1 Protein Synthesis (From Nucleus to Cytoplasm). 2 The Central Dogma .

1 Protein Synthesis (From Nucleus to Cytoplasm)

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

3 REPLICATION TRANSCRIPTION mRNA TRANSLATION PROTEIN DNA tRNA Amino Acids Ribosomes rRNA Protein 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 (4 3 = 64 possibilities), meaning that three nitrogenous bases, as a group, code for ONE amino acid. A singlet code could only code for 4 1 = 4 amino acids (yet, there are 20) thus proving inadequate. A doublet code could only code for 4 2 = 16 amino acids. A group of three mRNA nucleotides (bases) is called a CODON. In total, there are 64 (4 3 ) 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? 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: i.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) 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.

19 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.

21 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) 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

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

23 Three Steps of Translation 1.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)

24 A-site

25 2.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.

26 -- before it is bumped off of the ribosomes 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.

27 P-site A-site

28 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.

30 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 PUMPYOU UP! Especially you girly men!

1 Protein Synthesis (From Nucleus to Cytoplasm). 2 The Central Dogma .

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Transcript of 1 Protein Synthesis (From Nucleus to Cytoplasm). 2 The Central Dogma .