The Genetic Material-DNA- and it’s Role

• Where is the genetic material stored?

• Before 1930, this was not clear…

• Hammerling in Germany studied the small green algae, Acetabularia to find out

Acetabularia

Cap

Stalk

Foot

Acetabularia• Cut off cap….new one

grows• Cut off foot…no new

growth• Hammerling hypothesized

the information for growth & development resides in the foot.

• AND… the nucleus resides in the foot

Acetabularia…Transplantation Experiments

• In replacing caps and feet between species, Hammerling found that the nucleus-containing foot was the determining factor.

But there were problems with believing that the nucleus is the site for

genetic material

• It contains primarily DNA and there’s just not enough variability in DNA to explain all of life!!!

• So many still believed proteins must contain the genetic code…

Hershey-Chase Experiment

1952• Worked with viruses

that attacked bacteria…called Bacteriophage

• Composed of only protein coat & DNA

• Look like bizarre spacecraft...

DNA

Protein Coat

Bacteriophage

• Life cycle includes attacking bacteria cell and injecting its genetic material inside…producing thousands of new viruses

• Text pg. 201• If virus has only protein and

DNA, one is the candidate genetic material

• How to determine which?

DNAProtein Coat

Bacteria cell

Hershey-Chase Experiment 1952

In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to determine whether protein or DNA was the hereditary material.

By labeling DNA and protein with different radioisotopes, they would be able to determine which chemical (DNA or protein) was getting into the bacteria. And such material must be the hereditary material.

Since DNA contains Phosphorous (P) but no Sulfur (S), they tagged the DNA with radioactive Phosphorous-32. Conversely, protein lacks P but does have S, thus it could be tagged with radioactive Sulfur-35.

Hershey-Chase Experiment

• Alfred Hershey & Martha Chase labeled the Bacteriophage protein with radioactive Sulphur (35S) and the DNA with radioactive Phosphorous (32P)

• Later…The newly formed viruses inside the cell had only 32P label.

Therefore: DNA is the Genetic MaterialStructure of DNA: A Review…• Made of Nucleotide units:

– Ribose sugar (5C sugar)

– Phosphorous group (-PO4)

– Nitrogen-containing base (A,G,C,T)

• Nucleotides are linked together by covalent (phosphodiester) bonds

Text pg. 205

DNA Structure• Two anti-parallel nucleotide strands

held together by H-bonds• And twisted together to form a helical

structure

Text pg. 204

How DNA Replicates

A .. TG...CT..A

ATC GA T

T AG C 5’

5’

3’

3’

DNA Replication

• An enzyme termed DNA Polymerase synthesizes new DNA from original in a 5’ to 3’ direction (the end with a free –OH group)

• DNA replication results in two new complementary strands of DNA….

• One of each is from original and one new…..semi-conservative replication

3’

5’

New nucleotides canOnly add to this end

ATC GA T

T AG C

5’

ATC GA T

T AG C

5’

3’

3’

CTA

C

A

TG

DNA Polymerase

5’ 3’

DNA replication involves a great many building blocks, enzymes and a great deal of ATP energy. DNA replication in humans occurs at a rate of 50 nucleotides per second and ~500/second in prokaryotes.

Nucleotides have to be assembled and available in the nucleus, along with energy to make bonds between nucleotides. DNA helicase enzymes unzip the DNA helix by breaking the H-bonds between bases. Once the polymerases have opened the DNA, an area known as the replication bubble forks (always initiated at a certain set of nucleotides, the origin of replication).

New nucleotides are placed in the fork and link to the corresponding parental nucleotide already there (A with T, C with G). Prokaryotes open a single replication fork, while eukaryotes have multiple forks.

Since the DNA strands are antiparallel, and replication proceeds in the 5' to 3' direction on EACH strand, one strand will form a continuous copy. The top strandhere….

Text pg. 212

The Lagging Strand

…while the other, lagging strand will form a series of short pieces with gaps. These are called “Okazaki fragments” and require the use of other enzymes to complete the process.

Text pg. 212

Semi-conservative replication

Original DNA StrandOriginal DNA Strand

New DNA Strands

Proofreading

DNA must be faithfully replicated…but mistakes occur:

• DNA polymerase (DNA pol) inserts the wrong nucleotide base in 1/10,000 bases– DNA pol has a proofreading capability and can correct

errors• Mismatch repair: ‘wrong’ inserted base can be

removed• Excision repair: DNA may be damaged by

chemicals, radiation, etc. Mechanism to cut out and replace with correct bases

– Text pg 213

How does DNA carry the information for life?

• Small segments of DNA (sequences of nucleotides) code for specific proteins

• These segments of DNA are called genes• One gene codes for one protein…(mostly!)

• Hundreds-Thousands of genes per chromosome