A Level Biology: The Structure and Function of Nucleic Acids

Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are perhaps the most important of all biomolecules. 




Well, these nucleic acids contains the instructions that make every single living organism on the planet. 


DNA and RNA are polymers, composed of monomers called nucleotides, and it’s now been 67 years since Watson and Crick published their seminal paper proposing the molecular structure of nucleic acids in the prestigious journal ‘Nature’ on the 25th April 1953.


In this section entitled “The structure and function oNucleic Acids” you’ll learn about the basics of molecular genetics, beginning with the structure and function of nucleotides. From here you’ll be in a much better position to fully understand the structure and function of the most important of all biomolecules - DNA and RNA. Once you have learned all about these structures, you’ll go on to find out about the different types of RNA - in particular messenger RNA (mRNA) and transfer RNA (tRNA), which you must become very familiar since you’ll need to be able to describer the roles of each of these players in protein synthesis. To finish this section you have to learn about genes and non-coding DNA before revisiting how DNA is stored in eukaryotes, prokaryotes and the cellular organelles Mitochondria and Chloroplasts.

A Level Biology: Nucleic Acids - 

Nucleotides & Polynucleotides


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In this A-Level Biology Lesson "Nucleic Acids - Nucleotides and Polynucleotides" Following the Learning Outcomes you’ll learn that Nucleotides are the monomers of DNA and RNA. Nucleotides are made up from 3 “parts” - The Pentose Sugar, The Nitrogenous Bases (which can be classified as either Purines or Pyrimidines) and The Phosphate Group. You learn the Structure of a nucleotide and how Condensation Reactions Build Polynucleotides. The lesson will finish with a basic Comparison DNA and RNA.

As you’ll have learned from watching the lesson, nucleotides have three parts to them: -


1) A pentose sugar (5 carbon atoms in it). By convention the carbon atoms are numbered; 1’, 2', 3’ etc, read as "one prime", “two prime", 3 prime, etc. This convention allows us to distinguish each of the carbon atoms in the base. It is useful when identifying the pentose sugar and describing it to others… for example, if carbon 2' has a hydroxyl group (OH) attached, then the pentose sugar is ribose, found in RNA. However, if carbon 2' just has a hydrogen atom (H) attached, then the sugar is deoxyribose, found in DNA.


2) A phosphate group - which is negatively charged, and gives nucleic acids their acidic properties.


3) A nitrogenous base. There are five different nitrogenous bases (you don't need to remember their molecular structures), but you do need to be aware that of them contain the elements carbon, hydrogen, oxygen and nitrogen, and you’ll also need to know which are Purines and Pyrimidines. 


The nitrogenous bases are generally referred to by their first letters only, (but yes, you do need to learn their full names).


There are five nitrogenous bases: -


Adenine (A

Cytosine (C

Guanine (G

Thymine (T) (DNA only)

Uracil (U) (RNA only)


*Note: - The base thymine is only found in DNA and the base uracil is only found in RNA, so there are only four different bases present at a time in one nucleic acid molecule.


DNA contains - A, C, G and T

Whilst RNA contains A, C, G and U


It is also important to note, that informally when talking about nucleotides, we refer to them by the letter name of the nitrogenous base that makes them… for example, the nucleotide ‘A’ (contains the nitrogenous base Adenine - but this nucleotide is actually called “Adenosine” (which you’ll be familiar with from that super imprint molecule ATP (Adenosine triphosphate). 


When a nitrogenous base is attached to a Pentose sugar and Phosphate group, the whole molecule is called a nucleotide. So, what are the actual names of the nucleotides: - 


When the nitrogen’s base A (Adenine) is attached to a pentose sugar and phosphate group - you already know this nucleotide is called Adenosine.


When C (Cytosine) makes up the nucleotide, the nucleotide is called: Cytidine. 


When G (Guanine) makes up the nucleotide, the nucleotide is called: Guanosine.


When T (Thymine) makes up the nucleotide, the nucleotide is called: Thymidine.


and When U (Uracil) makes up the nucleotide, the nucleotide is called: Uridine.

It’s super useful to know this information (since you’ll no doubt read these terms in books, and even exam questions). But you already have a lot to remember and you will not be expected to use these names to answer exam questions. 


So why should you be aware of them?

