FROM GENE TO PROTEIN

YOU MUST KNOW…• THE KEY TERMS GENE EXPRESSION,

TRANSCRIPTION, AND TRANSLATION• HOW TO EXPLAIN THE PROCESS OF

TRANSCRIPTION• HOW EUKARYOTIC CELLS MODIFY

RNA AFTER TRANSCRIPTION• THE STEPS TO TRANSLATION• HOW POINT MUTATIONS CAN CHANGE

THE AMINO ACID SEQUENCE OF A PROTEIN

CONCEPT 17.1• GENES SPECIFY PROTEINS VIA

TRANSCRIPTION AND TRANSLATION

• GENE EXPRESSION – THE PROCESS BY WHICH DNA DIRECTS THE SYNTHESIS OF PROTEINS

• ONE GENE-ONE POLYPEPTIDE HYPOTHESIS – STATES THAT EACH GENE CODES FOR A POLYPEPTIDE

• TRANSCRIPTION – THE SYNTHESIS OF RNA USING DNA AS A TEMPLATE IN THE NUCLEUS

• mRNA IS PRODUCED DURING TRANSCRIPTION AND CARRIES THE GENETIC MESSAGE OF DNA TO THE RIBOSOME

• DURING TRANSCRIPTION, ONLY ONE STRAND OF DNA IS TRANSCRIBED CALLED THE TEMPLATE STRAND

• mRNA PRODUCED IS THE COMPLEMENTARY TO THE ORIGINAL DNA STRAND

• mRNA BASE TRIPLETS ARE CODONS AND ARE WRITTEN IN THE 5’ TO 3’ DIRECTION

• MORE THAN ONE CODON CODES FOR EACH OF THE 20 AMINO ACIDS

• TRANSLATION IS THE PRODUCTION OF A POLYPEPTIDE CHAIN USING THE mRNA TRANSCRIPT AND OCCURS AT THE RIBOSOMES

• INSTRUCTIONS FOR BUILDING THE POLYPEPTIDE CHAIN ARE WRITTEN IN A SERIES OF 3 NUCLEOTIDES CALLED A TRIPLET CODE (ANTICODON)

CONCEPT 17.2• TRANSCRIPTION IS THE DNA-

DIRECTED SYNTHESIS OF RNA

• RNA POLYMERASE IS THE ENZYME THAT SEPARATES THE 2 DNA STRANDS AND CONNECTS THE RNA NUCLEOTIDES AS THEY BASE-PAIR ALONG THE DNA TEMPLATE STRAND

• PROMOTER – THE DNA SEQUENCES WHERE RNA POLYMERASE ATTACHES

• TERMINATOR – THE DNA SEQUENCE SIGNALING THE END OF TRANSCRIPTION

• TRANSCRIPTION UNIT – THE ENTIRE STRETCH OF DNA THAT IS TRANSCRIBED INTO AN RNA MOLECULE

3 MAIN STAGES OF TRANSCRIPTION

• INITIATION• IN BACTERIA, RNA POLYMERASE

RECOGNIZES AND BINDS TO THE PROMOTER

• IN EUKARYOTES, RNA POLYMERASE I, THE SPECIFIC RNA POLYMERASE THAT TRANSCRIBES mRNA, CANNOT BIND TO THE PROMOTER WITH SUPPORTING HELP FROM PROTEINS

3 MAIN STAGES OF TRANSCRIPTION

• ELONGATION• RNA POLYMERASE MOVES ALONG

THE DNA CONTINUING TO UNTWIST THE DOUBLE HELIX

• NUCLEOTIDES ARE CONTINUALLY ADDED TO THE 3’ END OF THE GROWING CHAIN

• AS THE COMPLEX MOVES DOWN THE DNA STRAND, THE DOUBLE HELIX RE-FORMS, WITH THE NEW RNA MOLCULE STRAGGLING AWAY FROM THE DNA TEMPLATE

3 MAIN STAGES OF TRANSCRIPTION

• TERMINATION• AFTER RNA POLYMERASE

TRANSCRIBES A TERMINATOR SEQUENCE IN THE DNA, THE RNA TRANSCRIPT IS RELEASED, AND THE POLYMERASE DETACHES

CONCEPT 17.3• EUKARYOTIC CELLS MODIFY RNA

AFTER TRANSCRIPTION

• A 5’ CAP AND A POLY- A TAIL ARE ADDED AND FACILITATE EXPORT OF mRNA FROM THE NUCLEUS, HELP PROTECT THE mRNA FROM DEGRADATION BY ENZYMES, AND FACILITATE THE ATTACHMENT OF THE mRNA TO THE RIBOSOME

