Basis of DNA

Requirements of Genetic Material

1) it stores information

2) it can replicate itself

3) it can be transmitted to offspring

4) it allows for variation between individuals of a speciies

What did Thomas Hunt Morgan discover?

Normally, fruit flies have red eyes. He suddenly bred a white-eyed male mutant, and when he crossed it with normal red-eyed females:

• F1 generation: All offspring had red eyes → confirming red was dominant

• F2 generation (breeding F1 flies together): White eyes reappeared, but only in males

White eyes was linked to males, so the gene responsible must reside on the X chromosome—one of the sex-determining chromosomes. This was the first concrete evidence that genes are located in chromosomes.

Chromosomes

Heritable molecules made of DNA and protein that holds genes, which are the basic units of inheritance

What was Griffith’s Bacterial Transformation and what did it prove?

He had two strains of pneumococcal bacteria: R-cells which have no coating so it is not resistant to immune systems, and S-cells which have a smooth polysaccharide coat and is resistant to the immune system. In mice, a mixture of dead S-cells and living R-cells lead to death even though they were both non pathogenic.

It turns out that the R-bacteria was transformed into living S-bacteria through an uptake of an unknown genetic material from the dead S-cells.

What was the Avery–MacLeod–McCarty experiment and what did it prove?

Building upon Griffith’s work, they took S and R cells and prepared a cell-free extract from dead S-cells. This extract alone could still transform R-cells into S-cells, so the transforming principle was definitely a molecule within the extract. They then used a classic process of elimination, and discovered that destroying DNA also stops the transformation. Therefore, it is the uptake of DNA that causes the transformation of R cells into S cells.

What was the Hershey-Chase phage experiment and what did it prove?

In 1952, scientists still doubted that DNA was genetic material, and many believed that protein held genes.

• Hershey and Chase examined bacteriophages (viruses that infect bacteria) which are essentially just DNA wrapped in a protein coat — making them a perfect tool to separate the two candidates.

• Phages inject genetic material to reproduce, so all they had to do was examine which material was injected.

• Proteins contain the isotope ³⁵S and DNA contains the isotope ³²P. They found an abundance of ³²P inside the host and little ³⁵S, proving that DNA was indeed genetic material.

Watson and Crick’s Double Helix DNA Model

The DNA is a double helix consisting of:

• Two strands of nucleotides wound around each other in opposite directions (antiparallel) with either 5-P or 3-OH on the edges

• A sugar-phosphate backbone on each strand

• Nitrogenous bases pointing inward, pairing across the two strands like rungs on a twisted ladder

What are Chargaff’s Rules for DNA?

1) In a strand of DNA, the number of G bases = the number of C bases, and the number of A bases = the number of T bases

2) The composition of DNA varies from one species to another

How does the double helix model and chargaff's rules explain how DNA could be a structurally valid genetic material?

1) Encoding: Even though DNA only has 4 bases, the sequence of those bases along a strand can vary infinitely; 4 bases arranged in different orders along a strand billions of bases long can encode enormous amounts of information.

2) Replication: Because each base has only one possible partner, each strand serves as a template for easily building a new complementary strand.
3) Offspring: In sexual reproduction, DNA is packaged into chromosomes within sperm and egg cells, which combine during fertilization to pass genetic material from both parents to offspring.
4) Variation between individuals: With ~3 billion base pairs, each of which can be one of 4 bases, the number of possible DNA sequences is essentially infinite. Variation between individuals can be vast even while the overall structure and mechanism remain the same

How does DNA replicate?

1) Opening: An initiator protein unwinds a short stretch of the DNA double helix. Then, a protein known as helicase attaches to and breaks apart the hydrogen bonds between the bases on the DNA strands, thereby pulling apart the two strands.

2) Another enzyme called primase briefly attaches to each strand and assembles a short RNA snippet of about 5-10 nucleotides called a primer at which replication can begin.

3) The enzyme DNA-polymerase needed primase to create a foundation because it can only add onto 5-P → 3-OH strands. Primase creates the 3-OH for polymerase to build upon. After the primer is in place, DNA-polymerase wraps itself around that strand and catalyzes the attachment of new nucleotides to the exposed nitrogenous bases. The result is two new daughter strands.

Where does replication begin on DNA?

Replication begins on special sites called “origins of replication.” Prokaryotes have one, while eukaryotes have thousands. Helicase breaks the H-bonds at each origin, creating a replication bubble.

The point where bubble meets the still-intact double helix is called a replication fork. Each bubble has two replication forks moving in opposite directions — this is called bidirectional replication.

All the proteins involved in DNA replication

1) Helicase: binds to each helix to break hydrogen bonds
2) Topoisomerase: relaxes supercoiled DNA ahead of the replication fork so that replication may proceed
3) Single-stranded DNA binding proteins (SSBs): prevent parted strands from joining again by binding to them and stabilizing them
4) Primase: creates RNA snippets for polymerase to build upon
5) DNA ligase: joins okozaki fragments

What are the two strands made during DNA replication?

1) Leading Strand

• Synthesized on the template strand running 3'→5', which means the new strand runs 5'→3'

• Because this orientation allows polymerase to work in its natural direction, the leading strand is built continuously as the replication fork opens

• It is essentially one long, uninterrupted new strand

2) Lagging Strand

• Synthesized on the template strand running 5'→3'

• Since polymerase cannot work backwards, it has to synthesize in short fragments going away from the replication fork, waiting for more template to be exposed before starting each new fragment

• These fragments are called Okazaki fragment and each fragment requires its own RNA primer laid down by primase before polymerase can begin. Once all fragments are made, the RNA primers are removed, the gaps are filled in, and the fragments are joined together by DNA ligase into one continuous strand