Porth's Essentials of Pathophysiology, 4e

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Genetic Control of Cell Function and Inheritance

C h a p t e r 5

Synthesis

Forming protein

Translation. The process of translation involves taking the instructions transcribed from DNA to mRNA and transferring them to the rRNA of ribosomes located in the cytoplasm. It is the recognition of the mRNA codon by the tRNA anticodon that ensures the proper sequence of amino acids in a synthesized pro- tein. In order to be functional, the newly synthesized protein must be folded into its functional form, modified further, and then routed to its final position in the cell. 2

Amino acid

Peptide bond

Transfer RNA "head" bearing anticodon Ribosome

U

A

U

G

A

A

C

G G

C C C A

A

G AU

CU

C C G A A

C

C C U U U

G

G UUU

G G

U

U

C C U U

Direction of messenger RNA advance

Messenger RNA

Codon

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Translation Proteins are made from a standard set of amino acids, which are joined end to end to form the long polypep- tide chains of protein molecules. Each polypeptide chain may have as few as 100 to more than 300 amino acids in it. The process of protein synthesis is called translation because the genetic code is translated into the language of this polypeptide assembly.

Translation requires the coordinated actions of all three forms of RNA. It begins when mRNA contacts and passes through the ribosome, where it bonds to rRNA. As the RNA complex passes through the ribosome, tRNA trans- lates its codons into complementary anticodons , which determine which amino acids it then delivers to the rRNA of ribosomes for attachment to the growing polypeptide chain. The long mRNA strand usually travels through and directs protein synthesis in more than one ribosome

RNA polymerase

Transcription factors

Termination site with a stop codon

Transcription initiation site

TATA box

Promoter region

Transcription unit

A

FIGURE 5-4. Transcription of messenger RNA (mRNA) from DNA double helix. (A) Transcription of mRNA involves attachment of RNA polymerase along with transcription factors to a specific nucleotide sequence, theTATA (thymine-adenosine-thymine-adenosine) region on the promoter region of the DNA.Transcription moves along the transcription unit and terminates at the stop codon. (B) Transcription creates a complementary copy mRNA from one of the DNA strands in the double helix. DNA, deoxyribonucleic acid; RNA, ribonucleic acid.

Transcription

U

C A U A A G U A

mRNA

AT

T TC

AT G

C

DNA

G

C

U

B