DNA STRUCTURE AND REPLICATION

DNA A NUCLEIC ACID MADE OF TWO STRANDS OF NUCLEOTIDES WOUND TOGETHER IN A SPIRAL CALLED A DOUBLE HELIX NUCLEOTIDE COMPOSED OF THREE PARTS: ◦5-CARBON SUGAR CALLED DEOXYRIBOSE, ◦A PHOSPHATE GROUP, ◦AND A NITROGENOUS BASE

NITROGENOUS BASES FOUR DIFFERENT BASES: ◦ADENINE (A) ◦THYMINE (T) ◦GUANINE (G) ◦CYTOSINE (C) ADENINE AND GUANINE BELONG TO A GROUP OF COMPOUNDS CALLED PURINES (TWO RINGS) CYTOSINE AND THYMINE ARE KNOWN AS PYRIMIDINES (ONE RING)

QUESTION HOW MANY BASIC UNITS DOES DNA HAVE? WHAT ARE THEY?

NITROGENOUS BASES PHOSPHATE AND SUGAR PARTS OF NUCLEOTIDES FORM THE BACKBONE OF EACH STRAND IN THE DNA DOUBLE HELIX BASES EXTEND TOWARDS THE CENTER OF THE DOUBLE HELIX

HISTORY CHARGAFF’S RULES: ERWIN CHARGAFF, AN AMERICAN BIOCHEMIST, [A] = [T] AND [G] = [C]

HISTORY X-RAY EVIDENCE: EARLY 1950s, BRITISH SCIENTIST NAMED ROSALIND FRANKLIN BEGINS STUDYING DNA USES TECHNIQUE CALLED X-RAY DIFFRACTION TO GET INFORMATION ABOUT STRUCTURE OF DNA

HISTORY THE DOUBLE HELIX: SAME TIME AS FRANKLIN, FRANCIS CRICK, A BRITISH PHYSICIST, AND JAMES WATSON, AN AMERICAN BIOLOGIST WERE TRYING TO UNDERSTAND STRUCTURE OF DNA BY BUILDING 3-D MODELS EARLY 1953: WATSON SHOWN COPY OF FRANKLIN’S X-RAY PATTERN, USING CLUES FROM HER PATTERN, WATSON AND CRICK BUILD STRUCTURAL MODEL THAT EXPLAINS PUZZLE OF HOW DNA CARRIES INFORMATION AND HOW IT IS COPIED PUBLISH RESULTS IN APRIL OF 1953

DNA STRUCTURE DOUBLE HELIX LOOKS LIKE A TWISTED LADDER OR SPIRAL STAIRCASE WATSON AND CRICK DISCOVER THAT HYDROGEN BONDS COULD FORM BETWEEN CERTAIN NITROGENOUS BASES AND PROVIDE JUST ENOUGH FORCE TO HOLD THE TWO STRANDS TOGETHER DISCOVER PRINCIPLE OF BASE PAIRING, WHICH EXPLAINS CHARGAFF’S RULE ◦ADENINE ALWAYS PAIRS WITH THYMINE AND ◦GUANINE WITH CYTOSINE

QUESTION WHAT ARE THE FOUR NITROGENOUS BASES FOUND IN DNA?

DNA REPLICATION QUICK REVIEW! WHERE IS DNA FOUND? WHAT FORM IS IT IN WHEN IN THE NUCLEUS? WHAT IS CHROMATIN COMPOSED OF?

