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U N I T 1
Cell and Tissue Function
UNDERSTAND I NG
DNA-Directed Protein
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
Nuclear
envelope
Transcription
RNA processing
DNA
Pre-mRNA
mRNA
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.
