Proteins and mutations

In a nutshell

Proteins are made in the cell cytoplasm on ribosomes. Once made, they fold and have specific functions. For example, some proteins are enzymes and others are hormones. Mutations can occur that change the sequence of bases in an organism's DNA. These can impact the structure and function of the protein. 

Making proteins

Proteins are made in the cytoplasm of the cell on ribosomes. The ribosomes use the DNA code to make proteins. However, DNA is located in the nucleus and it is too big to leave the nucleus to get to the ribosomes.

The cell uses messenger RNA (mRNA) instead as it is a copy of the genetic material small enough to leave the nucleus. Then, the correct amino acids are brought to the ribosome by the correct carrier molecules. 

Protein function

A protein will fold into a unique shape after it is made. This shape helps it carry out its function. Some protein functions are listed below. 

Mutations

Definition

A mutation is a random change in the sequence of bases in an organism's DNA. Mutations happen all the time and can occur if replication doesn't happen properly. The chances of DNA mutating is increased under certain conditions. 

Example

Radiation can cause mutations in DNA.

These mutations can produce genetic variants which are different forms of the gene. The sequence of DNA bases codes for the sequence of amino acids that make up a protein. This means that mutations in a gene can lead to changes in the protein that it codes for. 

Most mutations won't impact the protein, some will change the structure slightly but not enough to change the function. But some mutations will cause the protein to have a different shape and this can change the function. 

Example

If the active site of an enzyme is changed, then the substrate might not be able to bind and the enzyme will not be able to function properly. 

Example

If non-coding DNA is mutated then gene expression can be changed.

Types of mutations

There are three main types of mutations you need to know about. 

Insertion

An insertion mutation is when a new base is inserted into the DNA sequence. Every three bases codes for an amino acid so this type of mutation will change the way the sequence is read and can impact more that one amino acid as it will have a knock-on effect on amino acids further down the sequence.

Example

In this example, an adenine is inserted between two thymines. This produces a TAT codon instead of the original TTG codon. As a result, a tyrosine amino acid is coded for instead of leucine. This also shifts the rest of the bases into the next codon. So, instead of being AGT and coding for a serine amino acid, the second codon is GAG and codes for glutamic acid. The same happens to the third codon which instead of producing a tyrosine will produce a leucine. 

At point (1.), the final T that was originally in the third codon has. been pushed to a fourth codon. This shows how a single insertion can have knock-on effects on amino acids further down the sequence.

Deletion

A deletion mutation is when a base is deleted from the DNA sequence. This is like the insertions as it can have knock-on effects further down the sequence.

Example

In this example, a guanine base is deleted. This produces an ATT codon instead of the original AGT codon as the first T in the third codon is bought into the second codon. As a result, an isoleucine amino acid is coded for instead of serine. This will also impact the following codon (1.). This shows how a single insertion can have knock-on effects on amino acids further down the sequence.

Substitution

A substitution mutation is when a random base in the DNA sequence is changed to another base.

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Example

In this example, a thymine base is replaced with a guanine. This produces a GTG codon instead of the original TTG codon. Unlike the other two mutation types, substitutions will only impact the codon in which the base is replaced. So here, the original leucine amino acid is replaced with a valine. However, none of the subsequent codons are impacted.