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DNA
The Code of Life
Important Facts
1. DNA is the basic substance of heredity
*Remember that heredity is the passing on
of traits from an organism to its offspring
2. DNA stores and passes on genetic
information from one generation to
the next
3. Chromosomes are made of DNA
Important Facts
4. Genes are found on chromosomal DNA
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DNA is a very large molecule with one
molecule making up each chromosome
Each gene is only a small portion of the
chromosome that it is on
Each chromosome carries many genes.
Therefore each DNA molecule carries
many genes.
Who Discovered the Structure of DNA?
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It was in 1953, while working in Cavendish Laboratory
in Cambridge, England, that the British-born Crick,
then 36, and American-born Watson, 24, hit upon the
famous double-helix structure -- like a twisted ladder -of deoxyribonucleic acid, or DNA.
Who Discovered the Structure of DNA?
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Watson and Crick used X-ray crystallography data,
produced by Rosalind Franklin and Maurice Wilkins at
King's College in London, to decipher DNA's structure.
For their work on DNA, Watson and Crick would later
share the Noble prize with Wilkins.
In what many see as an unfortunate injustice, Franklin
was not also awarded the Noble prize, perhaps
because she was already deceased by the time it was
awarded.
DNA X-ray Crystallography Image
Author: I.C. Baianu et al.
http://en.wikipedia.org/wiki/File:ABDNAxrgpj.jpg
The Structure of DNA
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The DNA molecule forms a double-helix
A double-helix looks something like a
twisted ladder
http://pixabay.com/en/dna-gene-genetic-helix-rna-148807/
What are the parts of a nucleic acid
molecule (DNA or RNA)?
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DNA is composed of three types of
molecules:
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A phosphate group
A nitrogen base
A sugar molecule
These three parts combine to form
nucleotides that are the building blocks of
the DNA molecule (similar to the way amino
acids make up proteins).
What are the parts of a nucleic acid
molecule (DNA or RNA)?
Parts of a DNA molecule:
To make RNA instead, substitute Ribose sugar
for the Deoxyribose sugar.
How do nucleotides form
DNA?
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The sugar and
phosphate groups
alternate to form the
sides of the “ladder”
The “rungs” or
“steps” are pairs of
nitrogen bases joined
by hydrogen bonds.
Image modified from:
http://commons.wikimedia.org/wiki/File:Difference_DNA_RNA-EN.svg
The Four Nitrogen Bases
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In DNA there are only
four nitrogen bases:
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Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
When the opposing pairs
join up Adenine always
joins with Thymine
and Cytosine always joins
with Guanine.
Credit: Madeleine Price Ball
http://commons.wikimedia.org/wiki/File:DN
A_chemical_structure.svg
What makes genes?
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The order of the nitrogen bases
determines the genes on a
chromosome. That is why DNA is said
to carry the genetic code – the code is
the order of the nitrogen bases.
Each chromosome has 50-250 million
base pairs.
Humans have about 2.9 billion base
pairs.
DNA Replication
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In order for DNA to carry the genetic code
there must be a way to accurately copy
that code each time a cell reproduces.
The copy must be an exact duplicate.
This is accomplished through the process
known as DNA replication.
DNA Replication
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First the DNA molecule “unzips” – the two
strands separate between the base pairs.
Next, two new strands are made by adding
nucleotides one at a time, matching up the new
nitrogen bases to the existing bases.
The end result is two, double strands that are
identical to the original one.
DNA Replication
Beginning Strand of DNA
Step 1: The two
sides of the DNA
molecule unzip
between the
nitrogen bases
Step 2: Free nucleotides match up
to the nucleotides of each strand.
Final Result: Two strands that exactly
match the original strand and each other.