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7/12/2019 1 BI 102 Lecture 11: Transcription and Translation Genetic Code • There are 4 nucleotide bases (A, G, C, T) • The sequence of nucleotide bases in DNA is a code for making proteins • What’s the primary business of any cell? • Proteins are built from amino acids • There are 20 amino acids • How do 4 nucleotide bases code for all 20 amino acids? Genetic Code • Combinations of 3 nucleotide bases form 64 unique codes • These combinations code for the 20 amino acids • Some redundancy Number of bases Total number of unique codes 1 base forms code pattern 4 1 = 4 unique codes (A, G, C, T) 2 bases form code pattern 4 2 = 16 unique codes 3 bases form code pattern 4 3 = 64 unique codes 4 bases form code pattern 4 4 = 256 unique codes Genetic Code • A group of 3 DNA bases that code for a single amino acid is called a triplet Genetic Code • DNA is too large, too precious to leave the nucleus • Protein-making machinery is in the cytosol • How does DNA direct protein synthesis from within the nucleus? Transcription • Solution: the cell makes a temporary copy in the form of mRNA • Process is called transcription 1 2 3 4 5 6

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Page 1: Notes Lecture 11 BI 102 Transcription and Translation · _ Y Z Z X Y a ^ ' v ] > ] v P µ ] ] d Z Ç u l '&W } } J t Z } Z ] o o Ç } µ } µ Z P v ] } M

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1

BI 102

Lecture 11:Transcription and Translation

Genetic Code

• There are 4 nucleotide bases (A, G, C, T)• The sequence of nucleotide bases in DNA is a code for making

proteins• What’s the primary business of any cell?

• Proteins are built from amino acids• There are 20 amino acids• How do 4 nucleotide bases code for all 20 amino acids?

Genetic Code

• Combinations of 3 nucleotide bases form 64 unique codes• These combinations code for the 20 amino acids

• Some redundancy

Number of bases Total number of unique codes

1 base forms code pattern 41 = 4 unique codes (A, G, C, T)

2 bases form code pattern 42 = 16 unique codes

3 bases form code pattern 43 = 64 unique codes

4 bases form code pattern 44 = 256 unique codes

Genetic Code

• A group of 3 DNA bases that code for a single amino acid is called a triplet

Genetic Code

• DNA is too large, too precious to leave the nucleus• Protein-making machinery is in the cytosol• How does DNA direct protein synthesis from within the nucleus?

Transcription

• Solution: the cell makes a temporary copy in the form of mRNA• Process is called transcription

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3 4

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Page 2: Notes Lecture 11 BI 102 Transcription and Translation · _ Y Z Z X Y a ^ ' v ] > ] v P µ ] ] d Z Ç u l '&W } } J t Z } Z ] o o Ç } µ } µ Z P v ] } M

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Transcription

• Steps in transcription1) Initiation2) Elongation3) Termination

Transcription

• Initiation• Chromatin unwinds to expose DNA of the gene to be transcribed• DNA “unzips”• Enzyme concept called RNA polymerase bind to the promoter region of the

gene of interest

Transcription

• Elongation• One DNA strand is used as a template

• Called the template or antisense strand• Other strand is called the coding or sense strand

• mRNA nucleotides are added to match the DNA bases on the template strand

Warning: many, MANY online resources have these strands labeled backwards

Transcription

• Termination• Enzymes and mRNA strand are released• DNA “rezips”

Transcription

• DNA triplets are recoded as mRNA codons• Contains the same nucleotide

bases as the DNA sense strand • Except T in DNA is replaced with

U in mRNA

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Page 3: Notes Lecture 11 BI 102 Transcription and Translation · _ Y Z Z X Y a ^ ' v ] > ] v P µ ] ] d Z Ç u l '&W } } J t Z } Z ] o o Ç } µ } µ Z P v ] } M

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mRNA Processing

• mRNA is not yet ready to direct protein synthesis• Three modifications must take place

