Unit 9: DNA and RNA
Section 2: DNA Replication
DNA and Chromosomes
- In prokaryotic cells, the DNA is located in the cytoplasm.
- Most prokaryotes have a single DNA molecule containing nearly all the cell’s genetic information.
- The DNA is circular and connects to itself, but it still has the double helix twist.
- 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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Chromosome Structure
1. Eukaryotic chromosomes contain DNA and protein, tightly packed together to form chromatin.
2. Chromatin consists of DNA tightly coiled around proteins called histones.
(For example, imagine a phone cord wrapped around some marbles.)
3. DNA and histone molecules form nucleosomes.
4. Nucleosomes pack together, forming a thick fiber which condense to form rod-like shapes. Put two rods together and you have the classic “X” shaped chromosome.
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.
- 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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Duplicating DNA
- Before a cell divides by mitosis, it duplicates its DNA in a process called replication.
- Replication ensures that each resulting cell will have a complete set of DNA.
KEY CONCEPT: During DNA replication, the DNA molecule separates into two strands, 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.
Label the following diagram.
How Replication Occurs
1. DNA replication is carried out by enzymes that “unzip” a molecule of DNA.
2. The principle enzyme involved in DNA replication is DNA polymerase.
3. DNA polymerase joins individual nucleotides to produce a DNA molecule and then “proofreads” each new DNA strand.
4. Hydrogen bonds between base pairs are broken and the two strands of DNA unwind.
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A collection of animations for this unit are available on my class website:
http://wahs-biology.wikispaces.com/DNAandRNA
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Section 2 Vocabulary
chromatin replication histone DNA polymerase
Review Questions
1. What is the name of the enzyme “unzipping” DNA?
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2. Why would it be important for DNA to copy itself? ______
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3. What bond holds the new complementary nucleotides together?
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4. What needs to happen before the DNA can begin replication?
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Nucleotide Base / Abbreviation / Complementary BaseT
C
A
G
Unit 8, Section 2, Page 5