Presentation on theme: "Control of gene expression. Every cell has at least one chromosome consisting of a DNA molecule. Each chromosome contains genes – pieces of a DNA molecule."— Presentation transcript:
Control of gene expression
Every cell has at least one chromosome consisting of a DNA molecule. Each chromosome contains genes – pieces of a DNA molecule – that are recipes for proteins. Genes can be turned on or expressed, or turned off not expressed. It is a waste of resources (energy) to express all genes at all times.
Prokaryotes -expression of genes as needed - dictated by environment
Prokaryotic Control Mechanisms The Operon
Prokaryotes control gene expression through operons An operon is a group of genes that contains all of the protein recipes needed for one particular function Ex: There are 5 enzymes recipes needed for the production of the amino acid tryptophan
Bacteria often group together genes with related functions ex. enzymes in a biosynthesis pathway Transcription of these genes is controlled by a single promoter when transcribed, read as 1 unit & a single mRNA is made Operon operator, promoter & genes they control
Overview of an Operon Regulator Gene (upstream of rest of operon; recipe for a protein called repressor protein) Promoter (where RNA Polymerase attaches) Operator Gene (on/off switch for operon) Structural Genes (contains recipes for proteins) Regulator Gene Promoter Operator Gene Structural Genes
There are 2 types of negative control operons: Inducible Operons - are normally turned off - an inducer turns them on Repressible Operons - are normally turned on - a repressor/corepressor turn off
Lactose operon What happens when lactose is present? Need to make lactose-digesting enzymes What happens when lactose is absent? No need to make lactose-digesting enzymes… waste of cells energy & resources
When the inducer – lactose – is absent, repressor protein binds to the operator gene preventing RNA Polymerase from transcribing the structural genes. The operon is turned off. Lactose operon
When lactose enters the environment, it binds to the repressor protein and inactivates it. This turns the operon on because nothing is attached to the operator gene so RNA Polymerase is induced and can now transcribe the structural genes.
lac operon The lac operon is an example of an inducible operon. The inducer which turns the operon on is lactose. So when lactose is present, the operon is on and when it is absent, the operon is turned off.
trp operon This operon responds to the cells need for the amino acid tryptophan. This operon is usually turned on as the cell needs a lot of trptophan.
trp operon What if the cell begins to stockpile this amino acid? Dont need to make tryptophan-building enzymes…therefore the operon needs to be turned off. Tryptophan binds allosterically to repressor protein
trp operon The trp operon is a repressible operon (normally on but can be turned off) This is an example of feedback inhibition.
Operon summary Repressible operon usually functions in anabolic pathways synthesizing end products when end product is present cell allocates resources to other uses Inducible operon usually functions in catabolic pathways, digesting nutrients to simpler molecules produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do
Control of Gene Expression in Eukaryotes
The BIG Questions… How are genes turned on & off in eukaryotes? How do cells with the same genes differentiate to perform completely different, specialized functions?
prokaryotes use operons to regulate gene transcription, however eukaryotes do not The controls that act on gene expression are much more complex in eukaryotes than in prokaryotes. A major difference is the presence of a nuclear membrane which prevents the simultaneous transcription and translation that occurs in prokaryotes (which is why control of genes in prokaryotes really has to be done by turning transcription on or off)
Whereas, in prokaryotes, control of transcriptional initiation is the major point of regulation, In eukaryotic cells, the ability to express biologically active proteins comes under regulation at several points.
The control points of gene expression can occur at any step in the pathway from gene to functional protein Control of Eukaryotic Gene Expression
The first point is called DNA packing
Imagine you have been given a string 3 feet long, which represents an unwound, human chromosome. The chromosome (DNA molecule) is wound around a group of 8 positively charged histone proteins (bind tightly to negatively charged DNA) forming what is called a nucleosome – the basic unit of DNA packing. Another histone links adjacent nucleosomes. These nucleosomes resemble beads on a string.
The nucleosome string is further coiled to produce a thicker structure called chromatin
Some of the chromatin is tightly packed – it contains genes that are seldom used (genes are not transcribed). This tightly packed chromatin is called heterochromatin. Genes that are transcribed (used) are more loosely packed into areas that loop out – called looped domains – and form what is known as euchromatin (true chromatin). Chromatin structure affects the availability of genes for transcription
In humans, 97% of the DNA is heterochromatin (does not encode proteins or RNA)! Some of this heterochromatin consists of repetitive sequences called satellite DNA. Satellite DNA is usually found at the centromeres and telomeres and abnormally long sequences can cause a variety of genetic diseases such as Fragile X syndrome.
Fragile X syndrome most common form of inherited mental retardation defect in X chromosome mutation of FMR1 gene causing many repeats of CGG triplet in promoter region: 200+ copies normal = 6-40 CGG repeats
The final product of DNA packing is the chromosome (shown here as a metaphase chromosome).
2. Modifying the Chromatin DNA Methylation – attaching methyl (CH 3 ) groups turns off genes. Example is Barr Body (extra X chromosome)
2. Modifying the Chromatin… Histone acetylation activates genes = on attachment of acetyl groups (–COCH3) to certain amino acids of histone proteins neutralizes their positive charges and they no longer bind to neighboring nucleosomes they change shape & grip DNA less tightly = unwinding DNA transcription proteins have easier access to genes
3. Transcriptional Control DNA must be unpacked and if gene is methylated, the methyl group must be removed. Proteins called transcription factors assist in this job.
4. mRNA Processing Introns must be cut out, exons sewed together, and a GTP cap and poly A tail added. Of course, the newly formed mRNA can also be destroyed at this point if the cell has changed its mind.
5. mRNA Leaves Nucleus A fully processed mRNA must leave the nucleus in order to be translated into protein. The large nuclear pores must be opened for its passage. Again, the cell can change its mind and not open the pores.
6. Translation Translation requires that ribosomes participate. Some mRNAs have an open later label (they are called masked mRNAs) and they are not translated right away. Examples are plant proteins needed to photosynthesis (go given in day). This step can also be blocked by regulatory proteins that prevent the attachment of ribosomes to the mRNA.
7. Posttranscription Controls Finally, the new protein must be folded to become active. The cell can withhold this folding or even degrade (breakdown) the protein at this step.
Molecular Biology of Cancer
Cancer is a Genetic Disease Normally the cell controls whether or not it divides to form more cells. Some of this control comes from proteins called growth factors. These growth factors come from other cells and stimulate a target cell to divide.
All cells contain genes called proto-oncogenes which give the internal signal to leave G1 of cell cycle and begin S. Proto-oncogenes can be defective or they can mutate. This promotes excessive cell division. Such genes are now called oncogenes The cell now divides continually
But there are still safeguards called tumor suppressor genes It is the job of these genes to produce a protein that inhibits cell division. An example of a tumor suppressor gene is p53 The loss or mutation of a tumor suppressor produces the same effect as an oncogene.
p53 gene Guardian of the Genome the anti-cancer gene after DNA damage is detected, p53 initiates: DNA repair growth arrest apoptosis – cell suicide almost all cancers have mutations in p53
Multiple mutations, however, are required for the development of cancer. This is called the multiple-hit hypothesis: several changes must occur at DNA level for cell to become fully cancerous including at least 1 active oncogene & mutation or loss of several tumor-suppressor genes Some of these mutations can be inherited, resulting in a predisposition to certain types of cancer