Biology, 9th ed,Sylvia Mader

DNA Structure & Function

Chapter 13

Ch. 12 - DNA -

Structure &

Function

DNA Structure DNA Structure & Function& Function 2

DNA as Genetic Material

Johann Miescher (1869) Johann Miescher (1869) Removed nuclei from pus cellsRemoved nuclei from pus cells Found they contained a chemical he called Found they contained a chemical he called

nucleinnuclein This was rich in phosphorus and had no sulfur; This was rich in phosphorus and had no sulfur;

thus it could not be a proteinthus it could not be a protein

Later scientists realized there were two types of Later scientists realized there were two types of nucleic acids: DNA (deoxyribonucleic acid) and nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)RNA (ribonucleic acid)


Frederick Griffith (1931)Investigated virulence of Investigated virulence of Streptococcus pneumoniaeStreptococcus pneumoniae in in

mice in following manner:mice in following manner:

1. S strain bacteria have a smooth capsule & are 1. S strain bacteria have a smooth capsule & are capable of killing micecapable of killing mice

2. R strain have no capsule & don’t kill mice2. R strain have no capsule & don’t kill mice

3. Injected heat-killed S strain bacteria into 3. Injected heat-killed S strain bacteria into mice; they did not diemice; they did not die

4. Injected mice with mixture of heat-killed S strain & 4. Injected mice with mixture of heat-killed S strain & live R strain. These mice had living S strain live R strain. These mice had living S strain bacteria & diedbacteria & died

Concluded that virulence passed from the dead strain Concluded that virulence passed from the dead strain to the living strain; transformation had occurredto the living strain; transformation had occurred

4

Griffith’s Transformation Experiment


Avery, MacLeod & McCarty (1944)

(Refer to transparency here first) (Refer to transparency here first)

Discovered that DNA is the transforming substance.Discovered that DNA is the transforming substance.

1. Took DNA only from the S bacteria and mixed it with R 1. Took DNA only from the S bacteria and mixed it with R bacteria. bacteria.

2. S strain DNA was then incorporated into 2. S strain DNA was then incorporated into genome of living R strain bacteria and they genome of living R strain bacteria and they were then were then transformed into S strain bacteria.transformed into S strain bacteria.

3. Enzymes that degrade proteins or RNA did not 3. Enzymes that degrade proteins or RNA did not prevent transformation while those that digest DNA prevent transformation while those that digest DNA did.did.


Reproduction of VirusesViruses consist of a protein coat (capsid) Viruses consist of a protein coat (capsid) surrounding a nucleic acid coresurrounding a nucleic acid core

Bacteriophages are viruses that infect bacteriaBacteriophages are viruses that infect bacteria


Hershey and Chase (1952)Hershey and Chase (1952) Did an experiment to determine whether the Did an experiment to determine whether the

bacteriophages inject the protein or DNA into the bacteriophages inject the protein or DNA into the bacteria.bacteria. Radioactively labeled the DNA core and protein capsid of Radioactively labeled the DNA core and protein capsid of

a bacteriophagea bacteriophage

1. Radioactive P (found in DNA & not in protein) was 1. Radioactive P (found in DNA & not in protein) was found inside cellsfound inside cells

2. Radioactive S (found in protein & not in DNA) was 2. Radioactive S (found in protein & not in DNA) was found mainly outside of cellsfound mainly outside of cells

Results indicated that DNA, not the protein, enters the Results indicated that DNA, not the protein, enters the hosthost

The DNA of the phage contains genetic information for The DNA of the phage contains genetic information for producing new phagesproducing new phages

8Hershey and Chase Experiments


Structure of DNA

DNA contains:DNA contains:Two nucleotides with purine bases. These are Two nucleotides with purine bases. These are double ring nitrogenous bases.double ring nitrogenous bases. Adenine (A)Adenine (A) Guanine (G)Guanine (G)

Two nucleotides with pyrimidine bases. These Two nucleotides with pyrimidine bases. These are single ring nitrogenous bases.are single ring nitrogenous bases. Thymine (T)Thymine (T) Cytosine (C)Cytosine (C)

10Nucleotide Composition of DNA


Chargaff’s RulesChargaff’s RulesThe amounts of A, T, G, and C in DNA:

Identical in identical twinsVaries between individuals of a speciesVaries more from species to species

In each species, there are equal amounts of:A & TG & C

All this suggests DNA uses complementary base pairing to store genetic information.

Human chromosome estimated to contain, on average, 140 million base pairs.

Number of possible nucleotide sequences 4^140,000,000.


Diffraction DataDiffraction Data

Rosalind Franklin:

Studied structure of DNA using X-rays.Found that if a concentrated solution of DNA is made it forms into a crystal like structure.

When X-rayed, an X-ray diffraction pattern results.

