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12–2 12–2 Chromosomes and Chromosomes and DNA ReplicationDNA Replication

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Prokaryotic DNAProkaryotic DNA Prokaryotic cells lack nuclei and many of Prokaryotic cells lack nuclei and many of

the organelles of Eukaryotesthe organelles of Eukaryotes DNA molecules are located in the cytoplasm DNA molecules are located in the cytoplasm

single circular DNA molecule = chromosome single circular DNA molecule = chromosome

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Eukaryotic DNA Eukaryotic DNA

• as much as 1000 times the amount as much as 1000 times the amount of DNA as prokaryotes of DNA as prokaryotes

Found in nucleus as chromatinFound in nucleus as chromatin number of chromosomes varies number of chromosomes varies

widely from one species to the widely from one species to the nextnext DrosophilaDrosophila cells have 8 cells have 8 giant sequoia tree cells have 22 giant sequoia tree cells have 22 Human have 46Human have 46

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DNA LengthDNA Length DNA is very long.DNA is very long. E. coli E. coli contains 4,639,221 base contains 4,639,221 base

pairs. The length of such a DNA pairs. The length of such a DNA molecule is roughly 1.6 mm molecule is roughly 1.6 mm

a a humanhuman cell's cell's DNADNA totals about totals about 2-3 meters in 2-3 meters in lengthlength (that’s just (that’s just one cell!)one cell!)

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Chromosome StructureChromosome Structure

Eukaryotic chromosomes contain Eukaryotic chromosomes contain both both DNADNA and and proteinprotein

DNA + Protein (histones) DNA + Protein (histones) ChromatinChromatin

DNA and histone DNA and histone beadlike beadlike structure called a structure called a nucleosomenucleosome

Nucleosomes pack with one another to Nucleosomes pack with one another to form a thick fiber, which is shortened form a thick fiber, which is shortened by a system of loops and coils. by a system of loops and coils.

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Can only see fibers during mitosis – Can only see fibers during mitosis – chromosomeschromosomes

tight packing of nucleosomes may help tight packing of nucleosomes may help separate chromosomes during mitosis separate chromosomes during mitosis

There is also some evidence that There is also some evidence that changes in chromatin structure and changes in chromatin structure and histone-DNA binding are associated histone-DNA binding are associated with changes in gene activity and with changes in gene activity and expression. expression.

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What do nucleosomes What do nucleosomes do? do?

able to fold enormous lengths of DNA able to fold enormous lengths of DNA into the tiny space available in the cell into the tiny space available in the cell nucleus nucleus

histone proteins have changed very histone proteins have changed very little during evolution—probably little during evolution—probably because mistakes in DNA folding could because mistakes in DNA folding could harm a cell's ability to reproduce harm a cell's ability to reproduce

http://http://www.johnkyrk.com/chromosomestructurwww.johnkyrk.com/chromosomestructure.htmle.html

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NucleosomeNucleosomess

Nucleosome Animation

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DNA ReplicationDNA Replication Each strand of the DNA double helix has Each strand of the DNA double helix has

all the information needed to all the information needed to reconstruct the other half by the reconstruct the other half by the mechanism of mechanism of base pairingbase pairing. .

each strand can be used to make the each strand can be used to make the other strand = other strand = complementary complementary strandsstrands

prokaryotes = begins at a single point, prokaryotes = begins at a single point, proceeds, often in two directions, until proceeds, often in two directions, until whole chromosome is replicated.whole chromosome is replicated.

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Bidirectional Circular DNA Bidirectional Circular DNA Replication in Bacteria Replication in Bacteria

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Eukaryotic Eukaryotic ChromosomesChromosomes

DNA replication occurs at DNA replication occurs at hundredshundreds of places. of places.

Replication proceeds in both Replication proceeds in both directions until each chromosome is directions until each chromosome is completely copied. completely copied.

The sites where separation and The sites where separation and replication occur are called replication occur are called replication forks. replication forks.

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Duplicating DNADuplicating DNA

Before a cell divides, it duplicates its Before a cell divides, it duplicates its DNA in a copying process called DNA in a copying process called replicationreplication..

Each resulting cell will have a complete Each resulting cell will have a complete set of DNA molecules. set of DNA molecules.

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During DNA replication, the DNA During DNA replication, the DNA molecule separates into two strands.molecule separates into two strands.

then produces two new then produces two new complementary strands following the complementary strands following the rules of base pairing.rules of base pairing.

Each strand of the double helix of Each strand of the double helix of DNA serves as a template, or model, DNA serves as a template, or model, for the new strand. for the new strand.

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For example, a strand that has the For example, a strand that has the bases TACGTT produces a strand with bases TACGTT produces a strand with the complementary bases ATGCAA. the complementary bases ATGCAA.

The result is two DNA molecules The result is two DNA molecules identical to each other and to the identical to each other and to the original molecule.original molecule.

Note that each DNA molecule resulting Note that each DNA molecule resulting from replication has one original strand from replication has one original strand and one new strand.and one new strand.

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How Replication OccursHow Replication Occurs carried out by a series of enzymes carried out by a series of enzymes two strands of the molecule unwind two strands of the molecule unwind

enzymes “unzip” a molecule of DNA enzymes “unzip” a molecule of DNA hydrogen bonds between the base hydrogen bonds between the base pairs are brokenpairs are broken

Each strand serves as a template Each strand serves as a template for the attachment of for the attachment of complementary basescomplementary bases

DNA Replication AnimationDNA Replication Animation

DNA to DNA animation DNA to DNA animation

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Enzymes in ReplicationEnzymes in Replication Helicase = unwinds and unzips the DNAHelicase = unwinds and unzips the DNA DNA replication enzyme is called DNA replication enzyme is called DNA DNA

polymerasepolymerase because it joins individual because it joins individual nucleotides to produce a DNA molecule, a nucleotides to produce a DNA molecule, a polymer. polymer.

DNA polymerase also “DNA polymerase also “proofreadsproofreads” each ” each new DNA strand, helping to maximize the new DNA strand, helping to maximize the odds that each molecule is a odds that each molecule is a perfect perfect copycopy of the original DNA. of the original DNA.

Ligase = binds the okazaki fragments Ligase = binds the okazaki fragments together on the lagging strand of new together on the lagging strand of new DNA DNA