1. 12-2 Chromosomes and DNA Replication
12–2 Chromosomes and DNA Replication p
12-2 Chromosomes and DNA Replication
Photo credit: Jacob Halaska/Index Stock Imagery, Inc.
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DNA and Chromosomes
DNA and Chromosomes
In prokaryotic cells:
DNA is located in the cytoplasm
a single DNA molecule contains 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
Eukaryotes:
many have 1000 times the amount of DNA as prokaryotes
DNA located in the cell nucleus inside chromosomes.
have a wide variety of number of chromosomes between species
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DNA and Chromosomes
Chromosome Structure
Chromatin - DNA and protein(histones), tightly packed together
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
What happens during DNA replication?
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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.
Replication forks = sites where separation and replication occur
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DNA Replication
Duplicating DNA 
Replication = duplication of DNA
Replication ensures that each resulting cell will have a complete set of DNA.
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DNA Replication
During DNA replication :
1. The DNA molecule separates into two strands
2. DNA then produces two new complementary strands following the rules of base pairing.
Each strand of the double helix of DNA serves as a template for the new strand.
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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
enzymes that “unzip” a molecule of DNA.
Hydrogen bonds between base pairs are broken causing the two strands of DNA unwind.
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DNA Replication
DNA polymerase = the principal enzyme involved in DNA replication
DNA polymerase:
joins individual nucleotides to produce a DNA molecule
then “proofreads” each new DNA strand.
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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