I. Proteins Are Made by Decoding the Information in DNA

A. Traits are determined by proteins, built according to instructions specified in DNA

B. RNA (ribonucleic acid) differs from DNA in three ways

• single-stranded rather than double-stranded
• ribose rather than deoxyribose 5-carbon sugar
• uracil rather than thymine as a pyrimidine nitrogenous base

C. Gene expression = transcription + translation

• transcription = copying DNA to RNA
• translation = building proteins using RNA

II. Transcription Transfers Information from DNA to RNA

A. DNA is divided into genes, which are used to make specific proteins

B. RNA polymerase is the active enzyme of transcription, adding and linking RNA nucleotides to synthesize RNA complementary to DNA

C. Transciption involves four steps:

• Step 1: RNA polymerase binds to the gene's promoter, a start signal sequence of DNA
• Step 2: RNA polymerase unwinds and separates the two strands of the DNA double helix
• Step 3: RNA polymerase adds and links complementary RNA nucleotides as it reads the DNA sequence of a gene
• Step 4: RNA polymerase reaches a stop signal on the DNA, releasing the RNA and RNA polymerase from the DNA

D. Transcription involves details

• transcription uses the DNA nucleotide sequence as a template for synthesizing RNA
• RNA polymerase links RNA nucleotides together with strong covalent bonds
• transcription in prokaryotic cells occurs in the cytoplasm because prokaryotes lack nuclei; whereas transcription in eukaryotic cells occurs in the nucleus
• Up to 100 RNA polymerase molecules might be transcribing the same gene at the same time

III. The Genetic Code is Written in Three-Nucleotide 'Words'

A. Different types of RNA are made during transcription, depending on the gene expressed

• Messenger RNA, or mRNA:
  • carries instructions for making a protein, and delivers it to a ribosome for translation
  • where the sequence of mRNA nucleotides determines the sequence of amino acids in a protein
  • Codons are three-nucleotide sequences, each corresponding to a specific amino acid
  • the genetic code matches each codon to a specific amino acid

• Transfer RNA, or tRNA:
  • carry specific amino acids from the cytoplasm to ribosomes
  • each tRNA has a specific anticodon on one end which corresponds to a specific codon in mRNA
  • ensuring that the proper amino acid be located in the proper sequence in a protein
• Ribosomal RNA, or rRNA:
  • rRNA works together with proteins to create the structure of ribosomes, the sites of translation, the process of building proteins
  • ribosomes are made of a smaller and a larger subunit which work together in translation
  • ribosomes have two attachment sites for tRNA, the P-site and the A-site
  • each ribosome temporarily holds an mRNA and two tRNAs matching codons with anticodons to synthesize protein with a specific amino acid sequence

B. Translation involves seven steps

• Step 1: the ribosomal subunits, the mRNA, and the tRNA carrying methionine bind together;
  • methionine is always the first amino acid, and its tRNA binds to the P-site of the ribosome
  • because the anticodon of the tRNA matches the codon of the mRNA
• Step 2: the tRNA carrying the amino acid specified by the codon in the A-site arrives, its anticodon matching the mRNA codon
• Step 3: a peptide bond forms between the adjacent amino acids in the P-site and A-site
• Step 4: the tRNA in the P-site detaches and leaves its amino acid behind
• Step 5: the tRNA in the A-site moves to the P-site;
  • the tRNA carrying the amino acid specified by the mRNA codon in the A-site arrives, as its anticodon matches the codon
• Step 6: a peptide bond is formed attaching the A-site amino acid to the P-site polypeptide;
  • the tRNA in the P site detaches and leaves its amino acid behind
• Step 7: the process is repeated until a stop codon is reached;
  • the ribosome complex then falls apart;
  • the newly made protein is released

C. Translation details

• as the mRNA moves across the ribosome, another ribosome may read the same mRNA, thus speeding up the rate of protein synthesis
• the genetic code is the same for all organisms, implying that the process is ancient, and that all life has descended from a common ancestor