IB Biology HLDNA, RNA & the Genetic CodePaper 1 & 2~12 min read
Structure of DNA & RNA
Every instruction for building a living thing is written with an alphabet of just four letters. To understand how that works, you first need to know the single building block those letters are made from — the nucleotide.
📘 What you need to know
DNA (deoxyribose nucleic acid) carries the genetic code in all living organisms, which is why the code is called universal.
DNA is mainly in the nucleus, where it forms chromosomes, and is also found in mitochondria and chloroplasts.
RNA (ribonucleic acid) is the main component of ribosomes and is also found in the nucleus and cytoplasm.
Both are polymers made of repeating units called nucleotides.
Each nucleotide = a pentose sugar + a nitrogenous base + a phosphate group, all joined by covalent bonds.
Bases are either purines (adenine, guanine — two rings) or pyrimidines (cytosine, thymine, uracil — one ring).
Nucleotides join by condensation reactions, forming phosphodiester bonds and releasing water. This builds the sugar–phosphate backbone.
DNA is double-stranded with deoxyribose and thymine. RNA is usually single-stranded with ribose and uracil.
DNA as the genetic material of life
DNA carries the genetic code in all living organisms. That is a bigger statement than it sounds — it means a bacterium, an oak tree and you all store information in the same chemical language. This is why the genetic code is described as universal.
DNA is mainly found in the nucleus, where it forms chromosomes.
It is also found in the chloroplasts and mitochondria of eukaryotic cells.
RNA is the main component of ribosomes, which are essential for protein synthesis. Some RNA is also found in the nucleus and cytoplasm.
What about viruses?
Certain viruses, such as SARS-CoV-2, carry RNA as their genetic material instead of DNA. These viruses cause diseases including COVID-19, Ebola, mumps and influenza.
Viruses are not considered living organisms for two reasons:
They cannot replicate by themselves — they are dependent on other living cells to reproduce.
They lack a cellular structure.
Components of a nucleotide
Both DNA and RNA are polymers — long molecules built from many repeating units. Those units are called nucleotides, and each one has exactly three parts:
Part
What it is
How to draw it
Pentose sugar
A sugar with 5 carbon atoms — deoxyribose in DNA, ribose in RNA
A pentagon
Nitrogenous base
A nitrogen-containing organic base with either 1 or 2 rings of atoms
A rectangle
Phosphate group
Acidic and negatively charged
A circle, often with a P inside
The base and the phosphate group are both covalently bonded to the sugar. The base attaches to carbon 1 and the phosphate attaches to carbon 5.
Pentagon = sugar, circle = phosphate, rectangle = base, solid lines = covalent bonds. These conventions are accepted in the exam.
The five bases
DNA uses adenine (A), guanine (G), cytosine (C) and thymine (T). RNA uses the same bases except thymine, which is replaced by uracil (U).
They also split into two groups based on their shape:
Group
Bases
Structure
Purines
Adenine, guanine
Two rings of atoms — larger
Pyrimidines
Cytosine, thymine (DNA), uracil (RNA)
One ring of atoms — smaller
🧠
Memory trick: “PURe As Gold”
PURines are Adenine and Guanine. Everything else — cytosine, thymine, uracil — is a pyrimidine. Also note that pyrimidine and pyramid both start with “py”, and both are the smaller, simpler shape.
Linking nucleotides together
Nucleotides join into chains to form DNA or RNA strands. The link is always the same: the phosphate group of one nucleotide forms a covalent bond to the pentose sugar of the next one.
This happens by a condensation reaction, which means a molecule of water is released each time a bond forms. The bond produced is called a phosphodiester bond.
Building the backbone
nucleotide + nucleotide → phosphodiester bond + H2O
Repeat this thousands of times and you get a sugar–phosphate backbone with a base hanging off every sugar. The finished polymer is called a strand, or a polynucleotide.
Because the sugar and phosphate are identical in every nucleotide, all the information is carried by the order of the bases.
Why only four letters is enough. There are just 4 bases, but they can be joined in any combination and any order, in molecules of any length. That gives an effectively unlimited number of possible sequences.
RNA structure
RNA follows all the same rules, with a few important differences:
RNA molecules are relatively short — from a hundred to a few thousand nucleotides.
RNA is usually a single-stranded polynucleotide.
The pentose sugar is ribose, not deoxyribose.
The bases are adenine, guanine, cytosine and uracil (instead of thymine).
Three types of RNA
Type
Full name
Job
mRNA
messenger RNA
Formed in the nucleus and carried to the ribosomes in the cytoplasm
tRNA
transfer RNA
Transports amino acids to the ribosomes during protein synthesis
rRNA
ribosomal RNA
Forms part of the structure of ribosomes
The carbon atoms in the pentose sugar are numbered from the right, in a clockwise direction. That numbering is not just trivia — it tells you which bonds form the backbone, and it gives the strand a direction (the 3′ and 5′ ends you will meet in the DNA Structure note).
Worked examples
WE 1
Describe the structure of a nucleotide
Describe the structure of a single DNA nucleotide. (3 marks)
Point 1: name the three parts
A nucleotide is made of a pentose sugar, a nitrogenous base and a phosphate group.
Point 2: identify them for DNA
In DNA the sugar is deoxyribose and the base is adenine, guanine, cytosine or thymine.
Point 3: say how they join
The base and the phosphate are both covalently bonded to the sugar — the base at carbon 1 and the phosphate at carbon 5.
Sugar + base + phosphate, joined by covalent bondsthree marks, three components — the easiest mark to lose is forgetting to say the bonds are covalent
WE 2
Explain how a polynucleotide strand is formed
Explain how individual nucleotides are joined together to form a polynucleotide strand. (3 marks)
Point 1: which parts join
The phosphate group of one nucleotide bonds to the pentose sugar of the next nucleotide.
Point 2: name the reactionCondensation reactions form these bonds, and a molecule of water is released each time.
Point 3: name the bond and the result
The bond formed is a phosphodiester bond, and repeating this builds the sugar–phosphate backbone of the strand.
Condensation reactions form phosphodiester bonds, releasing water“phosphodiester” and “condensation” are the two words examiners look for
💡 Exam tips
Learn the three nucleotide components as a set: sugar, base, phosphate. Almost every structure question starts there.
Use the standard shapes when drawing — pentagon sugar, circle phosphate, rectangle base. A large, clear drawing is easier to mark than a small one.
Solid lines for covalent bonds, dashed lines for hydrogen bonds. Never mix them.
Say deoxyribose for DNA and ribose for RNA. “Pentose sugar” alone often will not get the mark in a comparison question.
Remember that condensation releases water. That single detail is worth a mark on its own.
Purines have two rings; pyrimidines have one.
⚠ Common mistakes
Saying nucleotides are joined by hydrogen bonds. Within a strand the bonds are covalent phosphodiester bonds. Hydrogen bonds only join the two strands.
Writing that RNA has thymine. RNA has uracil instead.
Calling uracil a purine. It is a pyrimidine, like cytosine and thymine.
Saying the phosphate joins to the base. Both the base and the phosphate attach to the sugar, not to each other.
Describing viruses as living. They cannot replicate independently and have no cellular structure.
Forgetting DNA is also in mitochondria and chloroplasts, not only the nucleus.
Up next: The Basis of the Genetic Code — how the order of those four bases actually gets turned into a protein, and why the same code is used by nearly every organism on Earth.
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