Cell Physiology Protein Synthesis

Protein Synthesis DNA serves as a blueprint for making proteins Gene: DNA segment that carries a blueprint for building one protein or polypeptide chain Proteins have many functions Fibrous (structural) proteins are the building materials for cells Globular (functional) proteins act as enzymes (biological catalysts) © 2015 Pearson Education, Inc.

Protein Synthesis DNA information is coded into triplets Triplets Contain three bases Call for a particular amino acid For example, a DNA sequence of AAA specifies the amino acid phenylalanine © 2015 Pearson Education, Inc.

Protein Synthesis Most ribosomes, the manufacturing sites of proteins, are located in the cytoplasm DNA never leaves the nucleus in interphase cells DNA requires a decoder and a messenger to build proteins, both are functions carried out by RNA (ribonucleic acid) © 2015 Pearson Education, Inc.

Protein Synthesis How does RNA differ from DNA? RNA: Is single-stranded Contains ribose sugar instead of deoxyribose Contains uracil (U) base instead of thymine (T) © 2015 Pearson Education, Inc.

Role of RNA Transfer RNA (tRNA) Ribosomal RNA (rRNA) Transfers appropriate amino acids to the ribosome for building the protein Ribosomal RNA (rRNA) Helps form the ribosomes where proteins are built Messenger RNA (mRNA) Carries the instructions for building a protein from the nucleus to the ribosome © 2015 Pearson Education, Inc.

Role of RNA Protein synthesis involves two major phases: Transcription Translation We will detail these two phases next © 2015 Pearson Education, Inc.

Protein Synthesis Transcription Transfer of information from DNA’s base sequence to the complementary base sequence of mRNA Only DNA and mRNA are involved Triplets are the three-base sequence specifying a particular amino acid on the DNA gene Codons are the corresponding three-base sequences on mRNA © 2015 Pearson Education, Inc.

Protein Synthesis Example of transcription: DNA triplets AAT-CGT-TCG mRNA codons UUA-GCA-AGC © 2015 Pearson Education, Inc.

Figure 3.16 Protein synthesis. Slide 1 Nucleus (site of transcription) DNA Cytoplasm (site of translation) 1 mRNA specifying one polypeptide is made on DNA template. 2 mRNA leaves nucleus and attaches to ribosome, and translation begins. Amino acids mRNA Nuclear pore Nuclear membrane Correct amino acid attached to each species of tRNA by an enzyme Synthetase enzyme 4 As the ribosome moves along the mRNA, a new amino acid is added to the growing protein chain. Growing polypeptide chain Met 3 Incoming tRNA recognizes a complementary mRNA codon calling for its amino acid by binding via its anticodon to the codon. Gly Ser Phe Ala Peptide bond 5 Released tRNA reenters the cytoplasmic pool, ready to be recharged with a new amino acid. tRNA “head” bearing anticodon Large ribosomal subunit Codon Direction of ribosome advance; ribosome moves the mRNA strand along sequentially as each codon is read. Portion of mRNA already translated Small ribosomal subunit

Figure 3.16 Protein synthesis (1 of 2). Slide 2 Nucleus (site of transcription) DNA Cytoplasm (site of translation) mRNA specifying one polypeptide is made on DNA template. 1 Amino acids mRNA Nuclear pore Nuclear membrane Correct amino acid attached to each species of tRNA by an enzyme Synthetase enzyme

Protein Synthesis Translation Base sequence of nucleic acid is translated to an amino acid sequence Amino acids are the building blocks of proteins © 2015 Pearson Education, Inc.

Protein Synthesis Translation (continued) Steps correspond to Figure 3.16 (step 1 covers transcription) mRNA leaves nucleus and attaches to ribosome, and translation begins Incoming tRNA recognizes a complementary mRNA codon calling for its amino acid by binding via its anticodon to the codon. © 2015 Pearson Education, Inc.

Figure 3.16 Protein synthesis (1 of 2). Slide 3 Nucleus (site of transcription) DNA Cytoplasm (site of translation) mRNA specifying one polypeptide is made on DNA template. 1 2 mRNA leaves nucleus and attaches to ribosome, and translation begins. Amino acids mRNA Nuclear pore Nuclear membrane Correct amino acid attached to each species of tRNA by an enzyme Synthetase enzyme

Figure 3.16 Protein synthesis (2 of 2). Slide 4 Incoming tRNA recognizes a complementary mRNA codon calling for its amino acid by binding via its anticodon to the codon. 3 tRNA “head” bearing anticodon Large ribosomal subunit Codon Direction of ribosome advance; ribosome moves the mRNA strand along sequentially as each codon is read. Portion of mRNA already translated Small ribosomal subunit

Protein Synthesis Translation (continued) Steps correspond to Figure 3.16 As the ribosome moves along the mRNA, a new amino acid is added to the growing protein chain. Released tRNA reenters the cytoplasmic pool, ready to be recharged with a new amino acid. © 2015 Pearson Education, Inc.

Figure 3.16 Protein synthesis (2 of 2). Slide 5 4 As the ribosome moves along the mRNA, a new amino acid is added to the growing protein chain. Growing polypeptide chain Met Incoming tRNA recognizes a complementary mRNA codon calling for its amino acid by binding via its anticodon to the codon. 3 Gly Ser Phe Ala Peptide bond tRNA “head” bearing anticodon Large ribosomal subunit Codon Direction of ribosome advance; ribosome moves the mRNA strand along sequentially as each codon is read. Portion of mRNA already translated Small ribosomal subunit

Figure 3.16 Protein synthesis (2 of 2). Slide 6 4 As the ribosome moves along the mRNA, a new amino acid is added to the growing protein chain. Growing polypeptide chain Incoming tRNA recognizes a complementary mRNA codon calling for its amino acid by binding via its anticodon to the codon. 3 Met Gly Ser Phe Ala Peptide bond 5 Released tRNA reenters the cytoplasmic pool, ready to be recharged with a new amino acid. tRNA “head” bearing anticodon Large ribosomal subunit Codon Direction of ribosome advance; ribosome moves the mRNA strand along sequentially as each codon is read. Portion of mRNA already translated Small ribosomal subunit