Ch 5: Protein Synthesis

PHONETICS

• PHONETICS

• XTA-23-CU
• group student Sirojiddinova Zebo

TRANSCRIPTION

TRANSCRIPTION

• THE PROCESS OF MAKING AN MRNA STRAND FROM THE DNA GENE SEQUENCE
• 3 STEPS: INITIATION, ELONGATION, TERMINATION

INITIATION

• THE ENZYME RNA POLYMERASE ATTACHES TO THE DNA DOUBLE HELIX AT THE PROMOTER REGION.
• PROMOTER REGION: A SEQUENCE OF DNA “UPSTREAM” FROM THE GENE THAT CONTAINS A LOT OF A-T BASE PAIRS. (EASIER TO BREAK APART THAN G-C)

RNA POLYMERASE UNWINDS AND SEPARATES THE HELIX EXPOSING THE TEMPLATE STRAND.

• RNA POLYMERASE UNWINDS AND SEPARATES THE HELIX EXPOSING THE TEMPLATE STRAND.
• SINCE RNA POLYMERASE ONLY BINDS AT THE PROMOTER REGION, IT ALLOWS ONLY THE GENE TO BE TRANSCRIBED.
• IF THE RNA POLYMERASE RANDOMLY TRANSCRIBED DNA, THE CELL WOULD NOT MAKE THE CORRECT POLYPEPTIDE

• Rich in A-T

• Coding strand

ELONGATION

• ONCE THE HELIX IS OPEN, RNA POLYMERASE STARTS BUILDING A SINGLE STRANDED MRNA IN THE 5’-3’ DIRECTION USING RNA NUCLEOTIDES AND COMPLIMENTARY BASE PAIRING RULES.
• UNLIKE DNA REPLICATION, THE RNA POLYMERASE DOES NOT REQUIRE A PRIMER TO BEGIN BUILDING THE COMPLEMENTARY STRAND

ONLY ONE STRAND OF DNA IS USED AS A TEMPLATE FOR MRNA SYNTHESIS. THIS IS THE TEMPLATE STRAND.

• ONLY ONE STRAND OF DNA IS USED AS A TEMPLATE FOR MRNA SYNTHESIS. THIS IS THE TEMPLATE STRAND.
• THE OTHER DNA STRAND THAT IS NOT USED FOR TRANSCRIPTION IS THE CODING STRAND
• THIS CODING STRAND IS IDENTICAL TO THE MRNA STRAND EXCEPT THAT IT HAS THYMINE INSTEAD OF URACIL.
• NOTE: THE PROMOTER REGION DOES NOT GET TRANSCRIBED.
• *AFTER THE DNA IS TRANSCRIBED, IT IS REWOUND INTO ITS HELIX

TERMINATION

• THE MRNA STRAND IS SYNTHESIZED UNTIL RNA POLYMERASE REACHES THE TERMINATOR SEQUENCE AT THE END OF THE GENE.
• THE MRNA STRAND WILL DETACH FROM THE DNA TEMPLATE.
• THIS MRNA STRAND IS CALLED THE PRIMARY TRANSCRIPT
• RNA POLYMERASE IS NOW FREE TO BIND TO ANOTHER PROMOTER REGION AND TRANSCRIBE ANOTHER GENE

POST-TRANSCRIPTION

• AFTER TRANSCRIPTION, THE MRNA PRIMARY TRANSCRIPT MUST GO TO THE CYTOPLASM TO BE TRANSLATED.
• IN EUKARYOTES, BEFORE IT LEAVES THE NUCLEUS IT NEEDS A “CAP” AND A “TAIL” TO PROTECT IT AND PREVENT IT FROM BEING BROKEN DOWN.

5’ CAP

• A 5’ CAP IS ADDED TO MRNA.
• THIS IS A MODIFIED GUANINE NUCLEOSIDE TRIPHOSPHATE.
• THE CAP PROTECTS THE MRNA FROM DIGESTION FROM ENZYMES WHEN IT ENTERS THE CYTOPLASM

POLY-A TAIL

• A SEQUENCE OF ~ 200 ADENINE RIBONUCLEOSIDES IS ADDED TO THE 3’ END TO PROTECT THE MRNA FROM BREAKING DOWN
• THIS IS CALLED A POLY-A TAIL
• THE TAIL IS ADDED WITH THE HELP OF THE ENZYME POLY-A POLYMERASE

EXONS AND INTRONS

• IN EUKARYOTIC DNA, A GENE CONSISTS OF INTRONS AND EXONS
• EXONS: CODING REGIONS – THEY CODE FOR THE SPECIFIC PROTEIN
• INTRONS: NONCODING - “FILLER DNA” THAT DOES NOT CODE FOR PROTEINS.

• BEFORE MRNA GOES INTO THE CYTOPLASM, THE INTRONS MUST BE REMOVED FROM THE MRNA
• PROTEINS CALLED SPLICEOSOMES CUT THE INTRONS OUT AND JOIN THE REMAINING EXONS TOGETHER.

MRNA TRANSCRIPT

• ONCE THE CAP AND TAIL HAVE BEEN ADDED, AND THE INTRONS REMOVED, THE MRNA IS CALLED AN MRNA TRANSCRIPT
• IT CAN NOW GO TO THE CYTOPLASM FOR TRANSLATION

UNLIKE IN DNA REPLICATION, THERE IS NO QUALITY CONTROL ENZYME FOR THE MRNA TRANSCRIPT

• UNLIKE IN DNA REPLICATION, THERE IS NO QUALITY CONTROL ENZYME FOR THE MRNA TRANSCRIPT
• ERRORS IN MRNA, HOWEVER, ARE NOT TOO DETRIMENTAL SINCE MULTIPLE MRNA TRANSCRIPTS ARE MADE FOR EACH GENE. THEREFORE, SUFFICIENT AMOUNTS OF PROTEIN WILL BE PRODUCED BY THE CORRECT COPIES.

• THANK YOU FOR YOUR ATTENTION