(5) Molecular Basis of Inheritance -- History and Basic Structure

8/2/2019 (5) Molecular Basis of Inheritance -- History and Basic Structure

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A timeline

1928:*British scientist -- Frederick Griffithstudies bacteria looking for cause ofpneumonia

*found two specific strains or cultures ofbacteria that looked different whengrowing on petri dishes:-one grew in smooth-edged groups

-other one produced colonies thatwere rough and ragged aroundthe edges


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Why important?

Visual differences made it easy torecognize and distinguish between thestrains of bacteria

Also, Griffith found that:

*smooth-edged colonies of bacteriacaused disease

*rough-edged colonies were harmless


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Disease-causingbacteria (smooth

colonies)

Harmless bacteria(rough colonies)

Heat-killed, disease-causing bacteria(smooth colonies)

Control(no growth)

Heat-killed,

disease-causingbacteria (smoothcolonies)

Harmless bacteria(rough colonies)

Dies of pneumonia Lives LivesLive, disease-causing

bacteria (smooth colonies)

Dies of pneumonia

Section 12-1 Griffiths Experiment

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Results of Griffiths 1928 experiment:Discovery of process of Transformation

Somehow the heat-killed bacteria

had passed their disease-causing

ability to the harmless strain The harmless strain had been

transformed into a disease-causing

strain

Hypothesized that some factor

was responsible for this change


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Timeline cont

1944:

-American, Oswald Avery, continuedbacteria research of Griffith

-Knew were 4 types of organic compoundsthat make up all life

- used enzymes to destroy lipids,

carbohydrates, proteins, and RNA inan extract from the disease causingbacteria.


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Transformation still occurred, so obviouslythe molecules they had destroyed werenot responsible for transformation.

Only organic molecule left that had notbeen destroyed was DNA

When repeated experiment with DNA-destroying enzymes, no transformationoccurred.DNA was the key to heredity


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Bacteriophage with

phosphorus-32 inDNA

Phage infects

bacterium

Radioactivity inside

bacterium

Bacteriophage withsulfur-35 in proteincoat

Phage infectsbacterium

No radioactivity insidebacterium

Hershey-Chase ExperimentSection 12-1

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Race for the structure of DNA

~1940:-Erwin Chargaffdiscovers thatpercentages of A and T are equal inany sample of DNA; same is true for C

and G 1944:

- Linus Pauling discovers thatproteins can have a helical shape

1952:- Rosalind Franklin takes pictures of DNA molecule

using technique called X-ray diffraction, shows thatDNA has helical shape


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1951-1952:

-Maurice Wilkins works with X- raydiffraction and sees same

pattern as Franklin, shares infowith James Watson

April, 1953:

-James Watson and FrancisCrickbuild first model of DNA

(are awarded Nobel Prize in 1960s)

Fi J W d F i C i k


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Figure 5.x3 James Watson and Francis Crick


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Basic DNA structure

Exists as a double helix

Uprights made up of alternating deoxyribose(sugar) and phosphate groups

Bases are attached to the sugars

Bases in DNA are adenine, thymine, cytosine,and guanine

A pairs with T, C pairs with G and vice-versa A and G are purines larger, double rings

T and C are pyrimidines smaller, single rings

Fi 16 6 B i i i DNA


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Figure 16.6 Base pairing in DNA


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Base Pairings


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Erwin Chargaff (discovered base pairings)

Discovered that A and T nucleotides werefound in equal amounts and C and Gnucleotides were found in equal amounts.

Because of this we now know about thebase pairings between the purines (A andG) and the pyrimidines (T and C)


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Why does A always pair with T (or U),and G with C?

Distance between uprights is 2 nm

(nanometers)


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Chromosome Structure ofEukaryotes

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

Section 12-2

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DNA contains the information that a cellneeds to carry out all of its functions. In away, DNA is like the cells encyclopedia.

Suppose that you go to the media centerto do research for a science project. Youfind the information in an encyclopedia.You go to the desk to sign out the book,

but the media specialist informs you thatthis book is for reference only and may notbe taken out.


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CW/HW Assignment

1. Why do you think the media centerholds some books for reference only?

2. If you cant borrow a book, how can

you take home the information in it?3. All of the parts of a cell are controlled

by the information in DNA, yet DNA does

not leave the nucleus. How do you thinkthe information in DNA might get from thenucleus to the rest of the cell?

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(5) Molecular Basis of Inheritance -- History and Basic Structure