cell makes control what that cell does - the cell's function. The DNA in a living thing is a mix of the DNA from each of its parents.
DNA structure.
The "rungs" of the DNA ladder are each made of two bases, one base coming from each "leg". The bases connect in the middle. 'A' connects only with 'T', and 'C' connects only with 'G'. The connections are hydrogen bonds.
The reason that the bases can only pair up 'G' with 'C' and 'A' with 'T' is the number of hydrogen bonds each base can make. Adenine and thymine can each make two hydrogen bonds and cytosine and guanine can make three hydrogen bonds. It may seem that DNA would always be the same because the bases are always in fixed pairs, but DNA is a very big molecule and the pairs can come in any order. The code or plan for all the proteins that make a person, animal, or plant can be found by reading the order of the nitrogen bases in the DNA.
Copying DNA.
When DNA is copied this is called DNA replication. Briefly, the hydrogen bonds holding the two strands of the molecule together are broken and the molecule is split in half. This gives two single strands. New strands are formed by matching the bases (A with T and G with C) to make the 'missing' strands.
First, an enzyme called DNA helicase splits the DNA down the middle by breaking the hydrogen bonds. Then after the DNA molecule is in two separate pieces, another molecule called DNA polymerase makes a new strand that matches each of the strands of the split DNA molecule. Each copy of a DNA molecule is made of half of the original (starting) molecule and half of new bases.
Mutations.
Usually mutations are fatal for the cell or the organism - the protein made by the 'new' DNA does not work as it should, but also evolution is moved forward by mutations, when the new protein works better or has another good effect.
Protein synthesis.
DNA is what tells the cell how to make particular proteins. Proteins do most of the 'work' in cells and in the whole organism. Proteins are made out of smaller molecules called amino acids. To make a protein to do a particular job, the correct amino acids have to be joined up in the correct order.
Proteins are made in parts of the cell - organelles - called ribosomes. Ribosomes respond to a special codes called codons - groups of three base pairs which 'tell' the ribosome which amino acid to add to the protein next. Ribosomes are found in the main body of the cell, but DNA is only found in the nucleus of the cell. The codon is part of the DNA, but DNA never leaves the nucleus. Because DNA can't leave the nucleus, the cell makes a copy of the DNA, but in a new single strand form which is smalller and can get through the holes in the membrane of the nucleus - pores - in the nucleus and out into the cell.
This one-stranded copy of DNA is called RNA. This is messenger mRNA. The mRNA attaches to a ribosome where the protein is made. The ribosome works on three bases at a time - this is called a codon and 'codes' for a particular amino acid. Another sort of RNA called transfer RNA tRNA 'matches' the RNA codon (but with the opposite bases) and is linked to the right amino acid so that the correct amino acid is brought to the ribosome which links it on to the growing amino acid chain.
History.
DNA was first isolated - extracted from cells - by Swiss physician Friedrich Miescher in 1869, when he was working on bacteria from the pus in surgical bandages. The molecule was found in the nucleus of the cells and so he called it "nuclein".
In 1919 this discovery was followed by the discovery by Phoebus Levene of the base, sugar and phosphate nucleotide unit. In the 1950's, Erwin Chargaff found that the amount of thymine (T) present in a molecule of DNA was roughly equal to the amount of adenine (A) present. He found that the same applies to guanine (G) and cytosine (C).
A few years after Chargaff's discovery, a British scientist named Rosalind Franklin worked on crystals of DNA and how they diffracted beams of X-rays and found that an "X" pattern was produced which showed that the crystal was probably helix shaped. Francis Crick and James Watson were also working on the structure of DNA and working from Franklin's results and using models of the bases worked out the shape of the molecule. How Crick and Watson got Franklin's results has been much debated. Crick, Watson and Maurice Wilkins were awarded the Nobel Prize in 1962 for their work on DNA - Rosalind Franklin had died in 1958.
Visit Our HomePage