Nucleic acids


1. 🧬 DNA STRUCTURE

What is DNA?

DNA (deoxyribonucleic acid) is the molecule that:

  • stores genetic information

  • contains the instructions for making proteins

  • controls the functioning of cells.

Structure of DNA

DNA is:

  • double-stranded

  • shaped as a double helix

  • made of nucleotides

  • held together by hydrogen bonds between complementary bases.

A nucleotide consists of:

Phosphate + sugar + nitrogenous base

The sugar in DNA is deoxyribose.

DNA bases

You MUST know:

  • A = Adenine

  • T = Thymine

  • C = Cytosine

  • G = Guanine

Complementary base pairs:

A ↔ T
C ↔ G

A and T have 2 hydrogen bonds, while C and G have 3 hydrogen bonds.

2. 🧬 DNA VS RNA

DNA

RNA

Double-stranded

Single-stranded

Deoxyribose sugar

Ribose sugar

A, T, C, G

A, U, C, G

Long-term genetic storage

Helps make proteins

Mainly in nucleus

mRNA moves to cytoplasm

Self-replicates

Made from DNA when needed


⭐ Remember:

RNA has U instead of T.

RNA uses uracil (U) where DNA uses thymine (T).

3. 📍 WHERE IS DNA AND RNA FOUND?

DNA

Most DNA is found in the nucleus, making up the chromosomes.

DNA is also found in mitochondria.

mRNA

  • Made in the nucleus

  • Carries the genetic message from DNA

  • Moves into the cytoplasm

  • Attaches to a ribosome.

tRNA

  • Found in the cytoplasm

  • Carries amino acids to the ribosome.

You do NOT need to learn other types of RNA. The syllabus specifically says mRNA and tRNA only.

4. 🧬 DNA REPLICATION

Why does DNA replicate?

Before a cell divides, DNA must be copied so that each daughter cell receives an identical set of genetic instructions.

Where?

S-phase of interphase

Basic process:

  1. DNA unwinds.

  2. Hydrogen bonds between the bases break.

  3. The two DNA strands separate.

  4. Each original strand acts as a template.

  5. Free nucleotides attach to complementary bases.

  6. DNA polymerase joins the new nucleotides.

  7. Two identical DNA molecules are produced.

⭐ Semi-conservative replication

Each new DNA molecule contains:

1 original strand + 1 new strand

This is called semi-conservative replication.

Enzymes to remember:

  • Helicase → separates the DNA strands

  • DNA polymerase → builds the new DNA strand.

5. 🧪 PROTEIN SYNTHESIS

The basic sequence is:

DNA → mRNA → Protein

This is the central dogma.

There are two stages:

1⃣ Transcription2⃣ Translation

6. ✍ TRANSCRIPTION

Where?

📍 Nucleus

What happens?

  1. The required section of DNA unwinds.

  2. The DNA strands separate.

  3. RNA polymerase builds an mRNA strand using the DNA as a template.

  4. Complementary RNA bases are added.

Remember:

DNA → mRNA

Base pairing during transcription:

DNA

mRNA

A

U

T

A

C

G

G

C


The completed mRNA leaves the nucleus and moves to a ribosome in the cytoplasm.

7. 🧩 CODONS

The mRNA sequence is read in groups of three bases.

Each group of three is called a:

CODON

Each codon codes for a particular amino acid.

Example:

AUG | GCU | AAA | ...

Each triplet represents an instruction for an amino acid.

8. 🔗 TRANSLATION

Where?

📍 Ribosome in the cytoplasm

What happens?

  1. mRNA attaches to the ribosome.

  2. The ribosome reads the mRNA codon by codon.

  3. tRNA brings the correct amino acids.

  4. The anticodon on tRNA pairs with the complementary mRNA codon.

  5. Amino acids join together using peptide bonds.

  6. This forms a polypeptide/protein.

  7. The process stops when a stop codon is reached.

⭐ Know these terms:

Codon = 3 bases on mRNA

Anticodon = 3 complementary bases on tRNA

tRNA = carries amino acids

Ribosome = site of translation

Amino acids → peptide bonds → polypeptide/protein

9. 🧬 MUTATIONS

A mutation is a change in DNA.

The syllabus only requires:

  • substitution

  • insertion

  • deletion

It also requires you to understand point mutations and frameshift mutations.

Point mutation

A change involving a small number of bases.

Substitution

One base is replaced by another.

It may:

  • have no effect

  • change one amino acid

  • change the protein.

Frameshift mutation

Caused by an insertion or deletion of bases.

It shifts how the bases are grouped into codons.

Therefore, it can affect all codons after the mutation, potentially changing many amino acids in the protein.

Causes

Mutations can occur:

  • by chance

  • because of mutagens.

⭐ Easy memory:

Substitution = swap

Insertion = add

Deletion = remove

Insertion/deletion → frameshift

10. 🧬 MITOCHONDRIAL DNA

Mitochondria contain their own small amount of DNA called:

mtDNA

In humans:

  • mitochondria are inherited from the mother

  • mtDNA is passed from mother to child

  • it can therefore be used to trace maternal ancestry and relatedness.

Why is mtDNA useful for relatedness?

Because it follows the maternal line with little/no mixing from the father's DNA, differences that accumulate over generations can be used to compare maternal lineages.

11. 🧪 DNA FINGERPRINTING

What is it?

DNA profiling/fingerprinting is used to distinguish between individuals using DNA.

It uses highly variable, non-coding DNA.

