Chapter 18.2 and 18.3 Flashcards
Prokaryotes and eukaryotes alter gene expression in response to
their changing environment
In multicellular eukaryotes,
gene expression regulates the development and is responsible for differences in cell types
Ex. Muscle cell v. nerve cell
RNA molecules play many roles in
regulating gene expression in eukaryotes
Eukaryotic gene expression is regulated at
many stages
All organisms must regulate which genes are
expressed at any given time
In multicellular organisms regulation of gene expression is
essential for cell specialization
Almost all the cells in an organism are
genetically identical
Differences between cell types result from
differential gene expression, the expression of different genes by cells with the same genome
Abnormalities in gene expression can lead to
diseases including cancer
Gene expression is regulated at
many stages
Genes with highly packed heterochromatin are
usually not expressed
Chemical modifications to histones and DNA of chromatin
influence both chromatin structure and gene expression
In histone acetylation,
acetyl groups are attached to positively charged lysines in histone tails.
This loosens chromatin structure, thereby promoting the initiation of transcription
The addition of methyl groups (methylation) can condense chromatin; the addition of phosphate groups (phosphorylation)
next to a methylated amino acid can loosen chromatin
The histone code hypothesis proposes that
specific combinations of modifications, as well as the order in why they occur, help determine chromatin configuration and influence transcription
DNA methylation, the addition of methyl groups to certain bases in DNA, is
associated with reduced transcription in some species
DNA methylation can
cause long-term inactivation of genes in cellular differentiation
In genomic imprinting,
methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development
Although the chromatin modifications just discussed do not alter DNA sequence,
they may be passed to future generations of cells
The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called
epigenetic inheritance
Chromatin-modifying enzymes provide
initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery
Associated with most eukaryotic genes are multiple control elements,
segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcription
Control elements and the transcription factors they bind are critical to
the precise regulation of gene expression in different cell types
To initiate transcription,
eukaryotic RNA polymerase requires the assistance of proteins called transcription factors
General transcription factors are
essential for the transcription of all protein-coding genes
In eukaryotes, high levels of transcription of particular genes depend on
control elements interacting with specific transcription factors
Proximal control elements are located close to
the promoter
Distal control elements, groupings of which are called enhances, may be
far away from a gene or even located in an intron
An activator is a
protein that binds to an enhancer and stimulates transcription of a gene
Activators have two domains,
one that binds DNA and a second that activates transcription
Bound activators facilitate a
sequence of protein-protein interactions that result in transcription of a given gene
Some transcription factors function as
repressors, inhibiting expression of a particular gene by a variety of methods
Some activators and repressors act indirectly by
influencing chromatin structure to promote or silence transcription
Transcription alone dow not account for
gene expression
Regulatory mechanisms can
operate at various stages after transcription
Such mechanisms allow a cell to
fine-tune gene expression rapidly in response to environmental changes
In 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
The life span of mRNA molecules in the cytoplasm is a
key to determining protein synthesis
Eukaryotic mRNA is more long lived than
prokaryotic mRNA
Nucleotide sequences that influence the lifespan of mRNA eukaryotes reside in the
untranslated region (UTR) at the 3’ end of the molecule
The initiation of translation of selected mRNAs can be
blocked by regulatory proteins that bind to sequences or structures of the mRNA
Alternatively, translation of all mRNAs in a cell may be
regulated simultaneously
For example, translation initiation factors are
simultaneously activated in an egg following fertilization
After translation,
various types of protein processing, including cleavage and the addition of chemical groups, are subject to control
Proteasomes are
giant protein complexes that bind protein molecules and degrade them
Noncoding RNAs play
multiple roles in controlling gene expression
Only a small fraction of DNA codes for proteins, and
a very small fraction of the non-protein-coding DNA consists of genes for RNA such as rRNA and tRNA
A significant amount of the genome may be
transcribed into noncoding RNAs (ncRNAs)
Noncoding RNAs regulate gene expression at two points:
mRNA translation and chromatin configuration
MicroRNAs (miRNAs) are
small single-stranded RNA molecules that can bind to mRNA.
These can degrade mRNA or block its translation
The phenomenon of inhibition of gene expression by RNA molecules is called
RNA interference (RNAi)
RNAi is caused by
small interfering RNAs (siRNAs)
siRNAs and miRNAs are similar but
form from different RNA precursors
In some yeasts siRNAs play a role in
heterochromatin formation and can block large regions of the chromosome
Small ncRNAs called piwi-associated RNAs (piRNAs) induce
heterochromatin, blocking the expression of parasitic DNA elements in the genome, known as transposons
RNA-based mechanisms may also block
transcription of single genes
Small ncRNAs can regulate
gene expression at multiple steps
An increase in the number of miRNAs in a species may have allowed
morphological complexity to increase over evolutionary time
siRNAs may have evolved first, followed by
miRNAs and later piRNAs
