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PROTEIN SYNTHESIS
TRANSCRIPTION AND TRANSLATION
TRANSLATING THE GENETIC CODE
■ GENES: CODED DNA INSTRUCTIONS THAT CONTROL THE
PRODUCTION OF PROTEINS WITHIN THE CELL
■ FIRST STEP IN DECODING GENETIC MESSAGES IS TO COPY
PART OF THE NUCLEOTIDE SEQUENCE FROM DNA TO RNA
(RIBONUCLEIC ACID)
■ RNA MOLECULES CONTAINS CODED INFORMATION FOR
MAKING PROTEINS
STRUCTURE OF RNA
■ CONSISTS OF LONG CHAIN OF NUCLEOTIDES
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DIFFERENCES BETWEEN DNA AND RNA:
SUGAR IN RNA IS RIBOSE AND NOT DEOXYRIBOSE
RNA IS SINGLE STRANDED
RNA CONTAINS URACIL INSTEAD OF THYMINE
TYPES OF RNA
■ IN THE MAJORITY OF CELLS, RNA MOLECULES ARE
INVOLVED WITH ONE JOB ---- PROTEIN SYNTHESIS
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THREE MAIN TYPES:
MESSENGER RNA
RIBOSOMAL RNA
TRANSFER RNA
TYPES OF RNA
■ messenger RNA (mRNA) – RNA MOLECULES THAT CARRY COPIES
OF INSTRUCTIONS FOR ASSEMBLING AMINO ACIDS INTO
PROTEINS
– Serve as “messengers” from DNA to the rest of the cell
■ ribosomal RNA (rRNA) – RNA THAT ALONG WITH SEVERAL DOZEN
PROTEINS THAT MAKE UP A RIBOSOME
– Proteins are assembled on ribosomes
■ transfer RNA (tRNA) – TRANSFERS EACH AMINO ACID TO THE
RIBOSOME AS IT IS SPECIFIED BY CODED MESSAGES IN mRNA
TRANSCRIPTION
■ THE PROCESS THAT COPIES THE MESSAGE IN A GENE INTO A
MESSENGER RNA (mRNA) MOLECULE THAT PROVIDES THE
INSTRUCTIONS FOR MAKING A PROTEIN MOLECULE
– REQUIRES AN ENZYME CALLED RNA POLYMERASE
– RNA POLYMERASE:
■ BINDS TO DNA AND SEPARATES THE DNA STRANDS
■ USES ONE STRAND OF DNA AS A TEMPLATE THAT CAN BE
ASSEMBLED INTO THE CORRESPONDING STRAND OF RNA
RNA SPLICING
■ IN EUKARYOTES, THE mRNA TRANSCRIBED FROM A GENE IS MODIFIED
OR PROCESSED BEFORE IT LEAVES THE NUCLEUS
■ INITIAL mRNA HAS STRETCHES OF NON-CODING NUCLEOTIDE
SEQUENCES CALLED INTRONS
■ THE CODING REGIONS OF THE RNA TRANSCRIPT—THE PARTS OF A
GENE THAT REMAIN IN THE mRNA AND WILL BE TRANSLATED, OR
"EXPRESSED"—ARE CALLED EXONS
■ BEFORE THE RNA LEAVES THE NUCLEUS, THE INTRONS ARE REMOVED
AND THE EXONS ARE JOINED TOGETHER, PRODUCING AN MRNA
MOLECULE WITH A CONTINUOUS CODING SEQUENCE
THE GENETIC CODE
■ THE LANGUAGE OF mRNA INSTRUCTIONS
– CODE IS WRITTEN IN A LANGUAGE THAT ONLY HAS FOUR
LETTER
– READ THREE LETTERS AT A TIME, SO THAT EACH “WORD” OF
THE CODED MESSAGE IS THREE BASES LONG
■ CODON: THE THREE CONSECUTIVE NUCLEOTIDES THAT
SPECIFY A SINGLE AMINO ACID THAT IS TO BE ADDED TO THE
POLYPEPTIDE
THE GENETIC CODE
■ SINCE THERE ARE FOUR DIFFERENT BASES, THERE ARE 64
POSSIBLE THREE-BASE CODONS (4 X 4 X 4 = 64)
■ SOME AMINO ACIDS SPECIFIED BY MORE THAN ONE CODON
■ AUG IS THE “START” CODON
– CODES FOR THE AMINO ACID METHIONINE
■ THERE ARE THREE “STOP” CODONS (UAA, UAG, UGA)
– STOP CODONS DO NOT CODE FOR AN AMINO ACID
TRANSLATION
■ DURING TRANSLATION, OR PROTEIN SYNTHESIS, THE CELL USES
INFORMATION FROM mRNA TO PRODUCE PROTEINS.
■ Step 1: MESSENGER RNA TRANSCRIBED FROM DNA IN THE NUCLEUS
AND RELEASED INTO THE CYTOPLASM.
■ Step 2: mRNA MOLECULE IN THE CYTOPLASM ATTACHES TO A
RIBOSOME. AS EACH CODON OF mRNA MOVES THROUGH THE
RIBOSOME, THE PROPER AMINO ACID IS BROUGHT IN BY tRNA.
– EACH tRNA MOLECULE ONLY CARRIES ONE KIND OF AMINO ACID
– IN ADDITION TO AN AMINO ACID, EACH tRNA MOLECULE HAS THREE
UNPAIRED BASES CALLED ANTICODONS
TRANSLATION
■ Step 3: RIBOSOME FORMS A PEPTIDE BOND BETWEEN THE
FIRST AND SECOND AMINO ACIDS
– RIBOSOME ALSO BREAKS THE BOND THAT HELD THE FIRST
tRNA MOLECULE TO ITS AMINO ACID AND RELEASES THE tRNA
– RIBOSOME THEN MOVES ON TO THE NEXT CODON
■ Step 4: POLYPEPTIDE CHAIN CONTINUES TO GROW UNTIL
RIBOSOME REACHES A STOP CODON ON THE mRNA MOLECULE.
– NEWLY FORMED POLYPEPTIDE CHAIN THEN RELEASED ALONG
WITH mRNA MOLECULE
VIDEO
■ https://www.youtube.com/watch?v=h5mJbP23Buo
CENTRAL DOGMA OF BIOLOGY
PRACTICE
■ A CERTAIN GENE HAS THE FOLLOWING SEQUENCE OF NUCLEOTIDES:
– GACAAGTCCACAATC
■ FROM LEFT TO RIGHT, WRITE THE SEQUENCE OF THE mRNA MOLECULE
TRANSCRIBED FROM THIS GENE.
■ READING THE mRNA CODONS FROM LEFT TO RIGHT, WRITE THE AMINO
ACID SEQUENCE OF THE POLYPEPTIDE TRANSLATED FROM THE mRNA.
■ REPEAT STEP THREE, READING THE CODONS FROM RIGHT TO LEFT.