Genetics, evolution and ecosystems

• Mutation


  A change in the DNA that could result in the creation of a non-functioning protein
• Some mutations code for a protein that provides an advantage to an organism
• All alleles of genes are a result of a mutation
• Gene mutation

  A change in the base sequence of the DNA
• Gene mutations

  • They randomly occur during DNA replication
  • They are more likely to occur if exposed to mutagenic agents (high energy radiation, ionising radiation, chemicals)

  
• Mutation alters the gene

  It can result in a different amino acid sequence in the encoded polypeptide
• If the amino acid sequence changes, the protein will form hydrogen and ionic bonds in different places and fold differently, resulting in a non-functioning protein
• Not all mutations are harmful, sometimes the new protein may be beneficial, like antibiotic resistance in bacteria
• Some mutations are neutral, the same protein is still made due to the genetic code being degenerate
• Types of gene mutations

  • Base substitution
  • Base deletion
  • Base insertion
• Frameshift

  Removal or insertion of one base changes all of the subsequent codons, which is more harmful as multiple amino acids may be incorrectly coded for
• Transcription factors

  Molecules from the cytoplasm that enter the nucleus and bind to DNA to initiate transcription of genes

  
• In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcription factors move from the cytoplasm into the nucleus

  
• Turning on/off particular genes in a cell is what enables them to become specialised
• In prokaryotes, the function of transcription factors is to preserve resources to ensure that not all proteins are constantly produced
• Transcription in eukaryotes

  1. Transcription factor binds to DNA

  2. Transcription begins, creating mRNA

  3. mRNA is translated in cytoplasm to create protein

  
• Without the binding of a transcription factor, the gene is inactive and the protein won't be made
• Oestrogen

  A steroid hormone that can initiate transcription by binding to a receptor site on a transcriptional factor, causing it to change shape and bind to DNA
• Operon

  A group of simultaneously controlled genes that are either all expressed or not, more common in prokaryotes than eukaryotes
• Lac operon

  • A sequence of 3 genes in E.coli that aid with lactose digestion

  
• Regulation of lac operon

  1. LacI repressor protein inhibits transcription when no lactose is present

  2. Lactose binds to repressor, changing its shape and preventing it from binding to operator, allowing transcription

  3. cAMP increases transcription rate by binding to CRP

  
• Epigenetics

  Heritable changes in gene function without changing the DNA base sequence, caused by changes in the environment

  
• Epigenome

  A single layer of chemical tags on the DNA that impacts the shape of the DNA-histone complex and determines whether the DNA is tightly wound (won't be expressed) or unwound (will be expressed)

  
• Methylation

  Increased methylation of DNA inhibits transcription by preventing transcriptional factors from binding and attracting proteins that condense the DNA-histone complex
• Acetylation

  Decreased acetylation of histones inhibits transcription by making the DNA and histones more strongly associated and harder for transcription factors to bind
• Pre-mRNA

  A newly synthesized strand of mRNA before it has been modified
• Introns

  Sections of pre-mRNA that are removed by splicing
• mRNA modification

  1. Addition of 5' cap and poly-A tail to protect mRNA

  2. Removal of introns by splicing
• Alternative splicing allows a single gene to result in the creation of multiple proteins
• Proteins are further modified after translation, such as addition of non-protein groups and folding into 3D shapes
• Homeobox gene sequence

  Sequences of genes that create proteins regulating the expression of other genes involved in body formation in early development

  
• Hox genes

  A type of homeobox gene found in animals that are responsible for correct body development and positioning of body parts
• The order of Hox genes in the DNA is the order in which their effects are expressed on the organism
• Most organisms show symmetry in their body development, either radial or bilateral
• Mitosis

  Cell division process that results in an increase in the number of cells, leading to growth
• Apoptosis

  Programmed cell death that removes unwanted cells
• The cell cycle is controlled by genes to ensure new cells are only made when needed for growth and repair, to preserve energy and prevent tumour formation
• Tumour suppressor genes make proteins that stop the cell cycle, while proto-oncogenes make proteins that initiate the cell cycle
• Apoptosis occurs when a cell is detected to have an error or is too old to function, destroying the cell and recycling the resources
• Hox genes regulate mitosis and apoptosis, and the genes controlling them are impacted by external and internal stimuli, with the greatest effect during growth and development