Genes in Action
- 2. Gene FunctionStructural genesBecome part of the structure & functioning of an organismRegulatory genesControl the action of other genesie. “switch genes on or off”Or control rate of production of proteinsCan switch genes on or off in one of two ways
- 3. Two types of gene regulationRegulatory genes may code for a DNA-binding proteinThese have a positively charged binding site that will enable it to bind to DNAThey will bind to a region near a gene and directly turn it on or offRegulatory genes may code for a signalling proteinThis will bind to a receptor on the cell membraneGenes will be turned on or off via signal transduction
- 4. 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.
- 5. Homeotic GenesIn humans, homeotic genes fall in to 4 groups (Hox A,B,C & D).These encompas 39 genes spread over 4 chromosomesMalfunction of HOXC8 results in an extra pair of ribsMalfunction of HOXD13 results in an extra digit between digits 3 & 4 (often fused)
- 6. Gene StructureThe 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.
- 7. Gene StructureEnzymes 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 geneSTARTSTOPPromoterregionTerminator region
- 8. Gene ExpressionOur DNA is like the master plan for building an organismGenes 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 cellsThe mechanisms for making the products for which these genes code are in the cytoplasmHow does the message get out of the nucleus and in to the cytoplasm?
- 9. Gene ExpressionIn order to be expressed, DNA must be transcribed in to mRNA.ACATAGGCTTGTATCCGA
- 10. TranscriptionAfter the complimentary strand is unzipped, the RNA is written against the template strand of DNAACATAGGCTUGUAUCCGA12
- 11. Transcription: step-by-stepThe 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 unwindsAs 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 mRNAThe transcribed portion of the helix recoils once it has provided a template for the mRNA bases
- 12. Transcription: continuedOnce RNA polymerase reaches the terminator sequence a hairpin loop forms in the mRNA, causing it to be releasedPoly-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
- 13. Post-transcription modificationThe DNA in Eukaryotic genes is made up of ...Introns (non-coding sequences)Exons (coding sequences)Prokaryotic DNA does not contain intronsThe entire gene is copied during transcription, so it is necessary to the spice out the intronsExonExonExonIntronIntron
- 14. Post-transcription modificationIntrons 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 releasedThe remaining exons are then joined together.
- 15. One gene, multiple productsResearch reveals that a single gene is able to make a different product at different stages of developmentAlso, a single gene can make one type of product in one type of tissue and a different product in another type of tissue tissueHow is this possible?The human genome contains approx. 25,000 genesYet there are approx. 120,000 recognised protein-coding mRNA sequences.How is this possible?
- 16. Alternative splicing of pre-mRNAA) Intron retentionThe final product can look quite different if not all introns are spliced outB) Exon jugglingExons can be recombined in a anumber of different combinations
- 17. TranslationThe mature mRNA moves out of the nucleus, through a nuclear pore, in to the cytoplasmRibosome 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 mRNAThe AUG triplet is the “start” codon
- 18. What is tRNA?tRNA is a molecule consisting of a single strand of 76 RNA nucleotidesThe 3 nucleotides at one end form the anticodonThe other end forms a binding site for a specific amino acid moleculeAmino acyltRNAsynthase catalyses the linking of each amino acid to its carrier tRNA moleculeTranslation (continued)Each carrier molecule adds its attached amino acid to the base of the growing chainNot all codons code for a different amino acidThere are 64 different codons, that code for 20 amino acidsTranslation continues until a “stop” codon is reached
- 19. Codons with corres-ponding amino acidsThe genetic code is universal99.9% of species use the same triplet code for the same amino acid
- 21. Prokaryotes vs EukaryotesWhere transcription / translation occursEukaryotes: nucleus then cyctoplasmProkaryotes: cytoplasmSpeed at which it occursSlower in Eukaryotes due to necessity to move out to cytoplasm as well as time required to splice mRNALife span of mRNAProkaryotes: a few minutesEukaryotes: hours/days to allow time for p/t modificationRibosomesEukaryotic ribosomes are larger and have a different rRNA to protein ratio
- 23. Gene regulation in ProkaryotesCASE STUDY: THE LAC OPERONBacteria have groups of genes that are controlled together and are turned on/off as requiredThe LAC operon is a group of genes that produce the enzymes required to preak down lactose to glucose and galactoseThe bacterium only wants to produce these enzymes when lactose is present.
- 24. The LAC OperonUsually a repressor protein (produced by LAC regulatory gene) sits on the controlling regionWhen lactose enters the cell it binds to the repressor, and the repressor releases from the DNAThe LAC genes will now start transcribing mRNA, which will enter a ribosome and produce the 3 enzymes required for lactose metabolismWhen concentration of lactose in the cell decreases, the lactose is released from the repressor and it returns to inhibiting the operon
- 25. Not all genes produce proteinsInstead of mRNA, genes can also be transcribed as:tRNA: then move out in to the cytoplasm as a transfer moleculerRNA: then move in to the cytoplasm to form part of a ribosomeThe nucleolus is a region in the nucleus where rRNA is transcribed and stored until required
- 26. Mitochondrial DNAIn Eukaryotes, mitochondrial DNA (mtDNA) is a double-stranded circular moleculeIn 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.