How is information contained in DNA expressed?
DNA has two jobs-one is to replicate and the other is to provide information for making proteins. The process of making proteins is called protein synthesis. Protein synthesis is the result of gene expression. And the synthesis of different proteins is what makes us humans.
People are unique. How different does the DNA have to be to provide the diversity that exists among individuals? Scientists say that two people are different because approximately one DNA nucleotide per thousand nucleotides varies. That means there are about 3,000,000 places in a DNA strand where differences occur.
Proteins are the most complex and diverse of all molecules that make up the human body. Over half of the dry weight of the human body is protein. Protein molecules are very big. Small protein molecules have over 1,000 atoms, while large protein molecules may have over a million atoms. These atoms are organized into molecules called amino acids. The amino acids are the building blocks of proteins. A protein can consist of several hundred amino acids. Each species has its own set of proteins. Different cells in your body produce different proteins but have the same DNA.
Proteins do many things in your body. Your muscles are made up of proteins that can contract. Your blood has the protein hemoglobin that carries oxygen to your cells. Your blood has other proteins that cause it to clot. Still other proteins protect you from germs. Proteins, called enzymes, control practically all of the chemical reactions in your cells. The cartilage that forms your joints and gives shape to your ears and nose is a protein. Proteins can do so many different things because they have very different shapes. The specific sequence of amino acids in a protein determines its shape. If a protein loses its shape, it loses its function. When you cook foods, you denature the proteins that are in the plant or animal tissues in the food. Scientists say protein is denatured when it loses its shape.
In other units, you will learn about important protein molecules including hormones, hemoglobin, antibodies, and enzymes. Proteins are very important components of your body and of the bodies of all living things.
Paleontologists study the DNA codes of ancient people they discover. Using this information, they can determine the group to which a person belonged. They also can determine when and where the people lived. This information tells us a lot about ancient civilizations, their movements, their longevity, and much more. The discovery of DNA added a new dimension to the study of paleontology and archaeology.
A look at the Information Code
How does a DNA molecule, with only four nucleotides, contain the code for all the different proteins that are found in the human body?
Remember that proteins are made of individual chemical building blocks called amino acids. There are more than 20 different amino acids. Many people have compared amino acids to letters in the alphabet. Just as we put together the letters in the alphabet into different combinations and sequences to make words, the amino acids are put together in different combinations to make different proteins. The DNA molecule contains coded information that controls the order in which the amino acids are put together.
The code in DNA is a three-letter code. How can we read the DNA code? Each sequence of three nucleotides codes for an amino acid. For example, the DNA nucleotide sequence of thymine, cytosine, and adenine (TCA) is a code for the amino acid named serine. Cytosine-adenine-thymine (CAT) is a code for the amino acid named valine. The three nucleotides (triplet) on the DNA molecule that code for an amino acid act as a single unit.
Did You Know?
Genes vary in their length. Some are about 500 DNA-code letters long; others are more than 2,000,000 letters long!
Figure 5.1 The nucleotides thymine, cytosine, and adenine join together to make the code for the amino acid serine. Cytosine, adenine, and thymine make the code for valine. DNA codes for more than 20 amino acids. See the Did You Know? about mRNA, in the margin.
The mRNA uses the nucleotide base uracil instead of thymus, so the DNA code TCA becomes AGU in mRNA. The mRNA code AGU codes for the amino acid serine. Similarly, the DNA code CAT becomes GUA in mRNA and codes for the amino acid valine.
The sequence of nucleotides in DNA spells out code words for assembling amino acids to build proteins. These code words can be grouped together like a “sentence” or a message called a gene. The gene tells the cell to use specific amino acids in a specific order to make a particular protein, like hemoglobin.
