Lecture 12 - Translation

What is the role of DNA in protein synthesis? What is a gene?

• DNA is the master blueprint that holds the code for protein synthesis

  • DNA directs the order of amino acids in a polypeptide

• A segment of DNA that codes for a molecule that has a function is referred to as a gene

  • Think of DNA as a cookbook and a gene as a recipe in that cookbook

Describe the processing of mRNA. What are the three steps?

• Newly formed mRNA is edited and processed before translation can begin

  • Before processing, it is referred to as pre-mRNA

• 3 steps

  • 5’ cap - helps mRNA to be exported, prevents degradation of mRNA, assists ribosomal binding during translation

  • Poly-A-tail - Prevents degradation of mRNA

  • Splicing

What are the two parts of a gene?

• Genes are composed of exons and introns

  • Exons are part of gene that actually codes for amino acids

  • Introns are noncoding segments interspersed amongst exons

How are introns removed?

• Introns are removed by special proteins called spliceosomes, leaving only exon coding regions

What are some types and functions of proteins?

Contractile - actin and myosin

Structural - spiders silk

Receptor - rhodopsin

Enzymes - rubisco

Storage - ferritin

Transport - haemoglobin

Protection - Antibody

Hormonal - Insulin

What direction is mRNA read? Where are proteins synthesized from?

• mRNA is read in a 5’ to 3’ direction

• Proteins are synthesized from the amino to the carboxyl terminus

What determines the amino acid sequence? What is the triplet code?

• The amino acid sequence is determined by the specific order of nucleotides in the gene

  • Code consists of three sequential bases (triplet code) - nucleotides are read in groups of three

  • Each triplet in the mRNA specifies the code for a particular amino acid

What is the role of RNA in translation

• RNA is the “go-between” molecule that links DNA to proteins

  • All RNA is formed in the nucleus

What is the role of messenger RNA (mRNA)

• Single stranded

• Code from DNA template strand is copied with complementary base pairs, resulting in a strand of mRNA

  • Process is referred to as transcription

What is the role of transfer RNA (tRNA)

• Have special areas that contain a specific triplet code (anticodon) that allows each tRNA to carry only a specific amino acid

• Anticodon of tRNA will complement base-pair with codon of mRNA at the ribosome, adding its specific amino acid to a growing peptide chain

• All tRNAs have a CCA sequence at the 3’ end where the amino acid will attach

Describe the reaction that adds amino acids onto tRNA

• This reaction is catalyzed by aminoacyl tRNA synthetases

  • There are 20 of them. One for each amino acid-tRNA combination

• Amino acid is activated by a reaction with ATP

• Amino acid is transferred to the 3’ CCA terminus and AMP is released

What is role of ribosomal RNA (rRNA)

• Structure component of ribosomes, the organelle where protein synthesis occurs

• Along with tRNA, helps to translate message from mRNA into polypeptide

• Ribosomes are composed of RNA and proteins which fold together to form the final structure

What are the phases of translation

1. Initiation

2. Elongation

3. Termination

Describe initiation

• First a small ribosomal subunit binds with mRNA. Then the small subunit moves along the mRNA until it reaches the start codon (AUG)

• Proteins called initiation factors bring in the large subunit that completes the translation initiation complex

What are the different tRNA binding sites?

• P site - holds the tRNA carrying the growing polypeptide chain

• A site - holds the tRNA with the next amino acid to be added to the chain

• E (exit) site - releases the tRNAs from the ribosome

How does tRNA bind with accuracy?

Accuracy comes from:

• Correct codon/anti-codon pairing

• “Decoding center” in the small subunit (regonizes correct pairs and discriminates against mismatches)

• The large ribosomal unit catalyzes this reaction with the rRNA playing an important role

• Results in the amino acids being attached to the t-RNA in the A site