Protein Sythesis

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PROTEIN SYTHESIS

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Protein Sythesis. The Genome of a Eukaryotic Cell is within the nucleus. Proteins are synthesized in the cytoplasm . DNA does not leave the nucleus. Why? 1) To dangerous! If distorted, cleaved, or damaged  useless Possible death of organism 2) Proteins required in large amounts - PowerPoint PPT Presentation

Transcript of Protein Sythesis

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PROTEIN SYTHESIS

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The Genome of a Eukaryotic Cell is within the nucleus.

Proteins are synthesized in the cytoplasm

DNA does not leave the nucleus. Why?1) To dangerous!

If distorted, cleaved, or damaged useless Possible death of organism

2) Proteins required in large amounts Only 2 strands of DNA Can be many strands of different RNA chains

(coding for different proteins) Increases flexibility in carrying out various activities.

3) Leaving/re-entering the nucleus would be time-consuming.

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Central Dogma Transcription (copy from one medium to

another) DNA RNA

Translation (convert into a different language) RNA polypeptide

Dogma: “proclaimed as true without perfect proof”

Science is a study which examines ‘truth’ down to its core. As a scientist, you can never prove anything as perfectly true. You can observe and study

millions of different species or processes within species... up to the finest parts that make up matter. But a real scientist will never stop trying to find out more.

The more you learn, the more there is to know.

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RNA Ribonucleic Acid (RNA)

How does RNA differ from DNA again?

Three Kinds mRNA: messenger

Longer the gene, the longer the mRNA

tRNA: transfer Transfers appropriate AAto ribosome to build a PP Short: 70-90 ribonucleotides

rRNA: ribosomal Structural component of Ribosome.

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The Genetic Code There are _____ amino acids. How many nucleotides must code for one amino acid?

If only one nucleotide coded for one AA... 41 = 4 possible combinations Ex// thymine cysteine

If two nucleotides coded for one AA... ________ possible combinations Ex// AC cysteine

If three nucleotides code for one AA... _________ possible combinations Ex// CAU histidine

Codon: sequence of three bases in DNA or complementary mRNA that serves as a code for a particular amino acid.

More than one codon can code for a single amino acid (redundancy).

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Start Codon: signals initiation of translation. Only one in the genetic code Is usually AUG methionine

Stop Codon: does not code for an amino acid: signals termination of translation. Three of these in the genetic code UAA, UAG, UGA

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Seatwork/Homework Pg. 241, #1--13

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TRANSCRIPTION (5.3)Initiation RNA polymerase binds to specific part of DNA

that is to be transcribed helix opens. Specific. How?

Transcription factors help polymerase to bind these factors help control which genes are transcribed.

Bound upstream of the gene: promoter region. Usually high in T and A

Less energy to unwind helix RNA polymerase only recognizes the promoter region

(therefore, not random!) This region is not transcribed

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Elongation RNA polymerase builds mRNA in 5’ – 3’

direction Does not require a primer RNA poly. Uses one strand of DNA for mRNA

synthesis Template strand: strand of DNA used by RNA

polymerase to build complimentary mRNA. mRNA complimentary to this DNA strand

Coding strand: not being used for transcription. mRNA identical to the DNA strand (except for uracil)

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Termination mRNA synthesized until the end of gene is

reached Terminator sequence: sequence of bases at

end of a gene that signals the RNA polymerase to stop transcribing Differs between prokaryotes and eukaryotes mRNA dissociates with DNA RNA polymerase free to bind to another

promoter region.

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Posttranscriptional Modifications

The mRNA made by transcription is called the primary transcript.

Modifications need to be made to RNA before it can leave the nucleus CAPPING AND TAILING SPLICING

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Capping and Tailing 5’ cap: 7-methyl guanosine

Added to start of primary transcript Protects it from digestion by nucleases and

phosphatases Helps bind mRNA to ribosome

Poly-A tail: approx. 200 adenine ribonucleotides is added to 3’ end by poly-A polymerase. Slowly degraded over time When ‘used up,’ mRNA is degraded

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Splicing DNA of eukaryotic gene comprises of two

intermingling regions Exons: coding regions Introns: noncoding regions

If these regions are translated, protein will not function properly.

Removed from primary transcript by spliceosomes

Introns stay within nucleus degraded nucleotides recycled.

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Primary transcript capped/tailed and introns exiced mRNA transcript.

No quality control enzyme! Errors not as detrimental to the cell (multiple

copies of mRNA) If error made during transcription, protein

usually useless and will be degraded.

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Summary of Transcription

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Seatwork/Homework Page 249

#1-6, 8-9.

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TRANSLATION Initiation

Ribosome recognizes the 5’ cap in eukaryotes and binds to mRNA.

Elongation Ribosome moves along

mRNA in 5’-3’ direction. Amino acids are added to

the growing PP Codon AA.

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Reading Frame Reading Frame: how the mRNA is read.

Three possibilities

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The Role of tRNA tRNA: single stranded nucleic acid

Transfers AAs to ribosome Anticodon: sequence of three bases on one

arm of the tRNA. Recognizes the codon of mRNA (complimentary) If mRNA codon is AUA, anticodon is _____.

tRNA + amino acid = aminoacyl-tRNA. Aminnoacyl-tRNA synthetase: enzyme

responsible for adding AA to tRNA. At least 20 of these enzymes.

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What must be

the codon coding for tyrosine?

_____

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Wobble Hypothesis Every tRNA transfers only ONE specific AA. Therefore,

there must be AT LEAST ____ different tRNA molecules. Between 20-64 tRNAs: depends on organism.

Third base may differ between two codons, but may code for the same AA. Ex// UAU and UAC tyrosine.

If tRNA’s anticodon is UAU, can still bind to UAC codon. tRNA can recognize more than one codon by unusual pairing Advantage: correct amino acid may be added despite errors

made from DNA mRNA. WOBBLE HYPOTHESIS: the proposal that tRNA can

recognize more than one codon by unusual base pairing b/n 1st base of anticodon and 3rd base of codon.

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Elongation of the Polypeptide Start codon: _______. Every protein initially begins

with the AA _______________. Ribosome has two sites for tRNA

A (acceptor) site: site that tRNA brings amino acid to. P (peptide) site: peptide bonds are formed between

adjoining AAs. tRNA carryingMethionine enters P site first. Next appropriate AA will enter the A site.Met + AA peptide bind.

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a) Methionine is the start codon.

b) Second tRNA enters A site.

c) Ribosome TRANSLOCATES one codon over, and next tRNA enters A site.

d) Bond, translocation, and addition of new tRNA.

e) Repeat d. Stop codon does not code for an AA.

f) Release-factor protein helps dismantle the ribosome-mRNA complex. New PP is released!

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Termination of Protein Synthesis (as seen in step f) of previous slide)

Ribosome eventually reaches a stop codon. Ribosome stalls

Release Factor Protein: aids in the release of the PP chain from the ribosome. Subunits fall off of mRNNA.

Glycosylation: sugars may be added to PP.

Phosphorylation: addition of phosphates. PP may also be cleaved (cut) afterward

Ex// methionine may not be the first AA anyore.

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Summary of Translation

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Seatwork/Homework Page 254 #1-4, 6-7, 9