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Gene Expression DNA in cells controls all sorts of things such as the color of your eyes, the color of your hair, and whether or not you can digest milk. These characteristics are called traits. DNA also controls your responses to stimuli in the environment to keep you alive. For example, when you are frightened, the DNA tells your adrenal cells to produce adrenaline which will help you respond to the situation. DNA does this by using its blueprints to make different proteins at different times. A section of DNA that contains the instructions for making one specific type of protein is called a gene. There are about 30,000 genes in each human cell! Proteins are made up of amino acid molecules. There are 20 different amino acids molecules with names like isoleucine, valine, and phenylalanine. These amino acids can be put together in any combination based on the code in the DNA molecule. Proteins are usually 200-250 amino acids long!!!! There are thousands of different kinds of proteins and each has its own special job based on its structure. 1. DNA controls a person’s __________________________ and ________________________. 2. Gene = 3. How many genes do humans have in each cell? 4. What are the building blocks of proteins? 5. How many amino acids are there? Let’s take a look at the process of making a protein from the directions in DNA (called gene expression). The DNA code works in threes. This sequence of 3 bases in DNA is called a codon. This means that every three bases in a gene has the directions for adding one amino acid to the protein molecule being built. If you arranged the 4 bases (A, T, C, G) in groups of three in every possible combination, you would get a total of 64 possible combinations. So, this means that there are 64 codons in the genetic code. Realize that there are only 20 amino acids, so some of the 64 codons code for the same amino acid. Scientists have created a chart called a codon chart which shows all 64 codons and the amino acid that each adds to the protein being built. U C A G U Phenylalanine Phynylalanine Leucine Leucine Leucine Leucine Leucine Leucine Isoleucine Isoleucine Isoleucine Methionine Valine Valine Valine Valine (based on the code in RNA) C A Serine Tyrosine Serine Tyrosine Serine Stop Codon Serine Stop Codon Proline Histidine Proline Histidine Proline Glutamine Proline Glutamine Threonine Asparagine Threonine Asparagine Threonine Lysine Threonine Lysine Alanine Aspartic Acid Alanine Aspartic Acid Alanine Glutamic Acid Alanine Glutamic Acid 1. Define codon: 2. How many codons are there in the genetic code? 3. Write your hypothesis here: G Cysteine Cysteine Stop Codon Tryptophan Arginine Arginine Arginine Arginine Serine Serine Arginine Arginine Glycine Glycine Glycine Glycine U C A G U C A G U C A G U C A G The DNA molecule needs a smaller helper molecule to get its instructions out the ribosome. This molecule is RNA (ribonucleic acid). The RNA molecule is similar to the DNA molecule. However, RNA structure differs from DNA structure in three ways. Check out this graphic and see if you can figure out the three differences. Write them here. DNA RNA Be aware that there are three types of RNA, each having a particular function in the process of building a protein. As we learn about the process, each of these three molecules will be discussed. TRANSCRIPTION The process of gene expression occurs in two main steps. The first step is called transcription. During this step, the code in a gene is copied into RNA. Enzymes unwind the double helix and unzip it to expose the gene to be copied. Other enzymes match up RNA complementary base pairs to the exposed pairs creating an RNA molecule called messenger RNA (mRNA). The mRNA molecule then leaves the nucleus. Other enzymes help the DNA molecule to seal back up and reform the double helix. 4. What happens during transcription? 5. What type of RNA is made in this process? 6. Where in the cell does transcription occur? 7. This is a sequence of bases in a gene. Fill in the bases for a complementary segment of RNA to show the process of transcription. Don’t forget that RNA has U instead of T!! So U would match with A. DNA RNA A A T C G T C C G TRANSLATION The second main step of gene expression is called translation. The code carried in the mRNA is used to make a protein at the ribosome. Translation involves two other types of RNA – transfer RNA (tRNA) and ribosomal (rRNA). Each tRNA molecule has a sequence of 3 bases on one end called an anticodon. tRNA anticodons match up with the mRNA codons. This is an important part of code translation because the tRNA molecule carries a certain amino acid based on its anticodon. For example, a UAC anticodon will carry the amino acid named methionine. By matching the tRNA anticodon to the messenger RNA codon, the correct amino acid is put into place as the protein is built. The ribosomal RNA (rRNA) makes up the ribosome and helps the codon and anticodon join. So once mRNA arrives at the ribosome, tRNA anticodons join with each mRNA codons one at a time. Each sequence of three bases (codon) codes for one amino acid to be added onto the protein being made. The tRNA drops off its amino acid cargo at this point. Each new amino acid that is dropped off is joined to the rest to form a chain (which is the protein). The tRNA than leaves and picks up another amino acid based on the code in its anticodon. 8. What type of RNA acts as an enzyme and makes up the ribosome? 9. What type of RNA brings amino acids to the ribosome? 10. What happens during translation? 11. Where does translation occur in the cell? 12. Complete the online Gene Expression activities listed on the webpage.