Mutations
- Gene mutations are changes to the base sequence or quantity of DNA within a gene or section of DNA.
- Gene mutations occur spontaneously during the process of DNA replication.
- The mutation rate is increased by mutagenic agents, which are chemical, physical or biological agent that causes mutations e.g. UV light
Type of Mutation | Description |
Addition | Addition of one or more nucleotides |
Deletion | Removal of one or more nucleotides |
Substitution | A nucleotide is replaced by a different nucleotide |
Inversion | A sequence of bases is separated and then reattached in the inverse order |
Duplication | One or multiple bases are repeated |
Translocation | A piece of DNA breaks off and doesn't reattach to itself or its homologous pair. |
- Some mutations may only affect a single codon, changing a single amino acid in a protein, therefore the protein may remain functional. Other may have no effect on protein structure due to the genetic code being degenerate (silent mutation).
- Mutations such as insertions and deletions can cause frame shifts, changing all the codons and amino acids downstream from the mutation. This results in a unfunctional protein
Transcription Factors
- In eukaryotes, transcription of target genes can be regulated by DNA-binding proteins (transcription factors). They can be help RNA polymerase bind (activators) or prevent it binding (repressors),
- In prokaryotes, lactose induces the production of lactose permease and β-galactosidase. Which allows lactose to enter the cell and hydrolyses lactose to glucose and galactose, respectively.
- The lac I gene in the Lac operon codes for a repressor protein, which binds to the operator sequence, preventing RNA polymerase from binding to the promoter. When lactose is present, it binds to the repressor protein, allowing RNA polymerase to carry out transcription of the enzymes.
Post-Transcriptional Regulation
- In eukaryotes, gene expression can also be regulated after the gene has been transcribed but before it is translated- post-transcriptional modification
- Transcription results in the synthesis of pre-mRNA which must be modified to form mature mRNA, as it contains introns and exons
- Introns are sections of DNA that code for proteins
- Exons are sections of DNA that do not code for proteins
- Splicing occurs which involves the removal of introns and joining the exons back together. It is catalysed by an enzyme-RNA complex called a spliceosome.
- The process of splicing does not always just remove the introns from the pre-mRNA – some of the exons may also be removed
- Depending on which exons are removed, different combinations of mature RNA are formed (alternative splicing) This means that one gene can produce many different mRNAs and code for many different proteins.
Regulating Translation
- The final level of regulating gene expression occurs after the protein has been synthesised – this is post-translational control
- Proteins can be modified in many ways, including:
- Carbohydrate chains can be attached to form glycoproteins
- Lipids can be attached which target the protein to cell membranes
- Proteins can be activated by the addition of a phosphate group
- Many proteins can be activated by signalling from hormone.
Control of Body Plan Development
- DNA contains homeotic genes which regulate morphogenesis
- A subset of homeotic genes are called homeobox genes which contain a 180 base pair length of DNA called a homeobox
- The homeobox sequence is highly conserved in plants, animals and fungi
- The homeobox sequence codes for a specific sequence of 60 amino acids within the synthesised protein called a homeodomain
- The homeodomain sequence folds into a specific shape consisting of three α-helices
- The helix-turn helix shape allows the protein to bind to DNA and regulate the transcription of nearby genes
- The proteins that contain a homeodomain are therefore transcription factors
- A subset of homeobox genes are called Hox genes
- Hox genes are homeobox genes that are only found in animals. They involved in the correct positioning of body parts in an organism
- In some animal lineages, including vertebrates, Hox genes have been duplicated, resulting in multiple Hox clusters
- When a Hox gene is mutated, body parts end up developing in the wrong place on the body (e.g. legs in place of the antennae in flies) - these are called homeotic mutations
- Hox genes are expressed in early embryonic development along the anterior-posterior (head-tail) axis of the organism
- Hox genes show colinerality, where the order of the genes on the chromosomes matches their temporal order and spatial order of expression.
- Apoptosis is the process of carrying out programmed cell death. It is important to remove old, damaged or unwanted cells.
- Apoptosis involves enzymes breaking down the cytoskeleton, forming blebs. Chromatin condenses and the nuclear envelope and the DNA breaks up. The cell is broken into apoptotic bodies, which are phagocytosed by macrophages
- Apoptosis plays an important role during development, removing the surplus cells allowing different body parts to be shaped e.g. allowing separation of digits of hands and feet.
- Apoptosis is controlled by internal signals in response to stimuli such as cellular stress or by external signalling molecules such as cytokines which bind to the target cell to initiate apoptosis.