Warm-up- 5 minutesWarm-up- 5 minutesQuietly and on your own, answer the following questions in your notes for today. Be prepared to answer.1.In what part of the cell cycle does DNA replication occur?2.Define chromosome.3.Define nucleotide.

Homework & Homework & remindersreminders

1.Chi-square lab write-up due TOMORROW in the beginning of class. 1. ALL late write ups = automatic 50% and additional 10%

points for each day that it’s late. No excuses will be accepted.

2.Chapter 16 quiz on Wednesday. Start studying TODAY.

Today’s AgendaToday’s Agenda1. Warm up2. Chi-square lab overview. Due tomorrow.

Worksheet is bonus.3. Review midterm exams.4. Chapter 16 lecture.5. Start DNA replication diagram.

Chapter 16: The Molecular Basis of

Inheritance

Chapter 16 vocabulary Chapter 16 vocabulary listlist

List of need to know words are provided on the class wiki page. This is for you to work on for yourself.

Building a Structural Building a Structural Model of DNAModel of DNA

• After most biologists became convinced that DNA was the genetic material of life, the next challenge was to determine its structure.

• Rosalind Franklin produced a picture of the DNA molecule by using a technique called X-ray crystallography Franklin produced a picture of the DNA molecule using this technique

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Franklin’s X-ray diffractionphotograph of DNA

Rosalind Franklin

• Based on the images, two other scientists named Watson and Crick were able to determine that DNA molecules took a double helix shape.

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5 end

3 end

5 end

3 end

Space-filling modelPartial chemical structure

Hydrogen bond

Key features of DNA structure

0.34 nm

3.4 nm

1 nm

Sugar–phosphatebackbone

5 end

Nitrogenousbases

Thymine (T)

Adenine (A)

Cytosine (C)

DNA nucleotidePhosphate

3 endGuanine (G)

Sugar (deoxyribose)

• Watson and Crick built models of a double helix to match to the X-rays and chemistry of DNAo The side strands, or “backbones” of the DNA

molecule are made of a sugar (deoxyribose) paired with a phosphate.

o The deoxyribose backbones are joined together by a series of molecules called nitrogenous bases.

Nitrogenous BasesNitrogenous Bases• There are two types of nitrogenous bases:

o Purines• Much wider• Include adenine and guanine

o Pyramidines• Much narrower• Include cytosine and thymine

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Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: widthmatches data from X-rays

How do the four bases combine to form DNA?

• Watson and Crick reasoned that the pairing was more specific – o Adenine paired only

with Thymineo Guanine paired only

with Cytosine

Base Pairing to a Base Pairing to a Template StrandTemplate Strand

• DNA is a double-helix molecule made of two intertwining strands.

• The two strands of DNA are complementary, meaning each has a set of bases that corresponds with the other.

• In DNA replication, the molecule is be separated into its two strands.o Two new strands can be made from these

templates, duplicating the molecule.

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The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

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The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

The first step in replication is separation of the two DNA strands.

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The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

The first step in replication is separation of the two DNA strands.

Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

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The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

The first step in replication is separation of the two DNA strands.

Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

The nucleotides are connected to form the sugar-phosphate back-bones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

Origins of ReplicationOrigins of Replication• Replication begins at special sites called

origins of replication.o The two DNA strands are separated, opening up

a replication “bubble”o Each chromosome may have hundreds or even

thousands of origins of replicationo Replication proceeds in both directions from

each origin, until the entire molecule is copied

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In eukaryotes, DNA replication begins at may sitesalong the giant DNA molecule of each chromosome.

Two daughter DNA molecules

Parental (template) strand

Daughter (new) strand0.25 µm

Replication fork

Origin of replication

Bubble

In this micrograph, three replicationbubbles are visible along the DNAof a cultured Chinese hamster cell(TEM).

Elongating the DNA Elongating the DNA StrandStrand

• Enzymes called DNA polymerases catalyze the elongation of new DNA.

• The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells.

Proofreading and Proofreading and Repairing DNARepairing DNA

• DNA polymerases also proofread newly made DNA, replacing any incorrect nucleotides.

• Two types of repair:o In mismatch repair, the enzymes replace

incorrect bases with the correct ones.o In nucleotide excision repair, enzymes cut out

and replace entire stretches of DNA that are damaged.

Replicating the Ends Replicating the Ends of DNA Moleculesof DNA Molecules

• DNA polymerase has one significant limitation.• The enzyme has no way to complete one of

the ends.o Every time the DNA is copied, it becomes a little

shorter.

• Cells will divide countless times over the lifespan of an organism. How can DNA be protected, given this limitation?

• Eukaryotic chromosomal DNA molecules have at their ends repeating nucleotide sequences called telomeres.o Telomeres are DNA, but do not actually encode

for any traits.o Telomeres do not prevent the shortening of DNA

molecules, but they postpone it.

• Eventually, the telomeres are worn down and essential genes begin to be lost from the chromosomes.o This is one of the hypothesized

causes of aging.

• An enzyme called telomerase catalyzes the lengthening of telomeres in stem cells.o This enzyme cannot be produced

indefinitely due to an increasing risk of the cell growing uncontrollably (cancer)

Chromosome and Chromosome and chromatinchromatin

Activity 1: DNA replication

1. Draw figure 16.13 on page 314 and include the four main blue labels showed in the diagram, the RNA primer and label the 3’ and 5’ ends.

2. Draw figure 16.15 and include the 2 main blue labels. Also label the origin of replication, RNA primer, sliding clamp, DNA pol III, parental DNA, and the 3’ and 5’ ends.

3. Draw figure 16.16 and label the steps of synthesizing the lagging strand.

4. Summarize DNA replication starting from unwinding of the DNA to the end of synthesizing the lagging strand. Include the important proteins and enzymes. Include the appropriate 3’ and 5’ labels.

Activity 2: DNA & Chromosomes

1. Draw one nucleotide using the figure 16.5 on page 308 for reference. In your diagram, include the labels: nucleotide, phosphate, deoxyribose, and nitrogenous base

2. Draw figure 16.7 A on page 309. Include the labels: double helix, nitrogenous bases, adenine, guanine, thymine, and cytosine.

3. Diagram chromatin packing in shown in figure 16.22 on page 321. Include in your diagram the labels: DNA, double helix, histones, chromatin, and chromosome

4. When you are done drawing, you will write a comprehensive story about the molecular unit of inheritance by including all of the words above.