protein synthesis
DESCRIPTION
PROTEIN SYNTHESIS. DECEMBER 13, 2010 CAPE BIOLOGY UNIT 1 MRS. HAUGHTON. GENE. Mendel 1866 described a gene as a unit of inheritance Morgan defined it as the shortest segment of a chromosome which could be separated from adjacent segments by crossing over. GENE. - PowerPoint PPT PresentationTRANSCRIPT
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PROTEIN SYNTHESIS
DECEMBER 13, 2010CAPE BIOLOGY UNIT 1
MRS. HAUGHTON
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GENE
• Mendel 1866 described a gene as a unit of inheritance
• Morgan defined it as the shortest segment of a chromosome which could be separated from adjacent segments by crossing over.
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GENE
• A gene can also be described as the shortest segment of a chromosome responsible for the production of a specific product (protein).
• Genes are codes or blueprints for proteins.
• A piece of DNA that codes for a polypeptide chain.
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GENETIC CODE
• Watson and Crick proposed that genetic information might be stored in the form of a sequence of bases in the DNA molecule.
• It was shown that DNA was a code for the production of proteins.
• It then became clear that the sequence of bases in the DNA must be a code for the sequence of amino acids in a polypeptide chain!
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GENETIC CODE
• This relationship between bases and amino acids is known as the genetic code.
• Did the code really exist?• How was it to be broken?• How exactly was the code translated to
primary protein structure?
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TRIPLET CODON
• Four bases hence four nucleotides arranged on a polynucleotide strand making up a DNA strand.
• This “alphabet” arrangement is responsible for carrying the genetic code.
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TRIPLET CODON
• There are 20 common amino acids used to make proteins and the bases in DNA must code for them.
• If only one base determined the position of an amino acid in the polypeptide chain, then only _________amino acids would be in the chain.
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•41 = 4
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• If two bases coded for an amino acid, then only _________ amino acids would make up the polypeptide chain.
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42 = 16
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TRIPLET CODON
• Lets list the 16 possible combinations of bases if only pairs of bases (ATCG) were used.
• AT AC AG AA• TT TC TG TA• CC CA CG CT• GG GA GT GC
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TRIPLET CODON
• Obviously a code composed of three bases could incorporate all 20 amino acids into the structure of protein molecules.
• Such a code would produce ________ combinations of bases.
43 = 64Let’s look at
them
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PROOF OF TRIPLET CODON
• Crick in 1961 produced DNA mutations called frame-shifts by adding extra or deleting bases/nucleotides from the genetic code.
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• Adding or deleting one base (+ or -) led to a different polypeptide chain entirely.
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• Adding or deleting two bases (++ or --) led to a different polypeptide chain entirely.
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PROOF OF TRIPLET CODON
• But adding or deleting three bases (+++ or ---) did not cause a different chain to be made, only the deletion of a single amino acid from the chain and this did not usually affect the protein being made.
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FEATURES OF THE GENETIC CODE
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1
• The code is a triplet of bases.
• Theoretically, three bases represents an amino acid.
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2
• The triplet code is degenerate.
• Some amino acids are coded for by several codons.
• For many amino acids, only the first 2 bases appear to be significant so the number of amino acids is less than the number of available codons.
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3
• The code is punctuated.
• Three of the codons (e.g. UAA) act as full stops determining where the coded message to be transcribed must end. These are “stop codons” or “stop signals”.
• Other codons are start codons or signals (e.g. AUG which codes for the a.a. methionine)
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4
• The code is universal as all living organisms contain the same 20 common amino acids and the same five bases (ATCGU).
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5
• The code is not overlapping.
• E.g. AUUAUCGUUAGCCA is read • AUU AUC CGU UAG CCA and not
• AUU UUA UAU….. Or• AUU UAU AUC…. etc.
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HOMEWORK
• In 250 words or less, explain just how scientists eventually determined which three bases represented which amino acid or family of amino acids (breaking the code).
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PROCESS OF PROTEIN SYNTHESIS
DNA makes RNA and RNA makes PROTEIN which is responsible for
how we look and function
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• Protein synthesis is a two-stage process.
1. Transcription – the making of mRNA from DNA. A length of DNA (a gene) is copied into a mRNA molecule.
2. Translation – translating the base sequence in mRNA into an amino acid sequence in a protein.
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TRANSCRIPTION
• The mechanism by which the base sequence of a section of DNA representing a gene is converted into a complementary base sequence of mRNA.
• The DNA double helix unwinds by breaking the relatively weak H-bonds between the bases of the 2 strands exposing the single strand of the DNA.
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• Only one of the strands can be selected as a template for the formation of a complementary single strand of mRNA.
• This molecule is formed by the linking of free nucleotides under the influence of RNA polymerase according to the rules of base pairing between DNA and RNA.
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• When the mRNA molecules have been synthesized, they leave the nucleus via the nuclear pores and carry the genetic code to the ribosomes.
• When sufficient numbers of mRNA molecules have been formed from the gene, the RNA polymerase molecule leaves the DNA and the two strands zip up again reforming the double helix.
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TRANSLATION
• This is the mechanism by which the sequence of bases in the mRNA molecule is converted into a sequence of amino acids in a polypeptide chain.
• It occurs on ribosomes.
• Several ribosomes may become attached to a molecule of mRNA like beads on a string (polysome/polyribosome).
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• Each ribosome consists of a large and small subunit.
• The first two mRNA codons (a total of 6 bases) enters the ribosome.
• The first codons bind to the aminoacyl-tRNA molecule having the complementary anti-codon and which is carrying the first amino acid which is usually Met (AUG).
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• The second codon then also subtracts the aminoacyl-tRNA molecule showing the complementary anticodon.
• The function of the ribosomes is to hold in position the mRNA, tRNA and the associate enzymes controlling the process until a peptide bond forms between the adjacent amino acids.
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• Once the new amino acid has been added to the growing polypeptide chain the ribosome moves one codon along the mRNA.
• The tRNA molecule which was previously attached to the polypeptide chain now leaves the ribosome and passes back to the cytoplasm to be reconverted into a new aminoacyl-tRNA molecule.
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• This sequence of ribosome reading and translating the mRNA code continues until it comes to a codon signaling STOP.
• These terminating codons are UAA, UAG and UGA.
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• At this point the polypeptide chain, now with its primary structure as determined by the DNA, leaves the ribosome and translation is complete.
• As the polypeptide chains leave the ribosome they may immediately assume either secondary, tertiary or quaternary structures.
• If the ribosome is attached to ER, the protein enters it and is transported.
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SUMMARY OF TRANSLATION
1. Binding of mRNA to ribosome.2. Amino acid activation and attachment to
tRNA3. Polypeptide chain initiation4. Chain elongation5. Chain termination6. Fate of mRNA
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HOMEWORK
• In 100 words or less, explain what non-coding DNA is.
• In 100 words or less, explain what introns and exons.