CELL REPRODUCTION

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 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 t ransformation …… –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.