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12–2  Chromosomes and DNA Replication

12–2  Chromosomes and DNA Replication. Prokaryotic DNA. Prokaryotic cells lack nuclei and many of the organelles of Eukaryotes DNA molecules are located in the cytoplasm single circular DNA molecule = chromosome . Eukaryotic DNA . as much as 1000 times the amount of DNA as prokaryotes

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12–2  Chromosomes and DNA Replication

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  1. 12–2 Chromosomes and DNA Replication

  2. Prokaryotic DNA • Prokaryotic cells lack nuclei and many of the organelles of Eukaryotes • DNA molecules are located in the cytoplasm • single circular DNA molecule = chromosome

  3. Eukaryotic DNA • as much as 1000 times the amount of DNA as prokaryotes • Found in nucleus as chromatin • number of chromosomes varies widely from one species to the next • Drosophila cells have 8 • giant sequoia tree cells have 22 • Human have 46

  4. DNA Length • DNA is very long. • E. coli contains 4,639,221 base pairs. The length of such a DNA molecule is roughly 1.6 mm • a human cell's DNA totals about 2-3 meters in length (that’s just one cell!)

  5. Chromosome Structure • Eukaryotic chromosomes contain both DNA and protein • DNA + Protein (histones)  Chromatin • DNA and histone  beadlike structure called a nucleosome • Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils.

  6. Can only see fibers during mitosis – chromosomes • tight packing of nucleosomes may help separate chromosomes during mitosis • There is also some evidence that changes in chromatin structure and histone-DNA binding are associated with changes in gene activity and expression.

  8. What do nucleosomes do? • able to fold enormous lengths of DNA into the tiny space available in the cell nucleus • histone proteins have changed very little during evolution—probably because mistakes in DNA folding could harm a cell's ability to reproduce • http://www.johnkyrk.com/chromosomestructure.html

  9. Nucleosomes

  10. 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. • each strand can be used to make the other strand = complementary strands • prokaryotes = begins at a single point, proceeds, often in two directions, until whole chromosome is replicated.

  11. Bidirectional Circular DNA Replication in Bacteria

  12. 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.

  13. Duplicating DNA • Before a cell divides, it duplicates its DNA in a copying process called replication. • Each resulting cell will have a complete set of DNA molecules.

  14. 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, or model, for the new strand.

  15. For example, a strand that has the bases TACGTT produces a strand with the complementary bases ATGCAA. • The result is two DNA molecules identical to each other and to the original molecule. • Note that each DNA molecule resulting from replication has one original strand and one new strand.

  16. How Replication Occurs • carried out by a series of enzymes • two strands of the molecule unwind (helicase) • enzymes “unzip” a molecule of DNA • hydrogen bonds between the base pairs are broken • Each strand serves as a template for the attachment of complementary bases • DNA Replication Animation • DNA to DNA animation

  17. Enzymes in Replication • Helicase = unwinds and unzips the DNA • DNA replication enzyme is called DNA polymerase because it joins individual nucleotides to produce a DNA molecule, a polymer. • DNA polymerase also “proofreads” each new DNA strand, helping to maximize the odds that each molecule is a perfect copy of the original DNA. • Ligase = binds the okazaki fragments together on the lagging strand of new DNA

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