DNA Structure (AQA GCSE Biology)

• DNA is a polymer (a molecule made from many repeating subunits)
• These individual subunits of DNA are called nucleotides
• Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar

A nucleotide

• There are four different nucleotides
• These four nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached
• There are four different bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G)

Higher tier only

• The bases on each strand pair up with each other, holding the two strands of DNA in the double helix
• The bases always pair up in the same way:
  • Adenine always pairs with Thymine (A-T)
  • Cytosine always pairs with Guanine (C-G)

• This is known as ‘complementary base pairing’

DNA base pairs

• A sequence of three bases is the code for a particular amino acid
• The order of bases controls the order and different types of amino acids that are joined together
• These amino acid sequences then form a particular type of protein
• In this way, it is the order of bases in the DNA which eventually determines which proteins are produced

• The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) and the base pairs of each strand connect to form the rungs of the ladder
• It is this sequence of bases that holds the code for the formation of proteins

The DNA helix is made from two strands of DNA held together by hydrogen bonds

Higher tier only

• Proteins are made in the cell cytoplasm on structures called ribosomes
• Ribosomes use the sequence of bases contained within DNA to make proteins
• DNA cannot travel out of the nucleus to the ribosomes (it is far too big to pass through a nuclear pore) so the base code of each gene is transcribed onto an RNA molecule called messenger RNA (mRNA)
• mRNA can move out of the nucleus and attaches to a ribosome (the mRNA acts as a messenger between DNA and the ribosome)
• The correct sequence of amino acids are then brought to the ribosome and joined together
• This amino acid sequence then forms into a protein

Protein synthesis

Higher tier only

• A change in DNA structure may result in a change in the protein synthesised by a gene
• If there is a change in the order of the bases in a section of DNA (eg. in a gene), then a different protein may be produced
• This protein may not function in the same way as the original protein would have (before the change occurred in the DNA)

Higher tier only

• The ribosome ‘reads’ the code on the mRNA in groups of three
• Each triplet of bases codes for a specific amino acid
• Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order
• In this way, the ribosome translates the sequence of bases into a sequence of amino acids that make up a protein
• Once the amino acid chain has been assembled, it is released from the ribosome so it can fold and form the final structure of the protein

The triplet code of DNA (carried by mRNA) is read by the ribosome and amino acids are attached together in a specific sequence to form the protein

Higher tier only

• When the protein chain is complete it folds up to form a unique shape
• This unique shape enables the proteins to fulfil a specific function. For example, proteins can be:
  • Enzymes – proteins that act as biological catalysts to speed up chemical reactions occurring in the body (eg. maltase is an enzyme that breaks down maltose into glucose)
  • Hormones – proteins that carry messages around the body (eg. testosterone is a hormone that plays an important role in the development of the male reproductive system and development of male secondary sexual characteristics, such as increased muscle mass and growth of body hair)
  • Structural proteins – proteins that provide structure and are physically strong (eg. collagen is a structural protein that strengthens connective tissues such as ligaments and cartilage)

Higher tier only

• Mutations are random changes that occur in the sequence of DNA bases in a gene or a chromosome
• Mutations occur continuously
• As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for
• Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
• There are different ways that a mutation in the DNA base sequence can occur:

Insertions

• A new base is randomly inserted into the DNA sequence
• An insertion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs
  • Remember – every group of three bases in a DNA sequence codes for an amino acid

• An insertion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence

An example of an insertion mutation

Deletions

• A base is randomly deleted from the DNA sequence
• Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs
• Like an insertion mutation, a deletion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence

Substitutions

• A base in the DNA sequence is randomly swapped for a different base
• Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the group of three bases in which the mutation occurs; it will not have a knock-on effect

An example of a substitution mutation

Higher tier only

• Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
• However, a small number of mutations code for a significantly altered protein with a different shape
• This may affect the ability of the protein to perform its function. For example:
  • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
  • A structural protein (like collagen) may lose its strength if its shape changes

Higher tier only

• Not all parts of DNA code for proteins
• Some non-coding parts of DNA can switch genes on and off
• This means they can control whether or not a gene is expressed
• Variations in these areas of DNA may affect how genes are expressed
• if a mutation occurs in a section of non-coding DNA that controls gene expression, the expression of these genes may be altered or in some cases, the mutation may cause them not to be expressed at all