Chromosomes, Mitosis, Meiosis H. Smith

Terminology DNA exists in tightly wound structures called chromosomes. All animals have a set # of chromosomes. Chromosomes exist in pairs

Humans Humans have 23 pairs, or 46 chromosomes More chromosomes doesn’t necessarily mean an organism is more complex.

Why Chromosomes?

Cell Division 2 versions: – Mitosis: division of somatic (body) cells(46) – Meiosis: creation of gametic (sex) cells(23) – Cells with 46 chromosomes: diploid cells – Cells with 23 chromosomes: haploid cells

The Key Roles of Cell Division Mitosis: Development, Growth, & Repair of multicellular forms Meiosis: Creation of sex cells: Reproduction

The Mitotic Cycle Interphase  90% – G 1 = Grow, normal fx. – S = Synthesis of DNA – G 2 = Prepares for cell division Mitosis – PMAT Cytokinesis – cell pinches in 2

Interphase Growth Phase: DNA is replicated Nucleus is well defined in a nuclear envelope DNA is in the form of loosely packed chromatin fibers Accounts for 90% of cell cycle

Prophase Chromatin condenses into Chromosomes The nucleus and nuclear envelope begin to disappear Spindle forms Centrioles separate

Metaphase Chromosomes line up along the Middle Centrioles send out spindle fibers which attach to lined up chromosomes.

Anaphase Spindle fibers attach And pull chromosomes apart and to opposite sides of cell

Telophase Nuclear envelope forms at each pole Chromosomes uncoil Cells begin to divide.

Citokinesis Occurs after Mitosis is complete, is the division of the cytoplasm. End Product: 2 identical cells

Occurs within gonads (testes:ovaries) Meiosis produces sex cells – gametes (sperm:egg) Gametes have half the chromosomes (23) that somatic cells do (46) Meiosis reduces the number of chromosomes by one-half 5.6 Meiosis

Meiosis Meiosis I: normal division (like mitosis) Meiosis II: second division: results in haploid cells, with only 23 chromosomes: gametes

There are millions of possible combinations of genes that each parent can produce because of: – Random alignment of homologous pairs – Crossing over – Random Fertilization (70 trillion) Meiosis contributes to Genetic Variation

Crossing Over Homologous chromosomes line up and exchange segments of DNA.

*somatic cells *divide once  diploid *forms 2 identical cells *gametes *divide twice  haploid *forms 4 different cells (crossing over) mcgraw- hill.com/sites/ /stud ent_view0/chap ter12/animation s.html#

Controls in the Cell Cycle Checkpoints exist in the cell cycle Cell determines if cell is ready to enter next part of cell cycle 5.4 Cell Cycle Control and Mutation hill.com/olc/dl/120082/bio3 4a.swf

Cancer begins when the proteins that regulate the cell cycle don’t work, the cell divides uncontrollably – Mutations can be inherited or induced by exposure to U.V. radiation or carcinogens that damage DNA and chromosomes 5.1 What Is Cancer?

Chromosome terminology

Chromatin: loose DNA in the nucleus Unduplicated chromosome: Chromatid: tightly wound single copy of DNA segment Duplicated Chromosome: replicated, tightly wound DNA in nucleus, 2 copies of same DNA segment (made of 2 chromatids: sister chromatids)

Homologous Chromosomes You have 2 copies of each chromosome, one from Mother, one from Father. (they make up each “pair”) These matching chromosomes are called homologous chromosomes. The genes on these chromosomes will code for the same things (hair color) but will code for different versions (blonde vs. brown)

Crossing over Crossing over during meiosis (prophase I) occurs between these homologous chromosomes, leading to genetic variation.

Humans