Well learning A-level biology is all about understanding concepts, terminology and being able to apply this knowledge, and when you know the language everything becomes so much easier… 


For example: - now you know why ATP is called “Adenosine triphosphate”. Adenosine is the name of the nucleotide and since it has 3 phosphates its called triphosphate. 


In fact all nucleotides can have one, two or three phosphate groups. So for instance you can have adenosine monophosphate (AMP), adenosine diphosphate (ADP) and of course adenosine triphosphate (ATP). These nucleotides are very common in cells and have many roles other than just  being a component of nucleic acids. ATP as you’ll now by now, is used as an energy storage molecule, while AMP and GTP are used as chemical messengers.


What is Nucleotide Polymerisation?


Polymerisation is biochemical combination of monomers to form polymers, and nucleotides polymerise via condensation reactions between the 3' carbon of the sugar and an oxygen atom of the phosphate group which results in the formation of phosphodiester bonds. The phosphodiester bonds link the pentose sugar to the phosphate - resulting in the sugar-phosphate backbone. Since the nitrogenous bases do not take part in the polymerisation, the sugar-phosphate backbone is made and the nitrogenous bases extending out from it. A polynucleotide has a free phosphate group at one end, this end is called the 5' end (because the phosphate is attached to carbon 5' of the sugar), a free OH group at the other end, called the 3' end (because it's on carbon 3' of the sugar).


The terms 3' and 5' are often used to denote the different ends of a DNA molecule, and will be useful to know when describing how DNA replication take places, or explaining where primers bond during PCR


Ok, so now you know that nucleotides are super important and you’ve watched the lesson on nucleotides, completed the knowledge check PDF and read all this… it’s time to move on to learning about DNA, and a brief history of the key players involved in the “discovery” of this infamous biomolecule.

A Level Biology: Nucleic Acids - Nucleotides and Polynucleotides Revision Notes with Knowledge Check Questions and Answers pdf
A Level Biology: Nucleic Acids - Nucleotides and Polynucleotides Revision Notes Poster A3 PDF

A-Level Biology: "Nucleic Acids - Nucleotides & Polynucleotides"


Nucleotides are the monomers (mononucleotides) that make up Polynucleotides such as DNA and RNA.


Each Nucleotide is composed of 3 parts.

  1. The Pentose Sugar

  2. The Nitrogenous base (either Adenine, Thymine (DNA), Cytosine, Guanine or Uracil (RNA)

  3. The Phosphate Group (ion)



Notice Carbon number 2 of these Pentose Sugars.


Ribose Sugar (A-Level Biology)
Deoyribose Sugar (A-Level Biology) Pentose Sugars

Ribose, has an OH group at Carbon 2.

Deoxyribose, has a Hydrogen at Carbon 2.

The Nitrogenous Bases are either Purines or Pyrimidines.

Purines (A and G) have “2 Rings”

Adenine Nitrogenous Base (A-Level Biology) Purine Bases
Guanine Nitrogenous Base (A-Level Biology) Purine Bases

Pyrimidines (C, T and U) have only “1 ring”

Thymine Nitrogenous Base (A-Level Biology) Pyrimadine Bases
Cytosine Nitrogenous Base (A-Level Biology) Pyrimadine Bases
Uracil Nitrogenous Base (A-Level Biology) Pyrimadine Bases

The Phosphate group – Notice the OH groups, remember these are important for condensation reactions.

Phosphate Group (A-Level Biology)

The basic structure of a nucleotide:

Nucleotide Structure (A-Level Biology)

Nucleotides join to one another via Condensation Reactions.


An OH group - located on the Phosphate group, attached to Carbon number 5 of the Pentose Sugar, undergoes a condensation reaction with the OH group attached to Carbon Number 3 of the Pentose Sugar. This condensation reaction results in the removal of a water [H2O] molecule and the formation of a phosphodiester bond between the nucleotides. Many of these condensation reactions take place to form the Sugar-Phosphate Backbone of polynucleotides.

Polynucleotide showing phosphodiester bonds (A-Level Biology)

You also have to be able to compare and contrast DNA and RNA… These are useful points from this lesson to include in your answers…


DNA has the Pentose Sugar “Deoxyribose” and is made up of the Nitrogenous bases, Adenine, Thymine, Cytosine and Guanine.

RNA has the Pentose Sugar “Ribose” and is made up of the Nitrogenous bases, Adenine, Uracil, Cytosine and Guanine.

