6.5 Regulation of Gene Expression

Gene Expression

the process by which the information encoded in a gene is turned into a functional product (usually a protein, but also RNAs)

not all genes are expressed or turned on in a cell

the combination of genes expressed and the level of expression determine the phenotype of a cell or organism

ex. neuron vs muscle cell: same DNA, but different gene expression

ex. butterfly wings: the color pattern is due to cells expressing different genes at different levels across the wings

note: the environment can also impact phenotype

Constitutively Expressed Genes

transcribed at all times

ensures a continuous supply of essential products for basic cellular functions

ex. housekeeping genes in eukaryotes maintain basic cellular functions like glycolysis

Inducible Genes

transcription can be turned “on” or “off” base on environmental and internal cues

ex. rice plants activate genes in response to pathogens for protection

Regulation Benefits

prokaryotes and eukaryotes must be able to regulate gene expression

helps to save energy and resources, respond to changes in their environment, and develop & maintain different cell types (eukaryotes)

prokaryotes: regulation occurs primarily at the level of transcription

eukaryotes: regulation occurs at multiple stages, but many genes are regulated primarily at the level of transcription

Gene Regulation in Prokaryotes

Bacterial Gene Expression: gene regulation in prokaryotes occurs primarily at the level of transcription

includes operons and regulatory genes

Operons

cluster of related genes that can be regulated and transcribed together under a single promoter

can be repressible or inducible

3 parts: promoter, operator, and structural genes

Promoter

RNA polymerase binding site

Operator

the on/off switch

Structural Genes

code for related enzymes in pathway

Regulatory genes

produce regulatory proteins (like repressors and activators) that control the expression of other genes

usually separate from the operon

Repressors

regulatory proteins that reduce transcription when bound to operator

negative regulation

Activators

regulatory proteins that increase transcription when bound to DNA

positive regulation

Repressible Operon

transcription is usually on, but can be turned off (repressed)

on→off

ex. trp operon in E.coli

the trp operon in bacteria controls the synthesis of tryptophan (contains genes needed to make tryptophan)

the trp operon is normally “on” (synthesizing tryptophan

when tryptophan levels build up, tryptophan(a corepressor) binds to the repressor, changing its shape

the repressor can now bind to DNA to temporarily shut off transcription for tryptophan, so the cell does not waste energy

Inducible Operon

transcription is usually off, but can be turned on (induced)

off→on

ex. the lac operon in E.coli

has both positive and negative regulation

glucose is the preferred energy source, but if levels are low/absent, E.coli can switch to lactose

the lac operon includes 3 genes that encode enzymes for the use/breakdown of lactose

these genes are only expressed when lactose is present

Lac Operon: negative regulation

when there is no lactose present, the lac repressor is bound to the operator, which prevents transcription

when lactose is present, the cell converts lactose to allolactose, which acts as an inducer for the lac repressor(because it changes the shape and allows RNA polymerase to initiate transcription)

allolactose binds to the lac repressor→ repressor changes shape→detaches from operator

RNA polymerase can now bind to the operator, but only loosely until it has help from a protein

transcription is weak until there is positive regulation

Lac Operon: positive regulation

for strong transcription of the lac operon, RNA polymerase needs help binding to the DNA from a protein called CAP, BUT this protein needs to be activated first

when glucose is very low/absent, E.coli gets “hungry” and produces a small molecule called cAMP

cAMP binds to CAP, activating it

cAMP-CAP complex can now bind to the DNA, which enhances RNA polymerase binding, triggering high levels of transcription

Gene Expression in Eukaryotes

during early embryonic development, cells receive internal and external cues that lead to differentiation

think of these cues as the first instructions about which genes need to be turned on or off in the cells

cues establish early gene expression patterns

Internal cues: cytoplasmic determinants

External cues: induction

Differentiation

cells become specialized in their structure and function through the expression of different genes for tissue-specific proteins

Cytoplasmic Determinants

mRNA and proteins (including transcription factors) from the maternal egg cytoplasm that direct early animal development

unequally distributed in the cytoplasm of an egg cell

when a sperm fertilizes an egg. it forms zygote

as the zygote divides, daughter cells receive different cytoplasmic contents, which means each nucleus is exposed to different cytoplasmic determinants, and that leads to different patterns of gene expression

Induction

cell to cell signals in early development that cause a change in gene expression in nearby cells

one group of cells (inducers) send inductive signals to another group of cells (responder cells) via cell-cell contact or paracrine signaling(unit 4)

triggers signal transduction pathway in the responder cells that activate transcription factors, which change gene expression

some inductive signals act as morphogen

the induction of transcription factors during early development (from both internal/external cues) will turn genes on/off

some genes may code for other transcription factors, which leads to sequential gene expression, ensuring the correct order and timing of development

Morphogen

specific type of signaling molecules that diffuse through developing tissues, forming a concentration gradient

a cell’s response is dependent on the concentration of the morphogen it was exposed to

leads to differential gene expression between cells