The amazing DNA molecule is the source of constancy and variation among species, and is the foundation for the unity and diversity of all life on Earth.

DNA contains only four types of nucleotides, which are the building blocks of nucleic acids.

Each nucleotide in DNA is made of:

• a five carbon sugar• a phosphate group• and one of four possible nitrogen bases

Deoxyribose

Credit for the discovery of DNA’s structure was given to James Watson and Francis Crick in 1953. They were awarded the Nobel Prize in 1962 for this work.

The Brief History of The Brief History of DNADNA

• Discovery of the DNA double DNA double helixhelix

Frederick Griffith – Discovers that a factor in diseased bacteria can transform harmless bacteria into deadly bacteria

(1928)Rosalind Franklin - X-ray photo of DNA.

(1952)Watson and Crick - described the

DNA molecule from Franklin’s X-ray.

(1953)

• Griffith: bacterial work; transformation: change in genotype and phenotype due to assimilation of external substance, which he thought was protein, by a cell (1928)

• Avery: hereditary agent was DNA, not protein (1944) Avery’s group proved this by adding protein digesting

enzymes to the mix, and then DNA digesting enzymes, and observing the reactions. Exposure to DNA digesting enzymes blocked the transformation.

The

four

nitrogenous

bases in

DNA

are…

DNA Double HelixDNA Double Helix

P

P

P

O

O

O

1

23

4

5

5

3

3

5

P

P

PO

O

O

1

2 3

4

5

5

3

5

3

T A

G C

When guanine and cytosine bond, they form triple hydrogen bonds

H-bonds

When thymine and adenine bond, a double hydrogen bond is formed

There is no question of the importance of DNA in an organism, but in most cases, DNA works in conjunction with another important nucleic acid…RNA

The DNA molecule serves as a template to build the RNA molecule, as you’ll see when we discuss protein synthesis next lesson. How does the structure of these two molecules vary?

Each nucleotide in RNA contains:• a five carbon sugar (ribose)

In the 2’ position, a Hydroxyl group(Not present in the 2’ position in Deoxyribose)

• a phosphate group• a nitrogenous base (all the same ones as DNA, except the Pyrimidine thymine is replaced with uracil.)

Uracil is a pyrimidine

, as you can see from the structure

• Antiparallel nature: •Sometimes called “complementary”

sugar/phosphate backbone runs in opposite directions (Crick);

• one strand runs 5’ to 3’, while the other runs 3’ to 5’;

The nucleotides connect at the hydroxyl group of the 5 carbon sugar (at the 3’ end)

Very simply put, DNA functions in all living things as a storehouse of information about heritable traits.

Within each chromosome lies millions of nucleotides, and the arrangement of their respective nitrogenous base pairs is the code for that particular organism.Remember, all life, from the simplest prokaryote to the most complex eukaryote, contains the same four nitrogenous bases.

It is simply the sequence of base-pairs, and amount of DNA that differs from organism to organism!

Think of DNA as the “keeper of the code”, or the “recipe” for the organism that contains it. These banding patterns on chromosomes represent “genes”. Genes are regions on chromosomes that code for specific proteins.While many parts of that code are important parts of the “recipe”, some parts are simply “filler”, and are unnecessary, so far as we understand.These unnecessary sections are called introns, and they are sequences of DNA, within genes, which have no apparent purpose. Within the cells of multicellular animals and plants almost every gene has introns, but they’re fairly rare in prokaryotes, and unicellular eukaryotes.

Exons are the working or expressed segments of genes.

Both terms, "introns" and "exons," were coined by Harvard biologist Walter Gilbert in 1978

While DNA's function is to provide the code on which all life is based, it does not accomplish this by itself. Some RNA is fashioned from a DNA template, and is then moves to different locations in the cell where proteins are made, based on the information provided by the DNA. These proteins are then used by the cells to accomplish various tasks.

We have studied cellular reproductive processes, such as mitosis and meiosis already. During what stage of the cell cycle did DNA replicate?

“S” Phase of Interphase!!!

Exactly how does this happen?

• DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only in a continuous fashion.

A portion of the DNA double helix unwinds through the work of an enzyme called a helicase.

The section that opens up, is called a replication fork.

On the other strand, assembly is discontinuous because the exposed –OH group is the only place where nucleotides can be joined together.

DNA ligase then helps to join the fragments and wind the strands back up into a double helix.

When DNA replication occurs, it begins somewhere in the middle of the molecule, and not at one end. The “replication fork” is within the bubble.

As the new molecules are built, they progress in opposite directions from the point of origin.

DNA DNA ReplicationReplication• Semiconservative Model:Semiconservative Model:

1. Watson and Crick showed:Watson and Crick showed: the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand.

Parental DNA

DNA Template

New DNA

This model is referred to as semiconservative, because half of every newly constructed DNA molecule is old, and from the “parent” DNA.

1. Why is replication necessary? A---?

G---?C---?T---?A---?G---?A---?G---?C---?A---?G---?T---?

DNA Quiz

2. When does replication occur?

3. Use the complementary rule to create the complementary strand:

So both new cells will have the correct DNA

During interphase (S phase).

4. What does ribose have, that deoxyribose doesn’t?

A second hydroxyl group in the 2’ position5. How can DNA be both the

source of constancy and variety on Earth?Because all organisms contain the exact same four nitrogenous bases, but their arrangement and amount makes for the huge variety of life on Earth.