1. 12-2: Chromosomes and DNA Replication
Objectives:
What happens during DNA replication?

2. DNA and Chromosomes Don’t have a nuclei
Prokaryotic cells
Don’t have a nuclei
DNA molecules are located in the cytoplasm
Most have a single circular DNA molecule
Has nearly all of the cell’s genetic information

3. DNA and Chromosomes Cont’d
Eukaryotic
As much as 1000x’s more DNA than prokaryotes
Located in the cell nucleus
DNA is in the form of a number of chromosomes
Varies widely from one species to the next

4. DNA Length Surprisingly long Human DNA for 1 cell = about 6 feet
If you stretched all of your DNA from your cells, it would stretch to the moon and back 1500 times

5. Chromosomes Structure
Eukaryotic cell DNA is packed tightly
chromatin: DNA and protein tightly packed together
histones: DNA that is tightly coiled around proteins
Form a beadlike structure called nucleosome
Can fold enormous lengths of DNA into the tiny space available in the cell nucleus
During most of the cell cycle
Nucleosomes are dispersed in the nucleus so that individual chromosomes are not visible
During mitosis
Nucleosomes of each individual chromosome are drawn together,
Forms tightly packed chromosomes

6. Quick Question
What is chromatin?

7. Quick Answer
DNA and protein tightly packed together

8. DNA Replication
The double helix structure explained how DNA could be copied
Able to reconstruct the other half by the mechanism of base pairing
Complementary

9. Duplicating DNA Duplicates DNA 2 strands of the double helix separate
Replication
Ensures that each resulting cell will have a complete set of DNA molecules
2 strands of the double helix separate
New bases are added as each new strand forms
Base pairing

10. How Replication Occurs
DNA replication is carried out by a series of enzymes
Enzymes “unzip” a molecule of DNA
Hydrogen bonds between the base pairs are broken
2 strands of the molecule unwind
Template for the attachment of complementary bases

11. How Replication Occurs Cont’d
DNA replication
Involves enzymes and regulatory molecules
Enzymes are highly specific
DNA polymerase: principal enzyme involved in DNA replication
Joins individual nucleotides to produce a DNA molecule (polymer)
“Proofreads” each new DNA strand
Each molecule is a perfect copy of the original DNA

12. Exit Ticket •Answer the following questions. •Use complete sentences.
•You may NOT use your book.
• You MAY use your notes.
•This is a quiz grade.
What happens during DNA replication?
What happens during DNA replication??