Proteins – what they are and how they’re made
Proteins are the key working molecules and building blocks in all cells. They are produced in a similar two-step process in all organisms called protein synthesis – DNA is first transcribed into RNA, then RNA is translated into protein.
From Mendel to DNA
The rediscovery of Mendel's work in 1900 began a series of revelations about that would continue throughout the 20th century. What were some of these revelations?
From Mendel to DNA
The rediscovery of Mendel's work in 1900 began a series of revelations about that would continue throughout the 20th century. What were some of these revelations?
Upstream of individual genes, sequences of called promoters determine when proteins are produced and in what amounts.
What is a protein?
Proteins are the major ‘working molecules’ within every organism. Among their many jobs, proteins reactions, transport oxygen and defend organisms from . They’re also crucial building blocks of organisms. They are the major components of wool, and milk, they package up the DNA in chromosomes and they insulate the cells of the nervous system. In short, proteins are hugely important!
Proteins are made of large numbers of amino acids joined end to end. The chains fold up to form three-dimensional molecules with complex shapes – you could think of it as origami with a very long and thin piece of paper. The precise shape of each , along with the amino acids it contains, determines what it does.
Proteins: key examples on the Hub
All about enzymes
Enzymes are proteins. Enzymes have many functions in our bodies. An important feature of enzymes is their ability to catalyse reactions without being used up in the process. Can you think of any enzymes and their uses?
Enzymes are proteins. Many enzymes have useful applications in medical or industrial biotechnology. Find out more in the video clip: Improving enzymes
is a protein that regulates blood . Type 1 diabetics do not produce insulin. Find out more in the video clip: Type 1 diabetes.
Mussels hold fast to rocks and piles using their strong , which are made of protein. Find out more in the interactive:
Antibodies are proteins, find out more in the article:
Casein is the protein in milk that is used to make cheese. Find out more in the animated video:
factors are specialised proteins that control the production of other proteins. Find out more in the video clip: What controls apple flesh colour?.
The macrofibrils within wool are made of protein. Find out more in the interactive:
Proteins are expressed from genes
All organisms make proteins in essentially the same way. The process starts with a – the ‘instruction manual’ for constructing the protein. For this reason, the process of making a protein is also called .
Gene expression has two main stages: transcription and .
Transcription
Structures in the identify the start and end of a gene and read the DNA sequence between them (the order of A, C, G and T bases within the gene). A molecular message (an molecule) is produced that echoes the sequence of the gene itself. In most respects, mRNA looks similar to a single-stranded piece of DNA.
Translation
A ribosome receives the mRNA molecule and starts to build a chain of amino acids (a protein) that exactly matches the instructions within the mRNA. The ribosome ‘reads’ the mRNA sequence as a series of three- chunks or codons. Each tells the protein-making machinery which to add next.
Making a protein
Protein synthesis (or gene expression) is a two-stage process. In the first stage (transcription), an mRNA molecule is produced that echoes the DNA sequence of the gene being expressed. In the second stage (translation), a chain of amino acids is formed at the ribosome, following the instructions provided by the mRNA molecule.
The genetic code is essentially the same throughout nature
Remarkably, across all life, each codon has the same ‘meaning’ in any given cell (with a few minor exceptions). For instance, the codon AGA is an instruction to add the amino acid arginine to a growing protein – whether that protein is growing within bacterial cells or human cells. In other words, every cell follows the same rules to make a new protein.
The genetic code
The genetic code is the cell’s ‘instruction manual’ for producing a protein from an mRNA sequence. Three-base-long sections of mRNA (codons) are ‘read’ in sequence at the ribosome. Each codon corresponds to a particular amino acid, which is added to the growing protein chain.
See the article How to add foreign DNA to bacteria for more information.
Which proteins are made when – the power of a promoter
Within any cell, only a proportion of proteins are produced at any one time. Proteins that perform essential roles are produced constantly, while others are expressed only when they are needed. Cells also need large amounts of some proteins (such as the enzymes involved in continuous processes like transcription and translation) and smaller amounts of others (such as hormones). But how does a cell decide which genes to express and how much to make?
Transcription factor binding to DNA
Transcription factors are proteins that control the production of other proteins. Enzymes involved in the same pathway in the cell are often controlled by the same transcription factor.
Promoters are the sequences of DNA that determine when a gene is expressed. These sections of DNA sit in front of genes and provide a ‘landing site’ for transcription factors (proteins that switch gene expression on and off) and (the protein that reads DNA and makes an mRNA copy). Different sequences have different strengths, and genes with ‘strong’ promoters are expressed at a higher level than those with ‘weak’ promoters.
Promoters and apple flesh colour
Red-fleshed apple
The flesh of the new apple cultivar is red all the way through to the core.
At Plant & Food Research, Richard Espley and colleagues are exploring the role of promoters in determining whether apples have white or red flesh. The group found a (MYB10) that binds to the promoter of several genes that produce red pigment in apples, causing them to be expressed. There is far more MYB10 in red-fleshed apples than in white-fleshed apples, so these pigment genes are expressed at a higher level and make more red pigment.
Find out more in the article: Discovering what controls apple flesh colour.
Proteins and gene expression
These articles have further information about gene expression and proteins.
Useful link
Explore the range of resources from the Genetic Science Learning Center at The University of Utah, including interactives.
