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

• 1952 - 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• 1951 - Worked with a technique called X-ray

diffraction• Determined the helical nature of DNA

James Watson and Francis Crick• 1953 – Determined the structure of DNA

using Chargaff’s and Franklin’s data

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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

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

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Genes• The sequence of nucleotides in an organisms genome

is unique• Genes consist of sequences of nucleotides

Human Genome

• 3164.7 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

• Follows complementary base pair rules

• “Leading strand” – continuously synthesized• “Lagging strand” – synthesized in fragments

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Origins of Replication

Steps of DNA Replication

1. DNA helicase unzips the original DNA molecules by severing hydrogen bonds between nucleotides

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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

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