A Level Biology: Nucleic Acids - 

The Structure and Brief History of DNA


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In this A-Level Biology Lesson "Nucleic Acids - The Structure and Brief History of DNA"

00:00 Intro Screen / Learning Outcomes

01:30 A Super Brief History of DNA

03:09 DNA contains Purines and Pyrimidines

03:27 1950 - Chargaff's Rule

03:53 Wilkins and Franklin

04:40 1953

05:16 The DNA Nucleotide

06:03 Complementary Base Pairing requires DNA polynucleotide chains to run in Antiparallel directions...

07:30 The % of A is equal to T...

08:00 Antiparallel Structure of DNA

08:26 Hydrogen bonding...

09:14 All this leads Watson and Crick to Build their 3D model of DNA...

09:29 - Summary of DNA Structure.

A Level Biology: Nucleic Acids - The Structure and Brief History of DNA Revision Notes with Knowledge Check Questions and Answers PDF
A Level biology: Nucleic Acids - The Structure and Brief History of DNA Revision Notes Poster A3 PDF

DNA is a nucleic acid, (because it is found in the nucleus and is weakly acidic).


DNA is a stable polynucleotide, which means it is composed of many nucleotides and does not begin to denature until it is heated to approximately 86°C. So, DNA is much more stable than proteins! - remember many proteins begin to denature just above 40 °C.


DNA is long molecule and is wound around proteins called histones.

DNA is a polymer: the monomers are nucleotides.


Each nucleotide has three components: -

★ Sugar – deoxyribose – which is a 5-carbon sugar

Phosphate group

Nitrogenous Base – one of four nitrogen containing compounds: Adenine (A), Thymine (T), Guanine (G) or Cytosine (C).


The nucleotides are arranged in a double helix (a twisted ladder). The two sides of the 'ladder' are chains of alternating sugar–phosphate groups, while the ‘rungs’ of the  'ladder' are made from pairs of bases bonded together by hydrogen bonds.


It is important that, both for protein synthesis and DNA replication, the strands can separate and rejoin without damaging the molecule.


Only one part, of one strand of the DNA molecule, at any particular point in the double-stranded molecule – "the sense strand" – is used to make proteins. The other side serves to stabilise the molecule.


The Nitrogenous bases.

The four bases always bond (via hydrogen bonding) in the same way: –

A hydrogen bonds with T and

C hydrogen bonds to G.


So if you know the base sequence down one side of the DNA molecule, you know the other!

For example, if one strand reads: -

...then of course the the other (the complementary) side will read: -


The nitrogenous bases are held together by weak hydrogen bonds, you must also remember that there are 2 hydrogen bonds between A and T (i.e. A=T) and 3 hydrogen bonds between C and G (G ≡C).


These regular hydrogen bonds along the whole length of the molecule make DNA very stable.

To appreciate the importance of DNA (and genes) in an organism, it is also important to understand the significance of proteins: -

  • Enzymes are proteins that affect the rate of metabolism

  • Structural proteins make up an important part of the fabric of organisms, e.g. collagen, keratin and elastin

  • Antibodies that form a vital part of the immune system are proteins

  • Hormones, e.g. insulin, are proteins

  • Proteins form channels in cell membranes that control what passes in and out of the cell

  • Blood clotting involves many different proteins in a complex sequence of reactions

  • Surface proteins on a cell are unique in each organism, which helps the immune system to differentiate self from non-self and so defend against pathogens / foreign invaders.

Organisms are largely made from proteins, and proteins play many important roles in what happens in our bodies. Yet each individual starts life as a fertilised cell; little more than two sets of genes, made from DNA.


So, When revising A-Level biology be sure to consider the relationship between DNA and Protein


for example: -

The role of nucleic acids in protein and enzyme synthesis.

How the genetic code in DNA is used to build a protein: - i.e. Transcription and Translation.

A Level Biology: Nucleic Acids - 

The Structure of RNA (mRNA, tRNA and rRNA)


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00:00 Intro Screen / Learning Outcomes

01:27 RNA is similar to DNA

01:56 The 3 types of RNA (mRNA, tRNA and rRNA)

02:35 RNA nucleotide

03:00 RNA Summary

03:34 Compare and Contrast DNA and RNA

04:44 Final points you need to know...

A Level biology: Nucleic Acids - The Structure of RNA (mRNA, tRNA and rRNA) Revision Notes Poster A3 PDF