DEOXYRIBONUCLEIC ACID

DNADNA—deoxyribonucleic acid

A. Controls the production of proteins in a cell.

B. Every new cell that is developed needs an exact copy of the DNA from its parent cell.

C. Function: to store and use information to direct the activities of cell and to copy itself exactly for new cells that are created.

Structure of DNA

A. DNA is a polymer—it is composed of repeating subunits called nucleotides.

B. Nucleotides—have three parts:

a. Deoxyribose—a five carbon sugar molecule

b. Phosphate group

c. Nitrogen base

Structure of DNA

C. The sugar molecule and phosphate group are the same in every nucleotide, the nitrogen bases may be one of four.

a. Adenine

b. Guanine

c. Thymine

d. Cytosine

Structure of DNA

D. Adenine and Guanine belong to the class of organic molecules called the purines.—Double ring of carbon and nitrogen

E. Thymine and cytosine belong to the class pyrimidines—Single ring of carbon and nitrogen

The Double HelixA. Each nucleotide—deoxyribose,

phosphate and nitrogen base—bond to another nucleotide to form a long strand.

B. Two of these long strands bonded together to form a molecule of DNA.

a. The two strands twist around a central axis to form a spiral structure called a double helix.

The Double Helix

C. The DNA molecule looks something like a twisted ladder.a. The sides of the ladder would

be formed by alternating sugar and phosphate units, while the rungs are bonded pairs of nitrogen bases.

b. Rungs would be of the same length, because each is formed by a purine bonding to a pyrimidine.

The Double Helix

D. DNA has a right hand twist and each full turn consists of 10 base pairs.

The Double HelixE. The two strands of DNA are held together by

hydrogen bonds—form between a purine and a pyrimidine.

a. Adenine always bonds with thymine—joins with 2 bonds

b. Cytosine always bonds with guanine—joins with 3 bonds

EXAMPLE:

A T T C G G C T T A A A

The Double Helix

The Double Helix

F. Because of these bonds always forming together, the arrangement of the nitrogen bases along one strand of the DNA is said to complement the arrangement of the other bases—two strands are called complementary strands.

Replication of DNA A. The process of the duplication of

DNA is called replication.B. Two complementary strands of DNA

unwind, then each strand serves as the template or mold on which a new complementary strand is built.

C. This process would produce two new DNA molecules that are exact replicas of the original DNA.

The Process of Replication

1. Begins when an enzyme called DNA helicase attaches to a DNA molecule, moves along the molecule and unzips the two strands.

a. The helicase breaks the bonds between the nitrogen bases.

The Process of Replication

2. The now unpaired bases in each strand react with the complementary bases of nucleotides that are floating freely in the nucleus

a. The complementary bases bond with the bases in the DNA by forming new hydrogen bonds.

The Process of Replication

3. As each new set of hydrogen bonds link the two bases, an enzyme called DNA polymerase forms bonds that connect the sugar to the phosphate—which connects one nucleotide to another

The Process of Replication

4. The process results in two new DNA molecules—each having one “old” strand and one “new” strand of DNA.

The Process of Replication

5. Replication of DNA does not begin at one end of the molecule and proceed to the other—instead copying occurs simultaneously at many points of the molecule.

a. Enzymes join the individual segments of DNA to each other as they are copied.

Accuracy and Repair

A. The entire process of DNA replication occurs with great accuracy—one error per billion nucleotides.

B. Also DNA has a mechanism for repair—repair enzymes recognize and remove damaged nucleotides and replace them with new ones.