DNA Structure  Before the 1950’s, scientists had no idea what DNA looked like or what it was made of.  Who were the researchers who came up with the best model of DNA’s shape and structure?  Watson and Crick  What shape best describes DNA?  A Double Helix = two winding spirals

What is the monomer (or subunit) that DNA is made of?  The Nucleotide  Each nucleotide has three parts: Name them.  1. A Phosphate  2. A Deoxyribose (5-carbon sugar)  3. And a Nitrogen Base  How many bases are there?  4-- Name them.

Nitrogen Bases  G-C-A-T  Guanine  Cytosine  Adenine  Thymine

How do these nitrogen bases pair up?  They are…  Complementary. Define this.  Each nitrogen base has a match and will only pair up with that match.  G only bonds with.. CC  A only bonds with.. TT  REMEMBER “G-CAT”

Notice how the complements fit together like a puzzle.

DNA Structure  The Ladder Analogy: How do these parts look like a ladder?  What is the Backbone of the ladder?  The strong phosphate/sugar sides.  What make the rungs, or steps, of the ladder?  The nitrogen base pairs.

DNA contains a code. Where is the code located?  In the order of the nitrogen bases.  Why do we say it is “universal”?  Because this code is present in all living things but in a different order.  What sort of messages are carried by the code?  The code is a recipe for making proteins which carry out all the important work of cells that make life possible.  Examples: building muscle tissue, digesting food, all chemical reactions in the body.

What bonds hold the components of the DNA together?

The bonds between the sugars and phosphates are …  Covalent bonds  Weak or strong?  Strong  The bonds between the nitrogen bases are…  Hydrogen bonds  Weak or strong?  Weak

Now let’s talk about replication.  What is replication?  A process in which a copy is made of the DNA strand.  When in the cell cycle does this happen?  Before the cell can divide, it needs to double its DNA. This happens in the S (synthesis phase) of Interphase.

Name the steps in replication including the enzymes needed  STEP 1: Helicase breaks the hydrogen bonds between the nitrogen bases. This “unzips” or unwinds DNA into two strands. The nitrogen bases are now unpaired because they are separated from their complements.  STEP 2: DNA polymerase adds free nucleotides to the unpaired bases and new bonds form  STEP 3: DNA polymerase checks for and corrects errors in the base pairing.  END RESULT: Two identical molecules of DNA from the original.

Why do we call this process “semi-conservative”?  Conservation= saving and recycling  The original strands are re-used to make two identical molecules.  Each molecules will have a strand of orginal DNA and a strand of new DNA.  Now that replication has taken place, what happens next?  Cell division

What is Meiosis?  a process of cell division in which a reproductive cell divides into 4 haploid cells  How is it different from Mitosis?

MITOSISMEIOSIS Occurs in any type of body cell except reproductive. Results in diploid cells Has 4 phases PMAT End product= 2 identical cells Occurs in reproductive cells only. Results in Haploid cells. Has 8 phases with two separate cell divisions. End product= 4 cells that are genetically unique

Distinguish between: homologous chromosomes, sister chromatids, and tetrads.  Homologous chromosomes: chromosomes that are the same kind—in size, shape, and types of genetic info. They are not necessarily identical.  EXAMPLE(One Homologue may carry the blue eye color gene, while the other carries the brown eye color gene, but BOTH chromosomes carry an eye color gene)  Sister chromatids: two matching strands of DNA joined by a centromere  Tetrads: 4 chromosomes in Metaphase I  Two homologous pairs, side by side.

Name two sources of genetic variation in Meiosis.  1. Crossing over:  2. Random Alignment of Homologous chromosomes

What is crossing over?  The exchange of genetic material between two homologous chromosomes. This occurs during..  Prophase I.

What is the Random Alignment of Homologous Chromosomes?  The random order the homologous chromosomes line up in, side by side, on the equator during Metaphase I.  How is this “Independent Assortment”?  (Any one with any other one)  The homologous chromosomes can be on the left or right of the midline.  The combination of chromosomes on the left will be very different than the combination on the right which results in haploid cells at the end of Meiosis that are very different.

Meiosis can be divided into two cell divisions Meiosis I separates the Homologous Chromosomes At the end, two cells go on to Prophase II

Meiosis I: Separate the Homologues

Meiosis II  Meiosis II separates the sister chromotids.  The end result is 4 haploid cells

Compare: Haploid vs. Diploid DIPLOID HAPLOID Somatic cells reproductive cells 2 sets of DNA 1 set of DNA MOM + DAD combination Identical cells each is unique

Reproductive cells begin as:  Germ cells (cells that may germinate, like seeds, into a new organism)  When they are haploid cells they are called..  Gametes. Give examples of gametes:  Sperm and egg cells are gametes  When sperm and egg join, the result is..  a zygote (a fertilized egg)

Why is it important for Meiosis to produce haploid cells?  So that when sperm and egg join, the resulting zygote will have 2 sets of DNA and not 4.  Remember the law of segregation?  “have 2, pass 1”.  Meiosis allows for the passing of 1 set of DNA to the offspring

The zygote becomes… A baby-- Also known as “the offspring”

"You're unique. Just like everyone else..." -- Anon.  Explain how this quote relates to what we’ve learned about DNA.  Every living thing has a genetic code in the sequence of nitrogen bases.  You and every other organism are unique because there are differences inherited; These variations come from Meiosis and sexual reproduction: the reshuffling of genetic information in millions of combinations.