APB Unit 6.2 - Gene Expression & Regulation #GoBioRams

APB Unit 6.2 - Gene Expression & Regulation #GoBioRams

Translation

Chapter 17

Multiple Choice

Which is the first part of the Central Dogma?

• Transcription

• Translation

Multiple Choice

What is added to the 5' end of the pre mRNA?

• Poly-A

• Cap

• Polyadenylation signal

Multiple Choice

How is the pre mRNA made?

• 3' to 5'

• 5' to 3'

Multiple Choice

Which of these pieces is expressed or kept during RNA splicing?

• Exons

• Introns

Once the mRNA enters the cytoplasm.

• A ribosome attaches.

• If the mRNA contains a Special Signal Peptide, this is translated first by the ribosome.

• The special Signal Peptide is recognized by a Signal-Recognition Particle (SRP) and the mRNA & ribosome now will be dragged to the Rough Endoplasmic Reticulum.

Attached Ribosomes

translate the mRNA at the RER.

These proteins will be "shipped out" of the cell.

Once the mRNA enters the cytoplasm.

• A ribosome attaches.

• If the mRNA does NOT contain a Special Signal Peptide, the ribosome stays "free" and does not get dragged to the RER.

• Instead, multiple free ribosomes can translated the one mRNA at the same time = Polyribosomes or Polysomes.

Free Ribosomes

stay in the cytoplasm and translated the mRNA.

The proteins that are made in the cytoplasm are needed by the cell, which is why so many ribosomes may be translating the same mRNA at the same time.

Chaperonins and Proteasomes

• These are chemicals that make sure that proteins are folded correctly during translation.

• Proteasomes are chemicals that make sure that misfiled proteins are destroyed.

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1219577/

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3524306/

Structure of a Ribosome

which is made up of two subunits.

Each subunit is made up of proteins and rRNA.

Note the need to bring in a

tRNA "blue boot" that has a Met = Methionine in order to build the Ribosome and start Translation.

APE

Ribosomes "reads" the mRNA.

Translation terminates

when the STOP codon is reached.

A Release Factor is added.

The tRNA "blue boots"

tRNAs or Transfer RNAs are located throughout the cytoplasm.

Each one has an amino acid attachment site and an Anticodon.

How does each tRNA get paired with

the appropriate amino acid?

First, the anticodon must match the amino acid.

Enzymes that belong to the aminoacyl-tRNA synthatases bind the specific amino acid to the tRNA.

Is this a perfect matchy-matchy?

No, sometimes only the two first letters of the anticodon are matched with the appropriate amino acid.

These mistakes are called Wobbles.

More about mutations shortly.

The Central Dogma

Other Mutations

• Point Mutation

• Can cause a protein to behave very differently or maybe have no effect.

Nucleotide-pair substitution

Silent with no effect on the protein.

Missense many possible impacts on the protein.

Nonsense adding a STOP codon too soon.

Nucleotide-pair insertion or deletion

Frameshift causing immediate nonsense.

Frameshift causing extensive missense.

3 Nucleotide-pair deletion or insertion.

What, then, is a gene?

"A gene is a region of DNA that can be expressed to produce a final functional product that is either a polypeptide or an RNA molecule."

https://www.ncbi.nlm.nih.gov/pmc/articles /PMC5378099/

Review

• We discussed what happens to an mRNA when it enters the cytoplasm.

• If there is a signal peptide coded in the mRNA, once it is synthesized, a signal-recognition particle (SRP) in the cytoplasm will bind to it and drag the ribosome-mRNA complex to the RER when translation will be completed for the protein that will "leave" the cell.

Review

• If there is no signal peptide coded in the mRNA, the free ribosome will complete the translation process in the cytoplasm for the protein that will be used by the cell.

• These free ribosome-mRNA complexes usually are pretty "popular" so you could find polyribosomes or polysomes.

• Ribosomes are made up of two subunits of rRNA and proteins that contain an mRNA binding site for the Start Codon - AUG, which codes for methionine (Met).

Review

• The ribosome will read the mRNA in triplets and begins moving the mRNA through the APE sites.

• Translation terminates once a Stop codon is read (UAG, UAA, or UGA), which is when a release factor attaches.

• tRNA contains the anticodon and an amino acid attachment site.

• Tyrosyl-tRAN synthetase brings together the tRNA with its amino acid but sometimes only properly reads the first two letters of the anticodon which will produce a wobble effect.

Review

• Many mutations may appear in addition to the wobble - when only the two first letters of the triplet match.

• You can have a point mutation, ex. sickle cell anemia.

• You could have a nucleotide-pair substitution which could produce a silent, missense, or nonsense mutation.

• You could have a nucleotide-pari insertion or deletion which could produce a frame-shift mutation or 3 nucleotide-pair deletion.

Review

• So, what then, is a gene?

• Please review the definition that was provided by the textbook since this is a great topic for discussion.