DNA as Genetic Material – Structure

and ReplicationChapter 16

Warm-Up

• Working with your table partner, write as many things as you can remember about DNA on your whiteboard!!

• Share!!• Keep track on your whiteboard of your ideas

that have already been shared so you don’t repeat what someone else has already said and our discussion isn’t redundant…

Follow-Up

• On your sticky note, take 1-2 minutes to write your name and any questions you have about DNA – at the end of class, we’ll see if your questions have been answered and if not we’ll address them over the next few weeks!

• If no questions come to mind right away, think about DNA technology and how

Brief Overview of the Timeline of DNA

With your table partners…• You have pictures of the experiments/results of 4 key

experiments in the development of determining that DNA is the genetic material

1) Griffith 2) Avery, McCarty, & MacLeod3) Hershey & Chase 4) Chargaff

• For each experiment…• Briefly describe what occurred in the experiment (set-

up and results)• Determine what could be concluded about DNA from

that experiment

Griffith

Avery, McCarty, and MacLeod

Hershey and Chase

Chargaff

Rosalind Franklin

James Watson and Francis Crick

• Read the original paper that Watson and Crick reported presenting their model for DNA structure

• As you read, underline/highlight the information that you recognize either from our discussion today or from your previous exposure to learning about DNA [the language will be challenging – focus on the facts!]

• Then, briefly describe what data helped them build their model and the key structural components/conclusions they identified

Business

• Quest• Due the Thursday we get back• Some questions about scientists,

structure, and replication… others about protein synthesis

• You may get ahead over the break • Quiz over structure/replication the

MONDAY we get back!

Details of DNA Structure

From DNA to Chromosome

• A strand of human DNA is about 3 m long…

• How does it fit into all our cells??

• Supercoiling

chromosomenucleus

cell

DNA

Nitrogen

bases

histones

Proteins that DNA wraps

around

Details of DNA Structure• Nucleotides are the monomers of nucleic acids• 5 carbon sugar

• Ribose• Deoxyribose

• Nitrogen Base• Adenine• Thymine• Cytosine• Guanine• Uracil

• Phosphate

5’ Carbon

3’ Hydroxyl

1’

2’3’

4’

5’

Details of DNA Structure

• What do you notice about the 5’ and 3’ ends of the two strands?

• They’re ANTIPARALLEL!!

• Why? For the nucleotide bases to line up

5’ Carbon

3’ Hydroxyl

3’ Hydroxyl

5’ Carbon

Details of DNA Structure

• What holds the nucleotides together?

5’ Carbon

3’ Hydroxyl

3’ Hydroxyl

5’ Carbon

• Nucleotide Bases: Purines and Pyrimidines• PURINES

• “Aggies are Pure” – A and G are Purines which have 2 rings

• PYRIMIDINES• “TCU Cheerleaders build Pyramids” – T, C, and U (in RNA) are

Pyrimidines have one ring

Details of DNA StructureDetails of DNA Structure

Details of DNA Structure

What do you notice about the number of hydrogen bonds between the different bases?

5’ Carbon

3’ Hydroxyl

3’ Hydroxyl

5’ Carbon

CFU

• Which letter represents the bonds that will be broken when a strand of DNA gets replicated?

CFU

• What type of bonds are the bonds between the nucleotides?

A.CovalentB.IonicC.Van der WaalsD.Hydrogen

CFU

• Which letter represents the 3’ end of the DNA molecule?

CFU

• Which letter represents a purine?

CFU

• Which pair of nucleotides must be C and G?

CFU

• Which base pairs are most likely easier to break and why?

A.A-T because they are held together by 3 hydrogen bonds

B.C-G because they are held together by 3 hydrogen bonds

C.A-T because they are both purinesD.C-G because one is a purine and the

other is a pyrimidine

CFU

• Which letter represents thymine?

DNA Replication

Flashback!•When in the cell cycle does replication occur? •When does the cell check for mutations? •What should happen to the cell if mutations are detected? (2 things!)

DNA Replication

Meselson and Stahl

Meselson and Stahl

DNA Replication

DNA Replication

•Coordinated by a large team of enzymes! •helicases•polymerases• ligases•primases

DNA Replication

• Problem: Nucleotides can only be added to the 3’ end by DNA Polymerase III…

• Solution: Okazaki• Leading and Lagging Strands

• Leading Strand• Continuous synthesis

• Lagging Strand• Okazaki fragments• Joined by ligase

Parental DNA

3’

5’

5’

3’

Remember: DNA polymerase can only add nucleotides to the 3’ end, so DNA gets built in the 5’ 3’ direction!

DNA polymerase

Lagging strand

5’

3’

Okazaki fragments

Leading strand

Ligase

One piece of 5’ 3’

Many little pieces of 5’ 3’ linked

together later

Leading and lagging have the same origin of replication, but since DNA polymerase can only add on the 3’ end, the lagging strand has to start backwards and make little pieces to link together

Priming DNA Synthesis• DNA polymerase can only

extend an existing DNA molecule; it cannot start a new one• Short RNA primer is built

first on parent DNA by primase

• RNA primer later removed by DNA polymerase I

Priming DNA Synthesis

• Closer look…

DNA polymerase

Primase builds the RNA primer

Replaces RNA nucleotides with

DNAPrimase

DNA Replication

Model It!

• Nucleic Acids• Parent/template strands DNA (blue beads)• Daughter strands DNA (orange beads)• Primer RNA (white beads)

• Enzymes• Helicase (black pipe cleaner)• DNA polymerase III (yellow pipe cleaner)• Primase (white pipe cleaner)• DNA ligase (blue pipe cleaner)

Test your understanding…On some paper, write A – H and decide whether each letter represents the 3’ or 5’ end of DNA. Then, label the sections (A-B, C-D, etc) as “leading” or “lagging”

A

B C

D

E

FGH

3’

5’ 3’

5’

3’

5’3’

5’

A-B: Leading

C-D: Lagging

F-E: Leading

H-G: Lagging

Editing and Proofreading DNAWhy do we not always get cancer?

DNA can repair itself!!!

• Since DNA polymerase III does 1,000 base pairs/second, it makes a lot of errors

• DNA Polymerase I (only 20 bp/sec) excises mismatched bases, repairs the DNA, and removes the primer

• DNA polymerase I reduces error from 1 in 10,000 bp to 1 in 100 million bp!!

Problems at the end…• Ends of chromosomes are “eroded” with each

replication (don’t get fully copied)• Telomeres are expendable, non-coding sequences

at the ends of the DNA strand• short sequence of bases repeated 1000s of times

• TTAGGG in humans

Telomeres and Aging• In the absence of

telomerase, the telomere will become shorter after each cell division. When it reaches a certain length, the cell may cease to divide and die.

Putting it ALL together

• Summarize the roles of the key enzymes• Label the diagram showing the steps of DNA replication• DNA Structure – Questions and Practice

Summary of Replication Enzymes

Enzyme FunctionHelicase

Primase

DNA Polymerase I

DNA Polymerase III

Ligase

Unzips DNA (breaks H-bonds between nucleotides)

Builds RNA primer in leading strand and Okazaki fragments

Adds DNA nucleotides (20 bp/s); replaces RNA primer with DNA; repairs errors in DNA

Adds DNA nucleotides (1,000 bp/s)

Joins Okazaki fragments (using phosphate groups)

• In the diagram below, label the key enzymes and structures in DNA replication. Be sure to label 3’ and 5’ ends, too!