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