2. Recall : the 4 types of organic molecules Carbohydrates Lipids Proteins Nucleic Acids **** Monomer: Nucleotide Polymer: DNA/RNA

3. What is DNA? DNA : Deoxyribose Nucleic Acid FUNCTION : It’s primary function is to store and transmit information Specifically: How and when to make a protein STRUCTURE : Nucleotide (monomer) has 3 parts: - Sugar - Phosphate - Nitrogen Base

4. Discovery of DNA During the mid-1900’s, scientists all over the world were competing to see who could discover the structure of this genetic material In 1953, James Watson and Francis Crick published a paper suggesting a model for the structure of deoxyribonucleic acid They relied heavily (and somewhat unethically) on scientific evidence reported by other scientists Example: Rosalind Franklin’s X-ray crystallographic images “We’ve found the secret of life!” ~ Francis Crick (1916-2004)

5. Watson and Crick determined DNA to have the shape of a Double Helix (twisted ladder) Watson (left) and Crick (right)

6. More On Nucleotides… There are 4 different types of N-bases in DNA Therefore, there are 4 different DNA nucleotides Adenine (A) Guanine (G) ↑ PURINES These bases contain 2 carbon rings Cytosine (C) Thymine (T) ↑ PYRIMIDINES These bases contain 1 carbon ring The 4 different nucleotides differ only in their nitrogen-bases The sugar and phosphate groups are identical in each nucleotide

7. DNA vs. RNA DNA -Deoxyribose Sugar -Double-stranded - Bases A, T, G, C RNA - Ribose Sugar - Single-stranded - Bases A, U, G, C

8. Molecular Modeling: NUCLEOTIDES

9. Chargaff’s Data Originally it was thought that the bases A, T, G, and C occurred in all living things in the same repeated pattern, such as: … ATGC ATGC ATGC ATGC ATGC … If this were true, then DNA could not be the hereditary molecule because it would not be able to provide the variety needed for a molecule containing the genetic code. Following WWII, the research of biochemist Erwin Chargaff revealed the percentage of each base (A, T, G, and C) found in the DNA of various organisms. The following table contains some of his actual data…

10. Nitrogen Base Make-Up of Different Organismal DNA (by %) ORGANISMATGC Mycobacterium tuberculosis 15.114.634.935.4 Yeast31.332.918.717.1 Wheat27.327.122.722.8 Sea Urchin32.832.117.717.3 Marine Crab47.3 2.7 Turtle29.727.922.021.3 Rat28.628.421.421.5 Human30.929.419.919.8 Question: Do you observe any patterns in this data?

11. Complementary Base-Pairing DNA is said to have a “sugar-phosphate” backbone The rungs of the “ladder” are made of the N-bases The makeup of the rungs follows some specific rules: –Each rung contains 1 purine and 1 pyrimidine –A always pairs with T –G always pairs with C The pairing is the result of hydrogen bonding between the N-bases

12. TRY IT! Carefully copy the single-stranded sequence of DNA into your notebook. Then, directly below it, write the sequence of the complimentary strand that would bind to it. A C T G G C C T A T T → T G A C C G G A T A A G G T A C A A T G T A → C C A T G T T A C A T

13. DNA Replication Cells reproduce by dividing (in half) through the process of MITOSIS Before it divides, a cell must copy all of its DNA … WHY? How can this be done efficiently? (so that there are NO mistakes!!!) - The enzyme HELICASE unwinds the DNA molecule (hydrogen bonds break apart, creating 2 single DNA strands) - The enzyme DNA POLYMERASE binds to the single stranded DNA - DNA polymerase adds available nucleotides, one at a time, to the free strands (following Chargaff’s rules) - This results in two DNA molecules with identical base sequences - This does not happen at just one site, but at many sites simultaneously on the DNA molecule - Occasionally, the polymerase may make a mistake, changing the original base sequence (MUTATION)

14. CHROMOSOMES During mitosis (cell division), the DNA of eukaryotic cells is packaged into compact structures called CHROMOSOMES Chromosomes are: Rod-like structures Easily visible by microscopy DNA coiled around proteins Histones : Proteins that help maintain chromosome shape and tight packaging of DNA - Right before division, a chromosome consists of two identical halves - Each half is called a (sister) chromatid - The sister chromatids are joined by a structure called a centromere - When the cell divides, each daughter cell will receive one chromatid

15. Chromosome Numbers Each species has a characteristic number of chromosomes in each of its cells - Humans have 46 chromosomes - The fruit fly has 8 chromosomes - Horses have 64 chromosomes Diploid (2n) cells have 2 copies of each chromosome (1 from each parent) Haploid (1n) cells have only 1 copy In sexually reproducing organisms, a chromosome is either: Sex ChromosomeAutosome In humans, there are 2: XX = female XY = male In humans, there are 44 Actually, they are 22 pairs of homologous chromosomes

16. Chromosome Analysis KARYOTYPE A photomicrograph of all the chromosomes in a (dividing) cell Homologous pairs (the 2 copies of each chromosome a cell has) are arranged by size Used to analyze the genetic condition of an individual (that is, their sex and the possible presence of a disorder)