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Protein Synthesis

Part A :Transcription

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A second major function of DNA is its role in producing structural and functional proteins that an organism needs. This function is known as “PROTEIN SYNTHESIS”. The full process consists of two major steps. The first step is called “TRANSCRIPTION” the second step is called “TRANSLATION”

Your parent copies it onto a sticky note for you

I type this message on an e-mail home to a parent

Tonight’s homework: Read pp. 526 -528. dna structure quiz next day

Tonight’s homework:

Read pp. 526-528. DNA structure Quiz next day.

Many times students will remember the details of each process, but they forget the proper name for which process is which. That is big trouble! Try to remember that to TRANSCRIBE something, means to copy it out in the same language. See above.

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The second step in Protein Synthesis is called TRANSLATION. To “Translate” something, you convert it from one language into another.

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Now that we have established a good understanding of both terms, we will focus in on the first process of protein synthesis known as TRANSCRIPTION. During this process, DNA is used to make RNA. But RNA and DNA are both the same category of organic molecule, both are NUCLEIC ACIDS. So it is like staying in the same language.

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This process (transcription), just like replication, also takes place in the nucleus. It starts off fairly similar to replication in that the section of DNA (GENE) that codes for a particular protein must unwind and unzip.

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But then this Transcription process begins to differ from replication. Different enzymes called “RNA Polymerases” do the work, and rather than DNA Nucleotides that are floating around in the nucleoplasm, it is RNA nucleotides that will be used to build a new complimentary strand.

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This strand of RNA that is made during the transcription of a gene is called mRNA. This stands for “MESSENGER RNA”. DNA has the blue prints for building the protein, but the DNA cannot leave the nucleus. Proteins are built outside of the nucleus out in the cytoplasm. So mRNA acts somewhat as a counterfeit copy of the gene that can carry the message out to the protein factories.

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Try practicing some transcription by using “Strand I” as the template for building a complimentary strand of mRNA.

Strand I : ACA GGG ACC CAT TTA ACC CAG

Strand II : TGT CCC TGG GTA AAT TGG GTC

mRNA produced: _?____?____?_____?____?____?____?___

Write down your ANSWER:

UCU CCC UGG GUA AAU UGG GUC

Every three bases represents a genetic code word called a “CODON”. Each codon codes for a given amino acid, for example the first codon (UCU) in our mRNA codes for the amino acid – SERINE. Check out the Genetic Code table in your textbook for how to read this.

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Good news is that you do not have to memorize this table. It will be provided for you on tests. Now either use this one or the one in your book to write down the names for the amino acids for the other 6 codons you transcribed.

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In a real gene, the strand of DNA is extremely long, having thousands and thousands of bases in it. But not all parts of the Gene are important. There are pieces of the gene that are important called “EXONS” and pieces of the gene that are unimportant called “INTRONS”.

Before the newly made (primary) mRNA is shipped out of the nucleus it must be processed by RIBOZYMES. These Ribozymes (a bit like enzymes) cut out the useless Introns and splice back together the important EXONS to form what is now called “Mature” mRNA.

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Processing mRNA and the ROLE Of Polyadenine Tail

Primary mRNA is produced from the entire gene during Transcription.

AUG CCG CAG CAC CCC UUA CCC GGG

Exon #1 Intron #1 Exon #2 Intron #2 Exon #3

Mature processed mRNA must be made with the help of Ribozymes

AUG CCG CAC CCC GGG

Then a POLYADENINE TAIL is added on to preserve this mRNA before it leaves the nucleus

AUG CCG CAC CCC GGG AAA AAA AAA AAA

AAA AAA ….

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The Role of the Polyadenine tail is to protect the mRNA so that it will be used by a protein factory to build the given protein. Once enough of a given protein (ex Insulin) the polyadenine tail will be cut off, telling the protein factory to stop using that piece of mRNA. It will then be broken down

mRNA

Polyadenine Tail

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Now the mature mRNA exits the nucleus and heads out to a RIBOSOME(not a Ribozyme) protein factory where it will be read and translated into a specific protein.