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Why does a snake have 25 or more rows of ribs, whereas a mouse has only 13? The answer, according to a new study, may lie in "junk DNA," large chunks of an animal's genome that were once thought to be useless. The findings could help explain how dramatic changes in body shape have occurred over evolutionary history. Scientists began discovering junk DNA sequences in the 1960s. These stretches of the genome-also known as noncoding DNA-contain the same genetic alphabet found in genes, but they don't code for the proteins that make us who we are. As a result, many researchers long believed this mysterious genetic material was simply DNA debris accumulated over the course of evolution. But over the past couple decades, geneticists have discovered that this so-called junk is anything but. It has important functions, such as switching genes on and off and setting the timing for changes in gene activity. 

Early-response genes, which are important for synaptic plasticity, are "switched off" under basal conditions by topological constraints. Neuronal activity triggers DNA breaks in a subset of early-response genes, which overrides these topological constraints, and "switches on" gene expression.

Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy.

Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy.

Quick look: In its modern sense, epigenetics is the term used to describe inheritance by mechanisms other than through the DNA sequence of genes. It can apply to characteristics passed from a cell to its daughter cells in cell division and to traits of a whole organism. It works through chemical tags added to chromosomes that in effect switch genes on or off.

Sometimes, a genetic tweak can make a really big difference in an animal's appearance. That's what likely happened when the predecessors of modern snakes lost their legs, a process that started some 150 million years ago, two separate groups of scientists have discovered. Although the teams took very different approaches to solve the mystery of how those limbs vanished, both came up with similar results: Mutations in DNA located near a gene key to limb formation keep that gene from ever turning on, they report today.

video 2:47 Gene 'switches' are what really make humans tick

This case study focuses on the relationship between evolution and plasticity using a hands-on, inquiry-based approach. Students view examples from the literature that illustrate the difference between nature and nurture, or the relative contributions of genes and the environment in shaping phenotypes. Using the Trinidadian guppy system as an example, students learn about seminal work in the field in addition to exploring quantitative genetic techniques used to partition phenotypic variance between genes (G) and the environment (E). They use real data from one of the publications cited in the case to graph reaction norms illustrating GxE interactions at the family and population level. The inquiry-based approach means that students are introduced to new concepts in a stepwise fashion, and asked to develop and build their understanding using causal, explanatory evidence. The case concludes with an exercise in which students apply their knowledge to a real conservation problem in Trinidad and Tobago, where guppies are native. This case would be appropriate for an upper level biology, genetics, or evolution course.

A Cornell study offers further proof that the divergence of humans from chimpanzees some 4 million to 6 million years ago was profoundly influenced by mutations to DNA sequences that play roles in turning genes on and off.

An environmentally dependent method to excise particular genes and eliminate genetically modified organisms (GMOs) if they leave the lab, published this week (May 19) in Nature Communications, uses an inducible CRISPR/Cas9 genome-editing system to snip out vital pieces of the E. coli genome.

Almost every cell in your body has the same DNA sequence. So how come a heart cell is different from a brain cell? Cells use their DNA code in different ways, depending on their jobs. Just like orchestras can perform one piece of music in many different ways. A cell's combined set of changes in gene expression is called its epigenome. This week Nature publishes a slew of new data on the epigenomic landscape in lots of different cells. Learn how epigenomics works in this video. Read the latest research on epigenetics at http://www.nature.com/epigenomeroadmap

When you look at yourself in the mirror you may ask, 'How, given that all the cells in my body carry the same DNA, can my organs look so unlike and function so differently?' With the recent progress in epigenetics, we are beginning to understand. We now know that cells use their genetic material in different ways: genes are switched on and off, resulting in the astonishing level of differentiation within our bodies.