Protein synthesis

The copying of a gene's DNA base sequence into a complementary molecule of **mRNA**. It happens in the **nucleus**.

**RNA polymerase** — it unwinds the DNA and joins RNA nucleotides into mRNA.

In the **nucleus** (where the DNA is).

A single-stranded copy of a gene that carries its base sequence **out of the nucleus to a ribosome** to be translated.

**One** — only the **template strand** is read and copied.

**Uracil (U)** — RNA has no thymine.

Template **A → U**, template **T → A**, template **C → G**, template **G → C**.

RNA contains **uracil (U)** in place of thymine; so a template adenine is copied as **U**, never T.

It **carries the gene's code out of the nucleus to a ribosome**, where it is translated into a polypeptide.

Binds the gene, **unwinds** the DNA, reads **one template strand**, pairs and **joins free RNA nucleotides** into mRNA.

**RNA polymerase** → transcription (mRNA); **DNA polymerase** → DNA replication.

**No mRNA is made**, so there is nothing for the ribosome to translate — the protein cannot be built.

A group of **three** consecutive bases on mRNA that codes for **one amino acid** (or a start/stop signal).

**Three** — the genetic code is a **triplet** code, read in non-overlapping groups of three.

The set of rules linking each mRNA **codon** to the **amino acid** it specifies; it is nearly the same in all living things.

**64** — there are 4 bases and codons are 3 bases long, so **4 × 4 × 4 = 64**. This covers 20 amino acids plus start/stop.

Almost **all organisms** use the **same codons** for the same amino acids.

Because the code is shared, a gene from one species can be **read correctly** by another (e.g. a human gene placed in bacteria).

**More than one codon** can code for the **same amino acid** (e.g. UUU and UUC both code for phenylalanine).

**UUU and UUC** both code for **phenylalanine**; **UCU and UCC** both code for **serine**.

A base substitution that changes a codon to another codon for the **same amino acid**, so the **protein is unchanged**.

Because several codons code for the same amino acid, a base change can give a different codon that still specifies the **same** amino acid.

**Split it into non-overlapping threes**, look up each codon in turn, and keep the amino acids in **order**.

**Universal** = the same code in all organisms (between species); **degenerate** = several codons for one amino acid (within the code).

The process where a **ribosome** reads an **mRNA** and builds the matching chain of amino acids (a **polypeptide**).

At a **ribosome** (in the cytoplasm).

A group of **three mRNA bases** that codes for **one amino acid** (or a stop signal).

**Three** — the mRNA is read one **codon** (3 bases) at a time.

It **brings the correct amino acid** to the ribosome; its **anticodon** base-pairs with the mRNA codon.

The **three bases on a tRNA** that base-pair with a complementary **codon** on the mRNA.

Each tRNA **anticodon** pairs with the matching **codon**, and each tRNA carries only **one specific amino acid**.

A **peptide bond**, formed by the ribosome as each amino acid is added.

A **stop codon** — no tRNA matches it, so no more amino acids are added and the polypeptide is released.

**Bases = amino acids × 3** (three bases per codon); add 3 more for the stop codon.

30 × 3 = **90 bases** (93 if the stop codon is counted).

They are caught **part-way through synthesis**; ribosomes further along the mRNA have added **more amino acids**, so their chains are longer.

Translation is **making the polypeptide** at the ribosome. Transcription is making the **mRNA** in the nucleus.

The **chain of amino acids** produced by translation, which folds up to form a **protein**.