AP BIOLOGYChapter 16.2-16.3

BELLWORK:

What type of Model is DNA replication?

A. Conservative Model

B. Semiconservative Model

C. Dispersive Model

Explain the correct model.

The Basic Principle: Base Pairing as a Template

• The two sides of DNA are complementary to each other

• Semiconservative Model• During Replication each strand has one original strand and one newly made

strand

Alternative Models

• Conservative Model• Two parental strands reassociate after acting as templates

• Dispersive Model• Each strand of both daughter molecule contains a mixture of old and newly

synthesized DNA

• 1950’s Matthew Meselson and Franklin Stahl experimented and supported the Semiconservative Model of DNA

Experimental Design:

EXPERIMENT: Cultured E. coli for several generations in a medium

containing nucleotide precursors labeled with a heavy isotope of

Nitrogen. Then transferred the bacteria a to medium with only a

lighter isotope. Two DNA samples were taken at 20 minutes (1

replication) and 40 minutes (2 replications). Distinguished DNA of

different densities by centrifuging the extracted DNA

Experimental Setup:

Experimental Design

CONCLUSION: Results were compared to predictions for each model.

1. The first replication produced a band of hybrid DNA—this eliminated the conservative model

2. The second replication produced both light and hybrid DNA—this refuted the dispersive model AND supported the semiconservative model.

DNA replication is semiconservative.

Experimental Design:

MAKE A PREDICTION:

If Meselson and Stahl had first grown the cells in the lighter medium and then moved them into the heavier medium before taking samples, what would have been the results?

DNA Replication: A closer look

• Each human cell has 46 DNA molecules in it’s nucleus.• One long DNA molecule per chromosome

• This can be copied in just a few hours with very few errors

• More than a dozen enzymes and other proteins participate in replication

• More is known about replication in bacteria, but the process is fundamentally similar between prokaryotes and eukaryotes

Getting Started

• Replication begins at special sites• ORIGINS OF REPLICATION: short stretches of DNA having a specific sequence

of nucleotides

• In E. coli the DNA is circular and has a single origin• Proteins initiate replication by recognizing this specific sequence

• Open up a “replication bubble”

• Replication proceeds in BOTH DIRECTIONS until the entire molecule is copied

Origins of replication in E. coli

Origins of Replication in Eukaryotes

• Eukaryotic chromosomes have hundreds or thousands of replication origins

• Multiple replication bubbles form and eventually fuse• WHY is this important???

• Replication occurs in BOTH directions

Eukaryotic Replication: Important Terms

• REPLICATION FORK• Y-shaped region where the parental strands of DNA are being unwound

• HELICASES• Enzymes that untwist the double helix at the replication forks

• SINGLE-STRAND BINDING PROTEINS• Bind to the unpaired DNA strands to stabilize them—as it becomes unwound

the points ahead of the replication fork twist tighter

• TOPOISOMERASE• Breaks, swivels, and rejoins the parental DNA ahead of the replication fork,

relieving the strain caused by unwinding

Origins of Replication in Eukaryotes

Proteins involved in Initiating Replication

Synthesizing a New DNA strand

• DNA polymerases• Catalyze the synthesis of new DNA by adding nucleotides to a preexisting

chain

• Require a primer and a DNA template strand, along which complementary DNA nucleotides line up

• ENZYMES TO KNOW• DNA Polymerase III

• Adds a DNA nucleotide to the RNA primer and then continues adding DNA nucleotides to the growing end of the NEW DNA strand

• DNA Polymerase I

Synthesizing a New DNA Strand

• Each nucleotide added to a growing DNA comes from a nucleoside (sugar and base) with 3 phosphates• Same type of molecule as ATP

• The difference is the type of sugar (deoxyribose instead of ribose)

• Nucleosides that binding in DNA synthesis are reactive due to the phosphate groups (like ATP)

• As each monomer (single unit) joins DNA two phosphate groups are lost as pyrophosphate • Hydrolysis of pyrophosphate is a coupled EXERGONIC reaction that helps

drive the polymerization reaction.

Incorporating a Nucleotide into DNA strand

QUESTION:What is meant when it is said that each DNA strand has directionality?

Antiparallel Elongation

• The two DNA strands are different, giving each strand directionality, like a one way street.

• Antiparallel (opposite directions) can be compared to a divided highway

How does the antiparallel arrangement of the double helix affect replication?

• DNA polymerases can add nucleotides ONLY to the free 3’ end

• DNA only elongates in the 5’-3’ direction

Antiparallel Elongation

• Leading Strand• At the Replication fork DNA polymerase III begins in the replication fork ant

continuously adds nucleotides to the new complementary strand as the fork progresses

• Only one primers is required for DNA pol III to synthesize the leading strand

• Lagging Strand• Still only able to move in 5’-3’ direction

• DNA pol III must work AWAY from the replication fork

• Unable to by synthesized continuously—occurs in a segments• These segments are called Okazaki fragments

Synthesis of the Lagging Strand

• Primers• Each Okazaki fragment

must be primed separately

• DNA pol I assists in placing primers

• DNA ligase, joins the sugar-phosphate backbones of all Okazaki fragments

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Proofreading and Repairing DNA

• During DNA replication DNA polymerases proofread each nucleotide against its template

• If a mistake is found, the polymerase removes the nucleotide and then resumes synthesis

• MISMATCH REPAIR• Enzymes remove and replace incorrectly paired nucleotides that have

resulted from replication errors

• Each cell continuously monitors and repairs genetic material

Proofreading and Repairing DNA

• NUCLEASE• DNA-cutting enzyme

• Resulting gap is filled with nucleotides using the undamaged strand as a template

• NUCLEOTIDE EXCISION REPAIR• Helps to repair damage caused by UV rayws

Replicating the end of DNA Molecules• A small portion of the cell’s DNA that DNA polymerases can neither

replicate nor repair.

• At the very end of the strand there is no 3’ end available for nucleotide addition

• Eukaryotic chromosomal DNA molecules have special nucleotide sequences called telomeres that the end• These sections do not contain genes

• This protects organisms genes

• Telomeres do not prevent the shortening of DNA molecules, but postpone the erosion of genes

• TELOMERASE• Catalyzes the lengthening of telomeres in germs cells (not in somatic cells)

16.3 Chromosomes (figure 16.21 in text)

• In the eukaryotic cell DNA is precisely combined with large amount of protein.• Complex of DNA and protein is called CHROMATIN

• Histones are proteins responsible for the first level of DNA packing• Four Types (H2A, H2B, H3, and H4)

• Histones are very similar in all eukaryotes—promotes likelihood of evolutionary importance in organizing DNA

• Nucleosomes “beads on a string”• Consists of DNA wound twice around a protein core composed of two

molecules each of the 4 main histones

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Chromatin

• Chromatin undergoes drastic changes during the cell cycle and condenses during cell division.• This forms several thick metaphase chromosomes

• HETEROCHROMATIN—the chromatin seen during interphase

• EUCHROMATIN—less condensed and more dispersed