Putting the Code to Work to Make Protein
DNA is found only in the nucleus, but proteins are produced in the cytoplasm outside the nucleus. So the coded information of DNA must get from the nucleus to the cytoplasm somehow. How is this code of the DNA molecule read to make a protein? Three other molecules are involved in the process for DNA to transmit the code. The three chemicals involved are mRNA, tRNA, and ribosomes. RNA is a molecule very much like DNA. It is composed of nucleotides that form base pairs with DNA, but it is a Single-stranded molecule rather than a double-stranded helix. Some RNA is in the form of a long chain and travels between the nucleus and the cytoplasm. This RNA called mRNA carries the coded message from the DNA in the nucleus to the cytoplasm. The “m” stands for messenger. mRNA is the messenger that takes the DNA code to the cytoplasm where proteins are made. There are short pieces of RNA in the cytoplasm that carry or transfer a specific amino acid from the cytoplasm to sites where the proteins are assembled. These sites are called ribosomes. Amino acids are joined together to make a protein at the ribosomes. The amino acids are carried (or transferred) to the ribosomes where protein is assembled by RNA called tRNA. The “t” stands for transfer. Both mRNA and tRNA are made in the nucleus. Since there are 20 or more amino acids that are used to make proteins, there are more than 20 different tRNAs.
You could think of protein synthesis like a construction project. The ribosomes are the construction sites where the protein is made. The DNA is the architect that has the design for a specific protein. The mRNA is the general contractor. The tRNAs are the various suppliers that bring the raw materials to the construction site, or in this case provide the amino acids for making the protein. The mRNA and the tRNAs meet at the ribosome. The mRNA has the coded message from the DNA, and the tRNAs have the different amino acids. When an mRNA and a tRNA that matches a particular triplet code on the mRNA meet at the ribosome, an amino acid is joined to the growing protein.
- DNA produces mRNA.
- mRNA travels outside the nucleus to the cytoplasm and then attaches to a ribosome.
- When a tRNA, picks up an amino acid floating around the cytoplasm, it travels to a ribosome.
- On the ribosomes, the mRNA tells the tRNAs in what order to line up.
- The result is a protein-a chain of amino acids whose job is to tell cells how to do their job.
Activity 5-1: Making Protein
Where and how are proteins made in your cells? What is the role of DNA? What is the role of RNA? What is the role of the ribosomes? In this activity you write a script and role-play an important cell process to demonstrate the role of DNA in making RNA and the role of RNA in making proteins in your cells.
- Signs, string, rope
- Resource (Optional)
- Activity Report
Step 1 Write an original script or use the script provided in the Resource that describes the steps involved in making a protein.
Step 2 Share your script with the class. Determine which of the scripts will be used for class presentation. Your teacher will provide copies for the players.
Step 3 Create a human cell formed in bone marrow that makes hemoglobin by placing
- a long piece of string or rope in a circle on the floor to represent the cell membrane.
- a shorter piece of string or rope on the floor inside the cell membrane to represent the nucleus.
- several signs labeled “ribosome” on the floor outside the nucleus.
Step 4 Identify players and have them make and display their signs.
Step 5 Players take their positions and rehearse and perform the acts of the script.
c. Messenger RNA (mRNA)
d. Transfer RNA (tRNA) for Proline
Messenger RNA (mRNA)
e. Transfer RNA (tRNA) for Glutamic Acid 1
Transfer RNA (tRNA)/Proline
Transfer RNA (tRNA)/Glutamic Acid 1
f. Transfer RNA (tRNA) for Glutamic Acid 2
Transfer RNA (tRNA)/Glutamic Acid 2
Amino Acid Proline (reverse side: The)
g. Amino Acid Proline
Amino Acid Glutamic Acid 1 (reverse side: Protein)
h. Amino Acid Glutamic Acid 1
Amino Acid Glutamic Acid 2 (reverse side: Hemoglobin)
i. Amino Acid Glutamic Acid 2
3 Ribosome (placed in the cell)
Figure 5.3 Players/Signs Table.
- What is a protein?
- What is an amino acid?
- What is the difference between tRNA and mRNA? Where are they produced and where do they do their work inside the cell?