The Components and Structure of DNA DNA – deoxyribonucleic acid Deoxyribose – a 5 Carbon sugar in DNA DNA is made up of monomers called nucleotides Nucleotide – made of a phosphate group (PO4), a 5 carbon sugar (deoxyribose) and a nitrogen base 4 Nitrogen Bases in DNA Adenine Guanine Cytosine Thymine Notes modified from large unit Purines – double ring Pyrimidines – single ring

Nucleotides join together to form long chains The phosphate group and deoxyribose form the backbone of the chain (sides of the ladder) Nitrogen bases form the steps of the ladder Erwin Chargoff’s Rule The amount of adenine equals thymine The amount of guanine equals cytosine

Phosphate Group (PO4) 5 carbon sugar-Deoxyribose nitrogen base hydrogen bond nucleotide

History of DNA Rosalind Franklin and Maurice Wilkins worked on the structure of DNA based on x-ray crystallography (pictures of DNA by x-rays). James Watson and Francis Crick were also working on the structure of DNA These scientists were trying to be the first to discover and prove the structure of DNA because they knew it would be one of the most important discoveries in the 20th century.

1953 Watson and Crick proposed that DNA is made of two chains of nucleotides joined together by nitrogen bases Two bases are held together by hydrogen bond Two strands are complementary Complementary base pairing A and T C and G DNA is shaped in a double helix

Cytosine (C) pairs with Guanine (G) Adenine (A) pairs with Thymine (T)

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

The Importance of Nucleotide Sequences All living organisms are composed of A, T, C, G How can organisms be so different from each other if their genetic material is made of the same four nucleotides? Differences in organisms are from the sequence of the four different nucleotides and how many nucleotides The closer the relationship between two organisms the greater the similarity in their order of DNA nucleotides

The Importance of Nucleotide Sequences Scientists use nucleotide sequences to determine evolutionary relationships among organisms Nucleotide sequences can also be used to determine whether two people are related DNA from a crime scene matches the DNA of a suspected criminal

DNA and Chromosomes Eukaryotic DNA is found in the nucleus in the form of a number of chromosomes DNA molecules are very long.

Human cell contains more than 1 meter of DNA How is so much DNA folded into tiny chromosomes? Chromatin – DNA that is tightly coiled around proteins Histones – proteins DNA is wrapped around Nucleosome – two wraps of DNA around a histone

Replication of DNA Before a cell can divide by mitosis or meiosis it must first make a copy of its chromosomes DNA Replication – DNA is copied All organisms undergo replication

How DNA Replicates During replication each strand serves as a pattern to make a new DNA molecule The end result is the formation of two DNA molecules that are identical (duplicated chromosome)

Steps of Replication Enzyme, DNA helicase, breaks the hydrogen bonds between nucleotides, this “unzips” the DNA molecule Free nucleotides from the surroundings in the nucleus bond to the single strands. Enzyme, DNA polymerase glues the new strands together

3. This continues until the entire molecule has been unzipped and replicated. DNA polymerase proofreads the DNA to check for mistakes. 4. Each new strand formed is a complement of one of the originals or parent strand. 5. Two DNA molecules that are identical