Download DNA Structure and Function

DNA Structure and
Function
Nucleic Acids
• Monomer = nucleotides
• Structure = three parts: sugar,
phosphate, and nitrogen-containing
base
Functions
• Function #1: Nucleotide monomers can be
used as “energy currency”
• Examples = ATP / ADP
Functions
• Function #2: Stores
genetic information
(traits and
inheritance)
• Examples = DNA,
RNA
Nucleotide Structure
•
Nucleotides – the
building blocks of
nucleic acids
•
1.
2.
3.
Made of:
Phosphate
Sugar
Nitrogenous Base
Sugar-Phosphate Backbone
• Nucleotides
connected together
with what type of
bond?
• Alternating sugars
and phosphates
Nitrogenous Bases
• Four bases:
–
–
–
–
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Base Pairing
• Bases form hydrogen
bonds with each other
– A with T
– C with G
PURINE with?
PYRIMIDINE with?
DNA
C
G
T
A
Base
pair
C
Hydrogen bond
T
A
T
G
C
G
A
T
A
C
G
C
G
T
T
C
A
G
A
A
T
A
T
A
G
A
Ribbon model
T
C
T
Partial chemical structure
Computer model
3-D DNA Structure
• Based on the angle of
the bonds (remember
what a C with 4 bonds
looks like), DNA forms
a DOUBLE HELIX
DNA Structure
• Hydrogen Bonds
occur between
Nucleotide Bases
• the bonds between
which 2 bases are
stronger?
• thousands of bases,
thousands of bonds,
thousands of big
twists
Polarity
• Is DNA polar or
non-polar?
Determining that DNA is the
Genetic Material
• GRIFFITH: 1928
• "Transforming
factor”
Determining that DNA is the
Genetic Material
Head
DNA
Tail
Tail fiber
300,000
• HERSHEY –
CHASE: 1952
• Determined that
the heredity
material was DNA
not protein
• Studied the
bacteriophage T2
Determining that DNA is the
Genetic Material
Phage
Bacterium
Radioactive
protein
DNA
Batch 1
Radioactive
protein
Mix radioactively
labeled phages with
bacteria. The phages
infect the bacterial cells.
Batch 2
Radioactive
DNA
Empty
protein shell
Phage
DNA
Radioactivity
in liquid
Centrifuge
Agitate in a blender to
separate phages outside
the bacteria from the
cells and their contents.
Pellet
Centrifuge the mixture
Measure the
so bacteria form a
radioactivity in
pellet at the bottom of
the pellet and
the test tube.
the liquid.
Radioactive
DNA
Centrifuge
Pellet
Radioactivity
in pellet
Determining the Structure of
DNA
• CHARGOFF: 1949
• Different species
have different
amounts of A, T, C, G
• A always equals T
• C always equals G
Determining the Structure of
DNA
• FRANKLIN and
WILKINS: 1950’s
• X-ray crystalographic
determination that
DNA is a double helix
Determining the Structure of
DNA
• WATSON and
CRICK: 1953
• Double helix structure
of DNA
Determining the Structure of
DNA
• http://www.pbs.org/wgbh/nova/photo51/
DNA Structure in more detail
Two strands run in
opposite directions
DNA is ANTIPARALLEL
• 3’ and 5’ ends
• 3’ has OH group
• 5’ has phosphate
group
Nucleotide Assembly
Organization of DNA
• Prokaryotes
– Several million base pairs one circular piece
– Related genes grouped
together
– Mostly coding DNA
– Nucleus?
23
Organization of DNA
• Eukaryotes
– Billions of base pairs –
several linear
chromosomes
– Genes not grouped
– Mostly non-coding
DNA
– Nucleus?
24
DNA in the cell
• Humans have 3 billion
nucleotide base pairs
• Roughly 6 feet of
DNA in the nucleus of
each cell
• How does it all fit?
DNA is tightly packaged into
chromosomes
CHROMATIN - eukaryotes ONLY
• Histone proteins: the
spools around which
DNA winds
• 9 Histone proteins +
DNA = Nucleosome
– core of 8 histone
proteins
– DNA wrapped around
– 1 “H1” histone
Higher Chromatin Organization
And even higher chromatin
organization!
And the highest organization!
