The function of the genetic code is to control heredity, the passage of genetic characteristics to offspring. Yet, this does not tell us what specific function DNA actually performs inside of a cell.
Since, the nucleus of a cell is the control center, it must be the DNA of the nucleus that actually "manages" the cell. It was discovered in this century that the actual function of the DNA in a cell was to produce polypeptides of proteins, the structure of our bodies. (Most proteins consist of several polypeptides, which are polymer chains of amino acids).
It is the proteins of a cell that keep a cell functioning and alive. They act as enzymes to run chemical reactions, and are used as structural reinforcement to prevent the cell from falling apart.
Need for RNA
Yet, the DNA does not actually produce the protein itself. To prevent its own damage, it makes copies of itself which actually do the protein production. The copies are actually made in a chemical called RNA, another nucleic acid.
RNA is identical to DNA in many ways. It contains nucleotides and it is structurally very similar.
But, RNA differs from DNA in three basic ways.
- RNA is always single stranded, never double stranded.
- RNA contains all N-bases except thymine, it contains another N-base, URACIL, instead.
- RNA contains a different sugar - RIBOSE - instead of deoxyribose which is found in DNA.
Uracil vs Thymine
The nitrogen bases of RNA and DNA are shown below. DNA contains the nitrogen bases T,C,A,G while RNA contains the nitrogen bases U,C,A,G. Note that uracil and thymine are very similar, differing only by a carbon and 2 hydrogen atoms.
Deoxyribose vs Ribose
Deoxyribose differs from ribose by a single oxygen atom. Deoxyribose is missing the oxygen atom while ribose has the oxygen atom. Try and find the difference below.
A typical RNA nucleotide would look like the following (a structural formula and abbreviated form is provided):
DNA vs RNA
Below are the structural formulas of two nucleotides, one DNA and the other RNA. Can you determine which is which? Remember that DNA has deoxyribose, which has one less oxygen atom per molecule, than ribose.
We will use abbreviations to represent the nucleotides in nucleic acids. A typical RNA polymer strand would look like the following:
The DNA in the nucleus produces RNA polymer copies of itself, called messenger RNA (mRNA) which then actually move out into the cell's cytoplasm to attach to a cell's protein factories - the ribosomes. Thus the actual production of protein takes place outside the nucleus and the DNA is protected from the process. Messenger RNA serves as the intermediary between DNA and ribosomes in protein synthesis.
In the cytoplasm, ribosomes bond to the messenger RNA and produce polypeptides by bonding amino acids together according to the genetic message carried by the messenger RNA. In the cytoplasm there are special molecules called "transfer RNA molecules" (tRNA) that carry amino acids to the ribosomes. The ribosomes cannot produce polypeptides unless they have messenger RNA code and transfer RNA to bring amino acids to them.
Transfer RNA molecules are shaped like cloverleafs. They serve only to help ribosomes make polypeptides. They pick up amino acids that are floating around in the cytoplasm (acylation) and carry them to the ribosomes so the ribosomes can string them together and make polypeptides. Each tRNA can carry only one amino acid at a time. Once they have dropped off their amino acid at a ribosome, they are free to go and pick up another amino acid and carry it to another ribosome. tRNA recycle themselves this way.
Transcription and Translation
The production of mRNA in the nucleus of a cell is called TRANSCRIPTION. The production of a polypeptide by the interaction of mRNA, tRNA, amino acids, and a ribosome is called TRANSLATION.
Protein Synthesis Overview
The overall protein synthesis process involving transcription and translation works this way:
OVERALL PROTEIN SYNTHESIS
|TRANSCRIPTION = mRNA Production|
| 1) The DNA produces an RNA copy of itself called mRNA, short for "messenger RNA".|
2) The mRNA moves out of the nucleus and into the cytoplasm.
|TRANSLATION = Protein Synthesis|
| 1) The mRNA bonds to a ribosome.|
2) Various tRNA molecules bring amino acids to the ribosome.
3) A ribosome bonds amino acids brought to it by tRNA and produces a polypeptide.
