What Is DNA?
- DNA is the molecule that stores the instructions for building and maintaining an organism.
- These instructions determine traits such as eye color, enzyme shape, and blood type.
- All living things use DNA, which shows that life shares a universal genetic code.
Deoxyribonucleic Acid (DNA)
A long, double-helical molecule that stores genetic instructions.
Why Is DNA Important?
- DNA contains genes that give cells instructions for building proteins.
- Proteins carry out nearly every function in the body, so differences in DNA lead to differences in traits.
- Because DNA instructions are passed from parent to offspring, DNA is the basis of inheritance.
The gene controlling melanin affects how much pigment skin cells produce, influencing skin color.
What Is the Structure of DNA?
- DNA is made of two strands twisted into a double helix.
- Each strand is built from repeating units called nucleotides.
- There are four bases: A (adenine), T (thymine), C (cytosine), G (guanine).
- Bases pair using complementary base pairing:
- A pairs with T
- C pairs with G
- Hydrogen bonds hold these base pairs together, stabilizing the double helix.
| Nitrogen Base | Pairing base | Hydrogen Bonds between the base. |
|---|---|---|
| Adenine | Thymine | 2 |
| Cytosine | Guanine | 3 |
Nucleotide
A building block of DNA consisting of a sugar, a phosphate group, and a nitrogen base.
How Did Patterns Help Crick and Watson Discover DNA’s Structure?
- Rosalind Franklin used X-ray diffraction to reveal repeating patterns in DNA fibres.
- The pattern showed a regular spiral shape with a consistent width.
- Crick and Watson realised this constant width only made sense if:
- A paired with T
- C paired with G
- These insights allowed them to propose the correct double-helix model.
How Are DNA, Genes, and Chromosomes Related?
Gene
A section of DNA that codes for one protein.
- DNA is packaged into long structures called chromosomes.
- Each chromosome contains many genes.
The hierarchy goes: DNA → Chromosomes → Genes → Proteins → Traits
How Does a Cell Use DNA Instructions?
- Cells use DNA in two different ways:
- To build proteins: uses transcription then translation
- To make more DNA when preparing to divide: uses DNA replication
These are three separate processes, don't mix them up.
1. Transcription: Copying a Gene Into mRNA
Transcription
The process of copying a gene’s DNA sequence into mRNA.
- Transcription happens in the nucleus.
- The cell selects the gene it needs.
- The DNA strands separate at that gene.
- The cell builds a single-stranded mRNA copy of the gene.
- mRNA leaves the nucleus and travels to a ribosome.
mRNA
A temporary copy of a gene used to carry instructions to the ribosome.
2. Translation: Turning mRNA Into a Protein
- Translation happens at the ribosome.
- The ribosome reads the mRNA instructions.
- Every three bases code for one amino acid.
- Amino acids join to form a polypeptide chain.
- The chain folds into a protein with a specific shape and function.
Translation
The process of using mRNA instructions to assemble amino acids into a protein.
Enzymes, antibodies, hormones, and structural materials like hair are all proteins.
- DNA is a cookbook.
- mRNA is a copied recipe.
- The ribosome is the kitchen assembling the final dish.
- This flow is also referred to as the "central dogma"
3. DNA Replication: Making a Copy of DNA
- Replication prepares the cell for mitosis.
- The DNA double helix unzips, separating into two strands.
- Each original strand acts as a template, meaning it guides which new bases are added.
- Free nucleotides pair with exposed bases using A–T and C–G pairing.
- Two new DNA molecules form, each made of:
- one original (old) strand
- one newly built strand
Semi-conservative replication
Each new DNA molecule keeps (“conserves”) half of the original DNA and builds the other half new.
- Imagine you lost a page from a book, but you still have the original copy.
- You place a blank sheet next to the original and rewrite it line by line.
- The final result is one old page paired with one new, perfectly matched copy.
Because each copy is made this way, chromosomes entering mitosis are accurately duplicated, ensuring both daughter cells receive identical DNA.
- What are the three parts of a nucleotide?
- How does complementary base pairing ensure accurate replication?
- How do transcription and translation turn a gene into a protein?
- Why did Crick and Watson need patterns to build their DNA model?
- Why must DNA replicate before cell division?