Why non-coding DNA?

Because the non-coding regions are highly variable between individuals of the same species, making them useful for identification.

Uses

Know that DNA profiling can be used for:

  • forensic identification

  • paternity testing

  • tracing relatives

  • identifying bodies

  • studying populations.

Interpreting a DNA fingerprint

You need to be able to compare the banding patterns.

If two DNA profiles have matching patterns, this provides evidence that the samples came from the same individual or, in a parentage test, that the child's bands can be accounted for by the parents.

You do NOT need to learn the detailed laboratory process of DNA fingerprinting.

12. 🧪 PCRPCR = Polymerase Chain Reaction

PCR is used to make many copies of a small DNA sample.

This is important because sometimes only a tiny amount of DNA is available.

PCR makes it possible to obtain enough DNA for applications such as DNA profiling.

You should know the basic idea:

Small amount of DNA → PCR → many copies → enough DNA to analyse

13. 🧪 AIM 2 — SKILLS YOU MUST BE ABLE TO DO

These aren't just facts to memorise. You need to be able to apply the knowledge.

🧬 Modelling DNA/RNA

Be able to:

  • construct/identify a nucleotide

  • identify the bases

  • identify complementary pairs

  • identify DNA vs RNA

  • comment on whether a model is accurate.

🧬 Model protein synthesis

Be able to work through:

DNA → mRNA → codons → tRNA/anticodons → amino acids → protein


🧪 DNA extraction

You need to be able to:

  • compare two DNA extraction methods

  • decide which method is better

  • justify your answer.

⚠ Your PDF's “What to Learn” requires this skill, but the material provided does not clearly specify which two extraction methods you must compare, so I wouldn't add methods that aren't in your notes.

📊 Nucleotide data

You need to be able to:

  • read bar graphs

  • interpret numerical data

  • work with frequencies/percentages of DNA bases.

Remember the complementary relationship:

A = T

C = G

So if you know the percentage of one base, you can work out its complementary base.

🧬 DNA profiles

Be able to look at DNA fingerprint patterns and determine whether samples match or are related.

14. 🏆 WATSON & CRICK — WHAT YOU NEED TO KNOW

You do not need to memorise every detail of the history.

Your PDF specifically says the important things are the main scientists, what they discovered/contributed, approximately when, and how the work came together.

Important scientists:

Avery — 1944

  • Provided evidence that DNA carries genetic information.

Chargaff

  • Found that:

    • A = T

    • G = C

  • This helped Watson and Crick understand complementary base pairing.

Rosalind Franklin

  • Used X-ray diffraction.

  • Her evidence supported the double-helix structure.

  • Her work showed important information about the arrangement of DNA.

Maurice Wilkins

  • Also worked with DNA and X-ray evidence.

Watson & Crick — 1953

  • Used evidence from multiple scientists to construct their model of the DNA double helix.

  • Published their structure in 1953.

Why did they work so quickly?

There was competition between scientists trying to solve the structure of DNA.

Linus Pauling was also working on the problem, which encouraged Watson and Crick to work quickly to publish their model first.

⭐ Important point for an essay:

The discovery of DNA was not the work of Watson and Crick alone. Their model depended on evidence and discoveries made by several other scientists.

15. 💊 DRUGS & PROTEIN SYNTHESIS

Some drugs, particularly antibiotics, work by interfering with protein synthesis.

The idea you need to understand is:

Drug interferes with protein synthesis → bacteria cannot make the proteins they need → bacterial growth/function is disrupted.

The syllabus specifically expects you to understand how pharmacological drugs can interfere with protein synthesis to act as antibiotics.

16. 🌍 DNA IN EVERYDAY LIFE

You should understand that DNA is no longer just something studied in biology.

DNA technology is used in areas such as:

  • forensic investigations

  • paternity testing

  • tracing relatives

  • identifying individuals

  • studying populations

  • genetic technologies.

17. 🦠 WHY DOES MITOCHONDRIAL DNA EXIST?

This is an AIM 3 discussion question.

Mitochondria have their own DNA because mitochondria are thought to have originated from ancient bacteria that were taken into another cell and eventually became permanent organelles.

This is known as the endosymbiotic theory.

The important idea is:

Ancient bacterium → lived inside another cell → became a mitochondrion → retained some of its own DNA

Today, mtDNA contains genes important for mitochondrial function.

⭐ THE BIGGEST THINGS TO MEMORISE

If you're short on time, make sure you know these really well:

DNA

Nucleotide = sugar + phosphate + base

DNA = double-stranded

A ↔ T

C ↔ G

RNA

RNA = single-stranded

A ↔ U

mRNA = carries message

tRNA = carries amino acids

Protein synthesis

DNA → mRNA → protein

Transcription = nucleus

Translation = ribosome/cytoplasm

Codon = mRNA

Anticodon = tRNA

Replication

DNA separates → complementary nucleotides added → DNA polymerase → two identical DNA molecules

Semi-conservative = one old strand + one new strand

Mutations

Substitution = swap

Insertion = add

Deletion = remove

Insertion/deletion → frameshift

DNA technology

PCR = makes many copies of DNA

DNA fingerprinting = identifies/compares individuals using variable non-coding DNA

mtDNA

Mother → child

Used for maternal relatedness/ancestry

History

Franklin + Chargaff + Wilkins + other scientists → evidence → Watson & Crick model in 1953

Applications

DNA → forensics, paternity, relatives, identification

Antibiotics → interfere with protein synthesis