Molecular Biology I - Lecture 11

Molecular Biology I - Lecture 11

• Translation is the process where mRNA codons specify amino acid sequences to form polypeptides.
• Multiple polypeptides fold to create a functional protein.
• DNA stores the information needed to synthesize all proteins produced by an organism.
• Protein-encoding genes are also called structural genes.
• Messenger RNA (mRNA) is the RNA transcribed from protein-encoding genes.

Major Principles of Translation

• Enzymes are a type of protein that catalyze chemical reactions in cells.
• All proteins are encoded by genes.
• Many proteins do not function as enzymes, some proteins are composed of multiple polypeptides.
• One gene may encode multiple polypeptides due to alternative splicing.
• Alkaptonuria occurs when a person inherits two defective copies of the gene encoding homogentisic acid oxidase.
• Phenylketonuria (PKU) occurs when a person has two defective alleles for the gene encoding phenylalanine hydroxylase.

The Genetic Basis for Protein Synthesis

• The genetic material encodes proteins.
• Cellular proteins are needed at the right time, in suitable amounts in the correct cell type.
• The task of creating thousands of different proteins is complex.
• Metabolic pathways consist of a series of metabolic conversions, each step catalyzed by a specific enzyme.
• Enzymes are different proteins that catalyze specific chemical reactions.
• All proteins are encoded by genes and may not all function as enzymes.
• One gene may encode multiple polypeptide, due to alternative splicing.

The Genetic Code

• The genetic code is composed of 64 codons.
• Polypeptides are composed of 20 kinds of amino acids.
• A minimum of 20 codons is needed to specify all amino acids.
• A three-base codon system can form a code for 64 different codons.
• Degeneracy means more than one codon can specify the same amino acid.
• For example, the codons GGU, GGC, GGA, and GGG all specify the amino acid glycine.

The Polypeptide Chain

• Polypeptide synthesis is directional, following the order of mRNA codons.
• A peptide bond is formed between the carboxyl group of the last amino acid and the amino acid.
• A condensation reaction releases water during this process
• The newest amino acid to be added has a free carboxyl group.
• The first amino acid is at the amino-terminal end (N-terminus), and the final amino acid is at the carboxyl-terminal end (C-terminus).

Structure and Function of tRNA

• tRNA molecules recognize mRNA codons and carry the corresponding amino acid.
• Anticodons on tRNA molecules are complementary to codons in mRNA.

Aminoacyl-tRNA Synthetases

• tRNA molecules have a specific amino acid attached to their 3' ends.
• Aminoacyl-tRNA synthetases catalyze the attachment of amino acids to tRNA molecules.
• Synthetase enzymes are specific for each amino acid.
• The process involves amino acid, ATP, and a tRNA molecule to form a charged tRNA (aminoacyl-tRNA).

Ribosome Structure and Assembly

• Ribosomes are composed of large and small subunits.
• Ribosomes have structural rRNA and proteins.
• Bacterial ribosomes are 70S, while eukaryotic ribosomes are 80S.
• Ribosomal subunits are assembled in the cytoplasm in bacteria
• rRNA is synthesized in the nucleus, proteins are synthesized in the cytosol in eukaryotes.
• Ribosomes synthesize polypeptides at a specific location in the ribosome called the mRNA binding site.

Stages of Translation

• Translation proceeds in three main stages: initiation, elongation, and termination.
• Initiation: the mRNA, tRNA, and ribosomal subunits assemble to form a complex.
• Elongation: addition of amino acids.
• Termination: the polypeptide is released when a stop codon is reached.

This quiz covers the principles of translation in molecular biology, detailing how mRNA codons specify amino acid sequences to form polypeptides. It also examines the genetic encoding of proteins and related genetic disorders like alkaptonuria and PKU. Test your understanding of these fundamental concepts in protein synthesis.