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Genes in Action
 

Genes in Action

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    Genes in Action Genes in Action Presentation Transcript

    • Genes in Action
    • Gene Function
      Structural genes
      Become part of the structure & functioning of an organism
      Regulatory genes
      Control the action of other genes
      ie. “switch genes on or off”
      Or control rate of production of proteins
      Can switch genes on or off in one of two ways
    • Two types of gene regulation
      Regulatory genes may code for a DNA-binding protein
      These have a positively charged binding site that will enable it to bind to DNA
      They will bind to a region near a gene and directly turn it on or off
      Regulatory genes may code for a signalling protein
      This will bind to a receptor on the cell membrane
      Genes will be turned on or off via signal transduction
    • Homeotic Genes
      “Master genes” that control embryonic development in insects and vertebrates.
      A malfunctioning homeotic gene in flies may result in wings, legs, antennae and halteres being absent, or appearing in places that they should not.
    • Homeotic Genes
      In humans, homeotic genes fall in to 4 groups (Hox A,B,C & D).
      These encompas 39 genes spread over 4 chromosomes
      Malfunction of HOXC8 results in an extra pair of ribs
      Malfunction of HOXD13 results in an extra digit between digits 3 & 4 (often fused)
    • Gene Structure
      The arrangement of base pairs (c) in a piece of double stranded DNA (d) will determine many things, such as the coding and non-coding portions of DNA (a), and the length of a gene (b). Thus many representations of the same strand required.
    • Gene Structure
      Enzymes need to know when to start and stop reading a section of DNA.
      If the base pairs were a sentence, regulatory genes may be likened to capital letters and full stops.
      5’
      3’
      3’
      5’
      Regulatory
      gene
      START
      STOP
      Promoter
      region
      Terminator
      region
    • Gene Expression
      Our DNA is like the master plan for building an organism
      Genes are specific instructions on how to build one tiny part of the entire organism.
      Genes are located on the DNA, in the nucleus of our cells
      The mechanisms for making the products for which these genes code are in the cytoplasm
      How does the message get out of the nucleus and in to the cytoplasm?
    • Gene Expression
      In order to be expressed, DNA must be transcribed in to mRNA.
      A
      C
      A
      T
      A
      G
      G
      C
      T
      T
      G
      T
      A
      T
      C
      C
      G
      A
    • Transcription
      After the complimentary strand is unzipped, the RNA is written against the template strand of DNA
      A
      C
      A
      T
      A
      G
      G
      C
      T
      U
      G
      U
      A
      U
      C
      C
      G
      A
      1
      2
    • Transcription: step-by-step
      The enzyme RNA polymerase attaches to the DNA in the upstream (3’) region of the template strand on the promoter sequence.
      The double-stranded DNA helix unwinds
      As RNA polymerase moves down the strand, complimentary RNA bases are put down in a 5’ to 3’ direction.
      A methylated cap is added to the 5’ end of the mRNA
      The transcribed portion of the helix recoils once it has provided a template for the mRNA bases
    • Transcription: continued
      Once RNA polymerase reaches the terminator sequence a hairpin loop forms in the mRNA, causing it to be released
      Poly-A polymerase cleaves the end of the mRNA and synthesises a poly-A tail (adenine bases and proteins).
      A single stranded molecule called pre-messenger RNA (pre-mRNA) is produced
    • Post-transcription modification
      The DNA in Eukaryotic genes is made up of ...
      Introns (non-coding sequences)
      Exons (coding sequences)
      Prokaryotic DNA does not contain introns
      The entire gene is copied during transcription, so it is necessary to the spice out the introns
      Exon
      Exon
      Exon
      Intron
      Intron
    • Post-transcription modification
      Introns are removed by a spliceosome, which is made up of a bundle of protein factors called snerps (snRNP)
      The introns are coiled in to a shape called a lariat and released
      The remaining exons are then joined together.
    • One gene, multiple products
      Research reveals that a single gene is able to make a different product at different stages of development
      Also, a single gene can make one type of product in one type of tissue and a different product in another type of tissue tissue
      How is this possible?
      The human genome contains approx. 25,000 genes
      Yet there are approx. 120,000 recognised protein-coding mRNA sequences.
      How is this possible?
    • Alternative splicing of pre-mRNA
      A) Intron retention
      The final product can look quite different if not all introns are spliced out
      B) Exon juggling
      Exons can be recombined in a anumber of different combinations
    • Translation
      The mature mRNA moves out of the nucleus, through a nuclear pore, in to the cytoplasm
      Ribosome assembles around mRNA and sequence of bases is read in blocks of 3 bases known as triplets ( = 1 codon)
      A transfer RNA (tRNA) molecule with the complementary anticodon is brought in and attaches to the mRNA
      The AUG triplet is the “start” codon
    • What is tRNA?
      tRNA is a molecule consisting of a single strand of 76 RNA nucleotides
      The 3 nucleotides at one end form the anticodon
      The other end forms a binding site for a specific amino acid molecule
      • Amino acyltRNAsynthase catalyses the linking of each amino acid to its carrier tRNA molecule
    • Translation (continued)
      Each carrier molecule adds its attached amino acid to the base of the growing chain
      Not all codons code for a different amino acid
      There are 64 different codons, that code for 20 amino acids
      Translation continues until a “stop” codon is reached
    • Codons with corres-ponding amino acids
      The genetic code is universal
      99.9% of species use the same triplet code for the same amino acid
    • Prokaryotes vs Eukaryotes
      Where transcription / translation occurs
      Eukaryotes: nucleus then cyctoplasm
      Prokaryotes: cytoplasm
      Speed at which it occurs
      Slower in Eukaryotes due to necessity to move out to cytoplasm as well as time required to splice mRNA
      Life span of mRNA
      Prokaryotes: a few minutes
      Eukaryotes: hours/days to allow time for p/t modification
      Ribosomes
      Eukaryotic ribosomes are larger and have a different rRNA to protein ratio
    • Gene regulation in Prokaryotes
      CASE STUDY: THE LAC OPERON
      Bacteria have groups of genes that are controlled together and are turned on/off as required
      The LAC operon is a group of genes that produce the enzymes required to preak down lactose to glucose and galactose
      The bacterium only wants to produce these enzymes when lactose is present.
    • The LAC Operon
      Usually a repressor protein (produced by LAC regulatory gene) sits on the controlling region
      When lactose enters the cell it binds to the repressor, and the repressor releases from the DNA
      The LAC genes will now start transcribing mRNA, which will enter a ribosome and produce the 3 enzymes required for lactose metabolism
      When concentration of lactose in the cell decreases, the lactose is released from the repressor and it returns to inhibiting the operon
    • Not all genes produce proteins
      Instead of mRNA, genes can also be transcribed as:
      tRNA: then move out in to the cytoplasm as a transfer molecule
      rRNA: then move in to the cytoplasm to form part of a ribosome
      The nucleolus is a region in the nucleus where rRNA is transcribed and stored until required
    • Mitochondrial DNA
      In Eukaryotes, mitochondrial DNA (mtDNA) is a double-stranded circular molecule
      In humans, it encompasses only 16,568 base pairs and 37 genes in total.
      Apart from the genes coding for tRNA and rRNA, the rest are involved in cellular respiration.
      Mitochondrial DNA is inherited entirely along maternal lines.