1) 5’ capping2) 3’ polyadenylation3) mRNA splicing

mRNA Processing

• Enzymes in the cytosol rapidly degrade mRNA• Major defense against viruses

• Both ends of the mRNA molecule must be “capped” to protect it from these enzymes

mRNA Processing

• 5’ capping• Adds a G nucleotide by a unique type of bond• Prevents enzymes from breaking the bond and degrading the mRNA• Also marks which end of the mRNA represents the beginning of the protein

• Prevents the ribosomes from reading the strand backwards

mRNA Processing

• 3’ polyadenylation• Other end of the mRNA strand is trimmed of its final few nucleotides• Then capped with a tail of between 100 and 250 adenine nucleotides• Called the poly-A tail

mRNA Processing

• mRNA splicing• mRNA contains non-coding regions called introns• Protein-coding sections of mRNA are called exons• Introns are interspersed between the exons and must be removed

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mRNA Processing

• mRNA splicing• Introns are spliced out and degraded• Exons are joined together to create the final transcript

mRNA Processing

• Purposes of introns• Protects transcript

• Reduces chances that mutations will occur in coding regions of the transcript

• Allows multiple versions of a protein to be coded for by the same region ofDNA

• Exons can be spliced together differently to create multiple versions of the same protein

• Called splice isoforms or splice variants

Protein Synthesis

• Once a strand of mRNA has been processed, it is ready to be decoded to build a protein

• Process is called translation

• The genetic code is translated from the language of nucleic acids tothe language of proteins

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Page 5: Notes Lecture 11 BI 102 Transcription and Translation · _ Y Z Z X Y a ^ ' v ] > ] v P µ ] ] d Z Ç u l '&W } } J t Z } Z ] o o Ç } µ } µ Z P v ] } M

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Genetic Linguistics

• Think about different written languages• Do the same letter combinations mean the same thing in all languages

that use the same alphabet?

English French

OursPain

Pet

Genetic Linguistics

• What about the languages of DNA and proteins?• All cells use the same nucleotides and the same amino acids

• But will every species read DNA in the same way?• If we put a gene for a protein from one species into another, will it

make the same protein?

Genetic Linguistics

• Experiment• Some jellyfish glow green under UV light• Due to the presence of a protein called green fluorescent protein

(GFP)

Genetic Linguistics

• Experiment• What will happen if we take the gene that codes for GFP and put it

into bacteria?• Will the bacteria make GFP?• Or will they translate the genetic code differently?

+ = ?

Genetic Linguistics

• Experiment• They make GFP!

+ =

Genetic Linguistics

• What about other species?• How will they translate jellyfish DNA?

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Page 6: Notes Lecture 11 BI 102 Transcription and Translation · _ Y Z Z X Y a ^ ' v ] > ] v P µ ] ] d Z Ç u l '&W } } J t Z } Z ] o o Ç } µ } µ Z P v ] } M

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Genetic Linguistics

• They make GFP too!• What does this tell you about the genetic code?

Genetic Linguistics

• They make GFP too!• What does this tell you about the genetic code?• It’s universal!• An mRNA codon will be translated into the same amino

acid, no matter the species• We exploit this in so many, very cool ways

• Recombinant DNA technology

Translation

• How do we get from mRNA to protein?• Takes place on the ribosome• Involves all 3 types of RNA

• mRNA• rRNA• tRNA

Translation

• Ribosomes are constructed of rRNA and proteins• Composed of two subunits

• Small subunit• Large subunit

• The two subunits come together around an mRNA molecule

• Move along the mRNA strand as the transcript is read

Translation

• Take a look at transfer RNA (tRNA)• Note the anticodon loop at the bottom

• Group of 3 nucleotide bases that form complimentary base pairs with the codons of mRNA

Anticodon loop

Translation

• Take a look at transfer RNA (tRNA)• Note the amino acid attached to the top• A tRNA with a given anticodon always carries the same amino acid

Anticodon loop

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Translation

• Ribosome moves along mRNA to sequentially read each codon

• As successive mRNA codons are read, each is matched to itscorresponding tRNA anticodon

• Amino acid from the other end of tRNA is added to the growingprotein chain

Translation

• Amino acids are joined together to form a protein’s primary structure

• Takes about 20 seconds to form the whole protein

Translation

• The signal for where to begin translation is the mRNA codon AUG

• Called the START codon• Codes for the amino acid methionine• All proteins begin with methionine

