Download - Dna structure & central dogma
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DNA• DNA is often called the blueprint of life.
• In simple terms, DNA contains the instructions for making proteins within the cell.
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Why do we study DNA?
We study DNA for many reasons, e.g.,
• its central importance to all life on Earth,
• medical benefits such as cures for diseases,
• better food crops.
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Chromosomes and DNA
• Our genes are on our chromosomes.
• Chromosomes are made up of a chemical called DNA.
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Genetic material of cells…
• GENES – units of genetic material that CODES FOR A SPECIFIC TRAIT
• Called NUCLEIC ACIDS
• DNA is made up of repeating molecules called NUCLEOTIDES
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The Shape of the Molecule
• DNA is a very long polymer.
• The basic shape is like a twisted ladder or zipper.
• This is called a double helix.
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The Double Helix Molecule
• The DNA double helix has two strands twisted together.
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One Strand of DNA• The backbone of the molecule is alternating phosphates and deoxyribose sugar
• The teeth are nitrogenousbases.
phosphate
deoxyribose
bases
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Nucleotides
C C
C
OPhosphate
O
O -P OO
O
O -P OO
O
O -P OO
O One deoxyribose together with its phosphate and base
make a nucleotide.
Nitrogenousbase
Deoxyribose
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One Strand of DNA
• One strand of DNA is a polymer of nucleotides.
• One strand of DNA has many millions of nucleotides.
nucleotide
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Four nitrogenous bases
• Cytosine C
• Thymine T
• Adenine A
• Guanine G
DNA has four different bases:
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Two Kinds of Bases in DNA
• Pyrimidines are single ring bases.
• Purines are double ring bases.
C
C
C
C
N
N
O
N
CC
CC
N
N
N
N
N
C
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Thymine and Cytosine are pyrimidines
• Thymine and cytosine each have one ring of carbon and nitrogen atoms.
C
C
C
C
N
N
O
N
cytosine
C
C
C
C
N
N
O
O
thymine
C
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Adenine and Guanine are purines
• Adenine and guanine each have two rings of carbon and nitrogen atoms.
C
C
C
C
N
N
N
Adenine N
N
C
C
C
C
C
N
N
O
N
Guanine N
N
C
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Two Stranded DNA• Remember, DNA has two strands that fit together something like a zipper.
• The teeth are the nitrogenous bases but why do they stick together?
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C
C
C
C
N
N
O
N
C
C
C
C
NN
O
N
N
N C
Hydrogen Bonds
• The bases attract each other because of hydrogen bonds.
• Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA.
• The bonds between cytosine and guanine are shown here with dotted lines
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Hydrogen Bonds, cont.• When making hydrogen bonds, cytosine always pairs up with guanine
• Adenine always pairs up with thymine
• Adenine is bonded to thymine here
C
C
C
C
N
N
O
O
C
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Chargraff’s Rule:
• Adenine and Thymine always join together
A T
• Cytosine and Guanine always join together
C G
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DNA by the Numbers• Each cell has about 2
m of DNA.• The average human has
75 trillion cells.• The average human has
enough DNA to go from the earth to the sun more than 400 times.
• DNA has a diameter of only 0.000000002 m.
The earth is 150 billion mor 93 million miles from the sun.
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Genetic Diversity…• Different
arrangements of NUCLEOTIDES in a nucleic acid (DNA) provides the key to DIVERSITY among living organisms.
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The Code of Life…
• The “code” of the chromosome is the SPECIFIC ORDER that bases occur.
A T C G T A T G C G G…
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DNA is wrapped tightly around histones and coiled tightly to
form chromosomes
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DNA Replication• DNA must be copied
• The DNA molecule produces 2 IDENTICAL new complementary strands following the rules of base pairing:
A-T, G-C
•Each strand of the original DNA serves as a template for the new strand
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DNA Replication
• Semiconservative Model:
1. Watson and Crick showed: the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand.
.Parental DNA
DNA Template
New DNA
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(1961) Watson & Crick proposed…
• …DNA controlled cell function by serving as a template for PROTEINstructure.
• 3 Nucleotides = a triplet or CODON(which code for a specific AMINO
ACID)
• AMINO ACIDS are the building blocks of proteins.
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DNA Transcription
• DNA can “unzip” itself and RNAnucleotides match up to the DNA strand.
• Both DNA & RNA are formed from NUCLEOTIDES and are called NUCLEIC acids.
See p.301
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DNA Translation
• The cell uses information from “messenger” RNA to produce proteins
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For more detail let see the movie….