A Bloke down the pub told me...
What is DNA? The Secret of Life
What is DNA? It has been called the molecule of life. But it was not always so. Careful work by Gregor Mendel helped figure out that traits could be passed on from generation to generation, but he had no idea what these 'particles of heredity' were. When Crick and Watson first elucidated the structure of DNA in 1953 they walked into their local pub (the Eagle in Cambridge...in case you were wondering) and announced:
“We have found the secret of life”
But this brings us back to the central question: What IS DNA? Short answer:
DNA is a double-stranded polynucleotide that carries the genetic code. It is the unit of heredity. It is formed by hydrogen bonding between complementary base pairs on two antiparallel DNA strands which allow it to unzip easily. Twisting of the molecule results in a twisted double helix shape. It replicates semi-conservatively.
Everything you wanted to know about DNA in a single paragraph. But, as always with Biology, the beauty is in the detail - so please...read on!
Like proteins and carbohydrates, DNA is a polymer. Polymers are long molecules made up of similar subunits (monomers) chemically bonded together. But of all nature’s polymers, DNA is unique – it is the only molecule able to direct their own replication from monomers. Without this ability, life as we know it would never have existed . Without this ability, DNA could not be reproduced in every living cell. Without this ability, DNA could not be passed on to the next generation.
(NB: Whilst the Nobel Prize went to Crick and Watson (and Wilkins) for the discovery of DNA, the road to finding this molecule of life was over 150 years long, and was walked by many prominent minds. Each successive discovery - some small, others profound - brought us one step closer to DNA. See this link on the history of DNA for details )
Nucleotides for Nucleic Acids
What are Nucleotides? Simple - the monomer of all nucleic acids. Each one is made of three subunits:
- Phosphate group (forms the backbone, as we shall see later)
- Sugar molecule (ribose for RNA, deoxyribose for DNA)
- An Organic Nitrogenous base (called 'nitrogenous' because they contain nitrogen)
The three subunits undergo condensation reactions to form a single nucleotide - for each nucleotide made, two water molecules are produced. In any set of given nucleotides:
- The phosphate groups are identical
- DNA always contains deoxyribose (that's why it is called deoxyribo nucleic acid); RNA always contains ribose)
- There are only 5 possible bases: Adenine, Guanine, Cytosine, and Thymine in DNA; Adenine, Guanine, Cytosine and URACIL in RNA. A and G are larger bases, known as Purines; T/U and C are smaller and are known as Pyrimidines
For each question, choose the best answer. The answer key is below.
- True or False: Thymine, Uracil and Cytosine are the Purines
- Which of the following is NOT one of the three subunits of a Nucleotide?
- Phosphate Group
- Pentose Sugar
- Fatty Acid
- Organic, Nitrogenous base
- Which of these Nirogenous bases would you NOT expect to find in DNA?
- How many different nucleotides are possible in a molecule of DNA?
- How many different nucleotides are possible in a molecule of RNA?
- How many different types of Nirogenous base are there in Nucleic acids?
- Which base bonds with Cytosine
- Fatty Acid
Interpreting Your Score
If you got between 0 and 2 correct answers: Go back and Try again
If you got between 3 and 4 correct answers: Halfway - re-read some of the trickier facts and then go for 100%
If you got 5 correct answers: Good Job - Read each question carefully
If you got 6 correct answers: Genius!
If you got 7 correct answers: Genius!
How Do Nucleotides join together?
As previously stated, DNA is a polymer made up of monomers called nucleotides. These are joined together in a condensation reaction - so called because water is released as a by-product of the reaction. This occurs between phosphate group of one nucleotide and the pentose sugar of the next. This results in a repeating sugar-phosphate backbone. The bases project from this backbone into the centre of the molecule.
These chains of nucleotides bonded together are called nucleic acids. Nucleic acids will only bond to nucleotides containing the the same pentose sugar as they do . This prevents RNA nucleotides from sneaking into a DNA sequence, and vice versa.
