Lab 6 - Transcription & Translation Simulation



Background

“Central Dogma”, a phrase coined by Francis Crick of the famed Watson and Crick duo, is used to describe the flow of molecular information through a trio of macromolecules, DNA, RNA and protein. A gene is a nucleotide sequence in DNA to which a specific genetic function can be assigned such as the production of the pigment for eye color. Genes provide the “blueprint” for the production of proteins. Proteins are macromolecules that consist of one or more polypeptides that function as enzymes, hormones, antibodies or structural components of cells.

So how is information in nucleus-bound DNA expressed as proteins found throughout the cells? Information is passed on through the processes of transcription and translation. The nucleic acid sequence of a gene on the coding strand of DNA is used to create a complementary nucleic acid sequence messenger RNA (mRNA). Messenger RNA as the name implies, takes its genetic information or message out of the nucleus and into the cytoplasm of the cell. This transfer of information from DNA to RNA is the process of transcription. Transcription generates a single stranded mRNA complementary in sequence to the coding or sense strand of DNA. Two other RNA intermediates important to translation and protein synthesis are similarly transcribed from specific gene sequences, ribosomal RNA (rRNA) and transfer RNA (tRNA).

Once outside the nucleus, mRNA takes its genetic message from the nucleus to the ribosome, the site of translation and protein synthesis.

Translation converts the nucleotide sequence of the mRNA into a specific sequence of amino acids to produce a specific protein. Translation of mRNA is accomplished by translating a series of three (triplet) nucleotides, called a codon, into the corresponding amino acid as specified in the Genetic Code. (See Chart) The genetic code consists of the three letter codons present in the nucleotide sequence of mRNA, as read in the 5’-3’ direction down the strand. For example the first triplet found in all mRNA as transcribed from DNA is composed of the bases adenine, uracil and guanine (AUG) and translates to the amino acid methionine (Met), the translation initiator codon or start codon. Without the AUG sequence, translation will not take place.

The Genetic Code:

Proteins contain 20 different amino acids. The order of the base pairs in the DNA determines the order of amino acids making up the protein. DNA contains a combination of 4 bases: adenine (A), thymine (T), cytosine (C) and guanine (G). When DNA is transcribed into mRNA, T is replaced by uracil (U). Because DNA remains in the nucleus of the cell, the strand of mRNA goes out of the nucleus into the cytoplasm where it is met by the small subunit of the ribosome. mRNA has a three basepair codon that codes for the amino acid. tRNA reads the codon with its anticodon and delivers a specific amino acid that it carriers to the ribosome. The docking site located on a small subunit of the ribosome is called the platform, or P site. The P site is where all the protein building is initiated. At the P site a special initiator tRNA aligns with the start codon of the mRNA. Finally, a larger ribosome subunit binds around the smaller subunit and forms the protein building complex. Translation of the codon begins. As the codon is being translated, the tRNA delivers the amino acid coded for. Once the amino acid is delivered and attached with a peptide bond, the tRNA is released from the P site. The next amino acid is delivered, attached and the tRNA is released until the end codon is signaled for. There are three end codons that signal the end of protein called stop codons. Their codons are UAA, UGA and UAG.

Objective:

Students are “cellular spies”. In this “case” the students are given one important “clue”, a coding strand of DNA, and are to deduce the amino acid sequence found in the protein that this segment of DNA codes for. Working alone, the “spies” must transcribe nucleotide information in their coding strand to synthesize the complementary strand of mRNA. Remember, the base uracil in RNA is complementary to adenine and synonymous with thymine in DNA. Messenger RNA triplets are then translated into the corresponding amino acids. Students will then assemble their polypeptide in the order indicated by their transcribed mRNA. Students will submit a polypeptide strand, ie. beads, constructed from information contained in their coding strand of DNA to the teacher.

Please tape your DNA clue below.

Record the transcription of your “clue” (mRNA).

Record the translation of your “clue” (protein sequence)?

Answer the following questions.

1) How many amino acids long is your protein?

2) Did your “clue” include a stop codon?

3) Did your “clue” start with an initiator codon?

4) What would you expect to happen if your “clue” did not have an initiator codon?

5) What would happen if you made an error in transcribing your “clue” and you substituted a different nucleotide for the 10th nucleotide?

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