1. DNA Chromosomes and Replication

2. 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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3. E. Coli Bacterium 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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4. Many eukaryotes have 1000 times the amount of DNA as prokaryotes.
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.
*The # is not related to complexity. For instance humans have 46 chromosomes while a Chicken has 70.
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5. 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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6. Eukaryotic Chromosome Structure
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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7. DNA Replication Origins of replication
1. Replication Forks: hundreds of Y-shaped regions of replicating DNA molecules where new strands are growing.
Replication
Fork
Parental DNA Molecule 3’ 5’

8. DNA Replication Origins of replication 2. Replication Bubbles:
a. Hundreds of replicating bubbles (Eukaryotes).
b. Single replication fork (bacteria).
Bubbles

9. DNA Replication Strand Separation:
1. Helicase: enzyme which catalyze the unwinding and separation (breaking H- Bonds) of the parental double helix.
2. Single-Strand Binding Proteins: proteins which attach and help keep the separated strands apart.

10. DNA Replication Strand Separation:
3. Topoisomerase: enzyme which relieves stress on the DNA molecule by allowing free rotation around a single strand.
Enzyme
DNA
Enzyme

11. DNA Replication Priming:
1. RNA primers: before new DNA strands can form, there must be small pre-existing primers (RNA) present to start the addition of new nucleotides (DNA Polymerase).
2. RNA Primase: enzyme that polymerizes (synthesizes) the RNA Primer.

12. DNA Replication Synthesis of the new DNA Strands:
1. DNA Polymerase: with a RNA primer in place, DNA Polymerase (enzyme) catalyze the synthesis of a new DNA strand in the 5’ to 3’ direction.
RNA
Primer
DNA Polymerase
Nucleotide 5’ 3’

13. DNA Replication
2. Leading Strand: synthesized as a single polymer in the 5’ to 3’ direction.
RNA
Primer
DNA Polymerase
Nucleotides 3’ 5’

14. DNA Replication
3. Lagging Strand: also synthesized in the 5’ to 3’ direction, but discontinuously against overall direction of replication.
RNA Primer
Leading Strand
DNA Polymerase 5’ 3’
Lagging Strand 5’ 3’

15. DNA Replication
4. Okazaki Fragments: series of short segments on the lagging strand.
Lagging Strand
RNA
Primer
DNA
Polymerase 3’ 5’
Okazaki Fragment

16. DNA Replication
5. DNA ligase: a linking enzyme that catalyzes the formation of a covalent bond from the 3’ to 5’ end of joining stands.
Example: joining two Okazaki fragments together.
Lagging Strand
Okazaki Fragment 2
DNA ligase
Okazaki Fragment 1 5’ 3’

17. Question: DNA 5’-GCGTATG-3’
What would be the complementary DNA strand for the following DNA sequence?
DNA 5’-GCGTATG-3’

18. Answer:
DNA 5’-GCGTATG-3’
DNA 3’-CGCATAC-5’