7.3

Ribosomes are made of…

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