Chapter 14: RNA and Protein Synthesis

The Role of RNA

• Ribonucleic acid (RNA) is a nucleic acid composed of long chains of nucleotides

  • DNA encodes genetic instructions, RNA copies the instructions and carries out the production of proteins

Protein synthesis

Formation of proteins by RNA from information in DNA

• RNA controls the assembly of amino acids into proteins

• Occurs in two stages: transcription and translation

The Structure of RNA

• 5-carbon sugar: ribose

• Contains nitrogenous bases: adenine (A), guanine (G), cytosine (C), and uracil (U)

• Usually single stranded

• Much shorter, length of one gene

• 3 types of RNA found in cells, each with different roles in protein synthesis

Messenger RNA (mRNA)

Carries copies of genes (DNA)

Ribosomal DNA (rRNA)

Part of a ribosome, site of protein synthesis

Transfer RNA (tRNA)

Transfers amino acids to ribosome to make protein based on instructions in mRNA

Transcription

Process of copying a base sequence (gene) from DNA to RNA

• First stage of protein synthesis

• Occurs in nucleus of eukaryotic cells

• DNA acts as template

• Genetic instructions transcribed as complimentary mRNA (copy)

Steps of transcription

1. DNA strand unwinds at the beginning of a gene

   • Promoter: region of DNA with specific base sequences that mark the beginning of a gene

   • RNA polymerase: enzyme, binds to promoter and unwinds DNA

2. RNA polymerase builds mRNA

   • Template strand: strand of DNA that is read

     • Always the same side for a given gene

   • RNA polymerase adds complimentary bases

   • Pre mRNA stand is built from 5’ to 3’

3. Copying is complete

   • Terminal signal: stop, base sequence that marks the end of a gene

   • DNA, RNA polymerase, and pre-mRNA strand separate

   • RNA polymerase free to transcibe another gene

4. Pre-mRNA is edited

   • Pre-mRNA is cut and edited before being sent to next step in protein synthesis

   • Carried out by enzymes

   • Introns: portions of mRNA that are cut out and discarded

   • Exons: portions of mRNA that remain, spliced together to form mature mRNA

Genetic code

Rules that relate to how sequences of bases correspond to a particular amino acid

• The specific amino acids in polypeptides and the order they are joined determine outcome of gene

• Every gene its own “recipe”

• Assembly of protein based on instructions encoded in mRNA

Codon

Three consecutive bases that specify a single amino acid to be added to a polypeptide chain

• Start and stop codons used for promoters and terminal signals

  • Start: AUG (methionine, an amino acid)

  • Stop: UAA, UAG, UGA (not amino acids)

Translation

Sequence of codons in mRNA are used to assemble amino acids in ribosomes (make proteins) into a polypeptide chain

• Second stage of protein synthesis

• Occurs in the cytoplasm of eukaryotic and prokaryotic cells

• Involves mRNA, rRNA, and tRNA

Steps of translation

1. mRNA is read

   • Ribosome attaches to mRNA in cytoplasm

   • Codons pass through ribosome and are read by rRNA

2. tRNA and the amino acids

   • tRNA bind to the appropriate amino acid in cytoplasm

   • tRNA brings amino acid to ribosome

   • tRNA binds to mRNA

   • Anticodon: three unpaired bases on tRNA molecules that bind with mRNA codons

3. Ribosome assembles amino acids

   • Individual amino acids in ribosome are joined via peptide bonds

   • Amino acid chain grows to polypeptides

   • tRNA and mRNA move through ribosome as code is read and amino acids are joined

4. Completion of translation

   • Stop codon initiate completion

   • All tRNA and mRNA molecules exit ribosome

   • Polypeptide released from ribosome

Transcription factors

Proteins that regulate gene expression

• The same DNA is found in all cells of a given organism

• Not all genes need to be expressed in every tissue

  • Ex: the genes that code for liver enzymes don’t need to be copied and expressed in skin cells