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12–2  Chromosomes and DNA Replication

Learn about the differences in chromosome structure and DNA replication between prokaryotic and eukaryotic cells, including nucleosome formation, replication mechanisms, and enzymes involved.

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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 Nucleosome Animation

  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 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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