The pattern of DNA shows that it is a helix.

13X-Ray Diffraction of DNA


Watson and Crick Model (1953)Watson and Crick Model (1953)

Using data provided by Franklin’s X-ray diffraction and other knowledge about DNA, they eventually determined that DNA is a double-helix

Sugar-phosphate backbones make up the sides

Hydrogen-bonded bases make up the rungs. Complementary bases (A-T; C-G) pair up.

Model matched data of both Franklin & Chargaff

Received a Nobel Prize in 1962

15Watson/Crick Model of DNA

DNA Structure DNA Structure & Function& Function 16DNA Replication:

Replication = process of copying a DNA molecule

1. During DNA replication, each old DNA strand of the parental molecule (original double helix)

serves as a template for a new strand in a daughter molecule.

2. DNA replication is termed semiconservative replication because one of the old strands is conserved, or present, in each daughter DNA molecule.

DNA Structure DNA Structure & Function& Function 17Steps of Replication

1. Unwinding DNA replication begins at numerous points along linear chromosome called replication forks.

DNA unwinds and unzips into two strands. Weak hydrogen bonds between paired bases are broken.

A special enzyme, DNA helicase, unwinds the DNA.


Replication (cont’d)

2. Complementary base pairing Each old strand of DNA serves as a template

for a new strand New complementary nucleotides are positioned by process of complementary

base pairing A special enzyme, called DNA polymerase, helps to position the complementary base pairs


Replication (cont’d)

3. Joining The complementary nucleotides join to form new strands.

This is also helped by DNA polymerase

20Semiconservative Replication

of DNA


Meselson & Stahl’s experiment (1958)Meselson & Stahl’s experiment (1958)

Confirmed semiconservative replication theory

They grew bacteria in a medium containing heavy N-15 so only heavy DNAs were found.

Switched bacteria to N-14 medium.

After 1 division, only hybrid DNA was found

After 2 divisions, half the DNA is light & half is hybrid

These are the results expected if DNA replication is semiconservative.

22Meselson and Stahl’sDNA replication experiment

DNA Replication Video 23

DNA Replication Animation

http://www.courses.fas.harvard.edu

/~biotext/animations/replication1.html

http://www.courses.fas.harvard.edu/~biotext/animations/replication1.html


Details of DNA ReplicationDetails of DNA ReplicationCarbon atoms are numbered in the deoxyribose

molecule.

DNA strands are antiparallel. One of the strands runs from 3’ to 5’ in one direction, and the other strand runs from 3’ to 5’ in the opposite direction.

During replication, DNA polymerase has to synthesize the daughter strand in the 5’ to 3’ direction.

Why? DNA polymerase can only join a nucleotide to a free 3’ end of a previous nucleotide.


Details of DNA Replication (cont’d)Details of DNA Replication (cont’d)

This also means that DNA polymerase cannot start the synthesis of a DNA chain.

An RNA polymerase lays down a short amount of RNA, called an RNA primer, that is complementary to DNA.

Then DNA polymerase can join DNA nucleotides to the 3’ end of the growing daughter strand.



As helicase unwinds DNA, one parental strand runs in the 3’ to 5’ direction toward the fork. Thus, the new complementary daughter strand will be synthesized from the 5’ to 3’ direction. This strand is called the leading strand.

The other parental strand, however, is running in the opposite direction (3’ to 5’ AWAY from the fork). The daughter strand must begin at the fork and run in the opposite direction to the leading strand. This is called the lagging strand.

28Antiparallel Replication of DNA



Replication of the lagging strand is discontinuous.

It results in segments called Okazaki fragments.

While proofreading, DNA polymerase will remove the RNA primers and replace them with complementary DNA nucleotides.

DNA ligase will then join the fragments together.

30Antiparallel Replication of DNA

DNA Structure DNA Structure & Function& Function 31DNA Replication:

Prokaryotic

Prokaryotic Replication

Bacteria have a single circular loop

Replication moves around the circular DNA molecule in both directions. Takes about 40 minutes.

Produces two identical circles

Cell divides between circles, as fast as every 20 minutes

32Replication:Prokaryotic vs.

Eukaryotic


Replication Errors

Genetic variations are the raw material for evolutionary change

Mutation:A permanent (but unplanned) change in base-pair sequence Some due to errors in DNA replication. Proofreading occurs which eliminates most errors. Mistake rate is only 1 per 1 billion base pairs.

Others are due to DNA damage like UV radiation

DNA repair enzymes are usually available to reverse most errors


Videos for Chapter 13Videos for Chapter 13

I

http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swfDNA Replication Animation II

http://www.courses.fas.harvard.edu/~biotext/animations/replication1.html

http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf

Biology, 9th ed,Sylvia Mader

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