DNA History, Structure and Function

What is DNA? Deoxyribonucleic Acid long chain moleculeIt is a long chain molecule that carries the code for the characteristics of living things. genesSections of this chain are called genes.

DNA Code nitrogen basesThe code is made of molecules called nitrogen bases. –Adenine (A) –Thymine (T) –Guanine (G) –Cytosine (C) The code is represented in strings of letters that stand for the nitrogen bases Ex: ATTAAGGCATA TAATTCCGTAT

HISTORY Oswald Avery 1944 Discovered that DNA stores and passes on genetic information from one generation to the next

Chargaff’s Rules Before 1950 In any DNA, the number of A’s was the same as the number of T’s, and the number of C’s was the same as the number of G’s In other words: A must pair with T and G must pair with C A = T G = C

X-ray Crystallography Rosalind Franklin early 1950’s Used concentrated x-ray beams to take pictures of DNA molecules Could only get partial pictures She didn’t realize what the picture meant for the structure of DNA. A chemist with extensive knowledge on X-ray crystallography

James Watson chemistAn American chemist from Chicago Entered the University of Chicago at 15 He met Francis Crick at Cambridge University.

Francis Crick Born in England PhysicistA Physicist that studied proteins using x-ray diffraction form functionKnowing the form (structure) gives clues to the function of molecules.

Watson and Crick Had been trying to solve the puzzle of the structure of DNA : Watson viewed a copy of Franklin’s x-rays. One week later: One week later: they solved the puzzle together

GENOME The nucleus of a human cell contains 20,000 to 30,000 genes in the form of DNA called the GENOME. DNA Vocabulary

CHROMOSOMES CHROMOSOMES are coiled DNA and proteins. Humans have 23 pairs of chromosomes in the NUCLEUS of any one of your cells

GENE Segment of DNA that codes for an organism’s trait.

Structure of DNA shape of a double helix –a twisted ladder. made up of 4 nitrogen bases that bond together, with hydrogen bonds. –the "rungs" of the ladder. Nitrogen Bases are attached to a phosphate- sugar backbone –the “rails” of the ladder.

More on Structure double helixRight handed double helix nucleotidesMade of units called nucleotides: sugardeoxyribose –One 5 carbon sugar ring (deoxyribose) phosphate group –One phosphate group nitrogen base –One nitrogen base P S Thymine PhosphateSugar Nitrogen Base One Nucleotide

Nitrogen Bases PurinesPurines (longer): adenine and guanine PyrimidinesPyrimidines (short): thymine and cytosine These pair one short with one long –Complimentary bases Guanine ThymineAdenine Cytosine

Purines and Pyrimidines A and T are bonded by 2 hydrogen bonds. C and G are bonded by 3 hydrogen bonds. Pyrimidines (T and C) have a single ring structure. Purines (A and G) have a double ring structure.

Sides Alternating sugar and phosphate molecules Nitrogen bases are connected to the sugar molecules One nucleotide

What is a DNA Sequence? The particular order of the nitrogen bases is called the DNA SEQUENCE. The first letter of the base is used: Ex. A T G C C G A T C

DNA Replication ReplicateReplicate = to copy EnzymeEnzyme = a chemical that makes a reaction go faster DNA replicates with the help of an enzymes One nucleotide

Enzymes in DNA replication HELICASE unwinds parental double helix DNA POLYMERASE binds new nucleotides to form new strands LIGASE joins Okazaki fragments and seals other breaks in sugar-phosphate backbone

Replication Step 1: HELICASE unzips DNA down the middle by breaking Hydrogen bonds.

Step 2: DNA POLYMERASE moves along the 2 open parent strands (in 5’ to 3’ direction) and adds NEW complimentary nucleotides to each original parent strand. New DNA is proofread as it is made.

Step 3: Replication moves in opposite directions on each side -- only in the 5’ to 3’ direction. So, small DNA segments called OKAZAKI FRAGMENTS are made on the other strand.

Step 4: DNA LIGASE seals up all of the gaps in the DNA molecule, like the Okazaki fragments.

Semi-Conservative Model Each open side of the parental DNA acts as a template for a new complementary strand New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA