Molecular Biology Lecture 1

8/12/2019 Molecular Biology Lecture 1

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BIOL 321 - DNA Structure

Cristofre MartinDepartment of BiochemistrySt. George’s University



What is the genetic material?

The material responsible for hereditary informationmust have the following characteristics:

1. It must contain, in stable form, the information for

an organism’s cell structure, function, developmentand reproduction.

2. It must replicate accurately so that progeny cellshave the same genetic information as the parentalcell.

3. It must be ca able of chan e.



DNA was first identified in1868 by Friedrich Miescher, a

Swiss biologist, in the nuclei of pus cells obtained fromdiscarded surgical bandages.He called the substancenuclein , noted the presence of

phosphorous, and separated thesubstance into a basic part(which we now know is DNA)and an acidic part (a class ofacidic proteins that bind to

basic DNA).



Frederick Griffith (1928):The infection of S. pneumoniae in mice

and the transformation of bacteria.



Griffiths Experiments showed thatthere was a “transforming principle”from the killed virulent bacteria that

can alter the non-virulent bacteria to become virulent.

Griffith believed that this unknown“transforming principle” was protein.



Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

Transformation is blocked when DNA isdestroyed; therefore DNA is the genetic material.

Oswald Avery (1944)



Further experiments by Avery and his colleague supported

that the transforming factor was DNA.



The results of theOswald Avery’s (a

Canadian scientist)experiment suggestedthat DNA was thegenetic material.

At the time we did notknow very much aboutDNA.

Oswald Avery received Nobel Prize for hisresearch.


http://slidepdf.com/reader/full/molecular-biology-lecture-1 9/34Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

In 1953, AlfredHershey and MarthaChase published

experiments using bacteriophage viruses(viruses that attack

bacteria) that furtherdemonstrated that DNAis the genetic material.

Evidence from Bacteriophage



Lytic life cycle of a virulent phage



Hershey-Chase experiment demonstrating DNA isgenetic material

By radioactivelylabeling prot eins andnucleic acids (DNA)of bacteriophage itwas furtherdetermined that thehereditary material islocated on DNAmolecules.



Relative Proportions (%) of Bases in DNAOrganism A T G CHuman 30.9 29.4 19.9 19.8Chicken 28.8 29.2 20.5 21.5Grasshopper 29.3 29.3 20.5 20.7Sea Urchin 32.8 32.1 17.7 17.3Wheat 27.3 27.1 22.7 22.8Yeast 31.3 32.9 18.7 17.1E. coli 24.7 23.6 26.0 25.7

DNA hydrolysis experiments by Erwin Chargaff showedthat all double stranded DNAs contained 50% purine bases(A & G) and 50% pyrimidine bases (C & T).

Chargaff’s Rule:

A = T

C = G



An unknown piece of DNA contains 35%

adenine. What is the percentage of all othernucleotide in this same piece of DNA?

Answer:

Adenine = 35%Thymine = 35%

Cytosine = 15%Guanine = 15%

TOTAL = 100%




Structures of deoxyribose and ribose, and of thenitrogenous bases in DNA and RNA




em ca structures o an



Crystallographer, Dr.

Rosalind Franklin tookthe X-ray image of theDNA molecule thatWatson and Crick usedto deduce the structureof the DNA molecule.

Rosalind Franklin died of cancer in 1958 and thuswas unable to share in the award of the Nobel prizethat was awarded to Watson, Crick, and Wilkins in

1962




Molecular structure of DNAProperties of DNA:

1)The DNA moleculeconsists of two

polynucleotide chains

wound around each otherin a right-handed doublehelix.2) The two chains are

antiparallel. One strandis oriented in a 5’ to 3’way and the other strainis oriented 3’ to 5’.




Molecular structure of DNAProperties of DNA

(cont.):

4) The bases of theopposite strand are

bonded together byhydrogen bonds.

A-T base pairing forms twohydrogen bonds, while G-C

base pairing forms threehydrogen bonds




Structures of the complementary basepairs found in DNA

Properties of DNA(cont.):

5) Specific base pairing iscomplementary

such that A pairswith T, and G pairswith C.



Properties of DNA (cont.):

5) Specific base pairing is complementary such that A pairswith T, and G pairs with C.

Example 1:

Strand 1: 5’-TATTCCGA -

3’

Strand 2: 3’ -ATAAGGCT - 5’

Example 2:

A DNA strand contains 15% G and 35% T.Therefore, the same strand will contain 15% Cand 35% A.



Properties of DNA(cont.):

6) The base pairs are0.34 nm apart in the DNAhelix. A complete turntakes 3.4 nm; thereforethere are 10 bases per

turn. The externaldiameter of the helix is2nm.



Funny DNA trivia:

6 X 10 9 base pairs (bp)/human cell bp are 0.34 X 10 -9 meters (m) apart

so the length of DNA/cell is6 X 10 9 X 0.34 X 10 -9 m = 2 meters

109 =billion10 -9 =one billionth



Human karyotype

In order for the

extremely largeDNA molecules to

be packaged into a

tiny nucleus, theDNA is organizedinto physicalstructures calledchromosomes.



Chromosomes are composed ofchromatin.

Chromatin is the complex of DNA andchromosomal proteins.

The most important chromosomal proteins are called histones.




Nucleosome Structure

Various histone proteins bind witheach other along withDNA to form astructure called thenucleosome. Thenucleosome core is

composed of twomolecules each ofH2A, H2B, H3 andH4 histone proteins.



Nucleosomes can associate with each other toform a more compact structure that has been

termed, due to its size, the 30 nm fiber.



Electron

micrographof ametaphase

chromosomedepleted ofhistones



This figure shows theunraveled chromatin inthe upper inset with thedense protein core. Theindividual filaments areseen in the main

photograph and in thelower inset at higher

magnification.

The many different orders of chromatin packing that give



The many different orders of chromatin packing that giverise to the highly condensed metaphase chromosome

Two types of chromatin have been defined based onchromosome staining

properties:

Euchromatin - decondensedand transcriptionally active

Heterochromatin - condensedand transcriptionally inactive

Molecular Biology Lecture 1

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