Introduction to DNA (Deoxyribonucleic acid)

What do you know?

Scientists

Phoebus Levene 1920 – Determined the basic structure of nucleotides that make up DNA 5-carbon sugar, phosphate group, and a nitrogenous base

Erwin Chargaff Nitrogenous base composition % of adenine is equal to % of thymine % of guanine is equal to % cytosine Composition of DNA varies from species to species

Maurice Wilkins and Rosalind Franklin Worked with a technique called X-ray diffraction Determined the helical nature of DNA

James Watson and Francis CrickWatson 1953 – Determined the structure of DNA using Chargaff’s and Franklin’s data A

What is DNA?

DNA (Deoxyribonucleic acid) Nucleic acid Consists of monomers called nucleotides Stores genetic information, determines an organisms traits by synthesizing proteins Each organisms genome is unique

Structure of DNA

Double helix Consists of a double strand of nucleotides Two strands are anti-parallel – 5’ to 3’ – 3’ to 5’

Nucleotide Three parts of a nucleotide – 5 carbon sugar called deoxyribose – Phosphate group – A single nitrogenous base

Four Nitrogenous Bases Purines (double ring) – Adenine (A) – Guanine (G) Pyrimidines (single ring) – Thymine (T) – Cytosine (C)

Chargaff’s Rules A-T (2 hydrogen bonds) C-G (3 hydrogen bonds)

Sides of molecule - Alternating sugars and phosphates held together by strong covalent bonds Center – two nitrogenous bases held together by weak hydrogen bonds

DNA Structure Click on the hyperlink above to watch a short animation about the structure of DNA A

Genes The sequence of nucleotides in an organisms genome is unique Genes consist of sequences of nucleotides

Human Genome million nucleotides Average gene length is 3000 base pairs Largest gene is 2.4 million base pairs Approximately 20,000 – 25,000 genes

DNA Replication

Why is DNA replication important?

Why replicate? Each new cell created through cell division must receive an exact copy of the original cells DNA Occurs in the nucleus of cells

DNA Replication Semi-conservative – new DNA molecules have one original template strand and one new strand Semi-conservative Follows complementary base pair rules “Leading strand” – continuously synthesized “Lagging strand” – synthesized in fragments A

Origins of Replication

Steps of DNA Replication 1. DNA helicase unzips the original DNA molecules by severing hydrogen bonds between nucleotides A

2. New nucleotides are added to the template strands using DNA polymerase enzymes (complementary base pairing)

3. Okazaki fragments are pieced together by DNA ligase

DNA Replication C C T A A C G G T A C G A A T A