Hello Rakeshb,

Scientists have hypothesized that RNA played a key role in the origins of life. In 1986, Walter Gilbert hypothesized that there was once an “RNA world.” During this hypothetical period, RNA served as the genetic material and it also performed all the enzymatic activities necessary to replicate itself in early life forms. The RNA world hypothesis is based on the observation that RNA is able to perform two key tasks essential for life: 1) it can store genetic information, and 2) it can catalyze chemical reactions. As part of this view, as life evolved further, DNA became the genetic storage material in cells in place of RNA, and proteins replaced RNA as the main catalysts of the chemical reactions in cells. According to this hypothesis, cells still carry remnants of the RNA world. For example, the ribosome is an intermediate structure made up of both RNA and proteins, in which RNA catalyzes the synthesis of proteins.

So, why did DNA evolve to be the primary genetic material over RNA? According to the RNA world hypothesis, DNA evolved to store the genetic information instead of RNA because DNA is chemically more stable than RNA. Moreover, since DNA has a double-stranded structure, it has the ability to correct errors in replication. Scientists are currently researching RNA molecules to better understand their many functions and to support or refute the hypothesis that an RNA world once existed. Intriguingly, some scientists believe that a pre-RNA world existed before the RNA world and that even more simple molecules existed before RNA. Here are a few links to learn more about the RNA world and the pre-RNA world hypotheses. Happy reading!

To learn more about the “RNA world” and the various functions of RNA, check out the following links:
http://www.nature.com/scitable/content/the-rna-world-10239
http://www.nature.com/scitable/topicpage/rna-functions-352
http://www.ncbi.nlm.nih.gov/books/NBK26876/
http://news.sciencemag.org/sciencenow/2013/02/self-assembling-molecules-offer-.html
http://molbio.mgh.harvard.edu/szostakweb/publications/Szostak_pdfs/Trevino_et_al_2011_PNAS.pdf

Dear Abhijay,

This is an excellent question, and one that has taken researchers decades to answer. In short, our cells contain a number of different types of proteins that all work together to detect and fix all sorts of mutations, including those that accumulate through the normal process of cell division. These types of errors are called replication errors, and they occur when the enzyme that makes new DNA — DNA polymerase — makes a mistake and doesn’t catch it. Replication errors come in three forms: 1) mismatches, 2) improper nucleotide insertions, and 3) improper nucleotide deletions. Mismatches occur when DNA polymerase inserts the wrong nucleotide, and the DNA has to bend or become distorted to accommodate the resulting non-natural base pairing. Improper nucleotide insertions or deletions can occur when DNA polymerase incorrectly skips a base or inserts an extra copy of a base. When this happens, the DNA structure forms a sort of bubble, where the two ends are tethered together by correctly grouped nucleotides and the bubble is centered over the error. In the case of a nucleotide insertion, the bubble forms in the newly replicated DNA. In the case of a nucleotide deletion, the bubble forms in the old, parental DNA strand.

Fortunately, our cells possess back-up repair processes that step in when the DNA polymerase has made a mistake and moved on. As a whole, proteins called “DNA repair proteins” facilitate these processes, and they can be grouped according to the type of mutation they fix and how they operate. For instance, mismatch repair proteins are so named because they recognize and repair mismatched DNA. These DNA repair mechanisms are continuously working to find and correct new DNA mutations. It is for this reason that the estimated thousands of mutations that arise each time our cells divide do not affect us. There are in fact thousands of articles (and even an entire scientific journal, called DNA Repair) describing the DNA repair proteins and how they interact in all sorts of situations. We hope the information we’ve provided has helped answer your question and fueled your curiosity about how amazing our cells are!

Check out these links to some of our favorite articles about the DNA repair processes we’ve discussed:
http://www.nature.com/scitable/content/regulation-of-dna-repair-throughout-the-cell-16616
http://www.jbc.org/content/279/17/16895.full
http://www.nature.com/scitable/topicpage/dna-replication-and-causes-of-mutation-409

Hello Mandy,

A Punnett square is a grid that is used to view the potential combinations of parental alleles in the offspring of a genetic cross. You’re in luck, because there’s a concept page here at Scitable with a step-by-step explanation of how to draw and read Punnett squares for monohybrid and dihybrid crosses. This page also includes definitions of the terms you’ve listed above.

Good luck with your Human Biology studies!

Hello Krishna,

Your question about how pressure affects plants is very interesting. Even though it may sound like science fiction, there is interest about what happens to plants in lower atmospheric pressure because scientists wonder whether plants could grow well in a spacecraft or even on other planets, such as Mars!

It turns out that even though there are changes in gene expression for plants under low atmospheric pressure, plants do surprisingly well under these conditions. If oxygen levels are kept constant, then mung bean and lettuce seedlings grow just as well and may even get slightly larger than normal under low atmospheric pressure conditions! It will be interesting to see how space programs use this information in the future.

If you’d like to learn more the effect(s) of low atmospheric pressure on plants, check out the following links:
http://www.plantphysiol.org/content/134/1/215.full#ref-25
http://www.ncbi.nlm.nih.gov/pubmed/12583399
http://www.plantphysiol.org/content/86/1/19.full.pdf+html
http://www.ncbi.nlm.nih.gov/pubmed/17155885