SCIE1106 Study Guide - Final Guide: Permease, Lactose Permease, Metabolic Pathway
TOPIC FOUR: Regulation of Gene Expression and the Cell Cycle
Gene Regulation:
Gene regulation can be defined as the rules and mechanisms controlling the
expression of genes. Genes can either be constitutive or contingency.
Constitutive genes are always expressed and are essential for the basics of life,
for example DNA synthesis, replication and repair, and RNA and protein
synthesis. Contingency genes encode proteins that confer an advantage under
special conditions, for example, heat stress, pH stress or starvation. Gene
regulation is different for prokaryotes and eukaryotes:
Prokaryotes
Eukaryotes
Purpose of gene control
To provide maximum
growth
To regulate development
and differentiation
Challenges encountered
Continual environmental
changes
More constant
environment
Range of gene expression
Total switch-off is rare
Complete turning off of
genes is common
Production of mRNA
Coupled transcription
and translation.
Polycistronic mRNA
RNA in nucleus is
modified, mRNA is
translated in cytoplasm
Terminology:
The following terminology is used when discussing gene regulation:
• Promotor: A DNA sequence that RNA polymerase binds to, in order to
open the DNA double helix, and to begin synthesizing the mRNA
• Operator: A short region of DNA to which the repressor protein binds,
controlling the expression of the genes adjacent to it in the operon
• Repressor: A protein that binds to an operator sequence to prevent
transcription of the adjacent genes in the operon
• Polycistronic mRNA: RNA that has more than one coding protein and is
translated into a number of different proteins; formed when an operon is
transcribed
• Inducer: a small molecule that stimulates the synthesis of an inducible
protein
Genetic Organization of Bacterial Chromosomes:
In bacteria, genes are DNA sequences that code for a polypeptide, tRNA or rRNA,
represented as arrows. Genes are found on both strands of DNA and generally do
not overlap. They can be arranged singularly or in operons.
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Operons are groups of genes adjacent to each other on the chromosome that are
transcribed form a single promotor into a single mRNA molecule. Operons are
not found in eukaryotes.
Regulation of Gene Activity in Prokaryotes:
The reason why genes are regulated in prokaryotes is to satisfy the needs of the
cell but to avoid wasteful synthesis. As a result, the number of protein molecules
produced per unit time from active genes will vary from gene to gene, and vary
in response to the environment.
There 4 ways a cell can control the proteins it makes:
1. Transcriptional control: controlling when and how often a given gene is
transcribed, this is the most important point of control for prokaryotes
2. RNA processing control: controlling how the RNA transcript is spliced or
otherwise processed
3. Translational control: selecting which mRNAs in the cytoplasm are
translated by ribosomes
4. Post-translational control: selectively activating or inactivating products
of transcription after they have been made
Principles of Regulation
The first principle of regulation is coordinate regulation, which is the metabolic
pathway to convert one compound in another, for example:
a → b → c → d, regulated by 3 different enzymes.
In the cell, either all three enzymes are present, or none of the enzymes are
present. This is due to polycistronic mRNA, which encodes for all 3 enzymes. If
the particular mRNA is present, all 3 genes will be translated but if it is not
present, none of the genes are translated.
There are two different pathways to gene regulation – degradative (or catabolic)
pathways, and biosynthetic (anabolic) pathways. For degradative pathways, a
larger, complex molecule is broken down into a smaller, simpler end-product.
The availability of the starting molecule may determine whether enzymes in the
pathway are synthesized, and is usually the regulatory molecule. For
biosynthetic pathways, a precursor molecule is built up into a larger, more
complex end-product. The final product is often the regulatory model.
Regulation of genes can either be negative or positive. In negative regulation, the
repressor protein is present in the cell and prevents transcription. For positive
regulation, an activator binds to the DNA and allows no transcription. While
repressors are not present, a ligand can be, and its that ligand that determines
whether the activator will bind to the DNA.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Topic four: regulation of gene expression and the cell cycle. Gene regulation can be defined as the rules and mechanisms controlling the expression of genes. Constitutive genes are always expressed and are essential for the basics of life, for example dna synthesis, replication and repair, and rna and protein synthesis. Contingency genes encode proteins that confer an advantage under special conditions, for example, heat stress, ph stress or starvation. Gene regulation is different for prokaryotes and eukaryotes: Range of gene expression total switch-off is rare. Rna in nucleus is modified, mrna is translated in cytoplasm. Inducer: a small molecule that stimulates the synthesis of an inducible protein. In bacteria, genes are dna sequences that code for a polypeptide, trna or rrna, represented as arrows. Genes are found on both strands of dna and generally do not overlap. They can be arranged singularly or in operons.
