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1. DNA has the most basic code there is for making proteins and does not code for anything but proteins.

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2. DNA really codes for individual amino acids, the building blocks of proteins.

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1. DNA has the most basic code there is for making proteins and does not code for anything but proteins.

2. DNA really codes for individual amino acids, the building blocks of proteins.

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DNA is made of basic sub-units called nucleotides.

basePhosphate group

Deoxyribose- a sugarNucleotide base

3. The basic unit of DNA is a nucleotide made of a phosphate group, a sugar ring (deoxyribose) and a nucleotide base.

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T

The nucleotides get stacked, as you see at right.

nucleotide

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T

The nucleotides get stacked, as you see at right.

nucleotide 4 different types of bases.

They can be in any order.

They can be repeated too.

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2

3

4

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“Stacked” nucleotides

A little more realistic model

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Another model of DNA

Sugar-Phosphate Backbone

Nucleotide Bases- often called just “Bases”

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Three different models of DNA.

All are the same, drawn differently.

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Think Pair Share

1. For what does DNA code?

2. Does it code for anything else?

Proteins

No.

3. What is the most basic thing for which DNA codes?

An amino acid

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A model showing the “double helix” shape of DNA.

The bases are shown as A, C, G, and T.

They can be in any order.

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• DNA is made of a long chain of repetitive units.• The unit is the nucleotide.• The bases are different.• Everything else is the same.• A long chain of repetitive units covalently bonded into a

molecule is called a polymer.• Remember, a covalent bond is when atoms share

electrons, forming a molecule.• DNA is a giant molecule.

4. DNA is a polymer. A polymer is simply a large number of very similar chemical units linked together into a larger molecule.

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DNA HAS HYDROGEN BONDS

DNA also has a kind of bond we have not discussed since we talked about water.

This bond is called a hydrogen bond, and is much weaker than a covalent bond or an ionic bond.

Hydrogen bonds are shown as dashed lines in this model

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Hydrogen Bonds- review• Water (H2O) is polar-

because it is a covalently bonded molecule (shares electrons), and the oxygen “pulls” more strongly on the shared electrons.

+

+

-

•The result of this “pulling” is a slightly negative charge on the oxygen end of water, and a slightly positive charge on the hydrogen atoms.

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Hydrogen Bonds are Common in Biology

• Hydrogen bonds are common in many biological molecules and are present in DNA.

• That’s one reason why hydrogen, which often takes on a slightly positive charge, is useful in biological molecules.

• One hydrogen bond by itself is weak.• Many hydrogen bonds together are quite strong.• You may think of the hydrogen bonds in DNA as acting just

like a strong zipper.• As we will see later, DNA actually gets zipped and unzipped

all the time.

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Hydrogen bonds hold the two strands of DNA together.

Hydrogen bonds.

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Quick Review• DNA is composed of two strands of nucleotides

joined by hydrogen bonds and twisted into a double helix.

• A double helix is a “twisted ladder”• Every nucleotide in a strand of DNA contains three

parts: a sugar, a phosphate group, and a base.• The base can be one of four kinds: adenine,

thymine, cytosine, or guanine.• To understand more about the bases, continue.

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Nucleotide BasesTwo groups of bases: Purines and Pyrimidines.

Purines: The larger bases, adenine and guanine.

Pyrimidines: The two smaller bases, cytosine and thymine

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The bases “pair” with one another in DNA.

Adenine always pairs with….

Thymine

Guanine always pairs with….

Cytosine

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BASE PAIRINGThere are four bases in DNA, as you can see.

They make a chemical alphabet of four letters….

A , G, C, T.

Because A only pairs with T, and G with C, the “letters” on one strand of DNA tell the other side of the “ladder” what they should be.

This has several purposes. One is to help replace a damaged or lost base with the right one.

Another purpose is to make sure new copies are accurate, when DNA “replicates” or copies itself.

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A

C

A

G

G

T

T

G

C

C

Base pairing of DNA

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Nucleotide Bases• Two groups of bases: Purines and Pyrimidines.• Each group has two different bases:

– Purines: The larger bases, adenine and guanine.

