What is impressive about DNA is that each sugar molecule in the strand also binds to one of four different nucleotide bases.  These bases: Adenine (A), Guanine (G), Cytosine (C) and Thymine (T), are the beginnings of what we will soon see is a molecular alphabet.  Each sugar molecule in the DNA strand will bind to one nucleotide base.  Thus, as our description of DNA unfolds, we see that a single strand of the molecule looks more like this:

Each strand of DNA contains millions or even billions (in the case of human DNA) of nucleotide bases.  These bases are arranged in a specific order according to our genetic ancestry.  The order of these base units makes up the code for specific characteristics in the body, such as eye color or nose-hair length.  Just as we use 26 letters in various sequences to code for the words you are now reading, our body's DNA uses 4 letters (the 4 nucleotide bases) to code for millions of different characteristics.
        Each molecule of DNA is actually made up of 2 strands of DNA cross-linked together.  Each nucleotide base in the DNA strand will cross-link (via hydrogen bonds) with a nucleotide base in a second strand of DNA forming a structure that resembles a ladder.  These bases cross-link in a very specific order: A will only link with T (and vice-versa), and C will only link with G (and vice-versa).  Thus our picture of DNA now looks like this:

Artwork courtesy of Lisa Graf for MCET

        The specific base-pairing of DNA aids in reproduction of the double helix when more genetic material is needed (such as during reproduction, to pass on characteristics from parent to offspring).  When DNA reproduces, the 2 strands unzip from each other and enzymes add new bases to each, thus forming two new strands.  This process is illustrated in the Access Excellence DNA Replicating Itself page (just hit your browser's Back button to return here).
        Within this coil of DNA lies all the information needed to produce everything in the human body.  A strand of DNA may be millions, or billions, of base-pairs long.  Different segments of the DNA molecule code for different characteristics in the body.  A Gene is a relatively small segment of DNA that codes for the synthesis of a specific protein.  This protein then will play a structural or functional role in the body.  A chromosome is a larger collection of DNA that contains many genes and the support proteins needed to control these genes.

Protein Synthesis
        How does a gene code for a protein?  Protein synthesis is a 2 part process that involves a second type of nucleic acid along with DNA.  This second type of nucleic acid is RNA, ribonucleic acid.  RNA differs from DNA in two respects.  First, the sugar units in RNA are ribose as compared to DNA's deoxyribose.  Because of this difference, RNA does not bind to the nucleotide base Thymine, instead, RNA contains the nucleotide base Uracil (U) in place of T (RNA also contains the other three bases: A, C and G).

• Transcription: In the first step of protein synthesis, the 2 DNA strands in a gene that codes for a protein unzip from each other.  Similar to the way DNA replicates itself, a single strand of messenger RNA (mRNA) is then made by pairing up mRNA bases with the exposed DNA nucleotide bases.  The top column in the table below shows 6 bases in a DNA sequence.  Click on each DNA base to see the complementary mRNA base in the bottom column.

Remember that mRNA does not contain the base Thymine, so U is paired with each of DNA's A bases.
 
• Translation: After the mRNA is manufactured, it leaves the cell nucleus and travels to a cellular organelle called the ribosome (we will learn about the cell, nucleus and ribosome in the next lesson).  In the ribosome, the mRNA code is translated into a transfer RNA (tRNA) code which, in turn, is transfered into a protein sequence.  In this process, each set of 3 mRNA bases (the mRNA base triplet is called a codon) will pair with a complimentary tRNA base triplet (called an anticodon).  Each tRNA is specific to an amino acid, as tRNA's are added to the sequence, amino acids are linked together by peptide bonds, eventually forming a protein that is later released by the tRNA.  Using the mRNA strand we obtained above, you can generate the complimentary tRNA/amino acid sequence by clicking on the mRNA codons in the table below.

mRNA codon

 After the processes of transcription and translation are complete, we are left with a protein that consists of the chain:

Aspartic Acid -

Leucine

Although our 'protein' is only 2 amino acids in length, proteins normally consist of hundreds or thousands of amino acids.

• This Cell to Protein page allows you to input any DNA sequence and it will calculate the complementary mRNA sequence and the resulting amino acid chain.
• Genentech's Access Excellence pages have excellent tutorials on the History of DNA and Protein Synthesis.
• PBS has put together an outstanding, interactive DNA and Protein Synthesis tutorial.

 

 

And for those interested in even more DNA info.:

• The U.S. Department of Energy has put together a good description of the Human Genome Project, a research project designed to map the exact nucleotide base sequence of the entire human DNA genome!
• For those history buffs, the LionBook web site has put Watson and Crick's 1953 Nature article announcing the structure of DNA on-line.

Copyright © 1998-1999, All Rights Reserved, Anthony Carpi
The DNA double helix gif copyright Lisa Graf at MCAT
The DNA movie and CHIME molecule copyright A. Parrill & J. Gerway
"DNA Replicating Itself" copyright Access Excellence