Chemistry of Life » Nucleic Acids, RNA and DNA


There are two types of nucleic acids found in all living organisms. They are DNA and RNA. Every moment of the life of the cell is governed by the action of enzymes. Enzymes are proteins and the exact type of protein formed depends on the coded molecular message carried by the DNA. In higher organisms the DNA is totally protected in the nucleus of the cell which it never leaves. It is here that messenger-RNA molecules are synthesised. The best analogue is that the DNA molecule can be thought of as a negative photograph which creates a positive photograph namely the messenger-RNA molecule. Unlike DNA, the messenger RNA molecule is able to cross the membrane. It migrates to a RIBOSOME where the appropriate protein is synthesised. The word ribosome is the name given to a cellular ‘factory’ where proteins are made. At the ribosomes small amino acid molecules are fused together to an exactly genetically determined recipe to produce specific proteins. A randomly produced protein would be of no biological use whatsoever.

Both nucleic acids have very similar molecular achitectures. They are very large molecules which are made up of three types of subunits ribose (or deoxyribose), a phosphate group and one of four types of a nitrogen base. The ribose (or deoxyribose) units alternate in a long line with the phosphate groups. This may be hundreds of sub-units long. To each ribose or deoxyribose a subgroup is attached. In DNA the four subgroups are Adenine, Guanine, Cytosine and Thymine. The code is usually expressed as A,G, C and T. In RNA these four subgroups are Adenine, Guanine, Cytosine and Uracil.

Nitrogenous Base

Nit. Base

Short Abbreviation

Short Abb.

Found in

Adenine

A

Both

Cytosine

C

Both

Guanine

G

Both

Thymine

T

DNA

Uracil

U

RNA

The molecules are linear but take the form of a helix. Most RNA molecules form a single helix. DNA molecules form double helices joined jig-saw like by the bases. This explains the title of the book 'The Double Helix' by James Watson who, togethter with Francis Crick, Maurice Wilkins and Rosalind Franklin discovered the structure of DNA in 1953.

DNA is used to pass the genetic message on to future generations of living cells. Because DNA is an extremely stable molecule, the same messages can be passed from one generation to the next. 'Mistakes' (mutations) are rare but very occasionally a base change or a deletion can occur which leads to a mutation.
Besides its role in passing on the genetic message from one generation to the next, DNA also plays its part in the moment-to-moment work of all living cells. To ensure that the correct proteins are produced, molecules of messenger-RNA are synthesized in the cell nucleus on the surface of the DNA.
Messenger RNA molecules are single stranded. In both forms of nucleic acid the main spine of the molecule consists of a repeating sugar unit (Ribose or deoxyribose) joined by a phosphate group. Each sugar unit is connected to a nitrogen base. There are four mononucleotide nitrogen bases and it is these bases that carry the genetic code. The table below shows the full genetic code.

T-T

T-C

T-A

T-G

TTT Phe
TTC Phe
TTA Leu
TTG Leu

TCT Ser
TCC Ser
TCA Ser
TCG Ser

TAT Tyr
TAC Tyr
TAA Term
TAG Term

TGT Cys
TGC Cys
TGA Term
TGG Try

C-T

C-C

C-A

C-G

CTT Leu
CTC Leu
CTA Leu
CTG Leu

CCT Pro
CCC Pro
CCA Pro
CCG Pro

CAT His
CAC His
CAA Gln
CAG Gln

CGT Arg
CGC Arg
CGA Arg
CGG Arg

A-T

A-C

A-A

A-G

ATT Ile
ATC Ile
ATA Ile
ATG Met

ACT Thr
ACC Thr
ACA Thr
ACG Thr

AAT Asn
AAC Asn
AAA Lys
AAG Lys

AGT Ser
AGC Ser
AGA Arg
AGG Arg

G-T

G-C

G-A

G-G

GTT Val
GTC Val
GTA Val
GTG Val

GCT Ala
GCC Ala
GCA Ala
GCG Ala

GAT Asp
GAC Asp
GAA Glu
GAG Glu

GGT Gly
GGC Gly
GGA Gly
GGG Gly

The complex chemistry of RNA and DNA need not concern us in our overall understanding of genetics but it is worth noting that the amazing complexity of these compounds arises quite simply from the arrangement of only 5 types of atom namely carbon, oxygen, nitrogen, hydrogen and phophorus. The spine of RNA and DNA consist of ribose or deoxyribose alternating with phosphate groups contain just four types of atom carbon, oxygen, hydrogen and phosphorus. Adenine contains carbon, nitrogen and hydrogen atoms and the other three contain carbon, nitrogen, hydrogen and oxygen.

The genetic code consists of 64 'words'. Each 'word' is three letters long. Each letter is in reality a chemical subunit. Each letter is either thymine, cytosine, adenine or guanine. Thus there are three of these letters (chemical units) in each word. There are 64 ways of arranging four objects three at a time. Therefore the genetic code consists of 64 words. Three of these words TAA,TAG and TGA mean end of message (this may be the coding for a protein for example). One word ATG can either stand for beginning of a message of for the amino acid methionine. The other 60 words stand for amino acids. The technical term for a 'word' in the genetic code is a CODON. When the message is 'photocopied' in the cell from DNA to messenger RNA the new word is called an ANTICODON'. It has the same genetic 'meaning' as the CODON (just as Sie (German) and You (English), both mean the same thing). The message is then passed on to programme for a protein.


Somewhere
by
Ray Goodwin


Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know

Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see

Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands

Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are

One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face


The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.

Readers of this web site are invited to e-mail the author ( ray@lifeinthecosmos.com) and discuss their opinions of the topics dealt with and suggest any changes which they think may be helpful.

Life in the Cosmos Website
Version 01.00 - April 20, 2015.