DNA LENGTH DNA MOLECULES SURPRISINGLY LONG ◦E. COLI CHROMOSOME CONTAINS 4,639,221 BASE PAIRS AND IS 1.6 MM LONG ◦DNA MUST BE FOLDED INTO A SPACE ONLY ONE-THOUSANDTH OF ITS LENGTH ◦1000 TIMES AS MANY BASE PAIRS IN HUMAN CELLS AND MORE THAN 1 METER OF DNA

DNA LENGTH CHROMATIN CONSISTS OF DNA THAT IS TIGHTLY COILED AROUND PROTEINS CALLED HISTONES DNA AND HISTONE MOLECULES FORM A BEADLIKE STRUCTURE CALLED A NUCLEOSOME NUCLEOSOMES ABLE TO FOLD ENORMOUS LENGTHS OF DNA INTO THE TINY SPACE AVAILABLE IN THE CELL NUCLEUS

DNA STRUCTURE THESE CHARACTERISTICS ARE THE SAME FOR THE DNA OF ALL ORGANISMS. THE DNA OF DIFFERENT ORGANISMS DIFFERS IN THE SEQUENCE OF NUCLEOTIDES, AND THESE DIFFERENCES IN NUCLEOTIDE SEQUENCE ARE RESPONSIBLE FOR THE GENETIC DIFFERENCES BETWEEN DIFFERENT ORGANISMS.

QUESTION WHAT IS CHROMATIN? WHY WOULD IT BE ADVANTAGEOUS TO THE CELL TO HAVE ITS DNA MOLECULES TIGHTLY CONDENSED INTO NUCLEOSOMES DURING MITOSIS?

DNA REPLICATION STRUCTURE OF DNA EXPLAINED HOW IT COULD BE COPIED, OR REPLICATED PRODUCES TWO NEW DNA MOLECULES THAT ARE IDENTICAL TO THE ORIGINAL MOLECULE. EACH STRAND OF THE DNA DOUBLE HELIX HAS ALL THE INFORMATION NEEDED TO RECONSTRUCT THE OTHER HALF BY THE MECHANISM OF BASE PAIRING. DUE TO THIS, STRANDS ARE SAID TO BE COMPLIMENTARY.

DNA REPLICATION IN MOST PROKARYOTES, DNA REPLICATION BEGINS AT A SINGLE POINT IN THE CHROMOSOME AND PROCEEDS, OFTEN IN TWO DIRECTIONS, UNTIL ENTIRE CHROMOSOME IS REPLICATED. IN LARGER EUKARYOTES, REPICATION OCCURS IN HUNDREDS OF PLACES AND PROCEEDS IN BOTH DIRECTIONS UNTIL EACH CHROMOSOME IS COMPLETELY COPIED. SITES WHERE SEPARATION AND REPLICATION OCCUR ARE CALLED REPLICATION FORKS

QUESTION WHAT IS RESPONSIBLE FOR THE GENETIC DIFFERENCES BETWEEN DIFFERENT ORGANISMS?

DNA REPLICATION DNA HELICASE: UNWINDS AND UNZIPS DNA ◦SEPARATES TWO STRANDS BY BREAKING HYDROGEN BONDS BETWEEN COMPLIMENTARY BASE PAIRS ◦ATP NEEDED BY HELICASE TO MOVE ALONG DNA MOLECULE AND TO BREAK HYDROGEN BONDS ◦TWO SEPARATED STRANDS BECOME PARENT/TEMPLATE STRANDS FOR THE REPLICATION PROCESS

DNA REPLICATION DNA POLYMERASE: LINKS NUCLEOTIDES TOGETHER TO FORM A NEW STRAND, USING PRE-EXISTING STRAND AS A TEMPLATE ◦CREATES COMPLIMENTARY STRANDS ◦MOVES IN OPPOSITE DIRECTIONS ON EACH STRAND ◦PROOF READS THE COMPLIMENTARY BASE PAIRING ◦MISTAKES ARE VERY INFREQUENT, OCCURING APPROXIMATELY ONCE IN EVERY BILLION BASE PAIRS

PRACTICE REPLICATE THE STRANDS OF DNA SHOWN ATGCTAGCTAGTCGAT GCAATTGCATGCTAGT

EXIT TICKET EXPLAIN HOW THE DOUBLE HELIX STRUCTURE OF DNA CONTRIBUTES TO ITS FUNCTION.