Complementary Base Pairing
- Why are the strands on DNA called Anti-parallel?
The two strands of a DNA molecule are parallel because the space between them is taken up with nitrogenous bases. The strands are antiparallel because the strands run in opposite directions. Each pairing consists of a purine and a pyrimidine. This makes each pairing equal in width.
- What is complementary base pairing?
In DNA A only bonds with T (In RNA, A is complementary to U); C bonds with G. Think of DNA like a jigsaw puzzle: A and T fit together - you cannot force G or C to bond with A.These base-pairing rules occur because of the different structures of pyrimidines and purines. Erwin Chargaff discovered that in ANY sample of DNA, the amount of A = the amount of T; and the amount of G = the amount of C (Chargaff's Rule ).
- Why is DNA a double helix?
The shape of DNA is classic - two twisted ladders, winding round each other. The helix is (usually) right-handed.The helix also has a 'wide-groove' and a 'narrow-groove' The overall structure of the molecule is governed by molecular interactions such as van der Waals forces and Hydrogen Bonds. See the links for an animation
How Does DNA Store Information?
The same way as we store information - with an alphabet. Instead of 26 letters, DNA has 4. Instead of different length words, all words in DNA-ese are three letters long. The detail is in the length of the sentence
- There are four possible bases in DNA
- Each length of 3 bases codes for a single amino acid
- 4x4x4=64 combinations (much more than the 20amino acids in nature)
- Different strings of amino acids create different proteins.
- Proteins control, signal or provide structure to virtually all aspects of the body
- Still more variations can be made in the ER with post-translational modifications and differential splicing.
Put another way, it is the sequence of nitrogenous bases in the DNA that code for molecules that control all aspects of our existence.
Purine or Pyrimidine?
Unlocking the Secret of Life
Where Next? DNA
- Tour of the Basics
A confusion-free tour of the basics of Genetics. Well worth a look. Published by the University of Utah - recommended
- Why is DNA a helix?
A Java animation explaining that, far from miraculous or designed, DNA is just one of many macromolecules that is helical. An excellent resource
- Chargaff's Rule
A site that explains Chargaff's Rule in 12 seconds. Guaranteed. Published by Nature
- The Discovery of the Molecular Structure of DNA - The Double Helix
The details on the Nobel Prize awarded for the Discovery of DNA as the unit of heredity, and the structure thereof.
pinkhawk from Pearl of the Orient on December 03, 2012:
I have to absorb and digest these things! Thank you for sharing this informative and useful hub! ^_^
Dahlia Flower from Canada on February 09, 2012:
Purine problems somehow cause gout, from what I've been told. I've got to read this over and over and read more of your information. It's fascinating, but very challenging for someone who hasn't taken a biology class in decades. Thank you for SHARING.
STAR PUPIL on January 30, 2012:
haha hi sir its kenny this is helping me revise... Thanks
Rhys Baker (author) from Peterborough, UK on January 12, 2012:
You are very welcome - I hope this wasn't too complicated!
TroyM on January 12, 2012:
Thanks for informative article on DNA. Thanks!
Brandon Lobo on January 11, 2012:
Rhys Baker (author) from Peterborough, UK on January 11, 2012:
Thats not bad at all for a speed quiz :) Thanks for the feedback - very kind!
Brandon Lobo on January 11, 2012:
Hi I just got a 71% on your quiz. Would have got one more right if I wasn't in a hurry :)
You said to give you feedback so here it is - It's great
Rhys Baker (author) from Peterborough, UK on January 10, 2012:
Well said! Whilst the study of the effects of DNA is known as genetics, the study of DNA the molecule and how it works and replicates fits into biophysics quite nicely - as the BPS website says "All of Biology is fair game"
Deena from India on January 10, 2012:
X-ray diffraction(Biophysics)picture of DNA by Maurice Wilkins & Roslind Franklin provided the clue for explaining the structure of DNA by Watson & Crick.
Deena from India on January 10, 2012:
DNA is a genetic material, how it is related to biophysics?