1. 12-2 Chromosomes and DNA Replication
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DNA and Chromosomes
DNA and Chromosomes
In prokaryotic cells, DNA is located in the cytoplasm.
Most prokaryotes have a single DNA molecule containing nearly all of the cell’s genetic information.
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DNA and Chromosomes
Chromosome
Most prokaryotes, such as this E. coli bacterium, have only a single circular chromosome. This chromosome holds most of the organism’s DNA.
E. Coli Bacterium
Bases on the Chromosomes
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DNA and Chromosomes
Many eukaryotes have 1000 times the amount of DNA as prokaryotes.
Eukaryotic DNA is located in the cell nucleus inside chromosomes.
The number of chromosomes varies widely from one species to the next.
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DNA and Chromosomes
Chromosome Structure
Eukaryotic chromosomes contain DNA and protein, tightly packed together to form chromatin.
Chromatin consists of DNA tightly coiled around proteins called histones.
DNA and histone molecules form nucleosomes.
Nucleosomes pack together, forming a thick fiber.
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DNA and Chromosomes
Eukaryotic Chromosome Structure
Chromosome
Nucleosome
DNA double helix
Coils
Supercoils
Eukaryotic chromosomes contain DNA wrapped around proteins called histones. The strands of nucleosomes are tightly coiled and supercoiled to form chromosomes.
Histones
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DNA Replication
DNA Replication
Each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing.
In most prokaryotes, DNA replication begins at a single point and continues in two directions.
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DNA Replication
In eukaryotic chromosomes, DNA replication occurs at hundreds of places. Replication proceeds in both directions until each chromosome is completely copied.
The sites where separation and replication occur are called replication forks.
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DNA Replication
Duplicating DNA 
Before a cell divides, it duplicates its DNA in a process called replication.
Replication ensures that each resulting cell will have a complete set of DNA.
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10. Replication copies the genetic information.
A single strand of DNA serves as a template for a new strand.
The rules of base pairing direct replication.
DNA is replicated during the S (synthesis) stage of the cell cycle.
Each body cell gets a complete set of identical DNA.

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DNA Replication
New Strand
Original strand
Nitrogen Bases
Growth
Growth
During DNA replication, the DNA molecule produces two new complementary strands. Each strand of the double helix of DNA serves as a template for the new strand.
Replication Fork
Replication Fork
DNA Polymerase
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DNA Replication
How Replication Occurs
1. DNA replication is carried out by enzymes that “unzip” a molecule of DNA.
Hydrogen bonds between base pairs are broken and the two strands of DNA unwind.
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DNA Replication
Nucleotides pair with the exposed bases on the template strands.
The principal enzyme involved in DNA replication is DNA polymerase.
3. DNA polymerase bonds the nucleotides together.
DNA polymerase joins individual nucleotides to produce a DNA molecule and then “proofreads” each new DNA strand.
4. Two molecules of DNA identical to the original molecule result; each molecule contains one original strand and one new strand.
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14. The DNA molecule unzips in both directions.
nucleotide
The DNA molecule unzips in both directions.
DNA polymerase
new strand
nucleotide
original strand
new strand
Two molecules of DNA

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12–2
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12–2
In prokaryotic cells, DNA is found in the
cytoplasm.
nucleus.
ribosome.
cell membrane.
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12–2
The first step in DNA replication is
producing two new strands.
separating the strands.
producing DNA polymerase.
correctly pairing bases.
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12–2
A DNA molecule separates, and the sequence GCGAATTCG occurs in one strand. What is the base sequence on the other strand?
GCGAATTCG
CGCTTAAGC
TATCCGGAT
GATGGCCAG
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12–2
In addition to carrying out the replication of DNA, the enzyme DNA polymerase also functions to
unzip the DNA molecule.
regulate the time copying occurs in the cell cycle.
“proofread” the new copies to minimize the number of mistakes.
wrap the new strands onto histone proteins.
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12–2
The structure that may play a role in regulating how genes are “read” to make a protein is the
coil.
histone.
nucleosome.
chromatin.
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21. END OF SECTION