Gene Expression

Gene Expression

1. Mutations

   • Changes to the base sequence of DNA
     • Changes in the base nucleotide sequence
       • Can be caused by errors in DNA replication
         • Changes in sequence causes different polypetide formed
           • Changes final 3D protein structure
             • E.g. changes the structure of an enzyme preventing reactions
             • Can cause genetic disorders
   • Types of mutations
     • Substitution
       • One or more bases swapped
     • Addition/Deletion
       • One or more bases added/removed
         • Frameshift - shift the reading frame of codons in mRNA
     • Duplication/Inversion
       • One or more bases repeated/reserved
     • Translocation
       • Bases moved from one location in the genome to another
         • Movement within the chromosome or movement to a different chromosome
   • Not all mutations change order of AA
   • Mutagenic Agents
     • Increase rate of Mutation
       • UV light, Ionising radiation, viruses, chemicals
         • Acting as a base
           • Altering Bases
             • Some chemicals delete or alter bases
               • Changes structure of DNA
                 • Radiation can also change structure of DNA
                   • Causing problems during DNA replication
           • Base analogs can substitute for a base during DNA replication
             • Changing the base DNA sequence in the new DNA molecule
               • Can change the complementary base pairing, substitution mutations
   • Cancer
     • Mutations in genes can cause uncontrolled cell growth
       • Aquired mutations - mutations after fertilisation
         • If mutations in genes that control cell division
           • Can cause uncontrolled cell division
             • Uncontrol cell division results in a tumour (mass of abnormal cells)
               • Tumours that invade and destroy surrounding tissue called cancer
                 • Two types of genes that control cell division
                   • Proto-oncogenes
                     • Normally stimulates cell division by producing proteins that make cells divide
                   • Tumour suppressor genes

2. Stem Cells

   • Unspecialised cells that develop into other types of cells
     • Stem cells divide to become new cells which then become specialised
   • Embryo
     • Found in embryo to develop fetus and in adult to replace specialised cells
       • Embryonic stem cell Totipotent mature into any type of body cell
         • Only present in the in mammals for first few cell divisions of embyo
           • After first cell divisions turn into pluripotent
             • Pluripotent - specialise to any body cell but not cells in the placenta
   • Adult
     • Multipotent stem cells
       • Differentiate into some types of cells e.g. RBC / WBC
         • Found in bone marrow
     • Unipotent stem cells
       • Only differentiate into one type of cell
   • Become Specialised as different genes expressed
     • Stem cells only transcribe and translate part of their DNA

3. Epigenetic Control

   • Determine whether or not a gene is expressed
     • Gene is switched on or off
     • Works through attachment/removal of chemical groups (epigenetic marks)
       • To DNA or Histones but don't alter the base sequence
         • They instead alter how avaliable the molecules are to enzymes like polymerase

4. Transcription Factors (TF) control the transcirption of target genes

   • All cells carry the same genes but have different structures as different genes expressed
     • Eukaryotes have TFs move from cytoplasm to nucleus
       • They then bind to specific DNA sites at the start of target genes
         • Control expression by controlling the rate of transcription
           • TF: Activators - stimulate/increase the rate of transcription
             • By helping RNA polymerase bind to start of gene
             • TF: Repressors - Inhibit/decrease rate of transcription
               • Bind to start of gene preventing RNA polymerase from binding
   • Target genes - the genes they control the expression off
   • Oestrogen can initate the transcription of target genes
     • Oestrogen steriod hormone that binds to a TF called oestrogen receptor forming a complex
       • The complex moves from cytoplasm into nucleus, binding to specific target genes
         • Complex acts as an activator, helping RNA polymerase
           • Complex can act as a repressor depending on type of cell and target gene
   • RNA Interference