MOLECULAR GENETICS

YOU MUST KNOW…• THE STRUCTURE OF DNA• THE MAJOR STEPS TO REPLICATION• THE DIFFERENCE BETWEEN

REPLICATION, TRANSCRIPTION, AND TRANSLATION

• HOW DNA IS PACKAGED INTO A CHROMOSOME

CONCEPT 16.1• DNA IS THE

GENETIC MATERIAL

DISCOVERY THAT DNA IS THE GENETIC MATERIAL

• HERSHEY AND CHASE STUDIED BACTERIOPHAGES THAT ARE MADE OF ONLY DNA AND PROTEIN

• USED RADIOACTIVE ISOTOPE OF PHOSPHORUS TO TAG THE DNA AND RADIOACTIVE SULFUR TO TAG THE PROTEIN

• RESULTS DEMONSTRATED THAT ONLY THE DNA ENTERED BACTERIA INFECTED BY THE VIRUS, THE PROTEIN DIDN’T

DNA STRUCTURE• WATSON AND CRICK - CREDITED• FRANKLIN AND WILKINS AIDED TO

THEIR SUCCESS BY WORKING IN FIELD OF X-RAY

CHYSTALLOGRAPHY

(VISUALIZE MOLECULES

3-D)

FEATURES OF DNA• DOUBLE HELIX –

TWISTED LADDER• SIDES OR BACKBONE

ARE MADE OF 5C- SUGAR (DEOXYRIBOSE) AND PHOSPHATE

• RUNGS ARE MADE OF PAIRS OF NITROGEN BASES (A-T, G-C)

DNA• ANTIPARALLEL - LEFT SIDE OF THE

STRUCTURE IS IN THE READING POSITION BUT THE RIGHT SIDE IS OPPOSITE, UPSIDE-DOWN DIRECTION

• LEFT SIDE RUNS 5’ TO 3’, RIGHT SIDE RUNS 3’ TO 5’

CONCEPT 16.2• MANY PROTEINS WORK

TOGETHER IN DNA REPLICATION AND REPAIR

REPLICATION• MAKING OF DNA FROM AN EXISTING

DNA STRAND• DNA REPLICTION IS

SEMICONSERVATIVE

(EACH OF THE DAUGHTER MOLECULES HAS ONE

OLD STRAND AND ONE

NEW STRAND)

• REPLICATION OF DNA BEGINS AT SITES CALLED THE ORIGINS OF REPLICATION

• INITIATION PROTEINS BIND TO THE ORIGIN OF REPLICATION AND SEPARATE THE TWO STRANDING, FORMING A REPLICATION BUBBLE

• DNA REPLICATION PROCEEDS IN BOTH DIRECTIONS ALONG THE DNA STRAND UNTIL THE MOLECULE IS COPIED

• DNA POLYMERASES CATALYZES THE ELONGATION OF NEW DNA AT THE REPLICATION FORK

• DNA POLYMERASE ADDS NUCLEOTIDES TO THE GROWING CHAIN ONE BY ONE, WORKING IN A 5’ TO 3’ DIRECTION

• DNA REPLICATION OCCURS CONTINUOUSLY ALONG THE 5’ TO 3’ STRAND (LEADING STRAND)

• THE STRAND THAT RUNS 3’ TO 5’ IS COPIED IN SERIES OF SEGMENTS AND TERMED THE LAGGING STRAND

• THE LAGGING STRAND IS SYNTHESIZED IN SEPARATE PIECES CALLED OKAZAKI FRAGMENTS THAT ARE SEALED TOGETHER BY DNA LIGASE FORMING A CONTINUOUS DNA STRAND

FACTORS CONTRIBUTING TO THE ACCURACY OF DNA

REPLICATION• THE SPECIFICITY OF BASE PAIRING• MISMATCH REPAIR – SPECIAL REPAIR

ENZYMES FIX INCORRECTLY PAIRED NUCLEOTIDES

• NUCLEOTIDE EXCISION REPAIR – INCORRECTLY PLACED NUCLEOTIDES ARE EXCISED OR REMOVED BY ENZYMES TERMED NUCLEASES AND THE LEFT OVER IS FILLED IN WITH THE CORRECT NUCLEOTIDES

• DNA POLYMERASE CAN ADD NUCLEOTIDES ONLY TO THE 3’ END OF A MOLECULE• 5’ END OF THE DNA IS NOT COMPLETED• EVERY TIME A CHROMOSOME IS

REPLICATED FOR MITOSIS, A SMALL PORTION OF THE TIP OF THE CHROMOSOME IS REMOVED

• TO AVOID LOSING TERMINAL GENES, THE ENDS ARE “CAPPED” WITH TELOMERES (DO NOT CONTAIN GENES)

CONCEPT 16.3• A CHROMOSOME

CONSISTS OF A DNA MOLECULE

PACKED

TOGETHER WITH

PROTEINS

DIFFERENCE BETWEEN BACTERIAL DNA AND

EUKARYOTE DNA• BACTERIAL CHROMOSOME IS ONE

DOUBLE-STRANDED, CIRCULAR DNA MOLECULE WITH A SMALL AMOUNT OF PROTEIN

• EUKARYOTIC CHROMOSOMES ARE LINEAR DNA WITH LARGE AMOUNTS OF PROTEIN

EUKARYOTE CHROMOSOMES

• FIRST LEVEL OF PACKING IN CHROMOSOMES INVOLVES DNA WRAPPED AROUND PROTEINS CALLED HISTONES

• DNA/PROTEIN COMPLEX RESEMBLES BEADS ON A STRING AND IS CALLED A NUCLEOSOME

• THE STRING LOOPS AND FOLDS

• AS DNA BECOMES MORE HIGHLY PACKAGED, IT BECOMES LESS ACCESSIBLE TO TRANSCRIPTION ENZYMES

• THIS REDUCES GENE EXPRESSION