1 / 30

Protein Synthesis (From Nucleus to Cytoplasm)

Protein Synthesis (From Nucleus to Cytoplasm). The Central Dogma. www.video.sina.com.cn. Amino Acids. tRNA. TRANSCRIPTION. TRANSLATION. DNA. mRNA. PROTEIN. Ribosomes. REPLICATION. Protein. rRNA. The Central Dogma (in equation form). Initial Problem:

rehan
Download Presentation

Protein Synthesis (From Nucleus to Cytoplasm)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Protein Synthesis(From Nucleus to Cytoplasm)

  2. The Central Dogma www.video.sina.com.cn

  3. Amino Acids tRNA TRANSCRIPTION TRANSLATION DNA mRNA PROTEIN Ribosomes REPLICATION Protein rRNA The Central Dogma (in equation form)

  4. Initial Problem: DNA codes for proteins BUT DNA is confined to the nucleus and the ‘equipment’ to carry out protein synthesis is in the cytoplasm. (The equipment includes ribosomes, tRNA, and amino acids). Solution??? DNA can be “copied” into an RNA molecule (messenger RNA (mRNA)), which can travel from the nucleus to the cytoplasm carrying the instructions for building a protein. This DNA to mRNA copying process is known as transcription (see fig. 25.7 on p. 512).

  5. Visual Examples of Transcription

  6. Transcription • During this process, DNA serves as a template (guide) for the production of mRNA • The enzyme DNA helicase serves to unwind and ‘unzip’ the portion of the DNA double helix (ie. The GENE) that is to be transcribed (copied into mRNA). • Once this occurs, free-floating RNA nucleotides (within the nucleus along with free-floating DNA nucleotides) bond to the exposed bases of DNA through complementary base-pairing (forming hydrogen bonds). ** RNA nucleotides bind to only ONE exposed DNA strand.

  7. A (mRNA) binds with T (DNA). U (mRNA) binds with A (DNA). C (mRNA) binds with G (DNA). G (mRNA) binds with C (DNA).

  8. The first three mRNA bases are always AUG (called the start codon), which means that the first three DNA bases that are transcribed are TAC. • The last three mRNA bases are always one of UAG, UAA, or UGA (STOP codons), which means that the last three DNA bases transcribed are always one of ATC, ATT, or ACT. • The enzyme RNA Polymerase then works to join added mRNA nucleotides to each other (sugar-P-sugar-P etc…) through dehydration synthesis (producing water). • So, the mRNA nucleotides form spontaneous H-bonds with exposed DNA bases but then need enzymatic aid (RNA polymerase) to form the actual mRNA chain.

  9. DNA helicase continues to unwind/unzip the DNA until the gene that requires copying has been fully exposed. • The signal for DNA helicase to stop is when it encounters ATC, ATT, or ACT. A poly-adenine tail is added to one end of the finished mRNA while a guanine-based ‘cap’ is added to the other side to protect the molecule from cytoplasmic enzymes. Once this is finished, the mRNA moves into the cytoplasm, through a nuclear pore, and the DNA joins back together by reforming its complementary base-pairing H-bonds.

  10. START Codon STOP Codon

  11. The Genetic Code • Query: How is a sequence of nitrogenous bases on mRNA going to be used to code for a sequence of amino acids and hence, a protein??? • First of all, DNA is the universal code (ie. Every living thing has DNA, and DNA codes for proteins (through the use of mRNA)). • There are 20 amino acids in nature. • There are 4 different nitrogenous bases in both DNA and mRNA, and they serve as a ‘code’ for the amino acid sequence of proteins.

  12. The code reads as a TRIPLET CODE (43 = 64 possibilities), meaning that three nitrogenous bases, as a group, code for ONE amino acid. • A singlet code could only code for 41 = 4 amino acids (yet, there are 20) – thus proving inadequate. • A doublet code could only code for 42 = 16 amino acids. • A group of three mRNA nucleotides (bases) is called a CODON. In total, there are 64 (43) different codons. • A group of three DNA nucleotides (bases) is called a DNA triplet.

  13. There are 61 different codons that correspond to the 20 different amino acids (one of these 61 is AUG, which is the ‘start’ codon, that codes for the amino acid methionine). • The other three codons are called STOP codons, which terminate the formation of the polypeptide/protein chain. • These STOP codons DO NOT code for an amino acid. • The Genetic code is sometimes referred to as being redundant, because most amino acids are coded for by 2-6 different codons.

