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In this lab you will learn how living cells produce proteins. Protein synthesis requires two distinct processes, transcription and translation. You will have an opportunity to review both procedures as you “make a protein.” You will use your textbook and the information in this lab as a reference.

Part 1: Transcription and Translation Review^[1]

Introduction

As you know, DNA is a very long, thin molecule made of proteins and nucleotides. The DNA in one chromosome has 10s of millions of base pairs and hundreds or thousands of genes that code for a variety of different proteins. However an individual cell will only use a small portion of those genes in its lifetime. Imagine a mechanic who spends a lifetime fixing nothing but cars, but he or she is required nonetheless to carry around an entire library of repair manuals for everything from kitchen sinks to washing machines to light fixtures to computers and so on!

Now consider the location of Eukaryotic DNA. Eukaryotic organisms protect their DNA by storing it inside the nucleus. However, the protein making factories (ribosomes) are located in the cytoplasm outside of the nucleus. How does the cell solve this problem? It must send a “messenger” that carries a copy of the genetic information from the nucleus to the ribosomes in the cytoplasm.

Protein synthesis is a two-step process that involves two main events called transcription and translation.

In transcription, the DNA code is transcribed (copied) into mRNA. Once the mRNA is produced it moves out of the nucleus into the cytoplasm where it links up with ribosomes (protein making organelles) and begins churning out proteins. Before looking at translations let’s take a closer look at transcription.

Recall that DNA consists of a sugar-phosphate backbone with a nitrogenous base. There are 4 different bases in DNA abbreviated with the letters A,T,C, & G. The code contained in DNA derives from these 4 bases. We can think of them as letters in an alphabet that will spell different words.

In DNA code, a “word” is always 3 letters long and it specifies one of 20 amino acids. However, DNA is not directly involved in the translation process, instead mRNA is transcribed into a sequence of amino acids. When reading the mRNA, it is “read” in a series of three adjacent nucleotides.

In transcription, the DNA code is transcribed (copied) into RNA code, following rules similar to DNA replication we saw earlier except that Thymine (T) is replaced by Uracil (U).

Lab Questions

1. Transcribe the following DNA sequence into mRNA.
   A    T    C    G    T    C    C    A    A    A …(DNA strand). U    A    G   C     A    G   G    U    U    U    (mRNA strand)
2. Transcribe the following DNA sequence into mRNA. Draw a line separating  each codon (See the example above): T    A    G    C    A    G    G    T    T    T …. _________________________________

Transcription results in the formation of an mRNA molecule that carries the instructions for the specific protein to the ribosome where the information is “translated” into a sequence of amino acids to form a protein.

Now let’s look at the process of translation. Translation requires the instructions required to make the protein (mRNA), the required amino acids, and the ribosome (rRNA). Each mRNA codon corresponds to an amino acid that is transported to the RNA/ribosome complex by another special nucleic acid called tRNA. “T” stands for transfer. The ribosome essentially “reads” the RNA code and facilitates the linking of appropriate amino acids to make proteins.

Summary

1. DNA (in nucleus) transcribed to mRNA
2. mRNA leaves nucleus
3. mRNA enters cytoplasm
4. mRNA hooks up with ribosomes
5. Ribosomes scroll through mRNA
6. tRNA delivers amino acids to mRNA/ribosome complex
7. Enzymes link amino acids together to form a protein

Activity

There are only 4 letters in the mRNA code: U-A-C-G. How many possible combinations are there? In other words, how many “words” can you make with those 4 letters if any combination of letters is possible but all “words” are only 3 letters long? There are 64 possible combinations yet, there are only 20 amino acids (see the corresponding Genetic Code table in your Lab 6 homework. What does this mean about the how each amino acid is “spelled”? You will need to look this up.

Part 2: Transcription and Translation in Action

Now that you have read about transcription and translation, let’s see if you can translate and transcribe a gene. Please go to the University of Utah Genetics website and complete the activity entitled Transcribing and Translating a Gene.

Lab Questions

You will need to paste a screen shot of the completed activity in your homework.

1. Define transcription. Where does this process take place in the cell? Briefly explain why it is important for protein production.
2. Define translation. Where does this process take place in the cell? Briefly explain why it is important for protein production.
3. Identify the function of the following different types of RNA molecules:
   1. mRNA
   2. tRNA
   3. rRNA
4. Define a codon. Explain why it is important in protein production.
5. There are _____   possible codons using 4 letters with 3 letters per codon in any order. However, there are only 20 amino acids, and each codon “codes” for one amino acid. What does this mean (hint: look at table 1 below)?
   

   Table 1. Universal Genetic Code

6. The table below shows which amino acid corresponds with which codon sequence. Use the table provided to determine the specific amino acids for each of the codon sequences listed below the table.
   1. UAC: _________
   2. CAG: _________
   3. AGG: _________
   4. GAU: _________
7. List ALL of the codons for Valine:
8. Identify the Stop codon(s):
9. Methionine is the “Start” signal. Write its codon in the space provided.
10. Globin is a red blood cell protein that is responsible for oxygen transport. The amino acid sequence for a portion of the globin protein is Proline, Glutamic Acid, Glutamic Acid, Lysine. Write the mRNA sequence of the amino acids for these amino acids in the space below (note, you don’t have to write all possible mRNA combinations for each amino acid, simply choose one correct codon each amino acid specified above).
11. How many nucleotides would it take to code for the four amino acids in the above question. Explain your answer.