DNA Structure (AQA GCSE Biology): Revision Note

Nucleotides

• DNA is a chemical found in the nucleus of cells

• DNA structures can be described as follows:

  • it is a polymer (a molecule made from many repeating subunits)

  • it has individual subunits called nucleotides

  • it contains four different types of nucleotide, each of which contains:

    • the same sugar and phosphate group

    • one of four different bases attached to the sugar

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Bases: Basics

• The four different types of nucleotide differ in the base that is attached to their sugar molecule

• The four bases are:

  • A (adenine)

  • C (cytosine)

  • T (thymine)

  • G (guanine)

Base pairing

Higher Tier Only

• DNA molecules consist of two strands of nucleotides

• The bases on each strand pair up with each other, holding the two strands of DNA together

• The bases always pair up in the same way:

  • A-T

  • C-G

• This is known as complementary base pairing

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Coding for amino acids

• DNA functions as a code for the building of proteins in cells; a sequence of three bases is the code for a particular amino acid

• The order of bases controls the order of amino acids that are joined together

• The amino acid sequence then determines the type of protein that is produced

Double helix

• The phosphate and sugar sections of the nucleotides form the backbone of each DNA strand, and the base pairs connect the two strands together

• The two strands wind around each other, forming a double helix structure

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Protein synthesis

Higher Tier Only

• Protein synthesis is:

the process by which proteins are made in the cell cytoplasm

• The structure of the proteins made during protein synthesis is determined by the structure of the DNA molecules

  • The order of bases determine the order of amino acids in a protein

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DNA and phenotype

• The DNA of an organism determines its phenotype

• For coding DNA, a variation in the DNA sequence may change the sequence of amino acids and so alter the activity of the protein produced (most variants have no effect on the protein)

difference in DNA sequence → different proteins produced → different phenotype

• Some DNA does not code for proteins, and is known as non-coding DNA; this DNA affects phenotype by altering how genes are expressed

Changes to proteins

Higher Tier Only

• A change in DNA structure may result in a change in the protein synthesised from a gene

  • A change in a base gives a different set of three bases (triplet), which codes for a different amino acid. The protein chain then folds into a different shape; for an enzyme, the active site changes shape so the substrate no longer fits

• This protein may not function in the same way as the original protein

Function of ribosomes

Higher Tier Only

• Protein synthesis occurs at structures called ribosomes

• Specialised carrier molecules bring specific amino acids to the ribosomes, where they are attached to other amino acids in the order determined by the DNA triplet code

  • The DNA acts as a template for protein production

  • Each triplet of bases codes for a specific amino acid

• Once the amino acid chain is complete, it folds to form a three-dimensional structure that determines protein function

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Protein structure

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, e.g.:

  • enzymes – proteins that act as biological catalysts to speed up chemical reactions occurring in the body

  • hormones – proteins that carry messages around the body

  • structural proteins – proteins that provide structure and are physically strong

    • Collagen is a structural protein that strengthens connective tissues such as ligaments and cartilage

Mutations

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

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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

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Effects of mutations

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

Gene switching

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