The first thing to understand is that DNA consists of two strands of identical molecules called nucleotides (pronounced nuh-DEE-deez). Each strand contains four bases: A, T, C and G. These are abbreviated as ATG, CTG, GCG and GTC. They form pairs called adenine, thymine and guanine. The order of these nucleotides is what stores all the information needed to make you, you. (The letter “Y” is used in place of “U”.)
The strands are held together by a type of bond called a covalent bond. This is what keeps them from falling apart.
The bases are held together by two other types of bonds: hydrogen and hydrophobic. The first bond makes the pair more stable and allows them to stay together, while the second one causes an “avoidance” reaction.
This causes the strands to repel each other, so they are not pulled together.
This causes a twisting of the strands, forming what is known as the “double-helix”. This makes the information held within very hard to read.
It requires a “key” to unlock the DNA and make it readable. This key is another nucleotide, called guanine. (It’s abbreviated as G.)
Guanine has two major traits. First, it holds the strands together very tightly.
Second, it can bond with the base thymine. (Thymine is abbreviated as T.)
These two traits are what allow the information within your DNA to be read. When a guanine comes across a thymine, it bonds with it very tightly, so that the strands cannot be pulled apart.
This makes it very easy to read the information. The cell does this by using another type of nucleotide called RNA (pronounced R-N-A).
This has the same four bases, but they are abbreviated in a different way. Instead of ATGC, it is A, U, C and G.
Instead of thymine, it uses the base uracil (U).
When RNA comes across a guanine, it changes it into a different base. It does this by using the hydrophobic trait of guanine.
It converts it into a base called inosine, but it is written as I instead of G.
This can get confusing, so lets go through an example to help.
ATG becomes AUG, and the G turns into I. This is called “translation”.
The cell reads this sequence, and creates a protein. Making proteins is what cells do, after all.
The order of these bases tells the cell how to create the protein.
In this case, the sequence AUG means to add a group of molecules called an amino acid. (There are twenty different types, and each one is marked by a three letter abbreviation.
For example, the amino acid “valine” is abbreviated as VAL.)
If this sequence was CAA, it would tell the cell to stop adding amino acids. This is called a stop codon.
(Other stop codons are TAG, TAA and TGA. Notice that these all end in TAG, TAA or TGA.)
The cell reads the sequence from 5′ to 3′. Whenever it comes across a nucleotide with the bases A, C, G and U, it reads it as the first base in the codon.
(The first letter in the codon is always the base on the 5′ side of the double helix.) Whenever it comes across a nucleotide with the bases G or C, it reads it as the third base. (This is because G and C are bi-directional bases. They can be read as the first or third base in the codon, depending on where they are on the double helix. A is always the first base in the codon.)
For example, if the sequence was GUG, it would be read as GUA because G is the third base in the codon. If it was GAC, it would be read as GAA.
(G is still the third base in the codon.)
Whenever a stop codon is reached, translation is stopped.
The cell does this via a complex called the ribosome. (Ribosomes are written as RNS.
They are also called rRNA, but this can be hard to abbreviate, so ribosome is used instead.)
The ribosome reads the message from 5′ to 3′. It reads the codons in groups of three, and adds an amino acid if the next base in the codon is correct.
(A, C, G or U. Never a G or a C. This is where the bi-directionality of G and C comes into play. They are only read as the third base in the codon if they are on the 5′ side of the codon. If they are on the 3′ side, they are only read as the first base in the codon.)
Whenever a stop codon is reached, the ribosome pauses and the protein is released. This is called the “termination phase”.
The cell can only read the message in place if there is a start codon.
Sources & references used in this article:
- Crystal structure of parallel quadruplexes from human telomeric DNA (GN Parkinson, MPH Lee, S Neidle – Nature, 2002 – nature.com)
- Uracil-DNA glycosylase (UNG)-deficient mice reveal a primary role of the enzyme during DNA replication (H Nilsen, I Rosewell, P Robins, CF Skjelbred… – Molecular cell, 2000 – Elsevier)
- DNA markers reveal the complexity of livestock domestication (MW Bruford, DG Bradley, G Luikart – Nature Reviews Genetics, 2003 – nature.com)
- Routines and other recurring action patterns of organizations: contemporary research issues (MD Cohen, R Burkhart, G Dosi, M Egidi… – Industrial and …, 1996 – academic.oup.com)
- Epigenetics as a unifying principle in the aetiology of complex traits and diseases (A Petronis – Nature, 2010 – nature.com)