Porth's Essentials of Pathophysiology, 4e

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Cell and Tissue Function

U N I T 1

DNA-Directed Protein

UNDERSTAND I NG

Deoxyribonucleic acid (DNA) directs the synthesis of the many thousands of proteins that are contained in the different cells of the body. Although some of the proteins are structural proteins, the majority are enzymes that catalyze the different chemical reactions in the cell. Because DNA is located in the cell’s nucleus and protein synthesis takes place in the cytoplasm, a second type of nucleic acid—ribonucleic acid (RNA)—participates in the actual assembly of the proteins.There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) that participate in (1) the transcription of the DNA instructions for protein synthesis and (2) the translation of those instructions into the assembly of the polypeptides that make up the various proteins.

Nuclear envelope

Transcription. Transcription involves copying the genetic code containing the instructions for protein synthesis from DNA to a complementary strand of mRNA. Once mRNA has been processed, it diffuses through the nuclear pores into the cyto- plasm, where it controls protein synthesis. 1

DNA

Transcription

Pre-mRNA

RNA processing

mRNA