Download Cell Reproduction - Chromosomes and DNA

CELL REPRODUCTION
Chromosomes
Lesson Objectives
• Describe the coiled structure of chromosomes made of
DNA and proteins formed after DNA replicates and are
the form in which the genetic material goes through cell
division.
• Discover that DNA replication is semi-conservative; half
of the parent DNA molecule is conserved in each of the
two daughter DNA molecules.
• Outline discoveries that led to knowledge of DNA’s
structure and function.
• Examine the processes of DNA replication.
CHROMOSOMES
• Coiled structures made of DNA and
proteins
• Each chromosome is a single DNA
molecule
• When not dividing, chromosomes exists as
a grainy material called chromatin
• Chromosomes are created in eukaryotic
cells during the S phase of interphase
• Chromosomes are simpler structures in
prokaryotic organisms
Chromatids and the Centromere
• After replication  DNA condenses and coils
into the familiar X-shaped form of a chromosome
• Each chromosome consists of two identical
copies
– two copies are called sister chromatids
– attached to one another at a region called the
centromere.
Chromosome Numbers
DISCOVER OF DNA
STRUCTURE
• DNA is the genetic material
• It is an organic molecule
• For many decades, scientists thought that
proteins were the molecules that carry genetic
information
• DNA is actually classified as a nucleic acid and
carries the code of life
• The work of many scientists contributed to the
identification of DNA as the genetic material
Griffith Searches for the Genetic Material
• 1920s, Frederick Griffith
• Studied two different strains of a bacteria
– R (rough) strain  non-virulent
– S (smooth) strain  virulent, deadly
• Conclusion
– Killed S bacteria
transferred something to
R bacteria making
them virulent or deadly
Avery’s Team Makes a Major
Contribution
• 1940s, a team of scientists led by Oswald Avery
tried to answer the question raised by Griffith’s
results
• Conclusion
– DNA from killed
S bacteria
transformed the
live R bacteria
making them
deadly
Hershey and Chase Seal the Deal
• DNA being the genetic material was not widely
accepted at first
• 1950s, Alfred Hershey and Martha Chase did
experiments with viruses and bacteria
• Tagged the protein & DNA of
virus with radioactive
elements
• Only viral DNA transferred
into bacteria, protein not
transferred
• DNA is genetic material
Chargaff Writes the Rules
• Also 1950s Erwin Chargaff
– studied DNA from many different species
– interested in the four different nitrogen bases of
DNA: adenine (A), guanine (G), cytosine (C), and
thymine (T)
– found concentrations of the four bases differed from
one species to another
• BUT within each species
– the concentration of adenine = the concentration of
thymine
– the concentration of guanine = the concentration of
cytosine
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The Double Helix
• April 1953…shape of DNA molecule
discovered
– James Watson and Francis Crick
– DNA a double helix shape, like a
spiral staircase
• Discovery was based on the prior work
of Rosalind Franklin and other scientists,
who had used X rays to learn more about
DNA’s structure
• DNA double helix
consists of two
polynucleotide chains
running in opposite
directions
Each nucleotide consists
of a sugar (deoxyribose),
a phosphate group, and a
nitrogen base
Nitrogen bases
• Double ring with single ring
• Adenine & thymine
• Guanine & cytosine
DNA REPLICATION
• Occurs during the synthesis (S) phase of the eukaryotic
cell cycle
– begins when an enzyme unzips the DNA strands by breaking
the weak hydrogen bonds between complementary bases in
DNA exposing them.
– the nitrogen bases inside the molecule are ‘‘read” by another
enzyme and used to build two new “daughter” DNA strands
with complementary bases. A-T (adenine – thymine) & G-C
(guanine – cytosine)
– result, the two daughter molecules are both identical to the
parent molecule
– Each DNA molecule resulting from replication has one
original “parent” strand and one new “daughter” strand.
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DNA Replication is SemiConservative
• DNA replication of one helix of DNA results in
two identical helices
– original DNA helix is called the "parent" DNA
– two resulting helices are called "daughter" helices
• DNA replication is semi-conservative, meaning
that one parent strand is always passed on to the
daughter helix of DNA
Let’s Review
• Chromosomes are coiled structures made of …
– DNA and proteins.
• Chromosomes form
– after DNA replicates;
• prior to replication, DNA exists as …..
– chromatin.
• Human cells normally have 46 chromosomes, one from…
– each parent.
• The work of several researchers led to the discovery that DNA is
the genetic material.
• Griffith discovered the process of transformation ……
– DNA transferred from heat killed bacteria to live bacteria.
• Chargaff's rules state – base pairing rules
– amount of A = amount of T
– amount of G = amount of C.
• Watson and Crick discovered that DNA has a
– double helix shape, consisting of two polynucleotide
chains held together by bonds between
complementary bases. A-T, G-C
• DNA replication is semi-conservative …..
– half of the parent DNA molecule is present in each of
the two daughter DNA molecules.