Protein Synthesis

The Three T’sThe Three T’s

1. Transcription1. Transcription

2. Translation2. Translation

3. Termination3. Termination

Transcription in Eukaryotesie: Animal cells

Protein Synthesis in Prokaryotesie: single celled Bacteria

Where does All this Happen

Protein Synthesis

Nucleic Acids (DNA & RNA) carry the Nucleic Acids (DNA & RNA) carry the hereditary information.hereditary information.

This information is contained in codons.This information is contained in codons. What are Codons?What are Codons?

A codon is a set of three base pairs (A, T, C, G) that directs or codes for amino acids. (ex. GCU – Alanine)

How Many Amino acids?

There are 20 essential amino acids, however There are 20 essential amino acids, however they can be combined in they can be combined in any orderany order, just , just like the four nucleotides. This permits the like the four nucleotides. This permits the production of the many different proteins production of the many different proteins which let organisms grow and function. which let organisms grow and function.

Transcription

This process occurs in the nucleus.This process occurs in the nucleus. A section of DNA called a gene is unwound A section of DNA called a gene is unwound

and unzipped.and unzipped.

• A RNA copy of one of the DNA strands is made. This strand is made complimentary to the nucleotides on the the DNA Strand.

•DNA Bases: Adenine, Thymine, Cytosine, Guanine

•RNA Bases: Adenine, Uracil, Cytosine, Guanine

• DNA serves as a template for the synthesis of RNA.

•A regulatory protein binds to the promoter sequence.

•An enzyme (RNA Polymerase) binds to the promoter.

•Together they open the DNA double helix.

•RNA Polymerase proceeds down one strand moving in the 3’ to 5’ direction, as it does it assembles a complementary strand of RNA.

•Each ribonucleotide is inserted into the growing RNA strand following the rules of base pairing.

•Transcription stops when the termination sequence is reached.

•The completed RNA copy is now called messenger RNA or mRNA and carries the coded message to the ribosomes in the cytoplasm.

DNA Strand 1: AGCTATCGAGCAT

DNA Strand 2: TCGATAGCTCGTA

RNA copy: AGCUAUCGAGCAU

Transcription

RNA polymerase (enzyme) attaches to the promoter region of the DNA

Translation

The small subunit of the ribosome binds to a site on the The small subunit of the ribosome binds to a site on the 5’ 5’ endend of the start of the message strand of the start of the message strand

The ribosome moves The ribosome moves downstreamdownstream (5 (53’) until it 3’) until it encounters the start codon encounters the start codon AUGAUG

At this time, the large subunit joins and the “At this time, the large subunit joins and the “initiator t-initiator t-RNARNA” enters the scene” enters the scene

After this, the tRNA binds to the After this, the tRNA binds to the p sitep site on the ribosome on the ribosome The first amino acid that starts this sequence of peptide The first amino acid that starts this sequence of peptide

bonds is Methionine (one of the 20 a.a.’s)bonds is Methionine (one of the 20 a.a.’s) Eucaryotes:Eucaryotes: methionine methionine Bacteria:Bacteria: f Met (modified) f Met (modified)

T RNA

1. Ribosome attaches to mRNA and reads first codon. 2. A transfer RNA molecule (tRNA for short) brings the correct amino acid to the ribosome & drops it off. ex: AUG (codes for methionine) tRNA brings methionine to ribosome.

3. The ribosome moves on to the next codon and another tRNA brings the next amino acid. 4. The amino acids in the growing chain are linked together by a peptide bond. The growing chain is called a polypeptide or protein molecule. 5. When the ribosome reaches a STOP codon the polypeptide is released

and the mRNA falls off.

Elongation more specifically!

As another As another tRNAtRNA arrives with its associated a.a. arrives with its associated a.a. and binds to the next codon site, it becomes and binds to the next codon site, it becomes covalently linked to the incoming a.a. with a covalently linked to the incoming a.a. with a peptide bondpeptide bond

The initiator The initiator tRNAtRNA is released from the P site is released from the P site The ribosome moves downstream (5The ribosome moves downstream (53’) 3’) The more recently-arrived The more recently-arrived tRNAtRNA w/ its peptide w/ its peptide

moves to P site and opens the A site for the arrival moves to P site and opens the A site for the arrival of a NEW of a NEW tRNAtRNA and its corresponding a.a. and its corresponding a.a.

Termination

End of the message is a STOP codonEnd of the message is a STOP codon STOPSTOP codons: codons: UAA, UAG, UGAUAA, UAG, UGA Protein release factorProtein release factor sees these codons at sees these codons at

the A site and tells the polypeptide chain to the A site and tells the polypeptide chain to release from the ribosome siterelease from the ribosome site

Ribosome splits into its subunits until it is Ribosome splits into its subunits until it is time to make more protein!time to make more protein!

Termination

Check out this link for an animation on Protein Synthesis http://www.lewport.wnyric.org/http://www.lewport.wnyric.org/

jwanamaker/animations/Proteinjwanamaker/animations/Protein%20Synthesis%20-%20long.html%20Synthesis%20-%20long.html