MODERN GENETICS
 WHO DISCOVERED THE
STRUCTURE OF DNA?
 EVERYONE SEEMS TO KNOW
WATSON AND CRICK, BUT
WHO IS THIS?

MODERN GENETICS
 T.H. MORGAN OFFERED FIRST EVIDENCE OF WHAT
SUBSTANCE INSIDE LIVING THINGS CONTROLS
THEIR TRAITS
 LIKE MENDEL, THE
TRICK TO BEING
SUCCESSFUL LIED
IN WORKING WITH
AN ORGANISM THAT
COULD GIVE
MANY GENERATIONS
QUICKLY!!
 HENCE, FRUIT FLIES 
MODERN GENETICS
 T.H. MORGAN’S WORK LED TO THE FOLLOWING
UNDERSTANDING:
 CHROMOSOMES CONTROL SEX (GENDER)
 SEX-LINKED TRAITS
 GENE LINKAGE
MODERN GENETICS
 SEX DETERMINATION AND CHROMOSOMES
 A SPECIFIC PAIR OF CHROMOSOMES, KNOWN AS THE
SEX CHROMOSOMES, DETERMINES THE GENDER OF
ANY INDIVIDUAL
MODERN GENETICS
 SEX-LINKED
TRAITS
 OBSERVE
THE
PEDIGREE
CHART…
WHAT
PATTERN
DO YOU
NOTICE?
MODERN GENETICS
 SEX-LINKED TRAITS
 WHEN A TRAIT APPEARS TO BE DISPLAYED IN ONE
GENDER MORE FREQUENTLY THEN ANOTHER, AND
THE ONLY DIFFERENCE BETWEEN EACH GENDER IS
THE SEX CHROMOSOMES; THE TRAIT MUST BE
ASSOCIATED/LINKED TO THE SEX CHROMOSOMES
 X-LINKAGE VS. Y-LINKAGE
MODERN GENETICS
 EXAMPLES OF X-LINKED TRAITS:
 RED-GREEN COLORBLINDNESS
 HEMOPHILIA
MODERN GENETICS
 GENE LINKAGE
 IT WAS DISCOVERED THAT
GENDER AND TRAITS AREN’T
THE ONLY THINGS THAT CAN BE
LINKED…
 ANY PAIR OF TRAITS FOUND ON
THE SAME CHROMOSOME ARE
CONSIDERED LINKED
 WHAT DATA WOULD SUGGEST
THAT TRAITS ARE LINKED?
*HERE’S A HINT 
MODERN GENETICS
 IF GENES ARE LINKED, HOW CAN THEIR BE PEOPLE
WITH BROWN HAIR AND BLUE EYES?
 CROSSING OVER!!
 EXCHANGE BETWEEN
HOMOLOGOUS
CHROMOSOMES DURING
SYNAPSIS OF MEIOSIS
MODERN GENETICS
 RANDOM QUESTION…
 IN HUMANS, DOES HEIGHT
OCCUR IN JUST TWO FORMS;
TALL VERSUS SHORT?
 HOW CAN WE EXPLAIN THE
LARGE RANGE OF PHENOTYPES
FOR HUMAN HEIGHT?
 POLYGENIC INHERITANCE 
 MORE THAN ONE GENE
CONTROLS THE SAME TRAIT,
CREATING A LARGE NUMBER OF
POSSIBLE PHENOTYPES
MODERN GENETICS
 AFTER UNDERSTANDING ABOUT THE DIFFERENT
PATTERNS OF INHERITANCE, THIS STILL LEFT AN
UNANSWERED QUESTION…
WHAT WAS THE ACTUAL
SUBSTANCE THAT
CONTROLLED THESE
TRAITS TO BEGIN WITH?