– Pyrimidines: The two smaller bases, cytosine and thymine

• Adenine pairs only with thymine and guanine only pairs with cytosine.– A-T G-C

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Think Pair Share

4. Name the basic unit of DNA.

5. Name the 3 parts of this unit.

Nucleotide.

a. sugar

b. Phosphate group

c. Nucleotide Base

6. DNA is a large molecule, referred to as a _____. Made up of smaller units. Polymer.

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Think Pair Share

7. What 2 parts of the nucleotide do not change?

8. What part of the DNA molecule do these form?

Sugar and phosphate.

Edge of the ladder.

9. What part of the nucleotide is variable?

The nucleotide base.

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Think Pair Share

10. Name the 4 nucleotide bases.

11. Explain the rules for what order they can be in.

A G C T, adenine, guanine, cytosine, thymine.

Any order, any number of repeats.

12. What type of bond holds the edge of the ladder of the DNA molecule together?

Covalent.13. What type of bond holds the two strands of DNA together?

Hydrogen bonds.

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DNA REPLICATION

The process of copying DNA

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12. If you are a cell, you have the exact same DNA, in the exact same amount, as every other cell in the same organism.

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13. When cells divide, they have to give a copy of their DNA to each of the cells they will become. To do this, they must first copy their DNA.

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DNA molecules are built just like a ladder. To make a better model, like the one in the next slide, we just need to twist the ladder into a spiral.

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DNA Double Helix

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• The two sides of the ladder are complimentary to each other.

• In other words, the bases on one side “fit” the bases on the other.

• When replicating (copying) DNA, this is a key fact.

DNA REPLICATIONA

C

A

G

G

T

T

G

C

C

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DNA REPLICATION

• Practice: If the sequence of bases on one side of the “ladder” is ATTGCAT, then the sequence of the other side of the “ladder” must be ??????.....

•T A A C G T A

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This is a generalized picture of DNA replication

Replication requires many enzymes, and is highly complex.

Notice the dark colored strands in the diagram at left.

These are darkened to help you see that one original strand, and one complimentary strand make up each new copy of DNA.

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Hydrogen bonds are broken by enzymes.

New bases are brought in and pair with original bases.

Enzymes control this as well.

This creates two new strands, each paired with one old strand.

Hydrogen bonds broken.

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The diagram above is misleading in some ways.

The DNA strands do not break up into short pieces.

This is just a closeup of a part of an entire DNA molecule.

The two original strands determine bases to be added to the new strands.

Old OldNew

DNA polymerase I, II and III are the main enzymes that control DNA replication.

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Replication creates two exactly identical DNA molecules from one original.

When a cell divides, one copy goes to each new cell.

No information is lost!

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DNA replication occurs in both directions of the molecule, not just in one direction.

This electron micrograph of replicating DNA demonstrates that clearly.

The areas that look like rings are called replication bubbles, and show DNA replicating in two directions.

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Why Replicate DNA?• Replication occurs

just before a cell divides into two new cells.

• Both new cells need their own copies of DNA.

• Thus, DNA is replicated (copied).

Cell dividing into two new cells

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Think Pair Share

14. Describe the base pair rules.

15. Match these bases with their complement….

AGGCTCTAGC

A-T G-C

TCCGAGATCG

16. Is it possible for different cells, within the same organism to have different DNA information?

NO.17. Explain why or why not.

Every cell in an organism has the exact same copy of all chromosomes.

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A photo of a real chromosome

When DNA is replicated, each copy is then packed into a chromosome.

Here is a topic of great confusion for many students.

Model of a chromosome

Two exact copies of the same DNA molecule, packed up tightly.

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This diagram should help with the confusion.

An unreplicated chromosome is shown at top.

A replicated chromosome is shown in the next two models, just below it.

Photo of a real chromosome

Chromosome after cell division- single copy of DNA

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• We need to review the formation of a chromosome from chromatin to make all these different drawings make perfect sense.

• While you are reviewing, keep in mind that chromatin is replicated, then both copies of the chromatin are packed into a chromosome.

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• DNA exists in two general physical forms-

• Chromosomes…– Which are DNA in its

tightly coiled state– Formed when cell is

going to divide.

• Chromatin– DNA in its uncoiled

state.

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A

C

A

G

G

T

T

G

C

C

Chromatin- uncoiled DNA, wrapped around proteins called histones.

Chromatin- replicating

DNA

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Chromatin being coiled into a more compact form of DNA called a chromosome.

