Lecture 7 Flashcards
The different cell types of a multicellular organism contain the same DNA
Different cell types produce different sets of proteins
A cell can change the expression of its genes in response to external signals
Gene expression can be regulated at various steps from
DNA to RNA to protein
Differentiated cells contain all of the genetic instructions needed to direct the formation of a complete organism
This showed that nucleus of skin cells contain all of the genes necessary to grow a fully functioning organism
Gene expression in eukaryotic cells can be controlled at various steps
Most genes are under transcriptional regulation: Prevent transcription = saves energy; Energy saving and response time are inversely related
can repress or activate transcription (in eukaryotes, RNA processing is used (whether or not it’s spliced))
Most genes (both eukaryotes and prokaryotes) are regulated at step 1 (transcriptional control): Translation is one of the most energetically expensive processes, inactivating TRSANSCRIPTION will avoid this
1) transcriptional control
2) RNA processing contol
3) mRNA transport and localization control
4) mRNA degradation control
5) Translation control
6) Protein degradation control
7) Protein activity control
Transcription regulators bind to reguatory DNA sequences
A transcription regulator interacts with the DNA double helix
Both eukaryotic and prokaryotic have transcription regulators turn transcription on or off
These proteins INTERACT with DNA, but have NO enzymatic activity (can’t break it open)
Many Transcription regulators bind to DNA as dimers
Transcription regulators bin as dimers = ability to form combinations = able to regulate all 20,000+ gene
Usually heterodimers
Transcription switches allow cells to respond to changes in their environment
Repressors turn genes off and activators turn them on
A cluser of bacterial genes can be transcribed from a single promoter
Many bacteria organize their genes into strcutres known as OPERONS (only in bacteria, not eukaryotes)
ALL are regulated with the same control sequences
Either the ENTIRE operon is made or none of it is
Operons contain several different genes that are part of a biosynthesis pathway; has just ONE operator
Genes can be switched off by repressor proteins
Repressor proteins repress transcription
Cell where tryptophan is high do not need to make any; prtoein synthesis slows down, tryptophan binds to Trp repressor, repressor changes shape to bind to the operator = transcription off
Cell where tryptophan is low will need to make some: Cell has inactive repressor = RNA polymerase can attach
Repressors ONLY bind to the
Operator
Genes can be switched on by
Activator proteins
Activate processes, increase the efficiency of transcription
The lac Operon is controlled by an activator and a repressor
In E coli, lac operons primarily use glucose; in normal situation, lac operon is off; lac operon enablles cells to use lactose (E coli will usually want gloucose, uses lac operon to consume lactose ONLY when glucose is absent); lactose is a dissacharide = harder to use
Activator can ONLY bind to an activator binding site
Repressor can ONLY bind to operator
When glucose is abundant, lac repressor acts as an active repressor
Eukaryotic transcription regulators control gene expression from a distance
Eukaryotic transcription regulators help initiate transcription by recruiting chromatin-modifying proteins
In eukaryotes, gene activation can occur at a distance
Big distance from genes and regulatory sequences
2 types of transcription factors in eukaryotes:
1) General transcription factors (be with promoter and most will be the same)
2) Regulatory transcription factor (differ b/w cells; Do NOT interact with promoter; EITHER with enhancer or operator)
Transcription of OFF by default
One of the functions of activators is to restructure the DNA (i.e. modify histone tails: add negative functional groups, histones will become less positive)
Chromatin remodeling complexes move histone down
Eukaryotic transcriptional activators can rectruit chromatin-modifying proteins to help initiate
Gene transcription
Ways that transcription initiation can start; all triggered by transcriptional activator
The arrangement of chromosomes into looped domains keeps enhancers
In chek
Animal and plant chromosomes are arranded in
DNA loops
Both plants and animals: Enhancers pulled close to genes when they’re in the loop even though they are many bases away
Generating Specialized Cell Types: Eukaryotic genes are controlled by combinations of transcription regulators
- The expression of different genes can be coordinated by a single protiein
- Combinatorial control can also generate different cell types
Trancription regulators work together as a “committee” to control the expression of a eukaryotic gene
One of the biggest complexes in your cell; many proteins function
A single transcription regulator can coordinate the expression of many different genes
-One molecules can regulate several different genes
Combinations of a few transcription regulators can generate many cell types during development
-Precursor cell receives special signal to make protein; once it happens early on, ALL progeny cells will maintain that protein
Trancription regulators can be used to experimentally direct the formation of specific cell types in culture
A small number of transcription regulators can convert one differentiated cell type directly into another
-This happens in LAb, not nature
A combination of transcription regulators can induce a differentiated cell to de-differentiate into a puripotent iPS cell
- Genes introduced into fibroblast nucleus -> cells allowed to divide in culture -> iPS cell -> differentiate in culture
- Induce transcription factors in fibroblast to de-differentiate it (no longer a specialzed cell)
- De-differentiate fibroblast into a cell that has the ability to turn into anything
- IN nature, once a fcell has been differentiated, it stays as that cell type
- Fibroblast = type of cell common in extracellular matrix
Differentiated cells maintain their identity
A positive feedback loop can generate CELL MEMORY: generated by + feedback loop
-Parent cell: Gene A is not on (deactivated) -> signal from neighboring cell turns ON expression of gene -> transcription of self is activated and maintiant in ALL progeny cells (regardless of the presence of the signal) (cell “remembers” to transcribe the product without the presence of the signal)
Formation of 5-methylcytosine occurs by methylation of a cytosine base in the DNA helix
- Modification of DNA via methylation
- Add methyl to DNA, same segment in progeny cells will also be methylated (inherited)
- Methylate DNA = Add methyl group
- Continued in ALL progeny
- Method of REPRESSING TRANSCRIPTION
- Always methylating a C next to a G (NOT all CG pairs are methylated, these are the only ones that have the option to be)
- Newly synthesized strands will NOT be methylated
- Keeps transcripts off
- DNA methylation patterns can be faithfully inherited when a cell divides
- ALWAYS methylated on a cytosine next to a guanine; NOT all cytosines next to guanines will be methylated
- If you have it on one strand, the partner strand will also be methylated at this site
- Parent strand is methylated, newly synthesized strand is NOT
Hisonte modifications may be inherited by daughter chromosomes
- Histone modifications are also inherited
- Daughter cells receive about HALF of the nucleosomes; 1/2 will go through and modify the ones that aren’t modified; eventually all will be modified in the same way that the parent cell was
- CAN be inherited; when DNA replicated, daughter cells receive about half of those nucleosomes; modified ones attract modification proteins, unmodified ones will not
3 Ways special cell types can be generated
Cell memory
DNA methylation
Histone modification
