Chp. 12: DNA

DNA - History James Watson- American biochemist Francis Crick- English physicist Rosalind Franklin- English crystallographer

The Discovery Watson & Crick discovered that DNA was a double helix with chains running in opposite directions (1953). Franklin produced a photo by passing x-rays through DNA (showed it was a double helix)

Double Helix

DNA- Deoxyribonucleic Acid Made of Deoxyribose (sugar), phosphate groups, and a nitrogen base

DNA looks like a twisted staircase/ladder, double helix Nucleotides nucleic acid DNA (smallest) (largest) DNA looks like a twisted staircase/ladder, double helix RNA- Ribonucleic Acid

There are 4 possible nitrogen bases Adenine (A) Guanine (G) Cytosine ( C) Thymine (T) Discovered by Chargaff Nucleotides combine through condensation reactions.

The nitrogen bases are pairs that make up the rungs (steps) of the ladder. Adenine pairs w/Thymine (A-T) Cytosine pairs w/Guanine (C-G) These pairs are held together by a weak hydrogen bond.

The sides of the ladder are formed by the deoxyribose & phosphate groups. They are held together by covalent bonds

Importance of DNA Genetic material of living things is made of DNA Organisms are different because the order of nucleotides are different Use genetic information to determine evolutionary relationships. The more closely related they are, the more alike their nucleotide sequence. All Humans are 99.9 % identical. Humans and chimps DNA are 98.8% identical.

Human Genome Project Worked together to determine the nucleotide sequence for humans

DNA Replication DNA is copied during mitosis (Interphase) During replication each strand serves as a model to make a complementary strand. DNA Helicase (enzyme) breaks the hydrogen bonds that hold the nucleotides together, unzipping the DNA.

Replication cont… As the DNA unzips, free nucleotides that are in the nucleus bond to the single strands by base pairing. DNA Polymerase (enzyme) ensure the right bases go in the right spot As a result, each new strand formed is a complement of one of the original strands.

Forms 2 DNA molecules that are identical to the parent