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PROTEIN SYNTHESIS
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DNA and Genes
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DNA DNA contains genes, sequences of nucleotide bases
These Genes code for polypeptides (proteins) Proteins are used to build cells and do much of the work inside cells
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Genes & Proteins Proteins are made of amino acids linked together by peptide bonds 20 different amino acids exist
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Amino Acid Structure
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Polypeptides Amino acid chains are called polypeptides
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DNA Begins the Process DNA is found inside the nucleus
Proteins, however, are made in the cytosol of cells by organelles called ribosomes Ribosomes may be free in the cytosol or attached to the surface of rough ER
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Starting with DNA DNA ‘s code must be copied and taken to the cytosol
In the cytosol, this code must be read so amino acids can be assembled to make polypeptides (proteins) This process is called PROTEIN SYNTHESIS
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RNA
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RNA is the BLUEPRINT of the Master Plan
Roles of RNA and DNA DNA is the MASTER PLAN RNA is the BLUEPRINT of the Master Plan
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RNA Differs from DNA RNA has a sugar ribose
DNA has a sugar deoxyribose
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Other Differences RNA contains the base uracil (U) DNA has thymine (T)
RNA molecule is single-stranded DNA is double-stranded DNA
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Structure of RNA Like DNA, RNA is a polymer of nucleotides. In an RNA nucleotide, the sugar ribose is attached to a phosphate molecule and to a base, either G, U, A, or C. Notice that in RNA, the base uracil replaces thymine as one of the pyrimidine bases. RNA is single-stranded, whereas DNA is double-stranded.
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. Three Types of RNA Messenger RNA (mRNA) copies DNA’s code & carries the genetic information to the ribosomes Ribosomal RNA (rRNA), along with protein, makes up the ribosomes Transfer RNA (tRNA) transfers amino acids to the ribosomes where proteins are synthesized
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Messenger RNA Long Straight chain of Nucleotides Made in the Nucleus
Copies DNA & leaves through nuclear pores Contains the Nitrogen Bases A, G, C, U ( no T )
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Messenger RNA (mRNA) Carries the information for a specific protein
Made up of 500 to 1000 nucleotides long Sequence of 3 bases called codon AUG – methionine or start codon UAA, UAG, or UGA – stop codons
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Ribosomal RNA (rRNA) rRNA is a single strand 100 to 3000 nucleotides long Globular in shape Made inside the nucleus of a cell Associates with proteins to form ribosomes Site of protein Synthesis
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The Genetic Code A codon designates an amino acid
An amino acid may have more than one codon There are 20 amino acids, but 64 possible codons Some codons tell the ribosome to stop translating
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The Genetic Code Use the code by reading from the center to the outside Example: AUG codes for Methionine
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Name the Amino Acids GGG? UCA? CAU? GCA? AAA?
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Remember the Complementary Bases
On DNA: A-T C-G On RNA: A-U
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Transfer RNA (tRNA) Clover-leaf shape
Single stranded molecule with attachment site at one end for an amino acid Opposite end has three nucleotide bases called the anticodon
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Transfer RNA amino acid attachment site U A C anticodon
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Codons and Anticodons The 3 bases of an anticodon are complementary to the 3 bases of a codon Example: Codon ACU Anticodon UGA UGA ACU
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Transcription and Translation
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Pathway to Making a Protein
DNA mRNA tRNA (ribosomes) Protein
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Protein Synthesis The production or synthesis of polypeptide chains (proteins) Two phases: Transcription & Translation mRNA must be processed before it leaves the nucleus of eukaryotic cells
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DNA RNA Protein Eukaryotic Cell DNA Pre-mRNA mRNA Ribosome Protein
Nuclear membrane Transcription RNA Processing Translation DNA Pre-mRNA mRNA Ribosome Protein Eukaryotic Cell
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Transcription The process of copying the sequence of one strand of DNA, the template strand mRNA copies the template strand Requires the enzyme RNA Polymerase
