Download - DNA notes 2012.pptx [Read-Only] - LCPS
DNA
Definition
� Deoxyribonucleic acid (DNA) – molecule that contains genetic information that directs the activities of cells.
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directs the activities of cells.
� DNA contains the instructions cells use to make proteins.
�Helps make bones, muscle, new cells, etc.
History
� Rosalind Franklin - took x-ray pictures of DNA molecules which helped to determine its shape.
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determine its shape.
History
� James Watson and Francis Crick -built the first DNA model and are usually given credit for determining its shape.
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given credit for determining its shape.
Structure
� Monomers called
NUCLEOTIDES.
� 3 parts:
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� 3 parts:
� sugar (deoxyribose)
� phosphate
� nitrogen base
Structure� There are four different DNA nitrogen
bases:
�Purines - adenine and guanine
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�Purines - adenine and guanine
(double rings)
�Pyrimidines - cytosine and thymine
(single rings)
Structure
� The nitrogen bases always bond to the deoxyribose molecule (sugar) of the nucleotide.
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Purines Pyrimidines
Adenine Guanine Cytosine Thymine
Phosphate group Deoxyribose
Structure
� Nucleotides bond together to form a double stranded DNA molecule.
� DNA’s structure looks like a twisted ladder
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� DNA’s structure looks like a twisted ladder
�This structure is called the double helix.
� The deoxyribose and phosphate form the “rails,” of the ladder.
� Nitrogen bases bond together to form the “rungs,” of the ladder.
Structure
Nucleotide
Sugar-
phosphate
backboneKey
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Chargaff’s Rule
� Chargaff's Rule:
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� Chargaff's Rule:
�Adenine (A) = Thymine (T)
�Guanine (G) = Cytosine (C)
Nitrogen Base Pairing Rules
� ADENINE always bonds to THYMINE
� CYTOSINE always bonds to GUANINE
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Chemical Structure
� The nitrogen bases are
bonded with weak
hydrogen bonds to
hold the two DNA hold the two DNA
strands together.
� The structure and
shape of the nitrogen
bases allows them to
always bond to their
complement
Scientist Check Point
� The picture to the right shows an x-ray diffraction of DNA. The x-ray diffraction of DNA led to the idea that DNA —
� What Scientist Took this picture?
� Why are Watson and Crick famous?
Draw 2 strands, four
nucleotides each:
� Remember, each nucleotide consists of sugar, phosphate,
base!
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base!
� Label the sugars “S”, and the phosphates “P” … notice a pattern
� Notice: opposite directions!
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The Genetic Code
� The order of the nucleotides in creates a code used to make proteins.
� The arrangement of nitrogen bases (A-T-� The arrangement of nitrogen bases (A-T-G-C) determines the amino acids used to construct proteins
�Like the alphabet makes words
How Does It All Fit???
� All your DNA (end-to-end, from one cell) is about 2 meters long!
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� One cell’s nucleus is about 2 micrometers in diameter!
� So, how can 2m of DNA fit into 2µm of space?
Supercoiling!
1. DNA wraps
around
histone
proteins
� DNA + histone
= nucleosome2. Nucleosomes
form a coil
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proteins form a coil
3. Coils coil again
… This is a
SUPERCOIL!
Supercoiling!����
Supercoiling!����
A Chromosome Is Supercoiled DNA
Supercoiling!
Got It?
So, Again,
What is A Chromosome?
A chromosome is supercoiled
DNA
Making Connections