2 Every cell has at least one chromosome consisting of a DNA molecule 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.
3 expression of genes as needed Prokaryotesexpression of genes as needed- dictated by environment
5 Prokaryotes control gene expression through operons An operon is a group of genes that contains all of the protein recipes needed for one particular functionEx: There are 5 enzymes “recipes” needed for the production of the amino acid tryptophan
6 with related functions ex. enzymes in a biosynthesis pathway Bacteria often group together geneswith related functions ex. enzymes in a biosynthesis pathway Transcription of these genes iscontrolled by a single promoter when transcribed, read as 1 unit & asingle mRNA is madeOperon operator, promoter & genes they control
7 Overview of an Operon Regulator Gene (upstream of rest of operon; PromoterOperator GeneStructural GenesRegulator GeneRegulator 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)
8 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”
9 Lactose operon What happens when lactose is present? Need to make lactose-digesting enzymesWhat happens when lactose is absent?No need to make lactose-digesting enzymes… waste of cell’s energy & resources
10 Lactose operonWhen 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”.
11 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.
12 an example of an inducible operon. lac operonThe lac operon isan 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”.
13 This operon responds to the cell’s need for the amino acid tryptophan. trp operonThis operon responds to the cell’s need for the amino acid tryptophan.This operon is usually turned “on” as the cell needs a lot of trptophan.
14 trp operon What if the cell begins to stockpile this amino acid? Don’t need to make tryptophan-building enzymes…therefore the operon needs to be turned off.Tryptophan binds allosterically to repressor protein
15 This is an example of “feedback inhibition”. trp operonThe trp operon is a repressible operon (normally “on” but can be turned “off”)This is an example of “feedback inhibition”.
16 Operon summary Repressible operon usually functions in anabolic pathways synthesizing end products when end product is present cell allocatesresources to other usesInducible operon usually functions in catabolic pathways, digesting nutrients to simpler molecules produce enzymes only when nutrient isavailable cell avoids making proteins that have nothing to do
18 How are genes turned on & off in eukaryotes? The BIG Questions…How are genes turned on & off ineukaryotes?How do cells with the same genesdifferentiate to perform completelydifferent, specialized functions?
19 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)
20 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.
21 Control of Eukaryotic Gene Expression The control points of gene expression can occur at any step in the pathway from gene tofunctional protein
23 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.
24 The “nucleosome” string is further coiled to produce a thicker structure called chromatin
25 availability of genes for transcription Chromatin structure affects theavailability of genes for transcriptionSome 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).
26 Some of this heterochromatin consists of repetitive sequences called In humans, 97% of the DNA is heterochromatin (does not encode proteins or RNA)!Some of this heterochromatin consists of repetitive sequences calledsatellite DNA.Satellite DNA is usually found at the centromeres and telomeres and abnormally long sequences can cause a variety of genetic diseases such asFragile X syndrome.
27 Fragile X syndrome normal = 6-40 CGG repeats most common form of inherited mental retardationdefect in X chromosomemutation of FMR1 gene causing manyrepeats of CGG triplet in promoter region:200+ copiesnormal = 6-40 CGG repeats
28 The final product of DNA packing is the chromosome (shown here as ametaphasechromosome).
29 2. Modifying the Chromatin DNA Methylation – attaching methyl(CH3) groups turns “off” genes.Example is Barr Body (extra X chromosome)
30 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 nucleosomesthey change shape & grip DNA less tightly = unwinding DNAtranscription proteins have easier access to genes
31 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.
32 4. mRNA ProcessingIntrons 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.
33 5. mRNA Leaves NucleusA 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.
34 6. TranslationTranslation 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.
35 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.
37 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.
38 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 calledoncogenesThe cell now divides continually
39 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 p53The loss or mutation of a tumor suppressor produces the same effect as an oncogene.
40 “Guardian of the Genome” apoptosis – cell suicide p53 gene“Guardian of the Genome”the “anti-cancer gene”after DNA damage is detected, p53 initiates:DNA repairgrowth arrestapoptosis – cell suicidealmost all cancers have mutations in p53
41 “multiple-hit” hypothesis: 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 cancerousincluding at least 1 active oncogene & mutation or loss of several tumor-suppressor genesSome of these mutations can be inherited, resulting in a predisposition to certain types of cancer