Chapter 12 Notes, DNA, RNA,
and Protein Synthesis
THE DISCOVERY OF DNA
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BY THE EARLY 1900'S, SCIENTISTS KNEW THAT CHROMOSOMES WERE
RESPONSIBLE FOR TRAITS BEING INHERITED FROM PARENTS TO
OFFSPRING.
HOWEVER, THE KEY COMPONENT OF THE CHROMOSOMES THAT
ACTUALLY CONTAINED THE GENETIC INFORMATION REMAINED A
MYSTERY.
CHEMICAL ANALYSIS OF CHROMOSOMES TOLD THEM THAT THE GENETIC
MATERIAL HAD TO BE EITHER PROTEINS OR NUCLEIC ACIDS (DNA), BUT
THEY DIDN'T KNOW WHICH ONE WAS RESPONSIBLE FOR CARRYING THE
GENETIC INFORMATION.
GRIFFITH'S EXPERIMENT
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IN 1928, BRITISH BACTERIOLOGIST FREDRICK GRIFFITH PERFORMED AN
EXPERIMENT TO TRY TO DETERMINE WHAT THE GENETIC MATERIAL WAS.
GRIFFITH INJECTED TWO DIFFERENT STRAINS OF BACTERIA (STREPTOCOCCUS
PNEUMONIA) INTO MICE.
ONE STRAIN WAS COVERED IN A SUGAR COAT AND ONE WAS NOT.
HE CALLED THE STRAIN THAT WAS COVERED IN THE SUGAR COAT THE
SMOOTH OR S STRAIN.
HE CALLED THE STRAIN THAT LACKED THE SUGAR COAT THE ROUGH OR R
STRAIN.
THE RESULTS OF GRIFFITH'S EXPERIMENT
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THE SMOOTH STRAIN WAS THE VIRULENT (DISEASE CAUSING) STRAIN.
THE ROUGH STRAIN WAS NOT.
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THIS WAS THE RESULT OF HIS EXPERIMENTS
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MICE + SMOOTH (VIRULENT) STRAIN = DEAD MICE
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MICE + ROUGH (NONVIRULENT) STRAIN = LIVE MICE
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MICE + SMOOTH (VIRULENT) STRAIN AFTER THE SMOOTH STRAIN HAD
BEEN KILLED WITH HEAT = LIVE MICE
MICE + ROUGH (NONVIRULENT) STRAIN + HEAT KILLED SMOOTH
(VIRULENT) STRAIN = DEAD MICE
EXPLANATION OF GRIFFITH'S EXPERIMENT
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GRIFFITH CONCLUDED THAT A DISEASE CAUSING FACTOR WAS
TRANSFORMING THE ROUGH (NONVIRULENT) STRAIN INTO THE
SMOOTH (VIRULENT) STRAIN OF BACTERIA.
HERSHEY AND CHASE EXPERIMENT
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IN 1952, A BACTERIOLOGIST BY THE NAME OF ALFRED HERSHEY, AND A
GENETICIST BY THE NAME OF MARTHA CHASE PROVIDED CONCLUSIVE
EVIDENCE THAT DNA WAS IN FACT THE TRANSFORMING FACTOR.
THEIR EXPERIMENT INVOLVED A SPECIAL TYPE OF VIRUS CALLED A
BACTERIOPHAGE.
A BACTERIOPHAGE IS A VIRUS THAT ATTACKS BACTERIA.
THE BACTERIOPHAGE WAS IDEAL FOR THIS EXPERIMENT BECAUSE IT WAS
MADE OF THE TWO KEY COMPONENTS (PROTEIN AND DNA) WHICH
WERE THOUGHT TO BE POSSIBLE MOLECULES RESPONSIBLE FOR
INHERITANCE. AND VIRUSES CAN NOT REPRODUCE ON THEIR OWN.
HERSHEY AND CHASE EXPERIMENT
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HERSHEY AND CHASE USED A TECHNIQUE CALLED RADIOACTIVE LABELING TO
TRACE BOTH THE PROTEIN AND THE DNA OF THE BACTERIOPHAGE AFTER IT
INFECTED THE BACTERIA (E. COLI).
