DNA  DNA-Deoxyribonucleic Acid- molecule that contains genetic information.  Genes are segments of DNA. Chromosomes are strands of DNA.  In the 1950’s it was understood that DNA was the molecule that stored genetic information but the structure was unknown.

DNA

Structure of DNA  James Watson and Francis Crick discovered the structure of DNA.  Double Helix structure.  DNA and RNA are classified as nucleic acids.  Each strand of DNA is made of nucleotides- subunits of a nucleic acid.  Each nucleotide is made of a phosphate group, a five carbon sugar (deoxyribose), and a nitrogen-containing base.

Structure of DNA  There will be the same number of sugars, phosphates, and bases.  Sugar-phosphate and sugar-bases are held by strong covalent bonds.  Base pairs are held by weak hydrogen bonds.

Pairing between bases  Erwin Chargaff noticed that the amount of A=T and the amount of G=C.  In DNA Adenine is always paired with Thymine and Guanine is always paired with Cytosine.  One purine (two rings, A and G) is always paired with one pyrimidine (one ring-T and C).

Pairing between Bases  A - T, and G - C are complimentary base pairs  What is the significance?  The sequence on one strand will determine the sequence on the other strand.

DNA Replication "...It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” -Watson, J.D., and Crick, F.H.C "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" Nature, April 25, vol. 171: Watson, J.D., and Crick, F.H.C "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" Nature, April 25, vol. 171:

DNA Replication  DNA replication is the copying of DNA before cell division.  DNA Replication takes place in the S phase of the cell cycle before Mitosis and Meiosis  Jobs:  DNA is split.  Hold DNA apart.  Make new nucleotides.  Check for errors.

DNA Replication  DNA is split by an enzyme known as DNA helicase.  DNA is held apart by proteins at the replication fork.  The enzyme DNA polymerase moves along each strands, and adds the complimentary nucleotides until two new identical copies are produced. (semi-conservative)  DNA polymerase can backtrack to correct incorrect nucleotide base pairings.  lication/Dna%20Replication.htm lication/Dna%20Replication.htm lication/Dna%20Replication.htm

Meiosis  Meiosis is a form of cell division that produces gametes.  Homologous chromosomes are pairs that are similar is size, shape, and genetic information.  Before meiosis begins DNA is replicated.  Chromosomes are replicated and exist as two identically attached chromatids.

Stages of Meiosis I  Prophase I- Nuclear envelope breaks down, Homologous chromosomes pair, spindles form.  Metaphase I- Homologous chromosomes are moved to the equator.  Anaphase I- Spindle fibers separate the pairs of chromosomes, chromosomes are not split along the centromere.  Telophase I- chromosomes move to the poles, new nuclear membrane is formed, cytoplasm is divided.

Stages of Meiosis  Prophase II- new spindle forms, nuclear membrane breaks down.  Metaphase II- The chromosomes line up along the equator, attached at the centromere.  Anaphase II- each individual chromatid is pulled apart at the centromere  Telophase II- nuclear envelopes forms, spindle breaks down, cytoplasm splits, four haploid cells

Meiosis  Crossing over is the when a piece of a chromatid on one homologous chromosome is broken and exchanged with another, occurs in prophase I.  Spermatogenesis is meiosis in male testes that produce 4 sperm.  Oogenesis is meiosis in female ovaries that produces 1 egg and 3 polar bodies.