PROTEIN SYNTHESISTRANSCRIPTION 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

■ 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.