1 Protein Synthesis Protein Synthesis (From Nucleus to (From Nucleus to Cytoplasm) Cytoplasm)
Transcript
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1 Protein Synthesis (From Nucleus to Cytoplasm)
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2 The Central Dogma www.video.sina.com.cn
www.video.sina.com.cn
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3 REPLICATION TRANSCRIPTION mRNA TRANSLATION PROTEIN DNA tRNA
Amino Acids Ribosomes rRNA Protein The Central Dogma (in equation
form)
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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).
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5 Visual Examples of Transcription
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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.
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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).
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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.
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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.
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10 START Codon STOP Codon
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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.
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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.
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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.
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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
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15 iv. What was the DNA sequence of triplets that coded for
this mRNA? TTT CGA TGG CCA ATG CAG ATC
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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.
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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.
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18
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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.
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20
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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
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22 HINT: The anticodon will be the same as the original DNA
triplet except that a U will replace a T.
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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)
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24 A-site
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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.
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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.
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27 P-site A-site
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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.
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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!