• RNA SPLICING TAKES PLACE IN EUKARYOTIC CELLS

• LARGE PORTIONS OF THE NEWLY SYNTHESIZED RNA STRAND ARE REMOVED

• THE SPLICED OUT SECTIONS ARE INTRONS, THE SECTIONS THAT REMAIN ARE EXONS

• SPLICEOSOMES – SPLICE TOGETHER THE SECTIONS THAT REMAIN

RIBOZYME• WHEN RNA SERVES A CATALYTIC

ROLE IN THE EXCISING OF THE INTRONS AND JOINING OF EXON

• DID YOU KNOW…• FEWER THAN 25,000 GENES CAN

MAKE APPROXIMATELY 100,000 POLYPEPTIDES

• http://www.johnkyrk.com/DNAtranscription.html

CONCEPT 17.4• TRANSLATION

IS THE RNA-DIRECTED SYNTHESIS OF A POLYPEPTIDE

• tRNA FUNCTIONS IN TRANSFERRING a.a. FROM A POOL OF a.a. IN THE CELL’S CYTOPLASM TO A RIBOSOME

• THE RIBOSOME ACCEPTS THE a.a. FROM tRNA AND INCORPORATES THE a.a. INTO A GROWING POLYPEPTIDE

• EACH tRNA IS SPECIFIC FOR A PARTICULAR a.a. THE OTHER END HAS A NUCLEOTIDE TRIPLET CALLED AN ANTICODON WHICH ALLOWS IT TO PAIR WITH A COMPLEMENTARY CODON ON THE mRNA

• A CODON IS AN mRNA TRIPLET• RESULTS IN 64 DIFFERENT CODONS• AS CODONS ARE READ, ONE a.a. IS

ADDED TO THE CHAIN FOR EACH CODON READ

• THE RULES FOR BASE-PAIRING BETWEEN THE THIRD BASE OF A CODON AND THE CORRESPONDING BASE OF A tRNA ANTICODON ARE NOT AS STRICT AS THOSE FOR DNA AND mRNA CODONS

• RELAXATION OF BASE-PAIRING IS CALLED WOBBLE

• rRNA COMPLEXES WITH PROTEINS TO FORM THE TWO SUBUNITS THAT FORM RIBOSOMES

• RIBOSOMES HAVE 3 BINDING SITES FOR tRNA

• P SITE – HOLDS THE tRNA THAT CARRIES THE GROWING POLYPEPTIDE CHAIN

• A SITE – HOLDS THE tRNA THAT CARRIES THE a.a. THAT WILL BE ADDED TO THE CHAIN

• E SITE – THE EXIT SITE FOR tRNA

3 STAGES OF TRANSLATION• INITIATION – SMALL RIBOSOMAL

SUBUNIT BINDS TO mRNA – AUG• tRNA WITH ANTICODON UAC, WHICH

CARRIES THE a.a. METHIONINE, HYDROGEN BONDS TO FIRST CODON (HOLDS IT ALL TOGETHER)

• LARGE SUBUNIT OF RIBOSOME ATTACHES, ALLOWING THE tRNA WITH METHIONINE TO ATTACH TO THE P SITE. THE A SITE IS NOW AVAILABLE TO THE tRNA THAT WILL BRING THE SECOND a.a.

• ELONGATION • CODON RECOGNITION – THE CODON

IN THE A SITE IS MATCHED BY THE INCOMING tRNA ANTICODON

• PEPTIDE BOND FORMATION – THE INCOMING a.a. IN THE A SITE FORMS A PEPTIDE BOND WITH THE EXISTING CHAIN OF a.a. HELD IN THE P SITE

• TRANSLOCATION – OCCURS WHEN tRNA IN THE A SITE IS MOVED TO THE P SITE, AND THE tRNA IN THE P SITE IS MOVED TO THE E SITE, WHERE IT IS RELEASED. THE A SITE IS CLEAR AND THE PROCESS RESTARTS

• TERMINATION – A STOP CODON IN THE mRNA IS REACHED AND TRANSLATION STOPS

• A PROTEIN CALLED RELEASE FACTOR BINDS TO STOP CODON AND THE POLYPEPTIDE IS FREED FROM THE RIBOSOME

• POLYPEPTIDES FOLD TO ASSUME THEIR SPECIFIC CONFORMATION

• http://www.johnkyrk.com/DNAtranslation.html

CONCEPT 17.5• POINT MUTATIONS CAN AFFECT

PROTEIN STRUCTURE AND FUNCTION

POINT MUTATIONS• ALTERATIONS OF JUST ONE

BASE PAIR OF A GENE

• BASE-PAIR SUBSTITUTION – REFERS TO THE REPLACEMENT OF ONE NUCLEOTIDE AND ITS COMPLEMENTARY BASE PAIR IN DNA WITH ANOTHER PAIR OF NUCLEOTIDES

• MISSENSE MUTATIONS – SUBSTITUTIONS THAT ENABLE CODON TO STILL CODE FOR AN a.a. BUT IT MAY NOT BE THE CORRECT ONE

• NONSENSE MUTATIONS – SUBSTITUTIONS THAT CHANGE A REGULAR a.a. CODON INTO A STOP CODON, CEASING TRANSLATION

FRAMESHIFT MUTATIONS• INSERTIONS OR DELETIONS – OF

NUCLEOTIDE PAIRS IN A GENE• INTERFERE WITH CODON GROUPS

AND CAUSE THE mRNA TO BE READ INCORRECTLY

MUTAGENS• SUBSTANCES OR FORCES THAT

INTERACT WITH DNA IN WAYS THAT CAUSE MUTATIONS

• EX. X-RAYS AND OTHER FORMS OF RADIATION, CHEMICALS