Mr. Nichols
PHHS

Just a quick overview of what we’re going to cover…
 What ribosome is and what its subunits are
 The purpose of ribosome
 The process of protein synthesis, including:
 DNA to mRNA (transcription)
 mRNA to protein (translation)
 Initiation
 Elongation
 End of translation

Just a quick overview of what we’re going to cover…
 Structures of the two ribosome subunits



The larger subunit
The smaller subunit
RNA’s relation to their structure

What is ribosome?


Ribosome - protein synthesizer consisting of two subunits
Larger one, “50S”, is upper picture. Smaller is “30S”
(They look the same size here because of space restrictions.)

50S and 30S???
 Related to their respective sizes. Numbers actually measures of how quickly each subunit sinks to the bottom of a container of liquid when spun in a centrifuge
 One subunit smaller than other, but both are larger than average protein

A couple more nifty pictures…
 50S (left) and 30S. This time you can see them from different angles, through different style of picture

So what’s the purpose of ribosome?
 Ribosome basically a protein factory.
Subunits each have role in making of proteins
 To understand exactly what each subunit does, it’s necessary to walk through protein synthesis step by step

Protein synthesis


Process starts from
DNA through
“transcription”
“Translation” is where ribosome comes in.
Translation occurs when protein formed from code on mRNA
 Ribosome carries out the translation of the nucleotide triplets

Protein synthesis
 Chart - visual image of transcription and translation in protein synthesizing
 DNA and RNA have nucleotides that determine kind of protein
 3 nucleotides = 1 amino acid of a protein

Ribosome and RNA
 mRNA with code for proteins located at 30S subunit
 tRNAs responsible for carrying amino acids to mRNA. Each tRNA has own nucleotide triplet which binds to matching triplet on mRNA, ex., tRNA with code AAA (triple adenine) would match up with mRNA that has code UUU (triple uracil)

Initiation:
The first phase of translation
 Translation begins when mRNA attaches to the 30S
 tRNA comes and binds to mRNA where nucleotide code matches
 This triggers 50S binding to 30S. 50S is where all tRNAs will bind. Now we move on to elongation

Elongation:
The second phase
 Two binding sites on
50S: A site and P site, which aid in continuing translation
 First tRNA connected at
A site. Now moves to P site as another tRNA approaches
 Second tRNA binds to
A site

Elongation (continued)
 Peptide bond forms between amino acids of tRNAs (methionine and proline)
 First tRNA now detached from its amino acid, and it leaves ribosome.
Second tRNA still has proline and methionine attached

Elongation (continued)
 The tRNA left now moves to P site.
Ribosome ready to accept another tRNA and continue process


Each tRNA adds another amino acid to growing peptide chain
(thus “elongation”)
Eventually process has to finish, however…

End of translation
 Ribosome was moving along nucleotide triplets one by one
 Ribosome reaches
“stop codon,” peptide chain finished. Last tRNA leaves ribosome, leaving behind completed peptide chain

End of translation (continued)
 Ribosome separates from mRNA
 Ribosome subunits also separate, and will remain this way until another mRNA comes along to restart the process

Amino Acids of the Day!!!
Isoleucine Leucine
Both of these are considered “Essential Amino Acids” as your cells do not naturally produce them. Work very similarly in the body, most differences arise in what enzymes they’re included in.

Very Hydrophobic

Initiation: The first phase of translation
 Translation begins when mRNA attaches to the 30S
 tRNA comes and binds to mRNA where nucleotide code matches
 This triggers 50S binding to 30S. 50S is where all tRNAs will bind. Now we move on to elongation

Elongation:
The second phase
 Two binding sites on
50S: A site and P site, which aid in continuing translation
 First tRNA connected at
A site. Now moves to P site as another tRNA approaches
 Second tRNA binds to
A site

Elongation (continued)
 Peptide bond forms between amino acids of tRNAs (methionine and proline)
 First tRNA now detached from its amino acid, and it leaves ribosome.
Second tRNA still has proline and methionine attached

Elongation (continued)
 The tRNA left now moves to P site.
Ribosome ready to accept another tRNA and continue process


Each tRNA adds another amino acid to growing peptide chain
(thus “elongation”)
Eventually process has to finish, however…

End of translation
 Ribosome was moving along nucleotide triplets one by one
 Ribosome reaches
“stop codon,” peptide chain finished. Last tRNA leaves ribosome, leaving behind completed peptide chain

End of translation (continued)
 Ribosome separates from mRNA
 Ribosome subunits also separate, and will remain this way until another mRNA comes along to restart the process

Summary:
A ribosome is made from complexes of RNAs and proteins and is therefore a ribonucleoprotein. Each ribosome is divided into two subunits: the smaller subunit binds to the mRNA pattern, while the larger subunit binds to the tRNA and the amino acids.
When a ribosome finishes reading an mRNA molecule, these two subunits split apart. Ribosomes use Peptidyl transferase to link amino acids together is performed by the ribosomal
RNA.