Download Chromosomes and DNA Replication

Chromosomes and
DNA Replication
12-2
DNA & Chromosomes
Prokaryotes 
Lack nuclei and cellular organelles

Have single circular DNA molecule

Contains nearly all cell’s genetic information

Referred to as cell’s chromosome
Eukaryotes 
Have as much as 1000 times the amount
of DNA as prokaryotes

Not free in cytoplasm

Contained in nucleus in the form of a
number of chromosomes
DNA Length
DNA molecule is very long
Ex: E. coli DNA contains 4,639,221
base pairs, or ~1.6 mm long
Like trying to pack 300 m of rope into a
backpack
Chromosome Structure
Human cells contain almost 1000 times
as many base pairs as bacteria
The nucleus of a human cell contains >
1m of DNA
The composition of chromosomes
allows them to hold a lot of DNA
Chromosomes contain both proteins
and DNA

Form chromatin
Chromatin consists of DNA tightly coiled
around histone proteins

Forms beadlike structure - nucleosome
Nucleosomes pack with one another to
form a thick fiber, shortened by loops
and coils
Nucleosomes seem to make it possible to fold
enormous lengths of DNA
Histones have changed very little during
evolution
During most of cell cycle fibers are dispersed
- can’t see chromosomes
During mitosis, fibers are drawn together,
forming tightly packed chromosomes you can
see with a microscope
DNA Replication
When Watson and Crick discovered the
double helix, the structure explained how
DNA could be copied
Each strand of DNA has all the
information needed to reconstruct the
other half because of base pairing
Since each strand is used to make the
other half, the strands are said to be
complimentary
In eukaryotes, DNA replication begins at
100s of places along the molecule, and
continues until replication is complete

Sites known as replication forks
Duplicating DNA
During replication 
DNA molecule separates into 2 strands

Produces 2 new complimentary strands
following rules of base pairing

Each strand is a template for a new strand
DNA replication is carried out by a
series of enzymes
Enzymes “unzip” the DNA molecule,
creating replication forks

Breaks hydrogen bonds between bases
A pairs of with T, and G pairs up with C,
until the new strand is complete
Each strand created is composed of an
old strand and a new strand
The principal enzyme involved in this
reaction is DNA polymerase

Joins individual nucleotides to produce
DNA molecule

Proofreads each new molecule