RibosomeRibosome

a presentation by Erin Hussona presentation by Erin Husson

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

Still awake?It’s Pop Quiz Time!!!

Here’s a little quiz on what we’ve covered so far, just to keep your brain alert.

First question… What are the two subunits of ribosome known

as, and which is which? Answer: 50S (the larger one) and 30S (the

smaller one)

Another question…

(This one’s multiple choice!) The process of converting code on the mRNA

into a protein is called…? A. Transition B. Transduction C. Translation D. Transcription

And now…

That’s the end of the quiz (whew)! Any questions so far? Okay, that’s taken care of. And now, without

further ado, we move on to…

Structures of the subunits

50S and 30S besides difference in size, also have somewhat different structures

Both are rather complicated, since they’re much larger than average protein

We’ll cover structure of 50S first

The 50S subunit

This part of ribosome contains site where new peptide bonds are formed when proteins are synthesized

mRNA would be located horizontally in groove across middle

To help with protein synthesis, subunit uses adenine RNA nucleotide, shown by green dot in center

The 50S subunit

This part of ribosome contains site where new peptide bonds are formed when proteins are synthesized

mRNA would be located horizontally in groove across middle

To help with protein synthesis, subunit uses adenine RNA nucleotide, shown by green dot in center

The 50S subunit

50S made of two RNA strands: a long one, shown in an orangey color, and shorter one, shown in yellow

Blue things in the picture are a few of many proteins that have bound to ribosome’s surface

Many of proteins have long tails that go inside ribosome and keep the RNA strands in place

A bit more on 50S…

50S is quite rigid, folded and packed so well that it’s virtually immobile within its structure

Contrasts with structure of 30S, which we’ll now move to

The 30S subunit

Unlike 50S, 30S is fairly flexible It needs to have movable regions because,

when it shifts from one mRNA nucleotide triplet to another, movement is necessary to aid this process

30S controls flow of information during protein synthesis

The 30S subunit

30S finds an mRNA and then makes sure that each tRNA is matched up correctly on mRNA

It’s been suggested that mRNA enters through small hole in 30S (shown here in center of left side)

The 30S subunit

30S finds an mRNA and then makes sure that each tRNA is matched up correctly on mRNA

It’s been suggested that mRNA enters through small hole in 30S (shown here in center of left side)

The 30S subunit

mRNA then extends up into the place where translation occurs, located in cleft between top part, “head”, and bottom part, “body”

Structure and RNA

Recently discovered - about two-thirds of ribosome’s mass made up of RNA

Most important functions of ribosome performed by RNA. This has been found because of atomic knowledge of structures of 50S and 30S and their assemblage into 70S

(One might logically think it to be 80S, but it really isn’t)

Concluding…

Although a good amount of progress has been made in figuring out structures of 50S and 30S, there is probably still a lot that hasn’t been deciphered yet

It’s somewhat difficult to find out structures because, as mentioned earlier, they’re relatively large. Hopefully more will be discovered about them in the future

Guess what…It’s Quiz Time again!

This is the last one, I promise. It’s a piece of cake anyway…well, if you were paying attention, that is.

Ribosome is primarily made up of…? A. DNA B. RNA C. Proteins Glad that’s over, huh? Now to summarize

what we covered…

To summarize…

Ribosome: protein synthesizer consisting of two subunits, 50S and 30S

Protein synthesis starts from DNA, which carries the code for making the proteins. The DNA is converted to mRNA by “transcription”, and mRNA, in turn, is converted to protein by “translation”

Ribosome translates nucleotide triplets on mRNA into proteins

To summarize…

tRNAs carry amino acids to ribosome to be linked together by the ribosome. One by one, they bind to the ribosome, add their amino acid to the chain, and then depart

50S and 30S made primarily of RNA and are both larger than the average protein. 50S has rather rigid structure, while 30S has fairly flexible one. Not all about them has been discovered yet, but significant progress has been made in that respect

The EndThe End

That’s a wrap!That’s a wrap!

Any questions?Any questions?