AFTER THE VIRUS INFECTED THE BACTERIA WITH ITS OWN GENETIC MATERIAL,
HERSHEY AND CHASE MONITORED WHICH RADIOACTIVE MATERIAL WAS
INHERITED BY THE BACTERIA.
THIS WOULD IDENTIFY THE GENETIC MATERIAL AS PROTEINS OR DNA.
HERSHEY AND CHASE EXPERIMENT
THE STRUCTURE AND COMPOSITION OF DNA
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SCIENTISTS WERE NOW CONFIDENT THAT THEY HAD DISCOVERED WHAT
THE GENETIC MATERIAL WAS, BUT QUESTIONS REMAINED ABOUT THE
STRUCTURE OF DNA AND HOW DNA COMMUNICATED INFORMATION.
WHAT THEY DISCOVERED IS THAT DNA IS MADE UP OF NUCLEOTIDES. A
NUCLEOTIDE IS A SUGAR MOLECULE, A PHOSPHATE MOLECULE, AND A
NITROGENOUS BASE.
DNA STRUCTURE AND COMPOSITION
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IN THE DNA THERE ARE
FOUR DIFFERENT
NITROGENOUS BASES
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ADENINE
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GUANINE
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CYTOSINE
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THYMINE
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URACIL (IN RNA,
REPLACES THYMINE)
CHARGAFF'S RULE
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IN THE 1950S, ERWIN CHARGAFF DISCOVERED THAT IN EVERY
ORGANISM THE AMOUNT OF GUANINE AND CYTOSINE, AND THE
AMOUNT OF ADENINE AND THYMINE WAS NEARLY EQUAL. THIS IS
CALLED CHARGAFF'S RULE.
THE DOUBLE HELIX
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IN 1951, ROSALIND
FRANKLIN USED X-RAYS TO
PHOTOGRAPH DNA.
PHOTO 51 SHOWED THAT
THE DNA MOLECULE WAS IN
THE SHAPE OF A TWISTED
LADDER KNOWN AS A
DOUBLE HELIX.
THE DOUBLE HELIX
WATSON AND CRICK
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JAMES WATSON AND
FRANCIS CRICK USED DATA
FROM CHARGAFF AND
FRANKLIN'S PHOTO TO
BUILD THE FIRST ACCURATE
MODEL OF DNA.
THE STRUCTURE OF DNA
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DNA IS LIKE A TWISTED
LADDER MADE UP OF
ALTERNATING STRANDS
OF DEOXYRIBOSE AND
PHOSPHATE.
THE RAILS OF THE
LADDER ARE JOINED BY
THE BASES. (ADENINE,
GUANINE, CYTOSINE,
AND THYMINE)
COMPLEMENTARY BASE PAIRING
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EACH NITROGEN BASE
PAIRS UP WITH
ANOTHER BASE IN
WHAT IS KNOWN AS
COMPLEMENTARY
BASE PAIRING.
PURINE BASES PAIR
WITH PYRIMIDINE
BASES.
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•
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ADENINE AND GUANINE
ARE CALLED PURINES.
CYTOSINE AND THYMINE
ARE CALLED PYRIMIDINES.
ADENINE ALWAYS PAIRS
WITH THYMINE.
GUANINE ALWAYS PAIRS
WITH CYTOSINE.
PURINES AND PYRIMIDINES
COMPLEMENTARY BASE PAIRING
ORIENTATION OF THE DNA
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ANOTHER IMPORTANT FEATURE OF THE DNA STRUCTURE IS THE
ORIENTATION OF THE DNA STRANDS.
THE TWO STRANDS DNA ARE REFERRED TO AS ANTIPARRELLEL, MEANING
THEY RUN PARALLEL TO EACH OTHER, BUT IN OPPOSITE DIRECTIONS.
THIS ORIENTATION IS IMPORTANT TO UNDERSTAND BECAUSE IT EXPLAINS
HOW DNA REPLICATES.
ONE END OF THE DNA STRAND IS REFERRED TO AS THE 5' (FIVE-PRIME) END,
AND THE OTHER END IS REFERRED TO AS THE 3' (THREE-PRIME) END.
DNA ORIENTATION
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WE WILL DISCUSS THE
IMPORTANCE OF THIS
ORIENTATION LATER
HOW DOES DNA FIT INSIDE A CELL?