• Translation ends when a STOP codon is reached• Codes for no amino acid• Has no corresponding tRNA• Ribosome comes apart and releases mRNA, protein

Translation

• Translation animation

Universal Genetic Code DNA Mutation and Repair

• DNA is precious• If the DNA code is changed, the mRNA transcript will change,

could change the protein that is made• Just one change in a nucleotide base can be devastating

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DNA Mutation and Repair

• Many factors lead to DNA changes• Happens constantly• Is usually repaired

DNA Mutation and Repair

• Many different DNA repair mechanisms exist• Type used depends on the type of damage• Correct sequence determined by referencing

• Complimentary strand• Homologous chromosome

DNA Mutation and Repair

• Example: mismatch repair in newly synthesized DNA

DNA Mutation and Repair

• Example: homologous end joining• Uses homologous chromosome

to fill in the gap

DNA Mutation and Repair

• Rarely, DNA damage is undetected or improperly repaired• Called a mutation

• Several types of mutations exist• Point mutation• Addition• Deletion• Frameshift• Inversion• Translocation

DNA Mutation and Repair

• Let’s start by decoding a correct mRNA sequence from a normal gene

AAA – ACG – UGG – CGC - AAG

Amino acid 1 Amino acid 2 Amino acid 3 Amino acid 4 Amino acid 5

43 44

45 46

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Page 9: Notes Lecture 11 BI 102 Transcription and Translation · _ Y Z Z X Y a ^ ' v ] > ] v P µ ] ] d Z Ç u l '&W } } J t Z } Z ] o o Ç } µ } µ Z P v ] } M

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DNA Mutation and Repair

• Now let’s introduce a point mutation• Replace a single nucleotide with a different one

• Let’s replace the G at position 9 with a C

AAA – ACG – UGC – CGC - AAG

Amino acid 1 Amino acid 2 Amino acid 3 Amino acid 4 Amino acid 5

DNA Mutation and Repair

• This point mutation changed an amino acid• Lots of things could happen as a result…

DNA Mutation and Repair

• Nothing• Example: ACC to ACA• Both code for THR• No change in the protein

• Truncation• Example: UGG to UGA• UGG = TRP• UGA = STOP• Cuts the protein short

DNA Mutation and Repair

• Protein too long• Example: UGA to UGG• UGA = STOP• UGG = TRP• Stop codon isn’t made, so the protein is too long

• Protein isn’t made at all• Example: ATG to ATA• ATG = START• ATA = ILE• No START codon, so translation is never initiated

DNA Mutation and Repair

• Let’s go back to the original sequence and introduce an addition mutation

• Extra nucleotides are added

• Let’s add UU between positions 1 and 2

AUU – AAA – CGU – GGC – GCA - AG

Amino acid 1 Amino acid 2 Amino acid 3 Amino acid 4 Amino acid 5

DNA Mutation and Repair

• Let’s go back to the original sequence and introduce a deletion mutation

• Nucleotides are removed

• Let’s remove the C at position 5

AAA – AGU – GGC – GCA - AG

Amino acid 1 Amino acid 2 Amino acid 3 Amino acid 4 Amino acid 5

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DNA Mutation and Repair

• When a mutation changes how codons are grouped and read, it is called a frameshift mutation

• Changes every amino acid following the mutation

• Example: Take a sentence with all 3-letter wordsTHE DOG SAW THE CAT

• If we delete the E at position 3 and regroup the letters, it no longermakes sense

THD OGS AWT HEC AT• Which mutations in our previous examples produced a frameshift?

DNA Mutation and Repair

• Inversion and translocation• Large pieces of DNA (sometimes most of a chromosome) are

broken off and reattached elsewhere• Sometimes within the same chromosome• Sometimes to another chromosome

DNA Mutation and Repair

• May be benign• Entire genes and their promoters simply moved from one location

to another• Referred to as balanced• Frequently results in infertility, as gametes

are often unbalanced

DNA Mutation and Repair

• May result in significant impairment • Gene may be split into pieces, no longer code for a functional

protein• Example: most severe hemophilia is caused by an inversion of the

gene for a blood clotting protein

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