Study Guide

Eukaryotes Themes

  • Regulation

    Thirty percent of the transcription happening in one of your cells (right now) produces mRNAs, or messenger RNAs, that code for proteins. The other seventy percent? Those are called noncoding RNAs, or ncRNA, because they don't get turned into protein.

    That's a big chunk of RNAs.

    These RNAs include transcripts like tRNAs and rRNAs. However, it also includes RNAs that have crazy regulatory roles in your cell, contributing just as much function as proteins.

    Wanna hear about one?

    Some of your cells produce a really long ncRNA that is full of bologna. For that reason, it's been named HOTAIR.

    Kidding. HOTAIR is short for HOX Antisense Intergenic RNA. It's a piece of RNA that is transcribed from a HOX gene cluster, which are genes involved in development. It's transcribed in the opposite direction as the HOX genes, which is why it's called "antisense." The word intergenic refers to the fact that it's between genes.

    Normally, transcription factor proteins regulate the activity of RNA Polymerase. These proteins bind the DNA or each other, resulting in the increase or decrease of gene expression. As an RNA, HOTAIR is able to bind to some transcription proteins and regulate their function, just like a protein would. For a long time, scientists didn't know that RNA had those kinds of superpowers.

    It's like the RNA is saying to some proteins, "Come on down! The DNA is fine." The recruited protein transcription factors then take the "okay" signal, arriving at a specific chromosome location to modify its chromatin, or combo of nucleosomes and DNA. Without HOTAIR, some genes would be turned on, or expressed, when they should be silenced. Shhhh.

    HOTAIR was the first ncRNA found to affect DNA on a one chromosome while being transcribed from a different chromosome.


    This looks like a pair of eyeglasses, but it's actually a secondary structure of the HOTAIR ncRNA, which can basepair with itself to create a non-linear shape. The colors represent the sequence conservation among HOTAIR RNA. The more red, or conserved a nucleotide, the more important it might be to its function and interaction with other proteins or nucleic acids.

    HOTAIR is so important that it's no surprise its misregulation has been associated with cancer. In fact, scientists are beginning to understand how they can use the expression of HOTAIR and other ncRNAs to characterize cancer prognosis and possible treatments. For example, HOTAIR is transcribed at REALLY high levels in breast cancers that are advanced, compared to breast cancers in earlier stages.

    Pretty handy for a little RNA, right? Although HOTAIR functions to repress some specific genes, it's just one of many ncRNAs that can regulate genes and proteins in all different ways. New ones are being uncovered all the time, quickly becoming crowned as a previously underappreciated yet super important way to regulate biological function.

  • Structure and Function

    As humans, we inherit nuclear DNA from mom and pop. Only mama can give us her mitochondria and mitochondrial DNA.


    You can thank your mama for your mitochondria. Or not. The circular DNA reminiscent of bacterial DNA resides within the matrix of each one.

    What you should also know about mitochondria is that they are the agents behind The Mother's Curse.

    (Cue scary music and Edward Scissorhand-Zombies.)

    How can an innocent little structure, necessary to break down the delicious chocolate chip cookies from your loving mother, also give you the goose bumps when paired with a screechy organ tune?

    Each mitochondrion contains its own DNA from its pre-endosymbiotic days. That DNA helps the mitochondria perform all its important cellular respiration functions by coding for proteins specific for that process. It also turns out that the mitochondrial DNA also plays a major role in male aging and fertility, with little to no consequence in females. Scientists have found that certain mitochondrial mutations are associated with a shorter life span in males or decreased ability to reproduce in flies, yet the same exact mutations in females don't have the same effects. This may explain why males have shorter lifespans than females.

    That means that if the mitochondrial DNA from your mom contains "bad" mutations, they'll likely only affect you if you're male. Hence, the Mother's Curse really only applies to half of us. The male half. Aw, but don't worry guys. Mom didn't do it on purpose.

    It's also likely that these little structures are therefore behind the shorter life expectancy of males. Scientists don't exactly know how, but learning more about the mitochondrial genome, and the importance of each of its genes, will help us figure all this out. It won't stop moms all over the world from passing them down to their children anymore than it would stop yours from trying to feed you Brussels sprouts when you were three, but knowledge is power and Brussels sprouts aren't really all that bad.