Unit 16: Gene Regulation, Chromatin, and Epigenetics
How a cell decides which genes to switch on, from a bacterium sensing one sugar to the chromatin marks that lock in a whole expression pattern across divisions.
6 topics · 23 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The Lac Operon and Prokaryotic Gene Regulation
- How it worksHow does a repressor protein block transcription of the lac operon until lactose is around to remove it?
- How it worksHow does the presence of lactose flip the repressor off and let RNA polymerase transcribe the operon's genes?
- Common mistakeWhy isn't the lac operon transcribed as three separate genes with three separate promoters?
- Going furtherWhat happens to lac operon expression when glucose is also present alongside lactose?
Your turn1 of 2
The lac operon contains three genes needed for lactose metabolism, and all three are switched on or off together as a single unit in response to lactose availability. A student assumes each of these three genes must have its own separate promoter that gets regulated independently. Why isn't the lac operon transcribed as three separate genes with three separate promoters?Eukaryotic Transcription Factors and Enhancers
- How it worksHow does a transcription factor bind a specific DNA sequence and recruit RNA polymerase to a gene's promoter?
- How it worksHow does an enhancer loop DNA around to bring distant regulatory proteins into contact with a promoter?
- Common mistakeWhy isn't an enhancer required to sit right next to the gene it controls?
- Going furtherWhat happens to a gene's expression pattern when its enhancer is deleted but its promoter is left intact?
Your turn1 of 2
An enhancer sequence controlling a particular gene is found located tens of thousands of base pairs away from that gene on the chromosome. A student assumes a regulatory sequence must sit immediately next to the gene it controls in order to have any effect on it. Why isn't an enhancer required to sit right next to the gene it controls?Chromatin Structure and the Nucleosome
- How it worksHow does DNA wrap around a histone octamer to form a nucleosome, the basic unit of chromatin?
- Common mistakeWhy isn't DNA packaging into chromatin just a way to save space, with no effect on gene expression?
- Going furtherWhat happens to a gene's accessibility to transcription factors when its chromatin condenses into heterochromatin?
Your turn1 of 2
DNA is wound around histone proteins into nucleosomes, letting an enormous length of DNA fit inside a tiny nucleus. A student assumes this packaging exists purely to save physical space and has no further effect on which genes get expressed. Why isn't DNA packaging into chromatin just a way to save space, with no effect on gene expression?Histone Modification and the Histone Code
- How it worksHow does acetylation of histone tails loosen chromatin and make a gene more accessible to transcription?
- How it worksHow does methylation of a different histone residue instead help compact chromatin and silence a gene?
- Common mistakeWhy isn't the histone code a change to the underlying DNA sequence itself?
- Ties togetherHow does the histone code work alongside DNA methylation to lock in a cell's gene expression pattern across many divisions?
Your turn1 of 2
Two cells with identical DNA sequences carry very different patterns of histone acetylation and methylation, and these two cells go on to express very different sets of genes for the rest of their lives. A student assumes these histone modifications must actually be changes to the underlying DNA sequence itself. Why isn't the histone code a change to the underlying DNA sequence itself?DNA Methylation and Gene Silencing
- How it worksHow does DNA methyltransferase add a methyl group onto cytosine bases in a gene's promoter region?
- How it worksHow does that methylation recruit proteins that compact chromatin and block transcription factor binding?
- Common mistakeWhy isn't DNA methylation a mutation, even though it changes how a gene behaves?
- Worked exampleFind the percentage of CpG sites methylated in a region where 45 of 60 tested CpG dinucleotides show methylation.
Your turn1 of 2
Find the percentage of CpG sites methylated in a region where 40 of 81 tested CpG dinucleotides show methylation.X-Chromosome Inactivation and Genomic Imprinting
- How it worksHow does the Xist RNA coat one X chromosome and trigger it to condense into inactive heterochromatin?
- How it worksHow does genomic imprinting instead silence a gene based on which parent it was inherited from, rather than by random chance?
- Common mistakeWhy isn't X-inactivation the same mechanism as genomic imprinting, even though both silence one copy of a gene?
- Going furtherWhat happens to a calico cat's fur pattern because of which X chromosome gets inactivated in each patch of skin cells?
Your turn1 of 2
Both X-inactivation and genomic imprinting result in one copy of a gene (or an entire chromosome) being silenced while the other copy remains active. A student assumes the two processes must therefore be the same underlying mechanism. Why isn't X-inactivation the same mechanism as genomic imprinting, even though both silence one copy of a gene?