DNA replication

The process of copying a DNA molecule to make **two identical molecules**, done before a cell divides.

Each new DNA molecule is made of **one original (parental) strand and one new strand**.

Because **half** of each new molecule — one whole strand — is **conserved** from the original.

An **old strand** used as a pattern to build a new complementary strand.

By **complementary base pairing** — A pairs with T, and C pairs with G.

**One old (parental) strand and one new strand** — never two old or two new together.

**Conservative**, **semi-conservative** and **dispersive**.

They grew bacteria in **heavy ¹⁵N** (so all DNA was heavy), then switched them to **light ¹⁴N**.

Every molecule was half-heavy and half-light (one old + one new strand) — this **ruled out the conservative model**.

Some molecules were now fully light — this **ruled out the dispersive model**, leaving only semi-conservative.

That DNA replication is **semi-conservative**.

Each daughter molecule shows **one old (template) strand paired with one new strand**.

Copying a DNA molecule to make **two identical molecules**, each with **one old (template) strand and one new strand**.

It **unwinds and unzips** the double helix by **breaking the hydrogen bonds** between the paired bases.

The **hydrogen bonds** between the paired bases (A–T, G–C) that hold the two strands together.

It **adds complementary nucleotides** to a template strand, **building the new strand** and joining the nucleotides with covalent bonds.

**Covalent bonds** that join the nucleotides along the sugar-phosphate backbone of the new strand.

In the **5'->3' direction** — it adds new nucleotides only to the 3' end of the growing strand.

**Helicase** works first to open the helix; **DNA polymerase** follows to build the new strands.

An original (parental) strand whose base sequence is **read** to decide which nucleotides go into the new strand.

**Complementary base pairing**: A pairs with T, and G pairs with C.

**Hydrogen** bonds, **between** the two strands (between the paired bases).

**Covalent** bonds, **along** a strand (the sugar-phosphate backbone of the new strand).

Because each new DNA molecule keeps **one old strand and one new strand** — half of the original is conserved.

It **amplifies** DNA — makes **many copies** of a chosen piece of DNA from a tiny sample.

It roughly **doubles** every cycle, so the increase is **exponential** (about a billion copies after ~30 cycles).

**Denaturation → annealing → extension.**

The high heat **breaks the hydrogen bonds**, separating the double helix into **two single strands**.

**Primers bind (anneal)** to their matching sequence on each single strand, marking where copying begins.

**Taq polymerase** adds **nucleotides** to each primer to build a new **complementary strand** (72 °C is its optimum).

A **short single strand of DNA** that binds to a matching sequence and marks where copying should start.

It is **heat-stable (thermostable)** — it is **not denatured** by the ~95 °C step, so the same enzyme works every cycle.

From **Thermus aquaticus**, a bacterium that lives in **hot springs**, so its enzymes tolerate high temperatures.

It **separates DNA fragments by size** so they can be seen and compared as a pattern of **bands**.

The **smaller** fragments — they slip through the gel sieve more easily. (Small = far.)

Because DNA is **negatively charged**, so the electric field pulls it towards the **positive electrode**.

It shows **no band** — with no template DNA there is nothing to amplify (it checks for contamination).

**Fainter bands** — fewer cycles means **less DNA is made** (the amount roughly doubles each cycle).

The **whole of an organism's genetic information** — **all** of its DNA, every gene and every base.

**base ⊂ gene ⊂ chromosome ⊂ genome** — a base in a gene, a gene in a chromosome, all chromosomes make the genome.

**All** of the DNA — the genome is the complete set, not a single gene or chromosome.

**Every nucleated body cell** carries a complete copy of the whole genome.

It has **no nucleus**, so it carries no DNA to copy.

A technique that reads the **variable repeated regions** of the genome to **identify an individual** or test how closely two people are **related**.

The genes are almost **identical** between people, so they cannot tell individuals apart — the **variable repeats** can.

The **number of short tandem repeats** (variable, non-coding regions) at several places in the genome.

The two individuals are **closely related** — more shared patterns means a closer relationship.

**No** — genome size does **not** correlate with complexity; some simpler organisms have larger genomes than humans.

A **gene** is one length of DNA coding for a product; the **genome** is **all** of the DNA, containing thousands of genes.

Because **every nucleated cell** holds the whole genome, so even a single cell carries all of a person's DNA.