SCB Lecture 9: Protein Synthesis

What is Protein Synthesis?

Process of making proteins from DNA via RNA

What does Protein Synthesis mean?

Production of Proteins

Where does Protein Synthesis occur?

Cytoplasm

What are Genes?

Portions of DNA that provide genetic information necessary to build a protein

What is Gene Expression?

Production of a protein from a specific gene, determines what proteins are made

Where is DNA located?

Nucleus

What are the 3 key differences between RNA and DNA?

RNA composed of one strand

RNA has sugar ribose that contains additional oxygen atom'

RNA contains Uracil instead of Thymine

What are the two major steps in Protein Synthesis?

Transcription and Translation

Where does Transcription occur?

Nucleus

What happens in Transcription?

Gene is copied, creating a molecule of mRNA (messenger RNA).

Transcribes the DNA into a message.

What is the RNA copy during Transcription called?

mRNA, messenger RNA

What is mRNA?

Single stranded nucleic acid that carries a copy of the genetic code for a single gene out of the nucleus.

Are mRNA bound to the nucleus?

NO. Exits the nucleus through nuclear pores and enters the cytoplasm, then into a ribosome

What do Ribosomes do? What are they composed of?

Making of proteins, composed of rRNA and proteins

What is the first step in Transcription?

Transcription factors bind and activate promoter of a gene

What is the second step in Transcription?

Enzyme RNA Polymerase binds to the promoter (gene).

What is the third step in Transcription?

Using the template strand as a guide, RNA Polymerase uses the base pair rule to assemble free nucleotides, into a complementary strand of RNA (A,U,G,C, etc), creating mRNA strand.

After DNA is copied into mRNA, double helices go back together

What is the fourth step in Transcription?

RNA reaches a stop codon that signals the end of the gene. The enzyme RNA Polymerase detaches.

What needs to happen before “pre-mRNA” leaves the nucleus?

RNA Processing; Splicing.

What does Splicing do?

Removes non-coding regions from pre-mRNA transcript.

What is the removed segment called of the pre-mRNA transcription?

Introns

What is the segment that stays during Splicing?

Exons, remaining exons are pasted together

What happens after RNA Processing?

mRNA can exit the nucleus through nuclear pores into the cytoplasm to find ribosomes.

What is Translation?

Second step of protein synthesis, mRNA is converted into amino acids,

Process of synthesizing a chain of amino acids (polypeptide)

Where does Translation occur?

Ribosomes

What 2 things are Translation assisted by?

Ribosomes and Transfer RNA (tRNA)

Do Ribosomes have membranes?

Ribosomes are non-membrane bounded organelles

What are 3 characteristics of Ribosomes

• Consist of Large and Small subunits

• Composed of Ribosomal Proteins and Ribosomal RNA (rRNA). These two things compose the structure of the ribosome

• May exist free or attached to membranes (rough ER)

How many binding sites do Ribosomes have? What are the names?

A site (Aminoacyl)

P site (Peptidyl)

E site (exit)

What is the Aminoacyl (A) site? What does it do?

The Aminoacyl site binds to the incoming tRNA molecule

What is the Peptidyl (P) site? What does it do?

The Peptidyl site is where the Amino Acid is added to the growing protein

What is the Exit (E) site? What does it do?

The Exit site is where used tRNA departs

What is the function of tRNA? What is tRNA composed of/structure?

• Picks up specific amino acids and transfers it to the growing polypeptide chain.

• Composed of 1 amino acid, hydrogen bonds, RNA strand, and Anticodon

• 3 leaf clover formed by hydrogen bonds

What do Anticodons contain?

3 Nucleotides complementary to a specific mRNA codon.

What is on the opposite side of Anticodons in tRNA?

Amino Acid

What do mRNA codons specify for?

Amino Acids

ex. UUC specifies for Phenylalanine

What does the mRNA codon bind to?

Anticodon on tRNA

Does the Amino Acid on tRNA correspond to the anticodon or the mRNA codon?

mRNA Codon.

ex. mRN codon UUC specifies amino acid Phenylalanine.

What is one Met/Start codon?

AUG

What are 3 stop codons?

UAA, UAG, UGA

What happens during Translation?

Ribosome holds mRNA transcript between 2 subunits (large and small) to allow interaction with tRNAs.

What is the first step in Translation?

Initiation. tRNA binds to mRNA start codon (AUG) in the Ribosome.

Brings Large Ribosomal Unit and Small Ribosomal Subunit together.

What Amino Acid does tRNA carry when the anticodon binds to the mRNA start codon?

Methionine

What is the Large subunit composed of?

tRNA in the P site, E site, A site

What is the second step in Translation?

Another tRNA binds to open site A, with an anticodon complementary to next mRNA codon.

First amino acid joins Second amino acid by peptide bonds.

What is the third step in Translation?

Empty tRNA shifts from P site into E (Exit) site and leaves. tRNA that was in A site moves into P site.

Repeats as ribosome moves to next codon on mRNA.

What is Elongation in Translation?

When the ribosome keeps moving down to the next mRNA codon. Empty tRNA exits from the E site, and new tRNA comes into the A site.

When does Translation stop?

When ribosome reaches a stop codon (UAG, UAA, or UGA) and polypeptide is released.

What is Termination?

When Translation ends

What is Post- Transitional Modification?

Folding of peptide into proper 3-dimensional shape. Occurs in the Cytosol or Rough ER.

Where are polypeptides destined for Cytosol synthesized?

Free Ribosomes

Where are polypeptides destined for secretion/ insertion into organelles/ membranes synthesized?

Synthesized on Bound Ribosomes, modified in Rough Endoplasmic Reticulum