Chapter 25: DNA Structure

DNA

In the mid- twentieth century geneticists were determining that this was the genetic material

Bio chemists

Determined the structure of DNA and their efforts led to our knowledge of molecular biology

Able to store information for development, structure, and metabolism, and that it was stable

Before genetic material was known, it was known these qualities

Stability

This quality of genetic material made it so it able to be replicated with high accuracy and transmitted from generation

Fredrick Griffith

Discovery of genetic role or DNA began with research by him

Streptococcus pnenumoniae

Bacteria that Fredrick Griffith worked with that causes pneumonia in animals. He was trying to make a vaccine

• One strain appeared smooth and the other appeared rough

Smooth S strain

Appeared this way because it has a capsule to protect it

Rough R strain

Appeared this way because it lacked a capsule

The mice died

When mice were injected with S strain

They did not die

When mice were injected with the R strain

Heat- killed S strain bacteria

The mice were injected with this to determine if the capsule caused the virulence of the S strain, they did not die

The mice died and living S strain was recovered

Griffith injected the mice with heat killed S strain and live R strain in the final experiment

The S strain transformed the r strain with a substance producing a capsule to become virulent

Griffith’s conclusions about the final experiment when mice were injected with heat killed s strain and living r strain

Genotype

The change in phenotype of the r strain must be due to a change in their

Genetic material

The transforming substance was potentially

Both proteins and nucleic acids

Scientists recognized that genes are located in the chromosomes and chromosomes contain

DNA or protein

The debate between what genetic material was made of was whether or not it was

Oswald Avery

Along with his coinvestigators reported that DNA was the transforming substance and that DNA was the genetic material

• DNA from s strain bacteria caused r strain bacteria to be transformed and produce a capsule to be virulent

Results of Avery and his coinvestigators

DNase

Apart of Avery’s results, is an enzyme that digests DNA that prevents transformation from occurring, this supports the hypothesis that DNA is the genetic material

Protein digesting enzymes

Have no effect on the transforming substance nor does RNase suggesting that neither protein nor RNA is the genetic material since transformation still occurred

Alfred Hershey and Martha Chase

Demonstrated that DNA is the genetic material not proteins, and developed two experiments

Viral DNA labeled with 32P was found in the bacteria and not in the medium, it had entered the bacteria

Experiment 2

Viral protein in capsids labeled with 35S was found in the medium and not in the bacterium, it never entered the bacteria, only DNA entered

Experiment 2

James Watson and Francis H. C. Crick

Determined the structure of DNA

Structure of DNA

• Chain of nucleotides

• Phosphate, 5 Carbon sugar (pentose), and a nitrogen containing base

• Adenine, thymine, cytosine guanine

Adenine=Thymine

Cytosine=Guanine

DNA nucleotides

2 strands, long and thin, two nanometers, Helical (twisted), Repeating subunits

DNA consists of what

Adenine and guanine

Two purines

Thymine and Cytosine

Two pyrimidines

Purines

Have a double ring

Pyrimidines

Have a single ring

Polynucleotide

DNA is a … strand with a backbone of alternating phosphate and sugar groups

Bases

They are attached to the sugar and project to the side

Hydrogen bonding

Holds the DNA strands together between the bases

• They form complementary base pairs

2 hydrogen bonds

Adenine always pairs with thymine and is connected by

3 hydrogen bonds

Guanine always pairs with cytosine and is connected by

Purine

Always bonded to a pyrimidine

Ladder

Is resembled by unwound DNA helix

Sugar phosphate backbones

Make up the sides of the ladder and are also covalent bonds

Rungs

Complementary base pairs of the ladder and are also hydrogen bonds

Antiparallel

The orientation of the two DNA strands in opposite directions

• Sugars are also orientated differently

5 carbon atom

The uppermost on one strand and has a free phosphate

3 Carbon atom

The uppermost on the other strand and has a free sugar

DNA replication

The process of copying one DNA double helix into two identical double helices. The double stranded structure of DNA allows each original stand to serve as a template for a complementary strand

Semi conservative

DNA replication is this because each daughter DNA double helix consists of one new strand and the nucleotides and one old strand conserved from the parent DNA molecule

• Conserving at least half of the information

Two daughter DNA molecules

These will be identical to the parent molecule

Enzymes and Protein

Several of these are used in the process of DNA replication

Steps of DNA replication

• The enzyme DNA helices unwinds and “unzips” the double stranded DNA by breaking hydrogen bonds between paired bases

• New complementary DNA nucleotides fit into place along separated strands by complementary base pairing. These are joined and positioned by DNA polymerase

• DNA polymerase uses each original strand as a template

• DNA polymerase attaches to where the primer is ands reads the parent strand of DNA (nucleotide) and pulls out of the fluid surrounding this process, nucleotides that are complementary base pairs

DNA polymerase

• Attaches to where primer is and reads the parent strand on DNA (nucleotide) and pulls out of the fluid surrounding this process, nucleotides that are complementary base pairs

• Since DNA strands are antiparallel, this can only add new nucleotides to one chain, DNA synthesis occurs in opposite directions

Leading strand

Follows DNA helicase to the end

Lagging strand

Synthesized in Okazaki fragments and makes new strand of DNA in chunks

DNA ligase

• Connects to Okazaki fragments and seals breaks in the sugar phosphate groups.

• Forms covalent bonds

• The two double helix molecules are identical to each other and to the original DNA molecule