Saturday, January 15, 2011

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something related to Bio 5 Edexcel Scientific article A BIT...=))


AP Biology Chapter 18 Regulation of Gene Expression

PROKARYOTIC GENE EXPRESSION

·         Operator- segment of DNA that serves as a switch for gene expression
·         Operon- entire stretch of DNA required for enzyme production; several DNA sequences controlled as a unit
·         Repressor- protein that can switch off the operon by binding to the operator
·         Regulatory gene- gene located some distance from the operon it controls & has its own promoter
·         Corepressor- small molecule that cooperates with a repressor protein to switch an operon off; often a substrate

Figure 18.3 Page 353 (trp operon)

trp operon is a repressible operon – transcription is on until repressed by presence of substrate
lac operon is an inducible operon – transcription is off until activated by presence of substrate

·         Inducer- small molecule that inactivates the repressor

     Figure 18.4 Page 354 (lac operon)

·         Cyclic AMP (cAMP)- cyclic adenosine monophosphate; regulator in lac operon; accumulates when lactose is scarce
·         Activator – protein that binds to DNA and stimulates transcription of a gene

EUKARYOTIC GENE EXPRESSION

Transcriptional Control

·         Differential gene expression – expression of different genes by cells with the same genome
·         Histone acetylation – acetyl groups are attached to lysines in histone tails; acetylated lysines no longer bind to neighboring nucleosomes – transcription is easier

Histones can also be methylated – addition of methyl groups

·         Genomic imprinting – methylation permanently regulates expression of genes from either male or female parent
·         Epigenetic inheritance – inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence; alterations not in DNA, but chromatin structure
·         Control elements – segments of noncoding DNA that help regulate transcription by binding certain proteins
·         Enhancers – thousands of nucleotides upstream or downstream of a gene

Post Transcriptional Control

One gene can produce thousands of different proteins depending on mRNA processing

·         Alternative RNA splicing – different mRNA molecules produced from the same primary transcript; depends on which segments are ‘introns’ & which are ‘exons’

mRNA degradation – mRNA has a life span that differs b/t organisms – longer life spans help adaptability

Initiation of translation – inhibitory action of a protein – blocks translation
Protein activation – polypeptides must be processed into active final products
Protein breakdown – length of time a protein is functional; regulated by selective degradation

·         Proteasomes – protein complexes that recognize tagged proteins and degrade them

·         MicroRNAs (miRNAs) –  single strand RNA; capable of binding to complementary sequences in mRNA molecules; blocks translation
·         RNA interference (RNAi)- silencing of gene expression using double-stranded RNA to trigger breakdown of a gene’s mRNA
·         Small interfering RNAs (siRNAs)- small, single strand RNA; generated from double strand RNA; acts as an miRNA

·         Cell differentiation – process by which cells become specialized in structure and function
·         Morphogenesis – physical processes that give an organism its shape
·         Cytoplasmic determinants – maternal substance in egg that influence the course of early development
·         Induction – signals cause change within target cells
·         Pattern formation – cytoplasmic determinants and inductive signals contribute to spatial organization of tissues and organs
·         Positional information – cues that tell a cell its location relative to body axes and neighboring cells to determine how cell responds to signals
Fruit Fly Gene Specifics
·         Homeotic genes – control pattern formation in embryo, larva & adult
·         Embryonic lethals – mutations w/ phenotypes that cause death at embryonic or larval stage
·         Maternal effect gene – when mutant in mother results in a mutant phenotype in offspring
·         Egg-polarity genes – genes that control orientation
·         Morphogens – establish an embryo’s axes and other features of its form

Cancer and Genes
·         Oncogenes – cancer causing genes
·         Proto-oncogenes – normal versions of cellular genes
·         Tumor-suppressor genes – genes that encode proteins that help prevent uncontrolled cell growth
·         Ras gene – gene that encodes a protein that relays a growth signal from a growth factor receptor on the plasma membrane to a cascade of protein kinases, ultimately resulting in stimulation of the cell cycle
·         P53 gene – tumor suppressor gene

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