  14. / / / / / / • AAA GCU ACC GGU UAC GUC UAG mRNA sequence • Questions: i. How many codons present? ii. How many a. acids coded for? iii. What is the a. acid sequence? lysine, alanine, threonine, glycine, tyrosine, valine, STOP 7 6

  15. iv. What was the DNA sequence of triplets that coded for this mRNA? TTT CGA TGG CCA ATG CAG ATC

  16. Translation • The process of turning mRNA into a protein (ie. Translating the language of nitrogenous bases into the language of amino acids). • Recall that mRNA is constructed in the nucleus through the process of transcription, and is sent out of the nucleus through a nuclear pore. • Once mRNA enters the cytoplasm, it immediately associates with a ribosome (either a ‘free’ ribosome or a Rough ER ribosome). • The ribosome attaches to the mRNA at the guanine-based ‘cap’ that not only served as protection from enzymes, but acted as a ‘start here’ signal.

  17. Translation requires two other types of RNA: • rRNA (Ribosomal RNA) -- joins with ribosomal proteins (from nucleolus) to form ribosomes. -- produced in the nucleolus. -- one ribosome has two subunits: a. Large subunit (3 rRNAs and proteins) b. Small subunit (1 rRNA and proteins) -- the two subunits remain close together but do not actually attach until just prior to translation. -- rRNA is not involved in any coding or translating, it is purely structural.

  18. ii. tRNA (Transfer RNA) -- located in the cytoplasm and serve as ‘carriers’ of singular amino acids to the mRNA/ribosome complex. -- tRNAs carry one amino acid at one end, and a specific ANTICODON at the other end. -- this anticodon will ‘match-up’ with a complementary codon on mRNA (through complementary base-pairing) (fig. 25.8 p. 513). -- tRNA molecules are very specific (ie. A tRNA with a certain anticodon will ALWAYS be carrying the same amino acid). -- remember, though, that the translation table is translated from mRNA codons, not the tRNA anticodons.

  19. Eg. If the mRNA codon being translated is ACG, what anticodon and amino acid will the tRNA molecule, specific to this codon, be carrying? Ans. Anticodon = UGC Amino Acid = Threonine (need table) Eg 2. Codon = CAA, find anticodon and a. acid. Anticodon = GUU, A. acid = Glutamine Eg 3. DNA triplet = TTC, find mRNA codon, tRNA anticodon, and amino acid. mRNA codon = AAG, anticodon = UUC Amino acid = Lysine

  20. HINT: The anticodon will be the same as the original DNA triplet except that a U will replace a T.

  21. Three Steps of Translation • INITIATION -- the ‘cap’ of mRNA binds to the ribosome and the ribosome moves along the mRNA, ‘reading’ it, until it comes upon the ‘start’ codon, AUG. -- the tRNA with anticodon UAC binds to the AUG codon (through complementary base-pairing) at the A-site, and delivers the first amino acid Methionine. -- see handout (crude handout, that is)

  22. A-site

  23. ELONGATION (lengthening of the amino acid chain). -- firstly, the ribosome is large enough to accommodate two tRNAs at the same time  the ‘incoming’ tRNA (at the A-site) and the ‘outgoing’ tRNA (at the P-site). -- after the first tRNA binds to the mRNA codon, the ribosome shifts one codon (3 bases), thus exposing a new codon in the A-site which can then be bonded to by a new tRNA with the complementary anticodon. -- after this next tRNA binds, the ribosome shifts again and ‘bumps’ the tRNA in the P-site off of the ribosome.

  24. -- before it is bumped off of the ribosome’s P-site, the outgoing tRNA molecule always passes the amino acid chain, via dehydration synthesis, to the tRNA that is shifting from the A-site to the P-site. -- the liberated or bumped tRNA (now without an amino acid) will eventually pick up the same amino acid that it just ceded and will rejoin the group of tRNAs waiting to be ‘chosen’. -- amino acids are readily available in the cytoplasm. -- the ribosome continually shifts to accept more tRNA molecules so that the protein chain can grow one amino acid at a time.

  25. P-site A-site

  26. iii. TERMINATION -- occurs once the codon appearing in the A-site of the ribosome is a STOP codon. -- this STOP codon is recognized by the ribosome complex and a RELEASE FACTOR protein is summoned from the cytoplasm into the A-site. -- there is NO tRNA molecule for these codons. -- once the release factor protein binds to the STOP codon, the ribosome dissociates into its two subunits and falls off the mRNA (which is recycled). -- the peptide/protein chain is released by the tRNA in the P-site into the lumen of the Rough ER (if for export), or into a transition vesicle bound for the Golgi for modifications (if it is to remain in the cell). -- see fig. 25.9 p. 514, fig 25.11 p. 516 -- read translation summary on pp. 512-516.

  27. By simply eating protein, you can build new, custom proteins and maybe look like this: Yes, and once again, I am Hans; and I am Franz; and we want to PUMP…YOU UP! Especially you ‘girly’ men!

More Related