LESSON 1: FROM GENES TO PROTEINS

DNA is the genetic material of life. It is found in all organisms on the planet, and it holds the genetic information that each organism uses to produce the proteins necessary for life. Wherever it is found, whether in prokaryotic or eukaryotic cells, DNA exists in the iconic form of the double helix, and uses the same, universal genetic code that allows it to be translated into proteins.

The processes of producing proteins from DNA, known as transcription and translation, are also virtually the same in all organisms: DNA is transcribed into RNA, which is then translated into the amino acid sequence of a polypeptide. There are, of course, many details to these processes, including the enzymes that are utilized, the ways in which organisms control the processes, and modifications that must be made to polypeptides before they become active proteins. However, if you keep your eye on the basic processes, and keep in mind the overall importance of proteins to life, the details should fall into place as you travel from gene to protein.

Learning Objectives

• Come to grips with the importance of DNA to life on earth.

• Understand the basic chemical structure and properties of DNA.

• Be able to explain the universal nature of the genetic code.

• Understand the processes of transcription and translation, including how they are both similar and different in prokaryotic and eukaryotic organisms.

• Understand how and why polypeptides are modified after they are produced.

• Understand various ways in which prokaryotes and eukaryotes control protein production.

• Comprehend the significance of mutations that can occur in DNA.

Topics covered in this Lesson

• Properties of DNA
• Transcription
• Translation
• Protein Processing

Properties of DNA

The elucidation of the structure of DNA in the 1950’s ushered in a new era in biology, eventually opening up the disciplines of genomics and proteomics. DNA is found in all cells in the same structure, and it is truly the fundamental informational material for all of life. Think about how the structure of DNA fulfills all the requirements for storage and retrieval of information.

Learning Objectives

• Be able to describe the basic structure of the DNA double helix, including the important chemical bond types that help it maintain its shape.

• Be able to describe the basic structure of a nucleotide.

• Know what is meant by the 5’ and 3’ ends of strands of DNA.

• Understand what is meant by the ‘antiparallel’ nature of DNA.

• Know the four nitrogenous bases found in DNA and how they pair with each other.

• Understand the basics of what genes are and what the purposes are of the nucleotide sequences flanking genes.

• Understand how and when genes and DNA are packaged and unpackaged.

• Be aware of how much DNA is in your cells, and how this compares to other types of organisms.
   

Transcription

By now we all know that DNA carries all of our genetic information, but how is that information actually used by the cell? In this lesson you will find out how genetic information is “read” from the genes in an organism’s DNA to produce mRNA. After it is formed, that mRNA may be further processed before it is sent to the ribosomes for translation into a polypeptide.

Learning Objectives

• Know where in cells transcription occurs.

• Know the different types of RNA produced by transcription.

• Be able to define promoter region.

• Be able to describe the three stages of transcription.

• Understand that RNA polymerase reads the DNA template strand from 3’ to 5’ while the RNA transcript is built from its 5’ end to its 3’ end.

• Know where the energy that drives the condensation reactions that build mRNA come from?

• Understand the role of complementary base-pairing in transcription.

• Know how mRNA is processed in eukaryotic cells.

• Be able to define exon and intron, and how they are significant in eukaryotic RNA processing.
   

Translation

While transcription produces an RNA transcript of DNA, the actual production of polypeptides occurs during translation. In translation, mRNA and tRNAs carrying amino acids meet at the sites of protein synthesis, called ribosomes. At the ribosome, the genetic code is translated into specific sequences of amino acids that make up the polypeptide.

Learning Objectives

• Comprehend the universal nature of the genetic code.

• Be able to define codon.

• Understand what start codons and stop codons are.

• Know the roles of mRNA, ribosomes, tRNA and amino acids in the process of translation.

• Understand how a polypeptide is built, one amino acid at a time, in the different docking sites of the ribosome.

• Understand how tRNAs are ‘charged’ with amino acids.

• Know that ribosomes consist of a large and a small subunit.

• Be able to define polysome.
   

Protein Processing

The processes of transcription and translation, as we have seen, produce polypeptides – sequences of amino acids linked together by peptide bonds. The polypeptides produced by these processes, however, are usually not able to act as mature proteins until they are modified in some way. There are a number of common modifications made to polypeptides in cells, including moving the polypeptide to a specific area of the cell, cutting the polypeptide into smaller pieces, or adding various chemical groups to the polypeptide. The modifications to polypeptides are in many cases just as essential in protein production as transcription and translation.

Learning Objectives

• Understand why the polypeptides produced by translation are not “mature” proteins.

• Be able to define signal sequences, and understand their role in protein production.

• Know the location of translation in both eukaryotic and prokaryotic cells, and know why eukaryotic translation can start in one location and finish in another.

• Know that polypeptides are commonly modified by addition of chemical groups, such as phosphates or sugars, or by cleavage into two to several smaller polypeptides.

• Understand how the three dimensional shape of proteins, which is largely determined by the interactions of amino acid R-groups, is essential to the final role of the protein.

• Remember the various roles proteins play for cells, and why proteins are an essential part of life.

• Understand that proteins degrade over time, and what happens to their amino acids.