DNA: Structure and Function
The DNA Revolution
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1940s-1960s
Griffith & Avery—DNA transformed
pneumococcus bacteria.
Encouraged the study of prokaryotic
chromosomes.
Hershey and Chase—Bacteriophage
experiment. DNA from viruses is injected to
host bacteria cells.
Griffith and Avery
Hershey & Chase
Hershey & Chase
The Search for the Structure of the
DNA Molecule
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1951—Rosalind Franklin—X-ray
crystallography
Chargaff—Chargaff’s rules. Ratio of
nitrogenous bases in DNA. Complimentary
bases.
Watson & Crick--1953
Franklin
Chargaff
Watson & Crick Model of DNA
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Nucleotides
Sugar—Deoxyribose
Phosphate Group
Nitrogenous Bases
Adenine & Thymine
are complementary.
Cytosine and Guanine
are complimentary.
Purines—Adenine &
Guanine
Pyrimidines—Cytosine
& Thymine
5’ End—Phosphate side
3’ End—Other side
Watson & Crick Model
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DNA is composed of 2
chains of nucleotides that
form a double helix
shape.
The two strands are
antiparallel.
The backbone of the
DNA molecule is
composed of alternating
phosphate groups and
sugars.
The complimentary
nitrogenous bases
form hydrogen bonds
between the strands.
A is complimentary to T
and G is
complimentary to C.
DNA Model
DNA
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Functions
1. Storage of genetic information
2. Self-duplication & inheritance.
3. Expression of the genetic message.
DNA’s major function is to code for proteins.
Information is encoded in the order of the
nitrogenous bases.
Chromosomes
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Prokaryotic
Circular
Very small
1 chromosome per
cell
Some enzymes and
proteins are
associated with the
DNA.
Not housed in a
nucleus.
Eukaryotic
Linear
Fairly long
Several chromosomes per
cell.
Histone proteins---”spools”.
Same in all eukaryotes
Housed in a nucleus.
Nucleosome—2 loops of
DNA wrapped around 8
histone proteins.
Summary
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Describe the structure and function of DNA.
DNA Replication
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DNA replicates in a semiconservative way
1 strand is the “leading strand” (3’  5’)
1 strand is the “lagging strand” (5’  3’)
DNA Replication Steps
1. Helicase (protein) unzips the DNA
2. DNA polymerase (protein) binds to the strands
3. DNA polymerase adds the floating nucleotides to
the new DNA strand.
4. 2 new strands are made
Summary
Describe the process of DNA Replication
RNA
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3 types—mRNA, tRNA, rRNA
Ribose
Uracil replaces Thymine
Single stranded helix
Transcription
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RNA polymerase
Messenger RNA
mRNA
Unzipping of DNA helix.
Sense strand or template strand
Codons
Genetic code
Translation Vocabulary
1. Amino Acid- The basic building block of proteins
2. tRNA- a RNA molecule that carries the amino acid to the
ribosome
3. codon- a set of 3 nucleotides used to code for an amino acid
4. polypeptide- a string of many molecule (ex: protein is a
polypeptide of amino acids)
5. P site- where the tRNA detaches leaving behind the amino
acid
6. A site- where the tRNA arrives
Translation
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1.
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5.
Steps:
mRNA enters the cytoplasm from the nucleus
mRNA finds a ribosome
Ribosome starts translation by using the tRNA that
carries Methionine (AUG)
Protein chain elongates
Stop codons terminates the translation of protein
elongation