DNA

EUKARYOTIC CELLSDOUBLE HELIX

COILS AROUND PROTEINS(HISTONES)

HISTONES COIL MOLECULE INTO

SUPER COIL

SUPER COILS WIND UP TO FORM

CHROMOSOMES

CHROMOSOMES FORM CHROMATIN IN NUCLEUS

PRIMARY FUNCTION• STORES AND TRANSMITS GENETIC

INFORMATION BASED UPON CODE SEQUENCE

• CHEMICALLY CONTROLS THE SYNTHESIS OF ALL PROTEINS– ENZYMES THAT CONTROL CHEMICAL PROCESSES– DETERMINE PHYSICAL STRUCTURES OF ORGANISMS

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

– Thymine

NITROGEN BASESPURINES

BASES WITH TWO CARBON RINGS

PYRIMIDINESBASES WITH ONE

CARBON RING

BASE PAIR BONDINGONE PURINE BONDS WITH ONE PYRIMIDINE

FORMING A COMPLIMENTARY BASE PAIRADENINE ALWAYS BONDS WITH THYMINE FORMING

A-T OR T-A QUANINE ALWAYS BONDS WITH CYTOSINE FORMING

G-C OR C-G

THAT’S INTERESTING!• DNA LOOKS LIKE A LADDER

– SUGAR AND PHOSPHATE MAKES THE “FRAME”– BASES FORM THE “RUNGS”

• ONE SIDE OF MOLECULE MUST “FLIP” IN ORDER FOR HYDROGEN BONDS TO FORM BETWEEN BASES

• ALL BASE PAIRS, NO MATTER THE ORDER, HAVE THE SAME STRUCTURAL SHAPE SO THEY STACK ON TOP OF EACH OTHER AND TWIST FORMING THE DOUBLE HELIX

• SINCE A ALWAYS BONDS WITH T, AND C WITH G, THEIR AMOUNTS MUST BE EQUAL TO EACH OTHER

NATURE LOVES EFFICIENCY!DNA’S STRUCTURE ALLOWS THE MOLECULE TO REPLICATE ITSELF

QUICKLY, AS WELL AS, CODE FOR THE SYNTHESIS OF PROTEINS WITHIN THE ORGANISM THE SAME WAY EACH TIME.

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 fibers. At that time, little was known about the structure of DNA; though these photos didn't show the structure of the DNA, there were patterns on those images that could be used to determine the position of the DNA molecule's atoms. From these photos, Franklin determined that the DNA molecule must be long and thin and possibly a helix

James Watson and Francis Crick In 1951 James Watson and Francis Crick began to examine the DNA’s structure. Using previous X-ray diffraction photos of DNA fibers taken by Maurice Wilkins and Rosalind Franklin, they discovered that it showed an X shape... which is also the characteristic of a helix. In April of 1953, using this information, they came up with the double helix, the structure that is almost always associated with DNA.

DNA Replication

- Occurs during S phase of Interphase.- One strand of DNA has all of the information

needed to reconstruct the other half through the mechanism of base pairing.

- 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 the new strand.

DNA Replication

• Step 1: A portion of the DNA double helix is unwound and separated by the enzyme helicase.

DNA Replication

• Step 2: The enzyme DNA polymerase attaches to one strand of the DNA and begins synthesis of the leading strand in the 5’-3’ direction.

DNA Replication

• DNA replication is “semiconservative”.

• This means that one-half of each new molecule of DNA is old; one-half new.