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12–2Chromosomes and DNA

Replication

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Prokaryotic DNA

Prokaryotic cells lack nuclei & many organelles of Eukaryotes

Located in cytoplasm

single circular DNA molecule = chromosome

Eukaryotic DNA

~1000 times the amount of DNA as prokaryotes

Located in nucleus as chromatin

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DNA Length

DNA is very long

E. coli contains 4,639,221 base pairs. The length is roughly 1.6 mm

a human cell's DNA totals about 2-3 meters in length (that’s just one cell!)

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Chromosome Structure

Eukaryotic chromosomes contain both DNA and protein

DNA + Protein (histones) Chromatin

DNA + histone beadlike structure

called nucleosome

Nucleosomes pack to form thick fiber, shortened by system of loops & coils (chromosome).

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Nucleosomes

Nucleosome Animation

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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

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DNA Replication

Each strand of DNA double helix has all info needed to reconstruct the other half by mechanism of base pairing

Each strand can be used to make the other strand = complementary strands

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DNA Replication in Prokaryotes

• Begins at a single point

• Proceeds, often in two directions, until whole chromosome is replicated (bidirectional)

Replication in Eukaryotic

Chromosomes

• Occurs at hundreds of places

• Proceeds in both directions until each chromosome is completely copied

• Replication forks -sites where separation and replication occur

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Duplicating DNA

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

Ensures each cell will have a complete set of DNA molecules

During DNA replication:

DNA molecule separates into 2 strands

Then produces 2 new complementary strands following the rules of base pairing

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

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How Replication Occurs Carried out by a series of enzymes

1.DNA molecule unwinds

2.Enzymes “unzip” DNA by breaking the hydrogen bonds between base pairs

3.Free-floating complementary bases attach to the original (template) strands

DNA Replication Animation

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Enzymes in Replication Helicase - unwinds and unzips the DNA

RNA Primer – provides DNA polymerase a starting point

DNA polymerase – joins individual nucleotides to produce a DNA molecule (a polymer)

Also “proofreads” each new DNA strand; helps maximize odds that each molecule is perfect copy of the original DNA.

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

DNA to DNA animation

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CrashCourse: Replication

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