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USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A researcher conducts an experiment that studies epigenetic modifications in human cells. During this study, cells are cultured in vitro, and an epigenetic modification is made to Gene A. This change results in decreased production of the protein encoded by Gene A. Which of the following epigenetic modifications most likely took place?  

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Epigenetics is a process of gene regulation - turning genes on and off.

Think about it - you have about 37 trillion cells, and over 200 different types of cells in your body.

For example, there are muscle cells, for looking great at the beach as well as neurons that tell your muscles to flex when it’s time to show off.

And both muscle cells and neurons have the same origin and genetic material - meaning, 46 chromosomes, with each chromosome made up of a single DNA molecule.

Along that chromosome are sequences of DNA that code for genes, with thousands of genes on each one.

It makes sense that there would have to be a process to control all of those genes.

Now, it turns out, that DNA is a very long molecule - over 2 meters when fully stretched.

So to save space, DNA is wrapped around special proteins called histones.

Now - histones actually come in groups of 8 - 4 stacks of 2, like poker chips - and the DNA molecule wraps around each group of 8 histones twice, forming a nucleosome.

Different sections of DNA - meaning, different genes - wrap around different stacks of histones.

Finally, the nucleosomes are packed together even more tightly - resulting in chromatin which looks like threads of cotton-candy within the nucleus.

Now - a cell type boils down to what a cell does - and, in turn, what a cell type does boils down to the kind of proteins it makes to carry out its role.

Proteins are made based on genes - so our collection of genes, or genotype is actually like an incredible wardrobe - it contains something for every occasion.

And different cell types wear different attires.

For example, our muscle cells are usually doing the hard work of contracting and relaxing all day, so they would require the equivalent of athletic gear to do their job.

Posh neurons, on the other hand, might prefer a tuxedo to tend to their synapses in.

So, the muscle cell needs only certain parts of that wardrobe and the neuron needs a very different part of that wardrobe.

This is achieved through selectively activating or silencing certain genes.

The final appearance of how a cell looks depends on which genes are activated - and we call that the phenotype.

All of this happens through epigenetics - which specifically refers to mechanisms that can selectively activate or silence certain genes without modifying the nucleotide sequence of the gene.

Let’s start with histones. Histones can be influenced to either release their DNA or lock down their DNA, through chemical changes, like acetylation or methylation.


Epigenetics is the study of how environmental and lifestyle factors can change the way our genes are expressed without actually changing the DNA sequence. These epigenetic changes can be passed down from one generation to the next, which means that they can influence our health even if we don't have any direct descendants.

There are a number of different epigenetic mechanisms, but some of the most common ones include DNA methylation, histone modification, and microRNA expression. Each of these mechanisms can either promote or suppress gene expression, and they can be affected by things like diet, stress, exposure to toxins, and social interactions.


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