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22.6 Types of RNA 22.7 Transcription: Synthesis of mRNA22.8 The Genetic Code22.9 Protein Synthesis: Translation

Chapter 22 Nucleic Acids and Protein Synthesis

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Types of RNA

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Protein Synthesis

The processes involved in protein synthesis involve the formation of mRNA from DNA (transcription) and the conversion by tRNA to protein (translation).

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In transcription: A section of DNA containing the gene unwinds. One strand of DNA is copied starting at the

initiation point, which has the sequence TATAAA. An mRNA is synthesized using complementary

base pairing with uracil (U) replacing thymine (T). The newly formed mRNA moves out of the nucleus

to ribosomes in the cytoplasm.

Transcription: Synthesis of mRNA

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RNA Polymerase

During transcription, RNA polymerase moves along the DNA template in the 3’-5’direction to synthesize the corresponding mRNA.

The mRNA is released at the termination point.

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mRNA Processing The DNA of eukaryotes contains exons that

code for proteins along with introns that do not. The initial mRNA called a pre-RNA includes

the noncoding introns. While in the nucleus, the introns are removed

from the pre-RNA. The exons that remain are joined to form the

mRNA that leaves the nucleus with the information for the synthesis of protein.

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Removing Introns from Pre-mRNA

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Regulation of Transcription

A specific mRNA is synthesized when the cell requires a particular protein.

In feedback control, the end products speed up or slow the synthesis of mRNA.

In enzyme induction, high levels of a reactant induces the transcription process to provide the necessary enzymes for that reactant.

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Lactose Operon and Repressor The lactose operon consists of a control site

and the genes that produce mRNA for lactose enzymes.

When there is no lactose in the cell, a regulatory gene produces a repressor protein that prevents the synthesis of lactose enzymes.

The repressor turns off mRNA synthesis.

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Lactose Operon Turned Off

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Lactose Operon and Inducer When lactose is present in the cell, some

lactose combines with the repressor, which removes the repressor from the control site.

Without the repressor, RNA polymerase catalyzes the synthesis of the enzymes by the genes in the operon.

The level of lactose in the cell induces the synthesis of the enzymes required for its metabolism.

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Lactose Operon Turned On

RNA Polymerase

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Lac Operon Lactose Operon Movie http://www.dartmouth.edu/~cbbc/courses/

movies/LacOperon.html

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The genetic code: Is found in the sequence of nucleotides in

mRNA. Is a triplet of bases called codons along the

mRNA that codes for a particular amino acid. Contains different codons for all 20 amino

acids needed to build a protein. Contains certain codons that signal the “start”

and “end” of a polypeptide chain.

The Genetic Code

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The Genetic Code: mRNA Codons

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Suppose we want to determine the amino acids coded for in the following section of a mRNA.

5’—CCU —AGC—GGA—CUU—3’ According to the genetic code, the amino acids for

these codons are CCU = Proline AGC = Serine GGA = Glycine CUU = Leucine

The mRNA section codes for the amino acid sequence of Pro—Ser—Gly—Leu

Codons and Amino Acids

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tRNA Activation Each tRNA has a triplet

called an anticodon that complements a codon on mRNA.

A synthetase uses ATP hydrolysis to attach an amino acid to a specific tRNA.

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Anticodon

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Protein synthesis begins when a mRNA attaches to a ribosome.

On the mRNA, the start codon (AUG) binds to a tRNA with methionine.

The second codon attaches to a tRNA with the next amino acid.

A peptide bond forms between the adjacent amino acids at the first and second codons.

The first tRNA detaches from the ribosome and the ribosome shifts to the adjacent codon on the mRNA (translocation).

Initiation and Translocation

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After a polypeptide with all the amino acids for a protein is synthesized, the ribosome reaches the the “stop” codon: UGA, UAA, or UAG.

There is no tRNA with an anticodon for the “stop” codons.

Therefore, protein synthesis ends. The polypeptide is released from the ribosome

and is ready to function as an active protein.

Termination

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Protein synthesis. DNA words are three letters long. http://www.dnaftb.org/dnaftb/22/concept/

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