MODERN GENETICS
 THE GREAT DEBATE – PROTEIN VS. DNA
 BY THE EARLY 1900’S, SCIENCE HAD ALREADY
LEARNED OF THE EXISTENCE OF CHROMOSOMES AND
THAT THESE CHROMOSOMES WERE COMPOSED OF
BOTH PROTEINS AND NUCLEIC ACIDS
 LITTLE WAS KNOWN ABOUT NUCLEIC ACIDS, AND
PROTEINS WERE WELL UNDERSTOOD AT THAT TIME
 AS PROTEINS WERE FOUND TO EXIST IN EVERY
SUBSTANCE INVOLVED IN TRAITS, THEY WERE THE
FRONT-RUNNER FOR THE ACTUAL SUBSTANCE THAT
CONTROLLED TRAITS
MODERN GENETICS
 ENTER “THE MOUSE-KILLER”
 SIR FREDERICK GRIFFITH
MODERN GENETICS
 GRIFFITH’S EXPERIMENTS WEREN’T ENOUGH FOR
THE SKEPTICS…BUT EVEN THE SKEPTICS COULDN’T
DENY THE FINDINGS OF…
MODERN GENETICS
 HERSHEY AND CHASE EXPERIMENT
MODERN GENETICS
 ONCE IT WAS ESTABLISHED THAT DNA
CONTROLLED TRAITS, THERE WAS A MAD RUSH TO
FIND OUT THE STRUCTURE OF DNA…
 WHY IN THE WORLD WOULD SCIENTISTS CARE SO
MUCH ABOUT THE STRUCTURE OF DNA?
STRUCTURE DENOTES
FUNCTION 
MODERN GENETICS
 WHILE SCIENTISTS FIGURED OUT THE BUILDING
BLOCKS OF DNA, HOW THOSE BUILDING BLOCKS
GOT PUT TOGETHER ELUDED THEM…
MODERN GENETICS
 ENTER OUR
HERO’S
 WATSON AND
CRICK
 CREDITED WITH
IDENTIFYING /
DISCOVERING
THE STRUCTURE
OF DNA!!!
MODERN GENETICS
 DNA’S STRUCTURE
MODERN GENETICS
 DNA’S STRUCTURE
• DNA IS A DOUBLE HELIX (TWISTED
LADDER)
• ADENINE ALWAYS PAIRS WITH
THYMINE AND CYTOSINE ALWAYS
PAIRS WITH GUANINE
• EXPLAINED THE IMMENSE
COMPLEXITY OF TRAITS (HOW
DOES IT LEAD TO COMPLEXITY?)
• UNEXPECTEDLY EXPLAINED WHY
DNA CAN MAKE IDENTICAL COPIES
OF ITSELF (IDENTICAL OFFSPRING)
MODERN GENETICS
 DNA REPLICATION
MODERN GENETICS
 LEARNING THE STRUCTURE OF DNA STILL LEFT AN
UNANSWERED QUESTION…
 HOW WAS DNA THE SUBSTANCE THAT
CONTROLLED AN INDIVIDUAL’S TRAITS, IF
PROTEINS WERE WHAT WERE PRESENT THAT
ACTUALLY SEEMED TO CONTROL THE APPEARANCE
OF THE TRAIT?
 SOMEHOW, DNA HAD TO BE CONTROLLING THE
SYNTHESIS OF PROTEINS…
MODERN GENETICS
 WHERE IS DNA FOUND?
 WHERE DOES PROTEIN SYNTHESIS TAKE PLACE?
 DOES DNA EVER LEAVE THE NUCLEUS?
NO!!!
 HOW CAN DNA BE USED TO MAKE PROTEINS IF IT
NEVER GOES NEAR THE RIBOSOMES IN THE
CYTOPLASM?
MODERN GENETICS
 HOW CAN DNA BE USED TO MAKE PROTEINS IF IT
NEVER GOES NEAR THE RIBOSOMES IN THE
CYTOPLASM? (HINT - HOW CAN TWO PEOPLE
COMMUNICATE WHO NEVER SEE EACH OTHER AND
DON’T HAVE TELEPHONES?)
 THEY CAN WRITE TO EACH OTHER…WHICH IS
EXACTLY WHAT DNA DOES, IT WRITES A MESSAGE
TO BE SENT TO THE RIBOSOMES…
 ENTER OUR MESSENGER –
RNA!!
