From DNA to Protein

Proteins All proteins consist of polypeptide chains A linear sequence of amino acids Each chain corresponds to the nucleotide base sequence of a gene

The Path From Genes to Proteins 1. Transcription Enzymes use the base sequence of a gene as a template to make a strand of RNA 2. Translation Information in the RNA strand is decoded (translated) into a sequence of amino acids

Prokaryotes and Eukaryotes In prokaryotic cells (no nucleus) Transcription and translation occur in cytoplasm In eukaryotic cells Genes are transcribed in the nucleus Resulting mRNA is translated in the cytoplasm

Key Concepts: INTRODUCTION Life depends on enzymes and other proteins All proteins consist of polypeptide chains Chains are sequences of amino acids that correspond to sequences of nucleotide bases in DNA called genes The path leading from genes to proteins has two steps: transcription and translation

Transcription: DNA to RNA Two DNA strands unwind in a specific region RNA polymerase assembles a strand of RNA Covalently bonds RNA nucleotides (adenine, guanine, cytosine, uracil) according to the nucleotide sequence of the exposed gene

Three Types of RNA Messenger RNA (mRNA) Ribosomal RNA (rRNA) Carries protein-building codes from DNA to ribosomes Ribosomal RNA (rRNA) Forms ribosomes (where polypeptide chains are assembled) Transfer RNA (tRNA) Delivers amino acids to ribosomes

RNA and DNA Compared

RNA Base Pairing

Fig. 13.3, p.198

Fig. 13.3, p.198

Key Concepts: TRANSCRIPTION During transcription, the two strands of the DNA double helix are unwound in a gene region Exposed bases of one strand become the template for assembling a single strand of RNA (a transcript) Messenger RNA is the only type of RNA that carries DNA’s protein-building instructions

The Genetic Code Messenger RNA (mRNA) carries DNA’s protein-building information to ribosomes for translation mRNA’s genetic message is written in codons Sets of three nucleotides along mRNA strand

Genetic Code: RNA Triplets

From DNA to Polypeptide

tRNA and rRNA Function in Translation Transfer RNA (tRNA) Anticodon binds to mRNA codon Also binds amino acid specified by codon Different tRNAs carry different amino acids tRNAs deliver free amino acids to ribosomes during protein synthesis

tRNA

rRNA Ribosomal RNA (rRNA) and proteins make up the two subunits of ribosomes

Three Stages of Translation mRNA-transcript information directs synthesis of a polypeptide chain during translation Translation proceeds in three stages Initiation Elongation Termination

Initiation One initiator tRNA, two ribosomal subunits, and one mRNA come together as an initiation complex

Initiation

Elongation tRNAs deliver amino acids to the ribosome in the order specified by mRNA codons Ribosomal rRNA catalyzes the formation of a peptide bond between amino acids

Elongation An initiator tRNA carries the amino acid methionine, so the first amino acid of the new polypeptide chain will be methionine. A second tRNA binds the second codon of the mRNA (here, that codon is GUG, so the tRNA that binds carries the amino acid valine). A peptide bond forms between the first two amino acids (here, methionine and valine). The first tRNA is released and the ribosome moves to the next codon in the mRNA. A third tRNA binds to the third codon of the mRNA (here, that codon is UUA, so the tRNA carries the amino acid leucine). A peptide bond forms between the second and third amino acids (here, valine and leucine).

Termination The second RNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth mRNA codon (here, that codon is GGG, so the tRNA carries the amino acid glycine). A peptide bond forms between the third and fourth amino acids (here, leucine and glycine) Termination Steps d and e are repeated over and over until the ribosome encounters a STOP codon in the mRNA. The mRNA transcript and the new polypeptide chain are released from the ribosome. The two ribosomal subunits separate from each other. Translation is now complete. Either the chain will join the pool of proteins in the cytoplasm or it will enter rough ER of the endomembrane system (Section 4.8).

Termination Translation ends when RNA polymerase encounters a STOP codon in mRNA New polypeptide chain and mRNA are released Ribosome subunits separate from each other

Termination

Key Concepts: TRANSLATION During translation, amino acids become bonded together into a polypeptide chain in a sequence specified by base triplets in messenger RNA Transfer RNAs deliver amino acids one at a time to ribosomes Ribosomal RNA catalyzes the formation of peptide bonds between the amino acids

Mutated Genes and Their Protein Products Mutations are permanent, small-scale changes in the base sequence of a gene Common mutations include insertions, deletions, and base-pair substitutions

Common Gene Mutations

Some Causes of Mutations Natural and synthetic chemicals Cigarette smoke Environmental agents

Assembly of RNA on unwound regions of DNA molecule Transcription Assembly of RNA on unwound regions of DNA molecule mRNA rRNA tRNA mRNA processing proteins mature mRNA transcripts ribosomal subunits mature tRNA Convergence of RNAs Translation cytoplasmic pools of amino acids, ribosomal subunits, and tRNAs At an intact ribosome, synthesis of a polypeptide chain at the binding sites for mRNA and tRNAs Final protein Fig. 13.11, p.206