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Define gene expression.
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All Flashcards in Topic 4.5
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4.5.17 cards
Define gene expression.
The **transcription and translation** of a gene to make its **protein** (the gene's product).
What is a gene's 'product'?
The **protein** that the gene codes for — the end result of expressing it.
Every cell in your body has the same DNA. So why are cells different?
**Differential gene expression** — each cell type switches on a **different subset** of genes, so it makes different proteins.
What happens to a gene's protein if the gene is switched off?
The gene is **not transcribed**, so **no mRNA and no protein** are made — even though the gene is still in the DNA.
Define differential gene expression.
Different cell types expressing **different subsets** of the same genome, so each makes a different set of proteins.
What is cell differentiation, in terms of genes?
A cell settling into a **stable pattern** of which genes it expresses — a 'cell type' is just a particular set of switched-on genes.
A neuron and a red blood cell both have the haemoglobin gene. Why does only one make haemoglobin?
The red-blood-cell precursor **expresses** (switches on) the gene; the neuron keeps it **off**, so only the red blood cell makes the protein.
4.5.27 cards
What is the MAIN control point of gene expression?
**Transcription** — whether the gene is copied into **mRNA** at all.
What is a transcription factor?
A **protein** that binds a specific **regulatory sequence** in the DNA and controls whether a gene is transcribed.
Where do transcription factors bind?
To **regulatory sequences** in the DNA, such as the **promoter** or an **enhancer**.
What does an ACTIVATOR do?
It **helps RNA polymerase bind the promoter**, so transcription **starts** — the gene is switched **ON** (mRNA made).
What does a REPRESSOR do?
It **blocks RNA polymerase / the promoter**, so transcription is **prevented** — the gene is switched **OFF** (no mRNA).
Why do different cell types express different genes from the same DNA?
Because they contain **different sets of transcription factors**, so different genes are transcribed.
How can a hormone change which genes are expressed?
It can **act as, or switch on, a transcription factor** that binds specific genes and turns them on in target cells.
4.5.36 cards
Define epigenetics.
**Heritable changes in gene expression** that do **NOT change the DNA base sequence**.
What does DNA methylation do to a gene?
Methyl (**CH₃**) groups are added to the DNA (often at the **promoter**), which **blocks transcription** and **silences** the gene (switches it OFF).
How does histone modification control gene expression?
It changes how **tightly the DNA is packed**: **tightly packed = OFF** (hidden from RNA polymerase), **loosely packed = ON** (accessible).
Tightly packed DNA — is the gene ON or OFF?
**OFF** — condensed DNA is inaccessible, so RNA polymerase cannot reach the gene.
How is an epigenetic change different from a mutation?
A **mutation changes the base sequence**; an **epigenetic** change only changes **whether the gene is expressed** — the sequence is unchanged.
Are epigenetic marks permanent?
No — they are **reversible**, and they can also be **inherited** (copied to daughter cells).
4.5.47 cards
Name four environmental factors that can change epigenetic marks.
**Diet, stress, toxins and temperature** — each can add or remove marks and switch genes on or off.
How does the environment change gene expression?
It **alters epigenetic marks** (e.g. **methylation**) on top of the DNA, switching genes on or off — **without changing the base sequence**.
Are epigenetic changes inherited? How?
Yes — **through mitosis** to daughter cells (maintaining a differentiated state), and **sometimes across generations** to offspring.
Epigenetic change vs mutation — the key difference?
An **epigenetic change** alters the **marks** (base sequence unchanged, **reversible**); a **mutation** changes the **DNA base sequence** (usually **permanent**).
Why can identical twins end up different?
Same DNA, but **different environments change their epigenetic marks** over time → **same genotype, different phenotype**.
In one line, what does epigenetics explain?
How the **same genotype** can give **different phenotypes**, depending on the **environment** and the cell's **history**.
Define an epigenetic mark.
A **chemical tag on top of the DNA** (e.g. a methyl group) that changes **gene expression without changing the base sequence**.
4.5.57 cards
Define a mutation.
A **permanent change to the DNA base sequence**. It is heritable, **not normally reversible**, and can change the **structure** of the protein.
Define an epigenetic change.
A change in **gene expression** (via methylation / histone tags) **without altering the base sequence**. It is **reversible** and changes the **amount** of protein made.
Mutation vs epigenetic change — the one-question test?
Did the **DNA base sequence change**? **Yes = mutation**; **no (but expression changed) = epigenetic change**.
Which is reversible — a mutation or an epigenetic change?
An **epigenetic change** is reversible (a tag can be added/removed); a **mutation** is not normally reversible.
How does each affect the protein?
A **mutation** can change the protein's **structure**; an **epigenetic change** changes the **amount** of (normal) protein made.
How is gene expression measured?
By the **amount of mRNA (or protein)** a gene produces. **More mRNA = more highly expressed**; near-zero mRNA = the gene is switched off.
Same gene, different mRNA amounts in two cell types — what does it mean?
The cells differ in **expression**, not in their DNA — the same gene is read more strongly in one cell (often an epigenetic difference).
Topic 4.5 study notes
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