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Botany 1050

Introduction to Botany

Spring 2008

 


DNA

Mendel's crossing experiments established that particles or factors control the characteristics of organisms and these factors are transmitted from parents to their offspring. We now call these factors genes.

I. Chromosomal theory of inheritance

A. Boveri and Sutton (1902) proposed that genes are on the chromosomes

1. Genetic material must be in the nucleus because:

a. Nuclei fuse to form the zygote

b. In both plants and animals sperm cells consist of mostly just a nucleus

c. Sperm and egg make equal contributions to heredity of offspring

2. Genes must be on the chromosomes because the behavior of chromosomes during meiosis corresponds to the distribution of genes in the progeny

a. Both chromosomes and factors (alleles) are paired in diploid cells

b. Chromosomes and alleles of each pair separate during meiosis so gametes have one-half the total number

c. Chromosomes and alleles of each pair separate independently so gametes contain all possible combinations

d. Fertilization restores diploid chromosome number and paired condition for alleles in zygote

II. What is the genetic material?

A. Boveri and Sutton proposed that genes are on the chromosomes, but which part of the chromosomes? 

1. Chromosomes are made up of DNA and histone proteins. Therefore one of them must carry the hereditary information:

a. Pauling believed the histone proteins carried the genes, major flaw with Pauling's protein theory was that proteins have no mechanism for self-replication

b. Watson and Crick believed DNA carried the genes, proved to be correct

B. Genetic material

1. Must be able to store information used to control both the development and the metabolic activities of cells

2. Must be stable so it can be replicated accurately during cell division and be transmitted for generation

3. Must be able to undergo mutations that provide the genetic variability required for evolution

C. Previous knowledge about chemistry of DNA

1. Miescher (1869) - two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)

2. Nuclei acids are polymers of nucleotides:

a. Phosphate group - PO4

b. Five carbon sugar called ribose (or deoxyribose)

c. Nitrogenous base - ring structure containing C & N

3. There are five different nucleotides, classified by how many rings are present on the nitrogenous base:

Pyrimidines - have a single ring on their nitrogenous base:

a. Thymine (T) - found in DNA

b. Cytosine (C) - found in both DNA and RNA

c. Uracil (U) - found in RNA

Note that Uracil replaces Thymine in RNA.

Purines - have two rings on their nitrogenous base:

a. Adenine (A) - found in both DNA and RNA

b. Guanine (G) - found in both DNA and RNA

D. Griffith (1931) demonstrated bacterial transformation using virulent and non-virulent strains of pneumonia bacteria:

1. Mice injected with non-virulent strain lived

2. Mice injected with virulent strain died

3. Mice injected with virulent strain after it had been heated lived

4. Mice were with a mixture of the non-virulent strain plus the heated virulent strain died

5. Something from the heated virulent strain transformed the harmless non-virulent strain into a virulent strain

E. Finding the transforming substance

1. Avery, McCarty and MacLeod (1944)  followed up on Griffith's work by purifying and separating the components of the virulent strain 

a. Adding protein digesting enzymes to the extract did not stop the transformation

b. Adding DNA digesting enzymes to the extracts stopped the transformation

c. Therefore DNA was the transforming agent so it must be carrying the virulence gene(s)

F. Hershey and Chase (1952) - clearly demonstrated that DNA carried genes

1. Bacteriophage = virus that infects bacteria; consists only of a protein coat surrounding a nucleic acid 

2. Viruses reproduce by injecting their genes into a host cell which then make and liberate new viruses

3.  Hershey and Chase used bacteriophage T2 that infects the bacterium E. coli

4. The purpose of their experiments was to see which of the bacteriophage components---the protein coat or the DNA---entered bacterial cells and directed reproduction of the virus

a. In two separate experiments, they labeled the protein coat with 35S and the DNA with 32P

b. Viral coats are sheared away from bacterial cells; they are separated by centrifugation

c. Results: radioactive 32P alone is taken up by bacterial host and incorporated in virus reproduction

d. Confirmed that DNA carries the genetic material

III. Finding the structure of DNA

A. Chargaff (1949):

1. The amount of A, T, G, and C in DNA varies from species to species

2. In each species, the amount of A = T and the amount of G = C

B. Rosalind Franklin (1951) - produced X-ray diffraction photographs of DNA which indicated:

1. DNA is a helix

2. One part of the helix is repeated

C. Watson and Crick (1953) - propose the structure of DNA:

1. DNA is double helix; sugar-phosphate molecules on outside, paired bases on inside

2. Complementary base pairing  is the paired relationship between purines and pyrimidines in DNA, such that A is hydrogen-bonded to T and G is hydrogen-bonded to C

a. That's why the amount of A = T and the amount of G = C

3. Watson and Crick received the Nobel Prize in 1954 for their model of DNA

IV. DNA replication (recall S phase of Interphase)

A. Steps in DNA replication

1. Unwinding: old strands that make up the parent DNA molecule are unwound and weak hydrogen bonds between the paired bases are broken an enzyme

2. Complementary base pairing: free nucleotides present in nucleus bind with complementary bases on unzipped portions of the two strands of DNA, through process of complementary base pairing; process is catalyzed by DNA polymerase

3. Joining: complimentary nucleotides bond to each other to form new strands; each daughter DNA molecule contains an old strand and a new strand; also catalyzed by DNA polymerase

B. Meselson and Stahl (1958) - demonstrated semiconservative replication of DNA

1. DNA replication isemiconservative because duplication of DNA results in a double helix having one parental strand and one new strand

2. Grew bacteria in medium with heavy nitrogen (15N), then switched to light nitrogen (14N)

3. Density of DNA following replication is intermediate as measured by centrifugation of molecules

C. Replication errors sometimes occur

1. Ability to mutate is requirement for genetic material; base changes during replication are one way mutations occur

2. A mismatched nucleotide may occur once per 100,000 base pairs, causing a pause in replication

3. Errors in replication are minimized because DNA polymerase performs a proofreading function

4. Incorrect base pairs that survive the proofreading process contribute to gene mutations

V. How DNA is arranged into chromosomes

A. Each chromosome is made up of a single strand of DNA. If all the DNA within a human nucleus were stretched out it would be about 2 meters (6 feet) long

B. In order to fit the DNA into the nucleus it is complexed with histone proteins which act like a series of spools the DNA is wound around

 

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