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Chapter 13&14

BIOL 1000 Chapter 13 & 14 - Gene Structure and Expression.docx

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BIOL 1000
Jennifer Steeves

Chapter 13 and 14 13.1 – The Connection between DNA, RNA, and Protein • Genome – The entire selection of DNA sequences for a given organism • Gene – A unit containing the code for a protein molecule or one of its parts, or for functioning RNA molecules (tRNA and mRNA) • Gene Expression – Involves the synthesis of the product (e.g. protein). When synthesized, the protein encoded by the gene is “expressed” • The pathway from genes to proteins involves: 1. Transcription – A sequence of nucleotides in DNA is copied into a complementary sequence in an RNA molecule 2. Translation – The sequence of nucleotides in an mRNA molecule specifies an amino acid sequence in a polypeptide • The genetic code is a Triplet Code and essentially universal • Redundancy in the code helps protect against single-base substitutions or mutations  The polypeptide may not be altered by changing one nucleotide • Codes for products that are not translated also  i.e. tRNA, rRNA, and snRNA 13.2 & 14.2 – Transcription: DNA-Directed RNA Synthesis • The process by which information coded in DNA is transferred to a complementary RNA copy  mRNA is the complementary code of the DNA template  DNA is read 3’ – 5’  mRNA is created 5’ – 3’ 1. Initiation • RNA Polymerase II binds to the transcription factors at the promoter region; upstream  Together called the Transcription Initiation Complex  Transcription is slow if initiated with only these factors  TATAAAA sequence (TATA box) present; recognized by transcription factor  Collection of proteins that bind to the promoter region before RNA Polymerase • Promoter Proximal Region – Upstream of gene, contains Promoter Proximal Elements  Regulatory proteins binding to these elements may stimulate or inhibit the rate • Enhancer – More distant from the gene; contains regulatory sequences  Binding of Regulatory Protein may stimulate or inhibit the rate of transcription initiation Activation of Transcription • Activator Proteins bind to Promoter Proximal Elements and increase the rate of transcription • Other Activators bind to the enhancer and work with a Coactivator – large multiprotein complex • The DNA loops and the connection between the Activators at the Enhancer, Coactivator, and Proteins at the PPR and the Promoter creates the MAXIMAL transcription rate • Polymerase II shoots off and begins to unwind DNA and the transcription starts at the “Start Point” • Transcription Factors are Released Repression of Transcription • Repressors oppose the effect of activators; “battle” between the two signals • Some repressors bind to the Enhancer (same sequence as the activators) • Some bind near the activators and interact with them; preventing interaction with a Coactivator • Some bind to specific sequences and recruit Corepressors; inhibiting transcription initiation Combinatorial Gene Regulation • There is a combination of a few regulatory proteins in particular ways  The transcription of a wide array of genes can be controlled  A large number of cell types can be specified Coordinated Regulation of Transcription of Genes with Related Functions • All genes that are coordinately regulated have the same regulatory sequence associated with them • E.g. – Control of gene expression by steroid hormones in mammals • A single steroid hormone can regulate many different genes because all of the genes have an identical DNA sequence to which the hormone-receptor complex binds  A Steroid Response Element Regulating Chromatin • Transcriptionally active genes have looser chromatin structure than inactive ones • The change in chromatin structure involves chromatin remodelling  Specific Histone modifications; particularly in the region of a gene’s promoter • Method 1:  Activator binds to the regulatory sequence upstream of a gene’s promoter  Recruits a nucleosome remodelling complex  Uses the energy of ATP hydrolysis to slide the nucleosome exposing the promoter  Or restructures the nucleosome allowing transcription factors to bind • Method 2:  Activator binds to the regulatory sequence upstream of a gene’s promoter  Recruits enzyme that acetylates lysine amino acids in the tails of histones  The histones loosen their attraction to the DNA and the promoter is exposed  Reversed by deacetylation enzymes that remove the acetyl groups from histones 2. Elongation • RNA Polymerase II untwists DNA; approximately 10-20 bases exposed at a time • RNA Polymerase II adds RNA nucleotides to create RNA transcript in the 5’  3’ direction 3. Termination Eukaryotes • No terminator sequence – RNA nucleotide sequence called the polyadenylation signal sequence (near the 3’ end) is transcribed by RNA Polymerase II • Proteins bind to this sequence and cleave the RNA strand just downstream • Poly(A) Polymerase adds a poly A tail (50 to 250 adenine nucleotides) to newly created 3’ end • Creates pre-mRNA – not ready for translation Bacteria • RNA Polymerase II stops when terminator sequence is reached • mRNA is ready to be translated 13.3 & 14.3 – Processing of mRNAs in Eukaryotes • Regulation of masking proteins and proteins involved in modification can keep mRNA inactive • The 5’ G cap is soon added after transcription begins by a capping enzyme  Consists of a guanine-containing nucleotide that is reversed so that its 3’-OH group faces the beginning rather than the end of the molecule • 5’ Cap and the Poly-A Tail are important because they:  Facilitate export of the mRNA from the nucleus through the nuclear pore  Protect mRNA from degradation  Aid in attachment to ribosome for translation (protein synthesis)  Length of Poly-A Tail helps determine rate of translation • Untranslated Re
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