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JUST ONE STRAND OF DNA IN ONE CHROMOSOME CAN BE UP TO 245
MILLION BASE PAIRS LONG!
AND REMEMBER HUMANS HAVE 46 CHROMOSOMES
IT HAS BEEN ESTIMATED THAT IF ALL THE DNA FROM JUST ONE CELL OF A
HUMAN'S BODY WAS UNWOUND, IT WOULD STRETCH ABOUT 6 FT
LONG!
THAT MEANS THE DNA IN ONE CELL IS ABOUT 100,000 TIMES LONGER
THAN THE CELL ITSELF!
AND AMAZINGLY, IT ALL FITS INTO THE NUCLEUS, WHICH ONLY TAKES
UP ABOUT 10% OF THE CELL'S VOLUME!
HOW DOES DNA FIT INSIDE A CELL?
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SO HOW DOES ALL THAT INFORMATION FIT INTO A CELL?
DNA COILS TIGHTLY AROUND SMALL BALLS OF PROTEIN CALLED
HISTONES.
HISTONES AND PHOSPHATES FROM THE DNA COMBINE TOGETHER TO
FORM NUCLEOSOMES.
NUCLEOSOMES COMBINE TOGETHER TO FORM CHROMATIN FIBERS,
AND THE CHROMATIN FIBERS COMBINE TOGETHER TO FORM THE
CHROMOSOMES.
CHROMOSOMES AND HISTONES
SEMICONSERVATIVE REPLICATION
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WHEN WATSON AND CRICK CREATED THEIR MODEL OF THE DNA
DOUBLE HELIX, THEY ALSO PROPOSED A POSSIBLE WAY THAT DNA
MIGHT GET REPLICATED.
THE WAY THEY PROPOSED DNA GETS REPLICATED IS CALLED
SEMICONSERVATIVE REPLICATION.
IN SEMICONSERVATIVE REPLICATION, ONE OF THE STRANDS ALWAYS
GETS COPIED AND THE OTHER STRAND IS A COPY FROM THE ORIGINAL
PARENT OR TEMPLATE STRAND.
THE PROCESS IS SIMILAR TO HOW SOURDOUGH BREAD IS MADE. IN
ORDER TO MAKE IT YOU NEED A STARTER BATCH (ORIGINAL TEMPLATE).
SEMICONSERVATIVE REPLICATION
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SEMICONSERVATIVE REPLICATION OCCURS IN THREE STAGES:
UNWINDING, BASE PAIRING, AND JOINING
DURING UNWINDING, AN ENZYME CALLED DNA HELICASE UNWINDS
OR UNZIPS THE DNA DOUBLE HELIX.
AFTER THE STRANDS UNWIND, ANOTHER ENZYME CALLED DNA
POLYMERASE, ADDS NUCLEOTIDES TO THE NEW STRAND IN
COMPLEMENTARY BASE PAIRS.
SEMICONSERVATIVE REPLICATION
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BECAUSE THE STRANDS ARE ANTIPARALLEL, ONE OF THE STRANDS CAN
BE REPLICATED CONTINUOUSLY FROM ONE END TO THE OTHER. THIS
SECTION THAT IS REPLICATED CONTINUOUSLY IS CALLED THE LEADING
STRAND.
THE OTHER STRAND, CALLED THE LAGGING STRAND, HAS TO BE
REPLICATED IN REVERSE ORDER IN SECTIONS OF NUCLEOTIDES. THESE
SECTIONS OF NUCLEOTIDES ARE CALLED OKAZAKI FRAGMENTS.
SEMICONSERVATIVE REPLICATION
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THE OKAZAKI FRAGMENTS ARE THEN GLUED TOGETHER BY ANOTHER
ENZYME CALLED DNA LIGASE
SEMICONSERVATIVE REPLICATION
SEMICONSERVATIVE REPLICATION
THE CENTRAL DOGMA
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DNA CONTAINS A CODE THAT IS TRANSCRIBED AND TRANSLATED BY
ANOTHER NUCLEIC ACID CALLED RNA (RIBONUCLEIC ACID).
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RNA GUIDES THE SYNTHESIS OF PROTEINS.
