Mutations and gene editing

A **random change to the base sequence of DNA**.

They are the **source of new alleles** — the ultimate origin of all genetic variation.

By **changing the base sequence** of an existing gene, producing a new version (allele) of it.

**Substitution**, **insertion** and **deletion**.

One base is **swapped for a different base**; the total number of bases stays the same.

An extra base is **added** into the sequence; the total number of bases increases.

A base is **removed** from the sequence; the total number of bases decreases.

A shift in the reading frame so every codon downstream is read differently — caused by **insertion or deletion**.

Because it **does not change the number of bases** — the reading frame stays the same, so only one codon is affected.

**Count the bases**: same number (one letter different) = substitution; one more = insertion; one fewer = deletion.

Anything that **increases the rate of mutation** — for example **UV light**, X-rays or certain chemicals.

**No** — they can be harmful, neutral or beneficial; they are random changes.

Both are **random changes to the DNA base sequence** and both can produce a **new allele**.

Substitution **swaps** a base (number unchanged); insertion **adds** a base (number increases, causing a frameshift).

A mutation in a **gamete** (egg/sperm) or a gamete-forming cell. It **can be inherited** by offspring.

A mutation in any **body cell other than a gamete-forming cell**. It **cannot be inherited**.

A **germline** mutation — it is in a gamete (or gamete-forming cell), so it is passed to offspring through reproduction.

A **gamete-forming (germline) cell** — for example a cell in the **testis or ovary**, or a sperm or egg.

An **agent that increases the rate of mutation** — e.g. UV light, X-rays, or chemicals in tobacco smoke.

**Radiation** (UV light, X-rays) and **chemicals** (e.g. those in tobacco smoke).

A **mutagen that increases the risk of cancer** (for example the chemicals in tobacco smoke).

A disease in which body cells **divide uncontrollably**, forming a **tumour** that can invade and spread.

A mutation in a gene controlling **cell division** → the cell **divides uncontrollably** → mutations **accumulate** → a **tumour** forms.

It requires an **accumulation of several mutations** in the same cell line before division becomes fully uncontrolled.

Chemicals in smoke are **mutagens** → they cause **mutations** in lung-cell DNA (cell-division genes) → **uncontrolled division** → a **tumour**.

**No** — cancer arises from **somatic** mutations in body cells, so it is not passed to offspring (only an inherited *risk* can run in families).

A **base substitution** — one base in the haemoglobin gene is swapped for another.

**Haemoglobin** — the oxygen-carrying protein in red blood cells.

**Glutamic acid is replaced by valine** in the haemoglobin chain.

Just **one base** in the gene, which changes just **one amino acid** in the protein.

**Sickle haemoglobin** — the abnormal haemoglobin made by the sickle-cell allele. It sticks together into fibres when oxygen is low.

When oxygen is low, abnormal haemoglobin (HbS) **sticks together into fibres** that pull the cell into a rigid sickle (crescent) shape.

A mutation in which **one base in the DNA is replaced by a different base**.

The **observable characteristics** of an organism — here, the symptoms of sickle-cell anaemia.

They are **rigid** and get stuck, **blocking small blood vessels (capillaries)**.

Sickled cells **carry less oxygen** and are **destroyed faster**, so tissues get less oxygen and there are too few red blood cells.

Base substitution -> changed codon -> one amino acid changed (glutamic acid -> valine) -> abnormal haemoglobin -> sickled cells -> sickle-cell anaemia.

The gene is a **code**: one base change can change one codon, then one amino acid, then the **shape and behaviour** of the whole protein.

A change in the **number (or structure) of whole chromosomes**, rather than a change to the DNA bases of one gene.

The **failure of chromosomes (meiosis I) or sister chromatids (meiosis II) to separate** during meiosis, so both copies end up in the same gamete.

'Disjunction' = separating, so **non-disjunction = not separating**.

One gamete with an **extra chromosome (n + 1)** and another **missing that chromosome (n − 1)**.

Having an **abnormal number of chromosomes** — one too many or one too few — rather than a whole extra set.

Having **three copies** of a particular chromosome instead of the normal two.

**Chromosome 21** — three copies is called **trisomy 21**.

Chromosome 21 **fails to separate** in meiosis → a gamete gets an **extra copy** → **fertilisation** adds a third copy → **trisomy 21**.

The whole body grows from the single zygote by **mitosis**, so **every cell** inherits the extra chromosome.

A gene mutation changes **a few DNA bases** in one gene; a chromosome mutation adds or loses a **whole chromosome** and can be seen on a karyogram.

It **increases with age**, slowly at first and then **steeply** at older ages.

**Yes** — an extra or missing whole chromosome shows up as an extra (or absent) band, unlike a tiny gene mutation.

Deliberately changing an organism's DNA — for example by **adding a gene** from another organism or **editing** an existing gene.

A GM organism that carries a gene **transferred from a different species**.

**Cuts** DNA at a specific recognition sequence, often leaving short single-stranded **sticky ends**.

**Joins** two pieces of DNA by re-forming the **sugar–phosphate backbone** — it seals the gene into the vector.

A small loop of DNA (often a bacterial **plasmid**) that **carries a gene into a host cell**.

A single DNA molecule made by **joining DNA from two different sources** (e.g. a plasmid with a new gene added).

So both have the **same, matching sticky ends**, which are **complementary** and can base-pair together before ligase seals them.

Restriction enzyme **cuts** → DNA ligase **joins** (recombinant DNA) → vector **carries** the gene into the host → host **expresses** it.

The **uptake of the recombinant plasmid (vector) by a host cell**, after which the gene is expressed.

A **guide RNA** base-pairs with the chosen target sequence and leads the **Cas9** protein there to **cut** the DNA.

CRISPR **edits / knocks out a gene already in the cell**, rather than **adding** a foreign gene.

Higher **yield**, less crop lost to weeds/pests, less spraying, or more nutritious / drought-tolerant crops.

GM genes could **spread to wild plants**, long-term effects are **uncertain**, seeds are **patented/costly**, or there are **ethical** objections.

**Restriction enzyme cuts**; **DNA ligase joins**.

**PCR** (copies the DNA) then **gel electrophoresis** (separates the copies by size).

**Polymerase chain reaction** — it makes **millions of copies** of a chosen piece of DNA (amplification).

The **amplification (copying)** stage.

**Denaturation** (~95 °C), **annealing** of primers (~55 °C) and **extension** by Taq polymerase (~72 °C).

The DNA is heated to ~95 °C, which **separates the double helix into two single strands**.

The mixture cools to ~55 °C so that short **primers bind** to each single strand.

At ~72 °C, **Taq polymerase** adds nucleotides to build a new **complementary strand**.

Taq is **heat-stable**, so it survives the ~95 °C step that would destroy a normal enzyme.

It **doubles** — repeated cycling gives millions of copies.

It **separates DNA fragments by size** using an electric field.

Because DNA is **negatively charged**, so it is pulled toward the positive electrode.

**Smaller (shorter) fragments** — they slip through the gel more easily.