Chapter 11 DNA and Genes Section 1 DNA: the Molecule of Heredity

What Is DNA? DNA is a molecule that determines an organism’s traits by determining the structure of proteins. DNA is contained in the cell’s chromosomes.

Why Are Proteins Important? Proteins are responsible for structure in organisms (skin, hair, bone, muscles. Proteins called enzymes are responsible for all life functions because they control the rate of chemical reactions.

The Structure of Nucleotides DNA is a large molecule made up of subunits called nucleotides.

The Structure of Nucleotides Nucleotides are made of a phosphate group, a deoxyribose sugar and a nitrogenous base.

Nitrogenous Bases The four nitrogenous bases of DNA are: –Adenine –Guanine –Thymine –Cytosine

Structure of DNA Nucleotides join together to form long chains. Phosphate and deoxyribose form the back bone of the chain and the nitrogen bases stick out like teeth in a zipper. DNA is formed by two chains of nucleotides.

Watson and Crick In 1953, James Watson and Francis Crick were the first to show the structure of DNA.

Structure of DNA Watson and Crick said that DNA is shaped like a double helix. The double helix is two twisted chains of nucleotides.

Rosalind Franklin Watson and Crick base their findings on the research done by Rosalind Franklin.

Franklin’s work with x-ray crystallography showed the DNA had a double helix shape, that phosphate and sugar groups made the outside of the helix and nitrogen bases made up the center of the atom

Base Pairing Nitrogen bases are joined together in the double helix by weak hydrogen bonds. Adenine can only pair with Thymine. Cytosine can only pair with Guanine.

Complementary Base Pairs A-T C-G

Nucleotide Sequences Nucleotide sequences code for specific genetic information. The sequence A-T-T-C carries different information than the sequence T-A-C-T, even though the same nucleotides are present.

Nucleotide Sequences Organisms with similar DNA sequences are closely related. Scientists use nucleotide sequences to determine evolutionary relationships between organisms

Apes and chimpanzees have similar DNA nucleotide sequences.

DNA Replication When cells divide, the DNA in the chromosomes is copied so that each new cell can have chromosomes identical to the original cell. Chromosomes are copied by DNA replication

DNA Replication DNA molecule ‘unzips’ at base pairs. Free nitrogen bases attach to their base pair on the opened molecule. This continues until the entire molecule has been unzipped and copied.

DNA Replication