bio3

Chromosomes & DNA Replication

I. DNA & Chromosomes
• A. DNA is found in different ways depending on the
type of cell you are looking at
– 1. In prokaryotic cells DNA is located in the cytoplasm in
a single circular molecule known as a chromosome
– 2. In eukaryotic cells the DNA is located in the nucleus in
the form of many chromosomes
• B. DNA must be folded into at least 1/100 of its full
length to fit in any type of prokaryotic cell

II. Chromosome Structure
• A. In eukaryotic cells DNA has to be
folded even further in order to do
this chromosomes use the help of
proteins.
– 1. Histones – proteins that have tightly
wound DNA around them creating
chromatin.
• B. Together histones and DNA form a
structure known as nucleosome,
which gets further coiled into loops.
– 1. Only during mitosis are these fibers
drawn together forming the tightly
packed chromosomes. Which may help
separate chromosomes during mitosis

III. DNA Replication
• A. Each strand of the DNA double helix has the
information to reconstruct the other half by base
pairing.
– 1. Since each strand can be used to make the other the
strand are said to be complimentary.
• B. In prokaryotic cells replication occurs at one point
and proceeds in 2 directions until the entire
chromosome is replicated
• C. In eukaryotic cells replication occurs in many points
known as replication forks until each chromosome is
completely copied

IV. Eukaryotic DNA Replication
• A. DNA Replication – DNA molecule
separates into 2 strands, then
produces 2 new complimentary
strands following the rules of base
pairing. Each strand of the double
helix serves as a template.
• B. The 2 new DNA molecules each
have one old strand and one new
strand
• C. In order for replication to occur
DNA helicase “unzips” the DNA so
that DNA polymerase can join the
new nucleotides and proofreads to
ensure correct base pairing