DNA

• Deoxyribonucleic acid• A polymer• Monomers are nucleotides

Nucleotides• Building blocks of DNA• 4 types• Each type has three parts

–Ring shaped sugar called deoxyribose

–Phosphate group–Nitrogenous base

•Single or double ring of carbon and nitrogen atoms with functional groups

Nitrogenous bases• The four nucleotides found in DNA differ

only in their bases.–Thymine (T)–Cytosine (C)

• These are single ring structures called pyrimidines

-Adenine (A)

-Guaninie (G)

- are larger, double ring structures called purines

DNA Strands

• Nucleotides are joined to one another by covalent bonds that connect the sugar of one nucleotide to the phosphate group of the next.

This repeating pattern of sugar-phosphate-sugar-phosphate is called the “BACKBONE”

The nitrogenous bases are lined up along this backbone.

DNA Structure• Double Helix:

–Watson and Crick model DNA this way

–They created a new model in which two strands of nucleotides wound about each other.

–Forming a twisting shape called the DOUBLE HELIX

–Their model placed the sugar-phosphate backbones on the outside of the double helix and the nitrogenous bases on the inside.

–They hypostasized that the nitrogenous bases that aligned across the two strands formed hydrogen bonds.

Complementary Base Pairs

• Individual structures of the nitrogenous bases determine very specific pairing between the nucleotides of the two strands of the double helix.

• These pairing are due to the sizes of the bases and their abilities to form hydrogen bonds with each other

Adenine pairs with Thymine

Cytosine pairs with Guanine

While the sequence of nucleotides along the length of one of the two DNA strands can vary in a number of ways, the bases on the second strand of the double helix are determined by the sequence of the bases on the first strand.

Each base must pair up with its complementary base.

DNA replication • is the process of copying a double-

stranded deoxyribonucleic acid (DNA) molecule,

• a process essential in all known life forms.

• The general mechanisms of DNA replication are different in prokaryotic and eukaryotic organisms.

DNA replication• In a cell, DNA replication must

happen before cell division. • Prokaryotes replicate their DNA

throughout the interval between cell divisions.

• Eukaryotes, timings are highly regulated and this occurs during the S phase of the cell cycle, preceding mitosis or meiosis I.

DNA replication• Each DNA strand holds the

same genetic information.• Both strands can serve as

templates for the reproduction of the opposite strand.

• The template strand is preserved in its entirety and the new strand is assembled from nucleotides

Enzymes link the nucleotides together and form the two new DNA strands, called daughter strands.

This process of copying the DNA molecule is called

DNA replication

Replication of the Double Helix

• More than a dozen enzymes are involved in DNA replication.

• Each "incoming" nucleotide pairs with its complementary nucleotide on the parent strand.

•Enzymes called DNA polymerases (PAHL ih mur ayz ez) make the covalent bonds between the nucleotides of the new DNA strand.

•The process is fast and accurate •an error occurs in only about one of a billion nucleotides.

DNA replication begins at specific sites called origins of replication.

The copying proceeds outward in both directions, creating replication "bubbles".

The parent DNA strands open up as daughter strands grow on both sides of each bubble.

• A eukaryotic DNA molecule has many origins where replication can start at the same time.

• This shortens the total time to copy all the DNA. Eventually, all the bubbles merge

• End product: two double-stranded DNA molecules, each with one new and one old strand.

DNA replication occurs before a cell divides, ensuring that the cells in a multicellular organism all carry the same genetic information.

It is also the mechanism for producing the DNA copies that offspring inherit from parents during reproduction.

DNA replication

• The resulting double-stranded DNA molecules are identical; proofreading and error-checking mechanisms exist