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.
DNA and Genes• DNA contains genes, sequences of nucleotide bases• These genes code for polypeptides (proteins) 1
Protein Synthesis (Gene Expression)Proteins (Review)•Proteins make up all living materials
• Proteins are composed of amino acids – there are 20 different amino acids• Different proteins are made by combining these 20 amino acids in different combinations• Amino acid chains are called polypeptides.
• Proteins are manufactured (made) by the ribosomes
• Function of proteins:1.Help fight disease2.Build new body tissue3.Enzymes used for digestion and other chemical reactions are proteins (Enzymes speed up the rate of a reaction)4. Component of all cell membranes
Making a Protein—Transcription•First Step: Copying of genetic information from DNA to RNA calledTranscription Why? DNA has the genetic code for the protein that needsto be made, but proteins are made by the ribosomes—ribosomesare outside the nucleus in the cytoplasm. DNA is too large to leave the nucleus (double stranded), butRNA can leave the nucleus (single stranded).
The enzyme RNA polymerase binds to DNA and part ofDNA temporarily unzips and is used as a template toassemble complementary nucleotides into messengerRNA (mRNA). mRNA copies the template strand
• mRNA then goes through the pores of the nucleus with the DNA code and attaches to the ribosome.
Making a Protein—Translation•Second Step: Decoding of mRNA into aprotein/polypeptide chain is called Translation.•Transfer RNA (tRNA) carries amino acids from thecytoplasm to the ribosome.
These amino acids come from the food we eat. Proteinswe eat are broken down into individual amino acids andthen simply rearranged into new proteins according to theneeds and directions of our DNA.
• A series of three adjacent bases in an mRNA molecule codes for a specific amino acid—called a codon.• A triplet of nucleotides in tRNA Amino acid that is complementary to the codon in mRNA—called an anticodon.• Each tRNA codes for a different amino acid. Anticodon
• mRNA carrying the DNA instructions and tRNA carrying amino acids meet in the ribosomes.
• Amino acids are joined together to make a protein. Polypeptide = Protein
Ribosomes• Made of a large and small subunit• Composed of rRNA (40%) and proteins (60%)• Have two sites for tRNA attachment --- P and A copyright cmassengale 15
Step 1- Initiation• mRNA transcript start codon AUG attaches to the small ribosomal subunit• Small subunit attaches to large ribosomal subunit mRNA transcript copyright cmassengale 16
Ribosomes Large subunit P A Site Site mRNASmall subunit A U G C U A C U U C G copyright cmassengale 17
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 copyright cmassengale 18
Initiation aa2 aa1 2-tRNA 1-tRNAanticodon G A U U A Chydrogen A U G C U A C U U C G A bonds codon mRNA copyright cmassengale 19
Elongation peptide bond aa3 aa1 aa2 3-tRNA 1-tRNA 2-tRNA G A Aanticodon U A C G A Uhydrogen A U G C U A C U U C G Abonds codon mRNA copyright cmassengale 20
aa1 peptide bond aa3 aa2 1-tRNA 3-tRNAU A C(leaves) 2-tRNA G A A G A U A U G C U A C U U C G A mRNA Ribosomes movecmassengale one codon copyright over 21
peptide bonds aa1 aa4 aa2 aa3 4-tRNA 2-tRNA 3-tRNA G C U G A U G A AA U G C U A C U U C G A A C U mRNA copyright cmassengale 22
peptide bonds aa1 aa4 aa2 aa3 2-tRNA 4-tRNAG A U(leaves) 3-tRNA G C U G A A A U G C U A C U U C G A A C U mRNA Ribosomes movecmassengale one codon copyright over 23
peptide bonds aa5aa1 aa2 aa4 aa3 5-tRNA U G A 3-tRNA 4-tRNA G A A G C UG C U A C U U C G A A C U mRNA copyright cmassengale 24
aa1 peptide bonds aa5 aa2 aa3 aa4 5-tRNA 3-tRNA U G AG A A 4-tRNA G C UG C U A C U U C G A A C U mRNA Ribosomes movecmassengale one codon copyright over 25
aa5 aa4 aa199 Termination aa200 aa3 primary structureaa2 of a proteinaa1 terminator 200-tRNA or stop codonA C U C A U G U U U A G mRNA copyright cmassengale 26
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 aa5 aa3 aa4aa2 aa199aa1 aa200 copyright cmassengale 27
Messenger RNA (mRNA) start codon A U G G G C U C C A U C G G C G C A U A AmRNA codon 1 codon 2 codon 3 codon 4 codon 5 codon 6 codon 7protein methionine glycine serine isoleucine glycine alanine stop codon Primary structure of a protein aa1 aa2 aa3 aa4 aa5 aa6 peptide bonds copyright cmassengale 28
Use one of the codon charts on the next page to find the amino acidsequence coded for by the following mRNA strands. CAC/CCA/UGG/UGA GUG GGU ACC ACU___________/___________/___________/____________ AUG/AAC/GAC/UAA UAC UUG CUG AUU___________/___________/___________/____________
CAC/CCA/UGG/UGA Histidine Proline Tryptophan Stop___________/___________/___________/____________ 2nd Base1st Base 3rd Base