DNA, Chromatin, and Replication

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-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
-transformation (1928)
-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

deoxyribonucleic acid

What is it?

-essential a long molecule that is composed of nucleotides

Nucleotides contain

3 parts
-5 carbon sugar in a ring (Deoxyribose)
-phosphate group (PO4)
-nitrogenous base - 4 possibilites


-2 rings
-bigger than pyramidines
-symbols: A and G


-1 ring
-smaller than Purines
-symbols: C and T

DNA's backbone

-the sugar (deoxyribose & the phosphate group are bonded to eachother and alternate

Ladder rings=

Purines & Pyramidines

Ladder Uprights=

Phosphate group & sugar

Erwin Chargaff

Chargaff's Rule:
#[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

Double helix

-2 nucleotide polymers wound around eachother like a spiral staircase
-the nitrogenous bases fit on the inside (hydrophobic)

Base Pairing

-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)

DNA holds

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
Pyrimidine paring


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

2nd Level

-the histones & DNA form small beadlike structures (NUCLEOSOMES)
-1 nucleosome contains: 5 histones & DNA

3rd Level

-nucleosomes fold & pack on top of one another to create a thick fiber (Chromatin fiber)

4th Level

-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

DNA Replication

the process DNA goes through to copy itself

DNA Replication gives each cell...

a complete set of DNA (typically identical)

Step 1

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)

Step 2

DNA Polymerase attaches to the strands of DNA

DNA Polymerase

adds complementary nucleotides creating the new DNA strands in a specific direction (5' - 3')

Step 4

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

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