DNA

BEGIN THE CONSTRUCTION OF YOUR DNA MODEL… As you work, make note of any patterns or trends you can see structurally or chemically; list at least 5. Begin model here on the back of coloring worksheet 1. 2. 3. 4. 5.

EUKARYOTIC CELLS E D C B A A: DOUBLE HELIX COILS AROUND PROTEINS CALLED B: HISTONES HISTONES COIL MOLECULE INTO C: SUPER COILS SUPER COILS WIND UP TO FORM D: CHROMOSOMES CHROMOSOMES IN NUCLEUS FORM E: CHROMATIN E D C B A

PRIMARY FUNCTION STORES GENETIC INFORMATION IN BASE CODE SEQUENCES TRANSMITS GENETIC CODE FROM GENERATION TO GENERATION CHEMICALLY CONTROLS THE SYNTHESIS OF ALL PROTEINS CHEMICAL PROCESSES PHYSICAL STRUCTURES

DeoxyriboNucleic Acid Made of repeating monomers: NUCLEOTIDES 5-carbon sugar Phosphate functional group 1 of 4 Nitrogen containing bases Adenine Cytosine Guanine Thymine

NITROGEN BASES PURINES PYRIMIDINES BASES WITH TWO CARBON RINGS BASES WITH ONE CARBON RING Uracil only found in RNA

ONE PURINE BONDS WITH ONE PYRIMIDINE FORMING A COMPLIMENTARY BASE PAIR BASE PAIR BONDING ONE PURINE BONDS WITH ONE PYRIMIDINE FORMING A COMPLIMENTARY BASE PAIR ADENINE ALWAYS BONDS WITH THYMINE FORMING A-T OR T-A GUANINE ALWAYS BONDS WITH CYTOSINE FORMING G-C OR C-G

The ANTIPARALLEL structure of the DNA molecule…. 5’ 3’ 3’ 5’

THAT’S INTERESTING! NATURE LOVES EFFICIENCY! DNA LOOKS LIKE A LADDER SUGAR AND PO4 MAKES THE “FRAME” BASES FORM THE “RUNGS” ONE SIDE OF MOLECULE MUST “FLIP” IN ORDER FOR HYDROGEN BONDS TO FORM BETWEEN BASES, THE SIDES ARE ANTIPARALLEL ALL BASE PAIRS, NO MATTER THE ORDER, HAVE THE SAME STRUCTURAL SHAPE(isometric) THEY STACK ON TOP OF EACH OTHER THEN TWIST FORMING THE CHARACTERISTIC DOUBLE HELIX SINCE A ALWAYS BONDS WITH T, AND C WITH G, THEIR AMOUNTS MUST BE EQUAL TO EACH OTHER(Chargaff Principle) If 40% of the molecule is A-T, then how much of each base makes up the molecule? NATURE LOVES EFFICIENCY! DNA’S STRUCTURE ALLOWS THE MOLECULE TO BE REPLICATED QUICKLY CODE THE SYNTHESIS OF ALL PROTEINS PASS THE GENETIC CODE TO FUTURE GENERATIONS THE SAME WAY EVERY SINGLE TIME in ALL ORGANISMS. 20% A & 20% T, 60% C-G with 30% C & 30% G

Who figured all this out? Rosalind Franklin and Maurice Wilkins Maurice Wilkins and Rosalind Franklin were the first to obtain very good x-ray diffraction images of the DNA . At the time, little was known about the structure of DNA; though their x-ray photos didn't show the structure of the DNA, there were patterns on those images that could be used to determine the position of DNA’s nucleotides. From these photos, Franklin determined that the nitrogen bases were ‘inside’ the structure and molecule must be long, thin and possibly a helix

James Watson and Francis Crick Using the X-ray diffraction photos of DNA taken by Wilkins and Franklin, they also saw that they showed an ‘X’ shape...which does illustrate the signature of a helix. In April of 1953, using this information, together with the base pair’s isometric structure, they identified DNA’s double helix configuration. Watson: basepairing

DNA REPLICATION

DNA Replication - Occurs during S phase of interphase One strand of DNA has all of the information needed to construct the other strand through the mechanism of complimentary base pairing: A-T, T-A, C-G, G-C. During DNA replication, the DNA molecule is used to produce two new complementary strands. Each strand of the double helix serves as a template for each new strand. *Even though the DNA in eukaryotes is linear and circular in prokaryotes; the replication process is the same for both.

OLD REPLICATION MODELS “LADDER” MODEL DNA molecule “pried” apart Bases added Then sugar and phosphate added to hold bases in place

“ZIPPER” MODEL Enzymes “unzip” molecule down entire length Enzymes attach to each side Another enzyme attaches complimentary bases New strands build simultaneously in same direction until duplicate strands are formed

The ANTIPARALLEL structure of the molecule makes those models of replication impossible … 5’ 3’ 3’ 5’ That’s because polymerase enzymes can only work in one direction placing nucleotides “right-side up” 5’ to 3’

DNA Replication Step 3: DNA polymerase III attaches to Step 1: A portion of the parental DNA double helix is unwound and separated by the enzyme DNA helicase. This occurs in several places along the strand at replication bubbles Step 2: Proteins attach to the outside of the replication fork to stabilize the molecule Step 3: DNA polymerase III attaches to the leading(3’-5’) and lagging(5’-3’) strands with the help of an enzyme primase and an RNA primer.

Step 4: DNA polymerase III begins synthesis of the leading strand Step 4: DNA polymerase III begins synthesis of the leading strand. As the nucleotides are placed right side up…this strand is replicated continuously.

Step 5: The lagging strand must be assembled discontinuously in segments called Okazaki fragments because DNA Polymerase can only place nucleotides right side up and in one direction. Once a section is complete, a new enzyme DNA polymerase I removes the RNA primer and the two sides of the molecule are bonded together by DNA ligase. DNA ligase also ‘proof-reads’ the new strands and corrects errors

‘Semiconservative’ Replication OLD NEW NEW OLD DNA replication is “semiconservative”. This means that one-half of each new molecule of DNA is old; one-half new.