DNA-RNA-Protein Introduction

DNA carries the genetic information of a cell and consists of thousands of genes. Each gene serves as a recipe on how to build a protein molecule. Proteins perform important tasks for the cell functions or serve as building blocks. The flow of information from the genes determines the protein composition and thereby the functions of the cell.

The DNA is situated in the nucleus, organized into chromosomes. Every cell must contain the genetic information and the DNA is therefore duplicated before a cell divides (replication). When proteins are needed, the corresponding genes are transcribed into RNA (transcription). The RNA is first processed so that non-coding parts are removed (processing) and is then transported out of the nucleus (transport). Outside the nucleus, the proteins are built based upon the code in the RNA (translation).

The document has two levels, basic and advanced. This page is an introduction to both levels. You start at the basic level, then you can advance if you want more and deeper information.

DNA/Replication
As you might have noticed you are a mixture of your biological parents and preceding generations. Maybe you've got your mothers nose and your fathers eyes. How can that be? Something must carry that information between the generations.

DNA is the physical carrier of inheritance. It is like a giant book of information containing all the instructions for building and maintaining a living organism.
DNA

Replication followed by cell division is the answer to one of life's most interesting questions: How can the union of a single sperm and an egg become a five-trillion-cell baby, all containing the same DNA?

RNA-Transcription
We earlier imagined DNA as an instruction book. Let's even make it a reference book. When you need information about something you make a copy of the pages (genes) you're interested in, returning the book to the library. This way you don't have to risk losing or destroying the book.

In all eucaryotic cells DNA never leaves the nucleus, instead the genetic code (the genes) is copied into RNA which then in turn is decoded (translated) into proteins in the cytoplasm. Why? Wouldn't it be smarter if DNA itself was translated into proteins in the cytoplasm instead of using a RNA intermediate?
The answer, for many reasons, is no. One important reason is security. The cytoplasm is a dangerous environment for the DNA and the daily transcription of genes to proteins would be very harmful to the DNA, which has to stay intact in order to maintain life. Therefore, RNA works as a sort of throw-away version of DNA (like the copies from the reference book) - good for limited work but not for long-term storage. Another reason is to regulate the rate of protein synthesis. This will be further discussed in the section about protein-translation.

Processing
A newly made mRNA copy of a gene (pre-mRNA), must undergo three major modifications before it can leave the nucleus and be translated into a protein:

1) Capping - a special nucleotide is attached at the 5' end of the mRNA. It is believed that this modification is necessary for efficient initiation of protein synthesis and serves as stabilisation.

2) Poly(A)-tail - a special enzyme attaches a chain of 150-200 adenine nucleotides to the 3' end of the pre-mRNA directly after transcription. This is primarily believed to increase the stability and therefore prolong the lifetime of an mRNA molecule.

3) Splicing - The removal of noncoding sequences, introns, from the pre-mRNA to create mRNA, containing only the coding sequences for a protein. This process is carried out by the splicosome. The splicosome is a special complex made up by proteins and a catalytic kind of small nuclear RNA molecules, the snRNAs..

Transport
Since the DNA is transcribed into mRNA in the nucleus, and protein synthesis takes place in the cytoplasm, the mRNA has to exit the nucleus to the cytoplasm. The environment in the nucleus differs in many ways from that of the cytoplasm. To separate these two environments from each other the nucleus is enclosed by a double membrane, and the only connection to the surrounding cytoplasm is through channels called the nuclear pore complex (NPC).
When it is time for the mRNA to leave the nucleus, the mRNA is believed to be "tagged" by proteins which serve as export signals, directing the mRNA to the nuclear pore complex that the mRNA is to leave. The mRNA and it's bound export proteins then attaches to export receptors and the whole complex (RNA, export signal proteins and export receptors) is translocated through the nuclear pore complex. The mRNA is released into the cytoplasm and is immediately ready for the next step: translation

 
Protein / Translation
What do muscles, hair and the hormone insulin have in common? Right! They all consist of proteins.
Proteins are the building blocks and workers of our cells. Roll over the image to see a few examples of what proteins can perform in the human body. proteins

How does something so seemingly simple as DNA's long sequence, composed of only four different letters, get converted into the 100,000 or so different kinds of protein molecules that perform the daily work in our body? This is accomplished by the cell's protein-making factory and is called translation.