DNA: the molecule of heredity

What is DNA?

Although the environment influences how an organism develops, the genetic information that is held in the molecules of DNA ultimately determines an organism’s traits.

Living things contain proteins. Your skin contains protein, you muscles contain protein, and your bones contain protein mixed with mineral.

All actions, such as eating running and even thinking, depend on proteins called enzymes. Enzymes are critical for an organism’s function because they control the chemical reactions needed for life.

The structure of nucleotides

• DNA is a polymer made of subunits called nucleotides.

• Nucleotides have 3 parts: a simple sugar, a phosphate group and nitrogenous base

The structure of nucleotides

• In DNA, there are four possible nitrogen bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

Nucleotides join together. The phosphate group of one nucleotide is bonded with the deoxyribose of another nucleotide.

Nitrogenous bases

The structure of DNA

• DNA is composed of 2 strands that are held together by a hydrogen bond between nitrogenous bases. Think of teeth of a zipper holding the two sides of the zipper together.

• Hydrogen bonds can only form between specific paired nitrogenous bases called complementary base pairs.

1. (A)---------(T) 2. (C)---------(G)Hydrogen bond

Hydrogen bond

Nitrogenous base pairing

The structure of DNA

• The two strands of DNA twist together, and its shape is called a double helix.

The importance of nucleotide sequences

• In all organisms the DNA is made up of the four nucleotides. (A) (T) (C) (G)

• The sequence of nucleotides forms the unique genetic information of an organism.

• For ex. A nucleotide sequence of A-T-T-G-A-C carries different information from a sequence of T-C-C-A-A-A.

The importance of nucleotide sequence

• The closer the relationship is between two organisms, the more similar their DNA nucleotide sequences will be.

• The DNA sequences of a chimpanzee are similar to those of a gorilla, but different from those of a rosebush.

How DNA replicates

• The two DNA strands serve as a templates.

• Replication begins as an enzyme breaks the hydrogen bonds between bases that hold the two strands together, thus unzipping the DNA.

• As the DNA continues to unzip, nucleotides that are floating free in the surrounding medium are attached to their base pair by hydrogen bonding.

How DNA replicates

• The process continues until the entire molecule has been unzipped and replicated.