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 ■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 ■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 ■

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.