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DNA and the Genetic Code

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Title: DNA and the Genetic Code


1
DNA and the Genetic Code
  • Larry J. Scheffler

2
DNA
  • First isolated from the nuclei of cells in 1869
  • Oswald Avery (1944) presented evidence that
    suggested that nucleic acids were involved in the
    storage and transfer of genetic information.
  • Erwin Chargaff found that the DNA always contains
    the same relative amounts of certain pairs of
    amine bases. There are always equal amounts of
  • adenine and thymine
  • guanine and cytosine.
  • James Watson and Francis Crick in 1953 determined
    the structure of DNA as a double helix
  • Rosalind Franklin created the early X-ray
    diffraction pictures of DNA

3
Nuclei Acids
  • Nuclei acids fall into two classes,
  • DNA
  • RNA
  • RNA, or Ribonucleic Acid, is built on the
    ß-D-ribofuranose ring.
  • DNA, or deoxyribonucleic acid, is based on a
    modified ribofuranose ring in which the -OH group
    on the second carbon atom has been removed.

4
DNA Structure
  • DNA is made up of three units including
  • A ribose sugar

Ribose
Deoxyibose
Ribose and deoxyribose differ in that ribose has
an OH group on carbon 2 whereas deoxyribose has
only a hydrogen attached.
5
DNA Structure
  • DNA is made up of three units including
  • A ribose sugar
  • A phosphate group

Phosphate
Deoxyribose
The phosphate groups alternate with the ribose
sugar and are attached at carbon 3 and at carbon 5
6
DNA Structure
  • DNA is made up of three units including
  • A ribose sugar
  • A phosphate group
  • A nitrogen (amine) base

There are four different amine bases adenine,
thymine, cytosine, and guanine.
7
Nucleotide
  • The DNA strand is made up of alternating
    deoxyribose and phosphate groups with a nitrogen
    base attached as a side chain

Cytosine
8
DNA Structure
  • DNA is made up of three units
  • A ribose sugar
  • A phosphate group
  • A nitrogen (amine) base

The amine bases are side branches to a strand
made of alternating phosphate and deoxyribose
sugars These bases are attached at carbon1 of
the deoxyribose sugar
Adenine
Cytosine
9
DNA Structure
  • DNA is made up of three units
  • A ribose sugar
  • A phosphate group
  • A nitrogen (amine) base
  • These three molecules make up a nucleotide.
  • A DNA strand is a sequence of nucleotides.

Adenine
Cytosine
10
DNA Structure
  • DNA consists of two strands of nucleotides.
    These strands are wound together in a spiral
    known as a double helix
  • The amine bases hold the strand together
  • with a
  • sequence
  • of hydrogen
  • bonds

11
Complimentary Bases
  • Because of their size and ability to hydrogen
    bond, the amine bases exist in complimentary
    pairs in the DNA double helix
  • Adenine always bonds with Thymine and Guanine
    always bonds with Cytosine

12
Hydrogen Bond Alignment
  • The size and shape of the amine bases is such
    that hydrogen bonds can only form at specific
    sites
  • Adenine only bonds with Thymine
  • Guanine only bonds with Cytosine
  • Therefore they form complimentary base pairs

13
DNA Structure -- Hydrogen Bonding
  • Adenine and Thymine form a base pair

14
DNA Structure Hydrogen Bonding
  • Guanine and Cytosine

15
Base Pair Sequence
  • The sequences of bases appears to be random but
    in reality nothing is farther from the truth.
    The base pair sequence contains the code by
    which proteins are synthesized in the cell

16
DNA Structure
  • In the double helix of a DNA molecule, the two
    strands are not parallel, but interwoven with
    each other. 
  • The helix makes a turn every 3.4 nm, and the
    distance between two neighboring base pairs is
    0.34 nm. 
  • There are about 10 pairs per turn. 
  • The intertwined strands make two grooves of
    different widths, known as the major groove and
    the minor groove.
  • These grooves may facilitate binding with
    specific proteins.

17
DNA Shape
  • This color enhanced image taken by the Scanning
    Tunneling Electron Microscope shows a double helix

18
DNA Replication
  • In human beings there are 23 pairs of chromosomes
  • Chromosomes are effectively a very long DNA
    sequence. This DNA sequence replicates itself
    during cell division
  • As the DNA double helix partially unzips as the
    hydrogen bonds between the nitrogen bases are
    broken
  • Sugar and base units are picked up from the
    surrounding solution.
  • Since only A T and G-C combinations can occur
    the new strand is a complimentary replicate of
    the existing DNA

19
DNA Replication
  • When cells divide the DNA must is replicated
    exactly
  • As the DNA unzips new complimentary strands are
    formed.
  • These new strands are exact replicas of the
    previously existing strands

20
DNA and the Genetic Code
  • Genes are long sequences of DNA that code for the
    formation of proteins
  • Typical genes are often thousands of base pairs
    long
  • Not all of the DNA strand appears to have genetic
    information
  • The sequence for a particular gene is very
    specific.