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Template Strand
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Question: What would be the complementary RNA strand for the following DNA sequence? DNA 5’-GCGTATG-3’
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Answer: DNA 5’-GCGTATG-3’ RNA 3’-CGCAUAC-5’
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Transcription During transcription, RNA polymerase binds to DNA and separates the DNA strands RNA Polymerase then uses one strand of DNA as a template to assemble nucleotides into RNA
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Transcription Promoters are regions on DNA that show where RNA Polymerase must bind to begin the Transcription of RNA Called the TATA box Specific base sequences act as signals to stop Called the termination signal
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RNA Polymerase
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mRNA Processing After the DNA is transcribed into RNA, editing must be done to the nucleotide chain to make the RNA functional Introns, non-functional segments of DNA are snipped out of the chain
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mRNA Editing Exons, segments of DNA that code for proteins, are then rejoined by the enzyme ligase A guanine triphosphate cap is added to the 5” end of the newly copied mRNA A poly A tail is added to the 3’ end of the RNA The newly processed mRNA can then leave the nucleus
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Result of Transcription
New Transcript Tail CAP
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mRNA Transcript mRNA leaves the nucleus through its pores and goes to the ribosomes
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Translation Translation is the process of decoding the mRNA into a polypeptide chain Ribosomes read mRNA three bases or 1 codon at a time and construct the proteins
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Transcription Translation
Transcription occurs when DNA acts as a template for mRNA synthesis. Translation occurs when the sequence of the mRNA codons determines the sequence of amino acids in a protein. Translation
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Ribosomes Made of a large and small subunit
Composed of rRNA (40%) and proteins (60%) Have two sites for tRNA attachment --- P and A
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Step 1- Initiation mRNA transcript start codon AUG attaches to the small ribosomal subunit Small subunit attaches to large ribosomal subunit mRNA transcript
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Ribosomes Large subunit P Site A Site mRNA A U G C Small subunit
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Step 2 - Elongation As ribosome moves, two tRNA with their amino acids move into site A and P of the ribosome Peptide bonds join the amino acids
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Initiation G aa2 A U U A C aa1 A U G C U A C U U C G A codon 2-tRNA
anticodon A U G C U A C U U C G A hydrogen bonds codon mRNA
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Elongation G A aa3 peptide bond aa1 aa2 U A C G A U A U G C U A C U U
3-tRNA G A aa3 peptide bond aa1 aa2 1-tRNA 2-tRNA anticodon U A C G A U A U G C U A C U U C G A hydrogen bonds codon mRNA
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Ribosomes move over one codon
aa1 peptide bond 3-tRNA G A aa3 aa2 1-tRNA U A C (leaves) 2-tRNA G A U A U G C U A C U U C G A mRNA Ribosomes move over one codon
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peptide bonds G C U aa4 aa1 aa2 aa3 G A U G A A A U G C U A C U U C G
4-tRNA G C U aa4 aa1 aa2 aa3 2-tRNA 3-tRNA G A U G A A A U G C U A C U U C G A A C U mRNA
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Ribosomes move over one codon
peptide bonds 4-tRNA G C U aa4 aa1 aa2 aa3 2-tRNA G A U (leaves) 3-tRNA G A A A U G C U A C U U C G A A C U mRNA Ribosomes move over one codon
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peptide bonds U G A aa5 aa1 aa2 aa4 aa3 G A A G C U G C U A C U U C G
5-tRNA aa5 aa1 aa2 aa4 aa3 3-tRNA 4-tRNA G A A G C U G C U A C U U C G A A C U mRNA
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Ribosomes move over one codon
peptide bonds U G A 5-tRNA aa5 aa1 aa2 aa3 aa4 3-tRNA G A A 4-tRNA G C U G C U A C U U C G A A C U mRNA Ribosomes move over one codon
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Termination aa5 aa4 aa3 primary structure of a protein aa2 aa1 A C U C
terminator or stop codon 200-tRNA A C U C A U G U U U A G mRNA
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End Product –The Protein!
The end products of protein synthesis is a primary structure of a protein A sequence of amino acid bonded together by peptide bonds aa1 aa2 aa3 aa4 aa5 aa200 aa199
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Messenger RNA (mRNA) Primary structure of a protein A U G C aa1 aa2
start codon codon 2 codon 3 codon 4 codon 5 codon 6 codon 7 codon 1 methionine glycine serine isoleucine alanine stop codon protein Primary structure of a protein aa1 aa2 aa3 aa4 aa5 aa6 peptide bonds
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