How is development controlled Frogs Fertilized egg zygote Developing tadpoles Adult Underlying processes cell division and cell differentiation creates liver cells nerve cells and blood cells as well as other important cells organs for the body Genes control development and differentiation of cells Development is due to a program of gene expression Different genes are turned on in different cells at different times during development Regulation of gene expression is seen through turning on off genes to express a certain characteristic In a fertilized egg the cells begin to divide and genes are turned on to begin the creation of differing cells One egg with one cell will then divide into two cells which will divide into four cells and so on Genes that control head formation comes on tail formation comes on and therefore establishes anterior and posterior axis for the organism Secondly in a very simplified state the next cell division turns the genes on for the creation of the dorsal and ventral axis In each division MASTER REGULATORY GENES are either switched on or off GENES Gene A sequence of nucleotides in DNA that code for an RNA or protein So what is a gene and what does gene expression mean A gene is a stretch of bases in DNA that contain the information for making a single type of protein polypeptide 1 gene 1 polypeptide Gene expression is the production of a specific protein form the information stored in DNA Regulation of gene expression is the process by which cells regulate the production of specific proteins So a stretch of bases on DNA codes for the production of one polypeptide IMPORTANT Remember the Structure of DNA DNA is built of deoxynucloetides that are held together by phosphodiester bonds Four bases in DNA A T C G DNA is double stranded and are held together by hydrogen bonds that hold complimentary base pairs A to T and G to C together WHAT IS THE FLOW OF INFORMATION IN A CELL This is called the central dogma of genetics DNA RNA Protein DNA TRANSCRIPTION mRNA TRANSLATION Protein Most gene expression is controlled by regulating transcription This regulates whether a gene is on or off Not all transcription but a vast majority of regulation is from transcription How transcription is controlled is also how gene expression is controlled Genes are arranged linearly along the DNA molecule RNA polymerase makes RNA from DNA Transcription DNA molecules are opened up RNA molecule is synthesized off molecule and this associates to make proteins RNA polymerase is crucial in this process because it joins together RNA nucleotides to produce mRNA Ribosomes then use the triplet codes to join the correct amino acids The genetic code specifies the relationship between the triplet codons in RNA and the amino acids used in translation RNA Polymerase recognizes the beginning and the ends of each gene RNA polymerase has the job of finding the gene and transcribing it RNA polymerase has to know which genes to transcribe Therefore it has to be directed to the genes that need to be transcribed not to the ones that don t PROMOTER REGION RNA Polymerase recognizes this and binds to this promoter sequence and twists the DNA open eventually reaching the TERMINATOR sequence and falls off This synthesis is done from a 5 to a 3 end The promoter is a sequence of bases at the 5 end of a gene where RNA polymerase binds and initiates transcription REGULATION OF GENE EXPRESSION Which is the process by which the cell controls the production of a specific protein Control of transcription by RNA polymerase is the most important step The binding of RNA Polymerase to the promoter is regulated by TRANSCRIPTION FACTORS which are proteins Transcription factors REGULATE gene expression The TATA box is found in eukaryotes RNA polymerase recognizes and interacts with this to start transcription Normally transcription starts 20 bases down stream form the TATA box If there is a mutation in the TATA box sequence then it prevents transcription from occurring RNA polymerase by itself is NOT in charge because there isa s econd set of proteins TRANSCRIPTION FACTORS that control binding of RNA polymerase So how is development controlled Development is due to a program of gene expression Different genes are turned on off in different cells at different times during development Most gene expression is controlled by regulating transcription Transcription factors regulate gene expression Development is controlled by regulation of the activity of transcription factors that control the expression of genes that direct development Important proteins are the TRANSCRIPTION FACTORS These are the key players of gene expression and are referred to as MASTER REGULATORY GENES So for example if a fly has a defective copy of the bithorax gene instead of one set of genes has two sets of wings and two thoraxes what kind of protein does the bithorax gene code for Transcription Factor protein A TRANSCRIPTION FACTOR that is important in controlling development This mutation results in loss of expression in the gene that controls transcription Extra thoraxes are made so this mutation is in one of these MASTER REGULATORY GENES Regulation of Gene Expression in Bacteria Prokaryotes In bacteria gene expression is regulated in response to signals growth nutrients form the environment Key term to start thinking about here Lac Operon The goal here is to maximize growth rate Bacteria want to grow and divide as quickly as possible So many genes in bacteria are organized in functional units called OPERONS An operon is a group of genes with a single promoter which code for several different proteins needed for a metabolic pathway In bacteria some genes organize in clusters or OPERONS with a single promoter and start stop codons for multiple polypeptides Example trp Operon is needed for synthesis of the amino acid tryptophan Example 2 lac Operon is needed for the metabolism of the dissaccharie lactose The Lac Operon Has a promoter RNA polymerase binds Transcription terminator This can code for three different proteins Each protein has own start stop codon and instead of one you get three polypeptides These are Beta Galactosidase cleaves lactose into galactose and glucose Permease allows E Coli to take up lactose Transacetylase Lactose is a disaccharide breaks down into glucose and galactose If lactose is absent the operon is silent and not transcribed If lactose is present operon turns on and lactose can be metabolized So E coli prefers to grow on
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