DNA Structure and Replication

Figure 16.5 The double helix

DNA Q’s – Use diagram in previous slide1. Which diagrams show the double helix structure?

2. Examine Figure (b). It looks like a ladder. What two parts of a nucleotide form the sides (uprights) of the ladder?

3. Examine Figure (b) again. What part of a nucleotide forms the steps of the ladder?

4. Look at Figure (a). Determine the number of adenine, guanine, cytosine, and thymine nucleotides. Write them down. What do you notice about the numbers?

5. What connects the two strands of DNA?

6. What holds the nucleotides on the individual strands together?

7. How are the two ends of each strand different?

This slide shows how the pairing keeps the DNA double helix width uniform

This slide reviews complementary base pairing. Which pair makes more hydrogen bonds?

What happens if the DNA is not copied exactly or entirely?

• If not exactly, the sequence of bases will be different which could affect the organism because the different sequence would change the gene. Recall from the DNA Scissors lab that a gene is a segment of DNA that controls a trait.

• If not all is copied, some of the bases will be missing and the organism could lose some important sequence that controls all its life activities.

Figure 16.7 A model for Semiconservative DNA Replication

Click on the next few slides to view a simplistic version of DNA replication

Figure 16.7 A model for DNA replication: the basic concept (Layer 2)

Figure 16.7 A model for DNA replication: the basic concept (Layer 3)

Figure 16.7 A model for DNA replication: the basic concept (Layer 4)

• DNA replication begins at specific sites called origins of replication. There are (6) origins below.

Figure 10.5A

Parental strandOrigin of replication

Bubble

Two daughter DNA molecules

Daughter strand

DNA Replication: A closer look

More bubbles, faster the replication process

Figure 16.10 Origins of replication in eukaryotes

• Each strand of the double helix is oriented in the opposite direction

• This directionality causes the daughter strands to grow in opposite directions.

• The new strand always grows in a 5’ 3’ direction.

Figure 10.5B

5 end 3 end

3 end 5 end

P

P

P

PP

P

P

P

Directionality of DNA

LE 16-13

New strand

5 end

Phosphate Base

Sugar

Template strand

3 end 5 end 3 end

5 end

3 end

5 end

3 end

Nucleosidetriphosphate

DNA polymerase

Pyrophosphate

DNA Replication with Adding a Nucleotide

Notice how after we added the

Thymine nucleotide to the

new strand, the 3’ end still exists and

is ready for another

nucleotide. What would it be?

• How DNA daughter strands are synthesized

5 end

P

P

Parental DNA

Figure 10.5C

DNA polymerasemolecule

53

35

35

Daughter strandsynthesizedcontinuously

Daughter strandsynthesizedin pieces

DNA ligase

Overall direction of replication

53

• The daughter strands are identical to the parent molecule

Replication

Enzymes of Replication• Helicase – causes the DNA to unwind and open

• DNA polymerase III – allows for the adding of a nucleotide to the 3’ –OH end of the daughter strand

• Ligase – allows for the short DNA fragments to be joined to form one continuous piece of DNA

• Topoisomerase – enzyme that prevents the open DNA from twisting

• RNA primase – enzyme that puts the first nucleotide down to start the daughter strand

Proving DNA Replication is Semiconservative

• The next few slides illustrate the famous Messleshon and Stahl experiment. You are not responsible for understanding it for Honors Biology.

• Be able to describe the difference between semiconservative, conservative and dispersive replication.

Figure 16.8 Three alternative models of DNA replication

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 1)

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 2)

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 3)

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 4)

Adding Nucleotide Animation

DNA Replication Animation