Ribosomes and Protein Synthesis

Learning Objectives

  • Explain how the genetic code works.
  • Describe the role of the ribosome in assembling proteins.
  • Understand how molecular biology relates to genetics.

Genetic Blueprint

  • The blueprint for every polypeptide molecule that can be produced by a cell is contained within the DNA in the nucleus.

The Genetic Code

Codons
  • The genetic code is read in three-letter groupings called codons.
  • A codon is defined as a group of three nucleotide bases in messenger RNA (mRNA) that specifies a particular amino acid to be added to the polypeptide chain.
  • All living organisms read the genetic code in this way, three bases at a time. Examples of codons include AUG, AAC, and UCU.

Genetic Code Table

  • There are 64 possible three-base codons in the genetic code.
  • The reading of these codons is simplified by using a genetic code table.
  • The code table applies to most multicellular organisms, including animals and plants, although slight differences exist in the code as used by some microorganisms, as well as in the mitochondria and chloroplasts.

Reading Codons

  • To read codons from the genetic code table, one should start at the middle of the circle with the first letter of the codon and move outward. Example: CAC corresponds to Histidine.

Interactive Learning Activity

  • Activity: Provide three codons for a peer to identify the associated amino acid. Then, reverse the process by selecting amino acids and asking your peer to provide the codon or codons that represent them.

Start and Stop Codons

  • The methionine codon AUG serves as the start codon for protein synthesis.
  • There are three stop codons which end the process of translation: UAA, UAG, and UGA.

Tasks

Task 1
  • Watch the video and take notes. After watching, answer the following questions:   - What is the anticodon for the methionine codon, the start codon?   - What amino acid does the mRNA codon UUC bring to the polypeptide chain?   - If an mRNA codon has the bases CUA, what will the corresponding transfer RNA (tRNA) anticodon have?   - What is the product of transcription?   - What is the product of translation?   - Where do the amino acids that make up the protein come from?   - What is the central dogma of molecular biology?
Task 2
  • Interactivity assigned on Savvas: The genetic code.
Task 3
  • Assigned on Savvas: Analyzing Data: Track the code.

Translation Overview

  • Translation is defined as the process that produces proteins by decoding the sequence of mRNA codons.
  • Transcribed mRNA directs the translation process.

Translation and Transfer RNA (tRNA)

  • Translation starts when a ribosome attaches to an mRNA molecule.
  • tRNA molecules, carrying amino acids with them, bind to the mRNA codons using their anticodon.

Translation: Polypeptide Assembly

  • The ribosome helps form a peptide bond during the assembly of the polypeptide.
  • The ribosome breaks the bond holding the first tRNA molecule to its amino acid.

Translation: Completing the Polypeptide

  • The ribosome continues until it reaches a stop codon, which results in the release of the newly synthesized polypeptide and the mRNA molecule, thus completing the translation process.

Roles of RNA in Translation

  • All three major forms of RNA—mRNA, tRNA, and rRNA—are involved in the translation process.

Molecular Genetics

  • The central dogma of molecular biology indicates that information is transferred from DNA to RNA to protein.
  • Key processes include:   - Transcription occurring in the nucleus where DNA is transcribed into mRNA.   - Translation occurring in the cytoplasm where the ribosome synthesizes polypeptides from mRNA.

Gene Expression

  • When a gene (segment) of DNA code is utilized to build a protein, scientists refer to this as the gene being expressed.