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THIS IS PROCESS IS KNOWN AS THE CENTRAL DOGMA OF BIOLOGY.
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DNA IS TRANSCRIBED BY MESSENGER RNA.
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MESSENGER RNA CARRIES INFORMATION TO THE RIBOSOMES.
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RIBOSOMES (RIBOSOMAL RNA) AND TRANSFER RNA TRANSLATE THE
CODE TO MAKE THE PROTEINS.
THIS IS HOW GENES ARE EXPRESSED AS TRAITS.
THE CENTRAL DOGMA
WHAT IS RNA?
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RNA IS SIMILAR TO DNA. SOME DIFFERENCES ARE THAT RNA CONTAINS
THE SUGAR RIBOSE INSTEAD OF DEOXYRIBOSE.
ANOTHER DIFFERENCE IS THAT RNA USES THE NITROGEN BASE URACIL IN
PLACE OF THYMINE.
ANOTHER DIFFERENCE BETWEEN RNA AND DNA, IS THAT RNA IS SINGLESTRANDED WHILE DNA IS DOUBLE-STRANDED.
THERE ARE THREE MAIN TYPES OF RNA THAT PLAY A ROLE IN PROTEIN
SYNTHESIS THEY ARE MESSENGER RNA (MRNA), RIBOSOMAL RNA
(RRNA) AND TRANSFER RNA (TRNA).
MESSENGER RNA (MRNA)
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THE JOB OR ROLE OF MRNA IS TRANSCRIPTION.
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TRANSCRIPTION IS THE PROCESS OF COPYING THE DNA CODE.
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THIS IS THE ROLE OF MESSENGER RNA (MRNA).
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MESSENGER RNA ENTERS THE NUCLEUS, A SMALL PORTION OF THE DNA
STRAND IS COPIED. THEN THE MESSENGER RNA LEAVES THE NUCLEUS
AFTER COPYING DOWN A PART OF THE CODE TO MAKE A PROTEIN.
MESSENGER RNA AND TRANSCRIPTION
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AFTER THE DNA IS UNWOUND IN THE NUCLEUS, AN ENZYME COMES
ALONG TO ASSIST IN BASE PAIRING, CALLED RNA POLYMERASE.
RNA POLYMERASE ASSISTS MRNA IN RECORDING WHAT INFORMATION IS
FOUND ON A PORTION OF THE DNA STRAND.
MESSENGER RNA RECORDS THE CODE IN COMPLEMENTARY BASE PAIRS,
SIMILAR TO THE WAY DNA BASES ARE PAIRED DURING REPLICATION, EXCEPT
WHEN THE BASE PAIR ADENINE IS PAIRED, ADENINE PAIRS WITH URACIL
INSTEAD OF THYMINE.
MESSENGER RNA AND TRANSCRIPTION
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AFTER THE MRNA IS TRANSCRIBED, MRNA CAN LEAVE THE NUCLEUS
THROUGH NUCLEAR PORES AND ENTER INTO THE CYTOPLASM TO FIND
TRANSFER RNA (TRNA) AND RIBOSOMAL RNA (RRNA).
TRANSLATION AND TRANSCRIPTION
RIBOSOMES, TRANSFER RNA, AND
TRANSLATION
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AFTER A MRNA FINDS A RIBOSOMAL RNA (RRNA), THE CODE IS READ
AND TRANSLATED BY INTERPRETERS CALLED TRANSFER RNA.
TRANSFER RNA (TRNA) INTERPRETS THE CODE BY READING THE BASES IN
GROUPS OF THREE CALLED CODONS.
TRANSFER RNA MOLECULES EACH HAVE THEIR OWN ANTICODON THAT
ONLY MATCHES WITH A SPECIFIC CODON.
RIBOSOMES, TRANSFER RNA, AND
TRANSLATION
THE CODE
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THE DNA CODE IS READ
AS A THREE-BASE CODE
SYSTEM.
EACH CODON MATCHES
WITH A SPECIFIC
ANTICODON AND A
SPECIFIC AMINO ACID.
BY JOINING MULTIPLE
AMINO ACIDS
TOGETHER, PROTEINS
CAN BE ASSEMBLED.