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Chromatin

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An area of Confusion• Lets return to our “area of great confusion for

many students”.• If a chromosome is usually shaped like this….

When and why would it ever look like…

Copy 1

Copy 2

…this?

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This is a cell in the act of dividing. Each copy of the chromosome will be pulled away from the other half.

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Single copies of each DNA molecule, coiled tightly as a chromosome, are separated. THIS CELL IS DIVIDING.

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At right is a micrograph showing real chromosomes being separated, just as shown in the diagram above.

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These single copies of a DNA molecule, in chromosome form, do not stay that way for long.

The cell needs to “unpack” them, in order to read them and make proteins.

When a cell is done dividing, these will be unpacked and return to their chromatin state.

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• 14. One complete molecule of DNA is a chromosome- this is a change from the original definition we have learned. This can be a single copy of an original chromosome, which is what form exists during most of a cell’s life, or it can be a double copy, made just before a cell divides. Both are called a chromosome.

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A drawing of our new definition of a chromosome- a complete DNA molecule, in any form- chromatin or chromosome. This is a single copy of a DNA molecule as it exists most of the time, in order to be read.

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The unpacked “chromatin” form of a DNA molecule has copied itself here, just before cell division. Now there are two exact copies of the DNA molecule. But this is one molecule right now, so it is called a chromosome.

Centromere

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Each of the two identical DNA molecules, after replication, are called sister chromatids.

Sister chromatids

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A photo of a real chromosome

15. When DNA is replicated, each copy is a long molecule in its unpacked form- chromatin. To divide, the cell must package its chromatin molecules, AND the copy into compact chromosomes.

Replicated chromatin form- uncoiled.

Replicated chromosome form- coiled.

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• Play the FLV clip “mitosis” to show chromosome forms being separated during mitosis, just prior to actual cell division.

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The Genetic CodeAt right is a closeup model of DNA showing three bases on one strand, paired with their corresponding bases on the other.

Each series of three bases on a DNA molecule codes for one kind of amino acid in a protein.

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As we study transcription…

…..which means copying DNA code into mRNA code…

…and Translation

…which means making a protein with a ribosome, from mRNA…

…we will learn more details about the genetic code.

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Think Pair Share

18. A cell is getting ready to divide. What must it first do, before it can divide.

19. For the following items, identify as chromosome or chromatin…

Copy its chromosomes.

A

B

CD

Chromatin Chromosome Chromatin

Chromosome

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E F

G Chromosome

Chromosome

Chromosome

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Think Pair Share

20. Explain the basic physical difference between chromatin and chromosome.

21. For the following items, identify as before, during or after cell division.…

Chromatin is unpacked, can be read. Chromosome is packed up, unreadable.

A

B

CD

After

Before or after

Before Before

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E F

G During

Before

During

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Think Pair Share

22. The cells below are almost finished dividing. What will they be called when they are done?

Daughter cells.

23. Explain any differences in the set of chromosomes for each daughter cell.

There is none- they are exactly the same.

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Think Pair Share

24. Can the DNA in this form be read?

NO.

26. . Explain what must happen to this chromosome in order for the cell to make proteins.

It must be unpacked.

25. Explain why or why not.

It is in chromosome form- too tightly packed for enzymes to read it.

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Think Pair Share

27. . Name the monomer for which the bases (shown in the diagram below) code.

An amino acid.

28. Explain your answer.

Three bases in a row code for one amino acid.

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Think Pair Share

29. What process does this diagram represent?

Chromatin coiling into a chromosome.

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Think Pair Share

30. Name this structure.

Nucleotide.

31. Name the parts labeled 1, 2 & 3.

a. phosphate

b. sugar

c. nucleotide base

32. A large molecule made of

repetitive units is called

a(n) ______________

Polymer.

1

2

3

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BibliographyCummings, M. (2000). Human heredity, principles and issues (N. Horne, Ed.).

     Pacific Grove: Brooks/Cole, Thompson Learning.

DNA. (n.d.). The biology corner (dna , rna, dna to proteins, ). Retrieved June     3, 2006, from http://www.biologycorner.com/images.php

phenylketonuria. (n.d.). Online mendelian inheritance in man. Retrieved June 3,     2006, from Johns hopkins university Web site: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM

Cummings & Klug Concepts of Genetics 7th edition.

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