CELL REPRODUCTION

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 Sbacteriatransferred 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 Semi-Conservative • 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.

CELL REPRODUCTION