MODERN GENETICS
RNA VS DNA
MODERN GENETICS
 THE PROCESS OF DNA WRITING A MESSAGE IN THE
FORM OF RNA IS CALLED:
 DNA TRANSCRIPTION
 DNA  mRNA
 ALL WE HAVE IS A MESSAGE, WE HAVEN’T MADE A
PROTEIN YET…WHERE DO PROTEINS GET MADE?
 THE mRNA CAN GO VISIT THE RIBOSOME FACE TO
FACE 
MODERN GENETICS
 THE RIBOSOME USES THE mRNA AS THE
INSTRUCTIONS ON HOW TO MAKE A PROTEIN
 WHAT IS THIS PROCESS OF MAKING A PROTEIN
CALLED?
 PROTEIN SYNTHESIS
 WHAT DOES A PROTEIN LOOK LIKE?
 A LONG CHAIN OF AMINO ACIDS
 HOW DOES PROTEIN SYNTHESIS WORK?
 THE mRNA TELLS THE RIBOSOME WHAT THE ORDER OF
AMINO ACIDS SHOULD BE IN BUILDING A PROTEIN
MODERN GENETICS
 PROTEINS ARE MADE UP OF AMINO ACIDS; STUDIES
HAVE SHOWN THAT THERE ARE 20 DIFFERENT
AMINO ACIDS…
 THERE ARE ONLY 4 NITROGENOUS BASES IN mRNA;
A, U, C, G…HOW CAN 4 LETTERS = 20 DIFFERENT
BASES?
MODERN GENETICS
MODERN GENETICS
 I FORGOT TO TELL YOU…RIBOSOMES DON’T SPEAK
PROTEIN (IN OTHER WORDS THEY DON’T KNOW
WHAT AMINO ACID MATCHES A CODON)
 WE NEED A SPECIAL TRANSLATOR…
 ENTER tRNA
 SPEAKS BOTH CODON
AND PROTEIN, SO IT
“TRANSLATES” AND
“TRANSFERS” THE
CORRECT AMINO ACID
MODERN GENETICS
 LET’S LOOK AT IT
OVERALL
 PROTEIN
SYNTHESIS
 TRANSCRIPTION
 TRANSLATION
MODERN GENETICS
 REMEMBER, EVERY CELL IN YOUR BODY HAS THE
EXACT SAME DNA…WHY IS THAT?
 WE HAVE EXPLAINED HOW DNA ACTS AS THE
INSTRUCTIONS FOR MAKING PROTEINS…SO HOW
COME EVERY CELL IN YOUR BODY DOESN’T MAKE
THE EXACT SAME PROTEINS
 DIFFERENT CELLS USE DIFFERENT PARTS OF DNA
 WE CONTROLS/DETERMINES WHAT PARTS OF DNA
ARE USED?
 WE CALL THIS FIELD OF STUDY GENE EXPRESSION
MODERN GENETICS
 GENE EXPRESSION IS ONE OF THE MOST
COMPLICATED/LEAST UNDERSTOOD AREAS OF
GENETICS/SCIENCE TODAY
 THE SIMPLE EXPLANATION IS THAT MOST GENES
ARE LIKE LIGHT SWITCHES, THEY CAN BE ON OR
OFF…WHEN THEY ARE ON THEY ARE MAKING
PROTEINS, WHEN THEY ARE OFF, THEY AREN’T
MODERN GENETICS
 WHAT CAN TURN
THE SWITCHES ON
OR OFF?
 OTHER GENES
 ENVIRONMENTAL
FACTORS
 **MUST
REMEMBER – THE
ENVIRONMENT
CAN EFFECT GENE
EXPRESSION
MODERN GENETICS
 UNDERSTANDING GENE EXPRESSION MAY UNLOCK
CURES TO DISEASES SUCH AS CANCER
 ONCOGENES
MODERN GENETICS
 WHAT DO WE DO
WITH ALL THIS
INFORMATION ON
GENES?
 APPLIED GENETICS IS
NEXT!!