DNADeoxyribonucleic

acid

History of DNA

• Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA

• Proteins were composed of 20 different amino acids in long polypeptide chains

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History of DNA

• Chromosomes are made of both DNA and protein

• Experiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material

History of DNAHistory of DNA

• Rosalind Franklin took diffraction x-ray photographs of DNA crystals

• In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays. Their model of DNA was a double helix.

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DNA Structure• Double Helix - Helix is a shape

that twists like the threads of a screw.

• The two sides of the ladder are made up of molecules of sugar called deoxyribose, alternating with molecules known as phosphates.

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DNA Structure

• A long molecule made up of units called nucleotides.

• Each nucleotide is made up of three basic parts.– 5 carbon sugar– Phosphate group– Nitrogen base

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DNA Structure

• Each rung of the DNA ladder is made up of a pair of molecules called nitrogen bases.

• There are four kinds of Nitrogen Bases:

1.Adenine (A)2.Thymine (T)3.Guanine (G)4.Cytosine (C)

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ChargaffChargaff’’s Rules Rule• AdenineAdenine must pair with

ThymineThymine• GuanineGuanine must pair with

CytosineCytosine• The bases form weak

hydrogen bonds

G CT A

Chromosomes and DNA Replication

Chromosome Structure

• Eukaryotic chromosomes contain DNA and protein, tightly packed together to form chromatin.

• Chromatin consists of DNA tightly coiled around proteins called histones.

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Chromosomes & Genes

Duplicating DNA

• Before a cell divides, it duplicates its DNA in a process called replication.

• Replication ensures that each resulting cell will have a complete set of DNA.

What phase of cell division is this happening?

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DNA Replication

• How does it occur? – Step 1: Enzymes “unzip” a molecule

of DNA. This happens when the hydrogen bonds between base pairs is broken and the two strands of the molecule unwind.

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DNA Replication

• During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template for the new strand.

Nitrogen Bases

Replication Fork

DNA Polymerase

Replication Fork

Original strandNew Strand

Growth

Growth

Copyright Pearson Prentice Hall

DNA Replication

How Replication OccursDNA replication is carried out by enzymes

that “unzip” a molecule of DNA.Hydrogen bonds between base pairs are

broken and the two strands of DNA unwind.

DNA Replication

•The principal enzyme involved in DNA replication is DNA polymerase. •DNA polymerase joins individual nucleotides to produce a DNA molecule and then “proofreads” each new DNA strand.