Meet the chromosome
Chromosome Animation
• http://www.biostudio.com/demo_freeman_
dna_coiling.htm
Gene Expression
• Every cell in your body came
from 1 original egg and sperm
• Every cell has the same DNA
and the same genes
32
Gene Expression
• Every cell in your body came from
1 original egg and sperm
• Every cell has the same DNA and
the same genes
• Each cell is different, specialized
• Differences due to gene
expression
– Which genes are turned on
– When the genes are turned on
– How much product they make
33
Gene Expression
• DNA must be copied
when cells divide so
that DNA is the same
in all cells
DNA Replication
• The Watson-Crick model of DNA structure
suggested a mechanism for its replication
– DNA strands separate
– Enzymes use each strand as a template to
assemble new nucleotides into
complementary strands
A
T
A
T
C
G
C
G
G
C
G
A
T
A
T
A
T
Parental
molecule
of DNA
A
C
C
Nucleotides
Both parental
strands serve
as templates
T
A
T
A
T
G
C
G
C
G
C
G
C
G
C
T
A
T
A
T
A
T
A
T
A
Two identical
daughter molecules
of DNA
DNA Replication
• http://www.fed.cuhk.e
du.hk/~johnson/teachi
ng/genetics/animation
s/dna_replication.htm
DNA Replication Summary
• Helicase breaks hydrogen bonds to
separate strands
• DNA Polymerase pairs new bases and
creates two new strands Semiconservative
• Leading strand = continuous
• Lagging strand = segments connected by
ligase
DNA Replication in Eukaryotes
• Eukaryotic DNA is very long and linear
• replication begins at many points
DNA Replication in Prokaryotes
• prokaryotic DNA is circular
• replication begins at one point
Repair of Mistakes
• Error rate during replication: 1 in 100,000
nucleotides = about 3,000 mistakes per
replication (human)
• Error rate in completed daughter DNA
molecules: 1 in 10,000,000,000
nucleotides
Repair of Mistakes
• Enzymes proofread
DNA sequences
during DNA
replication and repair
damaged DNA
– Mismatch repair
enzymes replace
incorrectly paired
nucleotides
– Nucleotide excision
repair enzymes chop
out incorrect or
damaged bases
Repair of Mistakes
• When proofreading
and repair
mechanisms fail, an
error becomes a
mutation – a
permanent change in
the DNA sequence
When would the cell replicate
DNA?
• Any time it is going to divide so that new
cells have enough DNA
DNA Structure Review
• Covalent bonds on
sides connecting
nucleotides
• Hydrogen bonds
between bases
• A pairs with T
• C pairs with G
• 5’ end = phosphate
• 3’ end = OH on sugar
Cell Division
Cell Division
Questions to consider…
Where? Why? How?
Cell Division
• Eukaryotic cells utilize two types of cell
division:
• mitosis
• meiosis
Mitosis in everything else
Meiosis in sexual reproduction.
Why Divide?
• To replace dead or dying cell
• To produce more cells to enlarge the
organism
• Reproduction
Binary Fission
• asexual reproduction
used by most
prokaryotes and
protists
• Results in the
reproduction of a
livingcellby division
into two equal or
near-equal parts.
BINARY FISSION
BUDDING
BUDDING
SUMMARY OF MITOSIS
PARENT
CELL
GROWTH
SUMMARY OF WHAT THE DNA
DOES DURING MITOSIS
DNA in Mitosis
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to
daughter
cells
DNA Replication during Mitosis
• DNA  genes 
CHROMOSOME
• Count number of
CENTROMERES
• Sister chromatids:
copied chromosomes
attached at
centromere
DNA Replication during Mitosis
• KARYOTYPE: picture
of chromsomes
arranged into pairs
• Humans have 23
pairs (46 total)
chromosomes
•
http://en.wikipedia.org/wiki/List_of
_number_of_chromosomes_of_va
rious_organisms
DNA Replication
• The Watson-Crick model of DNA structure
suggested a mechanism for its replication
– DNA strands separate
– Enzymes use each strand as a template to
assemble new nucleotides into
complementary strands
A
T
A
T
C
G
C
G
G
C
G
A
T
A
T
A
T
Parental
molecule
of DNA
A
C
C
Nucleotides
Both parental
strands serve
as templates
T
A
T
A
T
G
C
G
C
G
C
G
C
G
C
T
A
T
A
T
A
T
A
T
A
Two identical
daughter molecules
of DNA
DNA Replication Summary
• Helicase breaks hydrogen bonds to
separate strands
• DNA Polymerase pairs new bases and
creates two new strands Semiconservative
• Leading strand = continuous
• Lagging strand = segments connected by
ligase