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.
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