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Gene Expression - Coggle Diagram
Gene Expression
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Transcription
Chromatin - modifying enzymes - provides initial control of gene expression by making a region of DNA either more/less able to bind to the transcription machinery
Eukaryotic Gene + Transcript - a cluster of proteins (transcription initiation complex) assembles on the promoter sequence at the "upstream" end of the gene - RNA Polymerase II then transcribes the DNA strand to create an mRNA strand - a 5' cap is added, and a poly-A tail on the 3' end is added as well - splicing of introns occurs - control elements (segments of noncoding DNA having particular nucleotide sequences that serve as binding sites for TFs) are critical to the precise regulation of gene expression
General Transcription Factors - act at the promoter of all genes - essential for the transcription of all protein-coding genes - they bind to a DNA sequence (like the TATA Box), but also can bind to other proteins, like other TFs and RNA Polymerase II
Specific Transcription Factors - in eukaryotes, high levels of transcription if these particular genes at the appropriate time + place depend on the interaction of control elements w/ another set of proteins, which are the specific TFs
Enhancers - proximal control elements are located close to the promoter and distal control elements (groupings of which are called enhancers) can be thousands of nucleotides upstream/downstream of a gene/intron - a gene can have multiple enhancers - in eukaryotes, the rate of gene expression can be increased/decreased by the binding of specific TFs to the control elements of enhancers
Researchers have discovered a DNA-Binding Domain (a part of the protein's three-dimensional structure that binds to DNA, and Activation Domains (bind other regulatory proteins/components of the transcription machinery, enhancing many protein-protein interactions)
TFs that Function as Repressors - some can bind directly to control element DNA, blocking activator binding - others can interfere with the activator itself so it can't bind the DNA - some can even promote silencing (repressors recruit proteins that remove acetyl groups from histones, leading to reduced transcription)
RNA Processing - Alternative RNA Splicing (different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns) - regulatory proteins specific to cell type control the intron-exon choices by binding to RNA sequences within the primary transcript - splicing also multiplies the number of possible human proteins, which may be related to the complexity of form
Translation
Initiation Stage - initiation of translation can be blocked by regulatory proteins that bind to specific sequences/structures within the UTR at the 5' or 3' end, which prevents the attachment of ribosomes
Eukaryotic Cell - "global" control involves activation/inactivation if one or more protein factors required to initiate translation
Life Span of mRNA molecules - bacterial mRNA molecules are typically degraded by enzymes within a few minutes - the short life span of the mRNA is the reason why bacteria can change their patterns of protein synthesis so quickly in response to environmental changes
After Translation - many proteins undergo chemical modifications that make them functional - regulatory proteins are activated/inactivated by the reversible addition of phosphate groups - cell surface proteins must also be transported to target destinations in the cell in order to function properly - selective degradation helps regulate the length of time each protein functions in the cell
Noncoding RNAs
MicroRNAs - small, single-stranded RNA molecules capable of binding to complementary sequences in mRNA molecules - capable of forming a complex with one or more proteins - the complex then either degrades the target mRNA or it can block its translation
Small Interfering RNAs - siRNA precursor RNA molecules are injected into a cell, the cell's machinery can process them into siRNAs that turn off expression of genes w/ related sequences (blocking of gene expression by siRNAs is known as RNA Interference)
Small Noncoding RNAs - can cause remodeling of chromatin structure - the siRNA system interacts with other, large noncoding RNAs w/ chromatin modifying enzymes to condense the centromere chromatin into heterochromatin
Piwi-associated RNAs (piRNAs) - induces the formation of heterochromatin, blocking expression of some parasitic DNA elements in the genome known as transposons
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