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DNA Transcription & Translation
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*How do genes work? We’ve seen how DNA is replicated, but still haven’t learned exactly how genes work! The first step in understanding how genes work is to know how to “decode” the DNA.
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Decoding DNA The decoder: RNA (Ribonucleic acid)
Long stranded like DNA, except: Ribose instead of Deoxyribose Uracil instead of Thymine Single stranded instead of double stranded
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Types of RNA The main job of all RNA types is protein synthesis (creation) 3 types: (*You will learn what each does later) Messenger RNA Transfer RNA Ribosomal RNA mRNA tRNA rRNA
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Protein Creation DNA holds the instructions for making proteins
Proteins do the cell’s work Two steps required for protein synthesis (creation) TRANSCRIPTION TRANSLATION
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Transcription TRANSCRIPTION is the process by which a DNA gene is “rewritten” into RNA
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Transcription A gene is only a segment of DNA
Only the gene is transcribed, not all of the DNA (like replication) DNA Transcription mRNA
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Transcription 3 phases: Initiation (“the beginning”)
Enzymes recognize appropriate gene in DNA Elongation (“the middle”) Enzymes copy gene into mRNA Termination (“the end”) Enzymes unbind from DNA
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Why re-write the instructions?
DNA is like the “master plan” for the construction of a house Too important to bring to the work site and risk being damaged or lost RNA is like an inexpensive, disposable copy of the master plan Used at the work site (the ribosomes)
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Transcription INITIATION
RNA POLYMERASE (RP) recognizes a promoter on the DNA sequence PROMOTER: base sequence that signals the start of a gene DNA is split by RP and one strand is used as a template
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Animation #1 Animation #2
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Transcription ELONGATION: RP makes an mRNA copy of the gene
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Transcription TERMINATION RP reaches TERMINATOR SEQUENCE
Signals it to unbind and stop transcription
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Transcription After transcription is done you end with one mRNA copy of a gene Some of the mRNA must be “spliced out” (removed) before we translate it
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mRNA Processing Pieces of the mRNA that don’t contain genes (called INTRONS) are removed by a SPLICEOSOME
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mRNA Processing Pieces of DNA that do code for genes (called EXONS) are left and pieced together
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The Genetic Code The mRNA strand is then used to make proteins in the process of TRANSLATION
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The Genetic Code The “language” of mRNA is known as the genetic code
The 4 “letters” (A-G-C-U) are used to write “words” that correspond to different amino acids
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The Genetic Code “Words”are written using 3 letters
Every 3 letters = 1 codon Every 1 codon = 1 amino acid
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The Genetic Code Because there are four different bases, there are 64 (43) different codon combinations Certain codons code for the same amino acid
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Translation Shortly after mRNA is transcribed, translation begins.
The sequence of bases in mRNA serves as instructions for the order in which amino acids should be joined
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Translation Translation: The decoding of an mRNA message and creation of a polypeptide chain (protein) RIBOSOMES are the factories that assemble the proteins
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Translation INITIATION mRNA in the cytoplasm attaches to a ribosome
ELONGATION The ribosome “reads” the codons and attaches the proper amino acids Stop codon Start codon
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Transfer RNA Transfer RNA brings the correct amino acids for the ribosome to use Has an ANTI – CODON on one end, and an amino acid on the other Anti-codon has a sequence complementary to an mRNA codon
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Translation TERMINATION
The ribosome continues along until it reaches a stop codon The amino acid chain is then released and allowed to fold into a protein Protein then performs its job within the cell
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Translation Translation can occur at many different spots along the mRNA strand, creating many proteins at one time.
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Let’s Try it ourselves Transcribe and translate the following DNA sequence: TAC GCA TGG AAT AUG CGU ACC UUA
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Translation Animations
Transcribe and Translate a Gene Why do fireflies glow?
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Let’s try it ourselves Transcribe and translate the following DNA sequence: TAC GCA TGG AAT AUG CGU ACC UUA Met – Arg – Thr – Leu
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Mutations Every now and then cells make mistakes.
Inserting incorrect bases Deleting bases Adding extra bases Mistakes that cause a change in genetic information are called mutations.
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Point Mutations Point mutations are mutations that affect one nucleotide. 3 different types: Substitution Insertion Deletion
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Substitution Occurs when the wrong base is added to a growing nucleotide chain Original DNA: TAC GCA TGG AAT Original mRNA: AUG CGU ACC UUA Original Protein: Met – Arg – Thr - Leu Substitution Original DNA: TAC GTA TGG AAT Original mRNA: AUG CAU ACC UUA Original Protein: Met – His – Thr - Leu
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Insertion Occurs when an extra base is added to a growing nucleotide chain Original DNA: TAC GCA TGG AAT Original mRNA: AUG CGU ACC UUA Original Protein: Met – Arg – Thr - Leu Insertion Original DNA: TAT CGC ATG GAA T Original mRNA: AUA GCG UAC CUU A Original Protein: Ile – Ala – Tyr – Leu
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Deletion When a base is deleted from a growing nucleotide chain
THE FAT CAT ATE THE RAT TEF ATC ATA TET HER AT
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Frameshift Mutations Mutations that cause a shift in the reading frame
Insertion Deletion More than one amino acid gets changed downstream TACGCATGGAAT TATCGTATGGAAT
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Original DNA: TAC GCA TGG AAT Mutated DNA: TAC GCC TGG AAT
Silent Mutations DNA changes that do not cause a change in the amino acid chain Original DNA: TAC GCA TGG AAT Mutated DNA: TAC GCC TGG AAT mRNA: AUG CGG ACC UUA Protein: Met – Arg – Thr - Leu
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