21
Gene Correspondance for Neuropilin-1
Species bp NP-1 Sense Primer Macaque
-- ACCCGCACCTCATTCCTACATCAATGAGTGGCTCCAAATA
GACCTGGGGGHuman 1422
ACCCGCACCTCATTCCTACATCAATGAGTGGCTCCAAATAGACCTGGGGG
Rat 1419 ACCCTCACCCCACCCATACATCAATGAA
TGGCTCCAAGTGGACCTGGGAGMacaque
AGGAGAAGATCGTGAGGGGCATCATCATTCAGGGTGGGAAGCACCGAGAG
Human 1472 AGGAGAAGATCGTGAGGGGCATCATCAT
TCAGGGTGGGAAGCACCGAGAGRat 1469
ATGAGAAGATAGTAAGAGGTGTCATCATTCAAGGTGGGAAGCACCGAGAA
Macaque AACAAGGTATTCATGAGGAAGTTCAAG
ATCGGGTACAGCAACAACGGCTCHuman 1522
AACAAGGTGTTCATGAGGAAGTTCAAGATCGGGTACAGCAACAACGGCTC
Rat 1519 AACAAAGTGTTCATGAGGAAGTTCAAGA
TCGCCTACAGTAACAATGGTTC Macaque
CGACTGGAAGATGATCATGGACGACAGCAAACGCAAGGCAAAGTCTTTTG
Human 1572 GGACTGGAAGATGATCATGGATGACAGC
AAACGCAAGGCGAAGTCTTTTGRat 1569
TGACTGGAAAATGATCATGGATGACAGCAAGCGCAAGGCTAAGTCTTTTG
Macaque AGGGCAACAACAACTATGACACACCTG
AGCTGCGGACTTTTCCAGCTCTCHuman 1622
AGGGCAACAACAACTATGATACACCTGAGCTGCGGACTTTTCCAGCTCTC
Rat 1619 AAGGCAACAACAACTATGACACACCTGA
GCTCCGGGCCTTTACACCTCTCMacaque
TCCACGCGATTCATCAGGATCTACCCCGAGAGAGCCACTCATGGCGGACT
Human 1672 TCCACGCGATTCATCAGGATCTACCCCG
AGAGAGCCACTCATGGCGGACTRat 1669
TCCACAAGATTCATCAGGATCTACCCCGAGAGAGCCACACATAGTGGGCT
-NP-1
Anti-sense Primer (reverse) Macaque
GGGGCCCCGAATGGAGCTGCTGGGCTGTGAAGTGGAAHuman
1722 GGGGCTCAGAATGGAGCTGCTGGGCTGTGAAGTGGAARat
1719 CGGACTGAGGATGGAGCTACTGGGCTGTGAAG
TAGAA
22
Protein Synthesis
  • DNA is found in the chromosomes which are found
    in the nucleus of the cell
  • DNA stores the genetic code for an organism
    through its sequence of the nitrogen bases
  • The genetic code is transferred via RNA to the
    ribosomes in the cytoplasm outside of the cell
    nucleus where protein in synthesized
  • The information required for protein synthesis
    is passed through a similar unzipping and
    replication process

23
RNA and Protein Synthesis
  • The transfer of information for building proteins
    is then accomplished by the RNA.
  • RNA is similar to DNA but there are some
    important differences
  • RNA is a single strand rather than a double helix
  • Deoxyribose is replaced with ribose
  • The nitrogen base thymine is replaced with Uracil

24
RNA
  • Ribose has a slightly different structure from
    deoxyribose
  • Ribose has an OH group on carbon 2 rather than a
    H as in deoxyribose

25
RNA
  • The structure of Uracil differs slightly from
    Thymine

Uracil
Thymine
26
Messenger RNA
  • Messenger RNA or mRNA copies and carries the
    genetic code from the DNA template within the
    cell nucleus to the ribosomes where proteins are
    synthesized.
  • It essentially aligns itself with the DNA and
    produces a complimentary copy

27
Transfer RNA
  • Transfer RNA acts as an amino acid carrier in the
    formation of proteins
  • Through a decoding mechanism it facilitates the
    addition of an amino acid to a peptide chain
    forming a protein
  • It directs the insertion of amino acids in the
    proper sequence in the poly peptide chain through
    sets of three nitrogen bases known as codons

28
RNA Codons
29
RT-PCR or Reverse Transcriptase Polymerase Chain
Reactions
  • RT-PCR was first developed by Cary Mullis for
    which he was awarded the Nobel Prize in
    Chemistry
  • Replicated DNA can be then separated and
    classified
  • This process allows the DNA from very small
    amounts of
  • cellular material to be replicated
  • DNA is extracted and broken down into smaller
    fragments
  • using restriction enzymes
  • The DNA is then replicated and separated using
    gel
  • electrophoresis

30
RT-PCR and DNA Replication
RT-PCR gel showing relative amounts of NP-1 using
cyclophilin as an internal standard.
31
DNA Sequencing
Lee Hood (Univ. of Washington invented a device
that can sequence the bases for a DNA fragment.
32
Forensic DNA Analysis
  • The DNA fragment contains codons that code for
    proteins
  • It also contains regions where there is no coded
    message in the base sequence
  • The application of a restriction enzyme cuts this
    part of
  • the DNA into fragments.
  • The sequence and hence the fragment sizes are
    unique
  • for each individual (Except for Identical Twins)

33
Forensic DNA Analysis
  • Used to identify
  • people in criminal
  • cases
  • Used to establish identity, paternity
  • and ancestry.
  • Used to study evolutionary
  • changes in species


34
Forensic DNA Analysis
  • DNA evidence is only as good as the person
    performing the tests. Care must be taken to
    guard against contamination for legal evidence to
    stand.
  • Chain of evidence rules
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