7.3

Ribosomes are made of…

\-Protein (stability)

\-ribosomal RNA (for catalytic activity)

Subunits of RNA

\-small→ mRNA binding site

\-large→ 3 binding sites: A, P and E sites

RIbosomes are found…

\-freely floating in the cytosol

\-bound to the rough ER (eukaryotes)

Ribosomes sizes

70S→ prokaryotes

80S→eukaryotes

Acceptor stem

Carries an amino acid

Anticodon

On the opposite side of the acceptor stem

\-Complementary to the mRNA codon for a particular amino acid

Bonds that hold together the clover shape

Hydrogen bons

T arm

Associates with the ribosome (via the E, P and A binding sites)

D arm

associates with the tRNA activating enzyme

Binding of an amino acid to the tRNA acceptor stem

\-enzyme binds ATP to the amino acid, forming an amino acid-AMP complex

\-The amino acid is coupled to tRNA and then AMP

Steps of translation

\-initiation

\-elongation

\-translocation

(-termination)

Large subunit binding sites in order

E P A, synthesis happends from A to E (starts from P)

Initiation

\-the small ribosomal subunit bind to the 5’ end of the mRNA and moves along it reaches a start codon (AUG)

\-appropiate tRNA molecule bind to the codon through the anticodon in the P side and forms a complex with the small subunit

Elognation

\-Second tRNA molecule pairs with the next codon in the ribosomal A site

\-A peptide bond is formed between the amino acids (covalent and condensation)

\-the tRNA on the P side deattaches from the amino acid

Translocation

\-the ribosome moves from a 5’ to 3’ memory

\-The deacylated tRNA moves to the E site and is released

\-Another tRNA molecule attaches to the next codon in A site and the process is repeated

Termination

\-Until a stop codon is reached process continues

\-The stop codon binds to a **release factor** that signals for the translation to stop

\-The polypeptide is released and the ribosome disassembles back into its two independent subunits

What happens after transcription in eukaryotes?

\-mRNA is transported from the nucleus via **nuclear pores** prior to translation by the ribosome

\-this requires modification to form mature mRNA

What happens after after transcription in prokaryotes?

\-Since they don’t have a nucleus transcription and translation do not need to be seperated

\-Ribosomes may begin translating the mRNA molecule while it is still being transcribed from the DNA template

\-This is possible because both transcription and translation occur in a 5’ → 3’ direction

Polysome (polyribosome) definition

\-a group of two or more ribosomes translating an mRNA sequence simultaneously

\-will appear as beads on a string (bead→ ribosome, string→ mRNA strand)

Proteins synthesised in free ribosomes

\-intercellular use within the cytosol

Proteins synthesised in bound ribosomes

\-made for secretion, membrane fixation or use in lysosomes

\-ribosome becomes bound to the ER

Primary Structure

\-the sequence of amino acids

\-formed by **covalent bonds** between amine and carboxyl groups

Secondary Structure

\-α-helices (spiral)

\-β-pleated sheets (folded)

\-**hydrogen bonding** between amine and carboxyl groups of non-adjacent amino acids

\-provides mechanical stability

\-Sequences that do not form either an alpha helix or beta-pleated sheet will exist as a random coil

Tertiary Structure

\-caused by interactions between R groups

\-includes hydrogen bonds, ionic bonds and hydrophobic interactions

\-Relative amino acid positions are important

Quaternary Structure

\-multiple polypeptides or prosthetic groups may interact to form a single protein

\-Quaternary structures may be held together by a variety of bonds

A prosthetic group

\-an inorganic compound involved in protein structure or function (e.g. the **heme group** in haemoglobin)

\-A protein containing a prosthetic group is called a **conjugated protein**

The start codon

AUG- methionine