DNA, the Central Dogma and the Genome
Where the DNA actually is ★★
A eukaryotic cell keeps DNA in more than one place, and the exam-relevant point is that there are three DNA-containing organelles, not one.
| Organelle | DNA | Note |
|---|---|---|
| Nucleus | Chromosomal DNA | Contains most of the cell's DNA; separated from the cytoplasm by a double layer of membrane |
| Mitochondria | Mitochondrial DNA | Mitochondria have a role in the oxidative degradation of nutrient molecules |
| Chloroplasts | Chloroplast DNA | Only in eukaryotic cells capable of photosynthesis — so not in humans |
The primer gives a calculation worth doing slowly, because it makes the scale real.
Every million bases occupies 0.34 mm of length. A human cell contains 6 × 10⁹ nucleotide pairs. Uncoil all the chromosomal DNA from a single cell and lay it in a line, and you have 204 cm — over two metres.
Two metres of thread packed into a nucleus a few micrometres across. That is the packing problem chromosomes exist to solve — and it is why DNA is wound, coiled and supercoiled rather than simply stored.
- Which three organelles contain DNA? → Nucleus, mitochondria, chloroplasts
- Which contains most of it? → The nucleus — chromosomal DNA
- How is the nucleus separated from the cytoplasm? → By a double layer of membrane
- How long is the DNA in one human cell? → About 204 cm, from 6 × 10⁹ nucleotide pairs
What a genome is ★★★
An organism's complete set of genetic instructions. Each genome contains all of the information needed to build that organism and allow it to grow and develop.
The primer's own analogy is a recipe book: our bodies are made of millions of cells, each with its own complete set of instructions. A skin cell and a liver cell contain the same set — which is the fact that makes development interesting, because identical instructions somehow produce different cells.
| Fact | Detail |
|---|---|
| Made of | DNA |
| The code | Determined by the order of the four nucleotide bases — adenine, cytosine, guanine and thymine: A, C, G, T |
| Structure | A twisted double helix |
If every cell carries the identical genome, why is a liver cell not a skin cell?
Because having the instructions and using them are different things. Every cell holds the whole recipe book; each cell type reads only certain pages. That selective reading is gene expression, and it is exactly what the central dogma in §3 describes.
So the genome is not a blueprint that is simply executed — it is a library, and what makes a cell what it is depends on which volumes it opens.
- Define genome. → An organism's complete set of genetic instructions, containing all the information needed to build it and let it grow and develop
- Do a skin cell and a liver cell have the same genome? → Yes — identical instructions
- What determines the code? → The order of the four bases A, C, G, T
- What is DNA's structure? → A twisted double helix
The central dogma ★★★
The process by which the instructions in DNA are converted into a functional product. It was first proposed in 1958 by Francis Crick, discoverer of the structure of DNA. It explains the flow of genetic information — from DNA, to RNA, to make a functional product, a protein.
The primer spells out the logic in three claims. DNA contains the information needed to make all of our proteins. RNA is a messenger that carries this information to the ribosomes. And the ribosomes serve as factories where the information is translated from a code into the functional product.
| Step | What happens |
|---|---|
| Transcription | DNA → RNA — the message is copied |
| Translation | RNA → protein — the code is read at the ribosome and a functional product is built |
The whole process by which DNA instructions become a functional product is called gene expression, and gene expression has those two key stages.
It is a reasonable question: if the instructions are in the nucleus and the factories are in the cytoplasm, why not move the factory to the instructions — or the instructions to the factory?
Because the DNA is the master copy, and there is only one of it. Two metres of irreplaceable instruction, from §1, kept behind a double membrane. Sending it out to the ribosomes would risk the only copy the cell has.
So the cell sends a transcript instead — disposable, made on demand, and discarded afterwards. That also gives the cell its control point: regulating how much RNA is made regulates how much protein appears, without ever touching the original.
- Who proposed the central dogma, and when? → Francis Crick, 1958
- State the flow of genetic information. → DNA → RNA → protein
- What is RNA's role? → A messenger carrying the information to the ribosomes
- What are the ribosomes? → The factories where the code is translated into the functional product
- Name the two stages of gene expression. → Transcription (DNA→RNA) and translation (RNA→protein)
Revision
| Question | Answer |
|---|---|
| Three DNA-containing organelles? | Nucleus · mitochondria · chloroplasts |
| Which holds most DNA? | The nucleus — chromosomal DNA |
| DNA length per human cell? | About 204 cm (6 × 10⁹ base pairs) |
| Define genome | An organism's complete set of genetic instructions |
| The four bases? | Adenine · cytosine · guanine · thymine (A, C, G, T) |
| DNA structure? | Twisted double helix |
| Central dogma — who and when? | Francis Crick, 1958 |
| The flow? | DNA → RNA → protein |
| Two stages of gene expression? | Transcription · translation |
- Which organelles contain DNA? → Nucleus, mitochondria and (in photosynthetic cells) chloroplasts
- Define genome. → The complete set of genetic instructions needed to build an organism and let it grow and develop
- State the central dogma and its author. → DNA → RNA → protein; proposed by Francis Crick in 1958
- Why does the cell use a messenger? → The DNA is the irreplaceable master copy; a disposable transcript is sent out instead