Final protein assembly
The polypeptide will then combine with other polypeptides and form a protein, which could reinforce the structure of the cell or act as an enzyme to speed up cellular reactions.
Symbols for Animation of Protein Synthesis
mRNA molecules are polymers like DNA, except that they are single stranded, not double stranded.
The figures below will be used to represent the DNA, mRNA, tRNA, amino acids, and ribosomes:
Protein Synthesis Animated:
Below is the first frame of the animation.
Be sure to study the parts before clicking the links to the animation below!!
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
To name the scientific processes you just viewed,
1- mRNA production is called TRANSCRIPTION, because it is a copy process. (to transcribe means "to copy")
2- protein production is called TRANSLATION, because it involves mRNA nucleic acid code being converted into protein amino acid sequences using ribosomes and tRNA molecules. (to translate means "to convert languages")
A "gene" is a piece of a DNA molecule that is responsible for storing the information for the production of a single type of polypeptide molecule. Thus, a single gene indirectly produces (via mRNA) a single type of polypeptide molecule of a protein.
1 DNA GENE ----> 1 Polypeptide
Genetic Code Explanation
In DNA molecules, there are strands or sequences of nucleotides. Specifically, it is the sequence of nitrogen bases (or nucleotides) in a gene that determines the types of polypeptide the gene will make. DNA in a cell contains a "genetic code" which is written in the sequences of NITROGEN BASES on the DNA strands.
Polypeptides and Nitrogen Base Sequences
Polypeptides are polymers consisting of chains of amino acids. It is the NITROGEN BASE SEQUENCE of a gene that determines the AMINO ACID SEQUENCE of a polypeptide. The nitrogen bases of DNA represent a "genetic alphabet" that determine the amino acid sequences of a protein.
Amino acids vs Codons
Proteins in general can consist of 20 different types of amino acids. There are only 20 different types of amino acids that exist in the biological world. There are 4 nitrogen bases in the DNA language, so they themselves could not represent the 20 different amino acids that exist. Some multiple of nitrogen bases --- "a word" --- must represent an amino acid.
Nitrogen bases vs Codons
It turns out that 3 DNA nitrogen bases or nucleotides represent a single amino acid of a protein. So, if a gene contained information for the production of a protein which was 50 amino acids long, the DNA molecule would have to be 150 nitrogen bases long in order to produce it.
Codons and Amino Acids
Three DNA N-bases of a gene representing an amino acid is called a CODON. A codon is a word in the genetic alphabet of N- bases. Since 3 N-bases represent an amino acid, the SEQUENCE OF CODONS OF A GENE REPRESENT THE SEQUENCE OF AMINO ACIDS OF A POLYPEPTIDE!
Gene and Polypeptide
Below is a strand of DNA, 12 nucleotides long. It has 4 codons or genetic words in it. Each stands for or represents an amino acid, and the sequence of the codons determines the sequence of the amino acids of the protein it is responsible for making.
There are 64 different possible DNA codons, 61 actually represent amino acids, the remaining 3 are reserved for special use and will be mentioned in the next program "gene expression". Use the translation table below to determine the meaning of several codons. Then use the decoder function below to test your ability to properly read the table.
To use the "codon wheel dictionary" below, you MUST BE USING AN mRNA CODON!! Find the first base of the mRNA codon in the middle circle (green). From the first base, move out to the 2nd mRNA base in the second circle-ring (dark-orange) and them move out to the 3rd mRNA base in the 3rd circle-ring (yellow-orange). The represented amino acid is in the fourth circle-ring (purple) just outside the 3rd base. For example, the DNA codon CTA transcribes into the mRNA codon GAU, which represents the amino acid "aspartic acid".
The genetic code is thus made up of codon sequences. Different genes produce different polypeptides because they have different codons which code for different amino acids.
Finally, each individual organism of a species differs slightly from another because of the slight differences in the codons of their genes. This causes their proteins to differ slightly in amino acid content, which causes differences in physical characteristics, such as hair color or eye color.