CoursesPhysiologyIntroductionThe cell and its functionsProtein synthesis

Protein synthesis

Learning objectives

After completing this study unit, you will be able to:

Describe the role of DNA and RNA in protein synthesis.
Compare and contrast transcription and translation.
Describe messenger RNA processing and post-translational modifications.

Introduction

Our DNA contains the information to make some of the most important molecules in the human body: proteins. This information is contained within sequences of DNA nucleotides, known as genes. The collection of the genes of an organism forms its genome. The translation of this genetic information into amino acid sequences follows a specific set of rules, called the genetic code.

DNA is in the nucleus, but proteins are synthesized on ribosomes in the cytoplasm. Since DNA molecules are too large to leave the nucleus, the cell synthesizes messenger RNA (mRNA) to direct protein synthesis in the ribosomes.

Protein synthesis can be divided into four phases:

Transcription: mRNA is synthesized from DNA
mRNA processing: Pre-mRNA is trimmed and modified
Translation: Ribosomes and transfer RNA link amino acids into a polypeptide
Post-translational modifications: Polypeptide becomes a functional protein

The synthesized proteins either remain in the cytosol or are tagged with sorting signals that direct them to their next destination.

Explore concepts

Genetic code

Messenger RNA codons can be translated into amino acids using the genetic code.

Phases of protein synthesis

Protein synthesis occurs in four sequential phases: transcription, mRNA processing, translation and post-translational modifications.

Transcription and mRNA processing

During transcription, RNA polymerase copies genetic information from DNA to messenger RNA. This pre-mRNA is trimmed and modified before it leaves the nucleus.

Translation and post-translational modifications

During translation, ribosomes use transfer RNA to link amino acids into a polypeptide, which is then folded to make it a fully functional protein in the post-translational modifications phase.

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Summary

Key points about protein synthesis
Genes	Gene: A segment of DNA that codes for a protein or another functional molecule like RNA Genome: The collection of genes of an organism Codon: A triplet of nucleotides that codes for an amino acid or a stop signal Genetic code: Rules to translate codons into amino acids
Transcription	RNA polymerase copies the DNA nucleotide sequence of a gene to mRNA. Location: Nucleus DNA-mRNA pairs: cytosine - guanine, guanine - cytosine, thymine - adenine, adenine - uracil
mRNA processing	Pre-mRNA is trimmed and modified. Location: Nucleus Introns: Non-coding regions of mRNA (removed) Exons: Coding regions of mRNA (joined together)
Translation	Ribosomes and tRNA build a polypeptide based on mRNA codons. Locations: Free ribosomes in cytosol; bound ribosomes in rough endoplasmic reticulum and nuclear envelope Anticodon: Triplet of bases at one end of tRNA; complements mRNA codon of the amino acid that the tRNA carries
Post-translational modifications	Polypeptide is modified and folded into a fully functional protein. Locations: Rough endoplasmic reticulum, Golgi apparatus, cytosol Tagging signals: appended to polypeptide to define its next destination