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DNA Replication, Transcription, and Translation

What is DNA replication?

When a cell divides, the DNA has to be duplicated, so that each of the two daughter cells has a complete set of DNA.  To do this, the pairs pull apart (think of it like unzipping a zipper). Each half of the unzipped chain is used to form a duplicate. Because A only binds with T and C with G, each newly formed half becomes an automatically correct copy.

How does DNA code for proteins?

To make proteins, the code in the DNA has to be translated to a language code that can make amino acids line up in the proper order. The process is a little more complicated than it might seem. DNA does not make proteins directly. There are two major in-between steps.

Step 1:  DNA Transcription

The first step is to transcribe, or copy, the code in DNA to another compound called "messenger RNA". You can think of DNA like a zipper that unzips when it is time to read the code. This is similar to changing a computer program (DNA code) into a written statement (messenger RNA). Remember, DNA does nothing unless the code is read. Loose talk about having certain genes, such as genes for alcoholism, schizophrenia, sexual orientation, and other conditions is very misleading. What matters is whether or not genes are "read," and that depends on many poorly understood aspects of the environment inside of cells.

DNA Transcription ImageMessenger RNA (mRNA) is a small molecule (green in diagram to the right) that copies the message of the DNA code. The message can be copied because mRNA has base pairs that preserve the DNA code. mRNA is just like DNA except that a base called Uracil is substituted for Thymine. Uracil performs the same function of matching with adenine.

When a gene is actively making protein, it "unzips" partially so that base pairs separate, and each member can can match specific base pairs in the mRNA. Thus, mRNA preserves the code in DNA. The key point is that RNA chains of bases are smaller than DNA and they can move out of the nucleus into other parts of the cell.

Step 2: RNA Translation

The second step in coding is to translate the code in mRNA to
protein-code language. The "language" of proteins is the sequence of amino acids.

In stringing together different amino acids to make a protein, the RNA code must be able to deal with the 20 known amino acids. If the code were based on only one  base compound (A or U or C or G), then the code could only translate for four amino acids.  If a PAIR of bases compounds served as the code, then more amino acids could be specified, but still not enough to account for all 20 amino acids that appear in proteins. Thus, mathematically we know that it must take at least three nuclei acids to code 20 amino acids. Experimentally, that has been demonstrated, and a Nobel Prize was awarded for that discovery.

Thus short RNA segments of only three nucleotides can act as carriers for amino acids.   Messenger RNA making these three-segment RNAs, known as transfer RNA (tRNA).  The tRNA joins three amino acids together in the right sequence, as specified by order of the three base compounds in the tRNA. This process is kept orderly because the RNA is anchored in membranes, known as endoplasmic reticulum

The final protein product is typically moved into another membrane system, an organelle called Golgi apparatus, which "fine tunes" the proteins by adding certain sugar residues.

Click here to see an animated cartoon of how specific tRNA molecules attach to specific amino acids and link them together to make proteins.

A neat site that explains these things in a different way can be found by clicking here.

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