-contains all the genetic info in an organism
-both stores and transmits the genetic information
Functions of DNA (3)
1. carry info. from one generation to the next
2. source of the info for protein synthesis
3. easily copied for the purpose of cellular replication & to pass to offspring
-infected mice w/ bacterial cells
-disease causing bacterial cells killed the mice
-dead disease causing bacterial cells did not kill the mice
-harmless bacterial cells did not kill the mice
-dead disease causing bacterial cells mixed w/ living harmless bacteria killed the mice
-proved that DNA was what caused the transformation in Griffith's experiment (1947)
-used radioactive markers to determine that genetic info. was passed by DNA and not proteins
DNA stands for
What is it?
-essential a long molecule that is composed of nucleotides
-5 carbon sugar in a ring (Deoxyribose)
-phosphate group (PO4)
-nitrogenous base - 4 possibilites
-bigger than pyramidines
-symbols: A and G
-smaller than Purines
-symbols: C and T
-the sugar (deoxyribose & the phosphate group are bonded to eachother and alternate
Purines & Pyramidines
Phosphate group & sugar
#[A] = #[T]
#[C] = #[G]
Made the width of the DNA "ladder rings" equal
Watson & Crick used...
Rosalind Franklin's x-ray crystallography (in England),
Chargaff's Rule, and
Hydrogen bonding to...
create the double helix
-2 nucleotide polymers wound around eachother like a spiral staircase
-the nitrogenous bases fit on the inside (hydrophobic)
-Cytosine always goes w/ Guanine
-Adenine always goes w/ Thymine
Base pairing is always a mix of...
a purine (A or G) w/ a pyramidine (T or C)
-2 bonds w/ A and T
-3 bonds w/ G and C
the hydrogen bonds are
strong enough to hold the double helix together
(yet weak enough to unzip)
a consistent shape
Why is DNA's width consistent?
b/c it puts a Purine (2 rings) w/ a Pyramidine (1 ring)
What would create an inconsistent width
Purine pairing, and
a long string of DNA that contains all the genes & many of the proteins associated w/ DNA
How does it all fit into a cell?
It must be tightly packed & well organized to fit in the nucleus (eukaryotes) and cell (prokaryotes)
the DNA and proteins involved in the packaging of the DNA
1st level of packaging
-the DNA is tightly wrapped around proteins (HISTONES)
-the attraction is electrostatic the + charged histone & - charged DNA
-the histones & DNA form small beadlike structures (NUCLEOSOMES)
-1 nucleosome contains: 5 histones & DNA
-nucleosomes fold & pack on top of one another to create a thick fiber (Chromatin fiber)
-chromatin fiber loops and folds to shorten itself
When a cell divides...
each daughter cell receives a complete set of chromosomes
-each new cell has a complete set of the DNA code
Before a cell can divide, what must happen?
the DNA must be copied
so there are 2 sets ready to be distributed to the new cells
What does the structure of DNA allow?
one strand to be a template for the other
What can you do with one strand?
make the complementary (matching) strand
the process DNA goes through to copy itself
DNA Replication gives each cell...
a complete set of DNA (typically identical)
DNA is separated into 2 strands by enzymes that break the H-bonds between the base pairs
enzyme that separates the DNA
What is the function of enzymes?
lower the energy required to break the bonds
makes the process possible
-normally, strands would not separate b/c it is not favorable (in terms of energy)
DNA Polymerase attaches to the strands of DNA
adds complementary nucleotides creating the new DNA strands in a specific direction (5' - 3')
DNA Polymerase also proofreads the new strand to
make sure the complementary bases are in the right place
How is DNA organized?
in an antiparallel manner
How is it possible that it only takes a few hours to replicate a complete set of human DNA?
use multiple polymerases