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Chromosome Structure and Cell Division

This chapter discusses the structure of chromosomes and the process of cell division, including mitosis and meiosis. It covers topics such as chromatids, centromeres, and the different phases of mitosis. Additionally, it explores the importance of meiosis in reducing the number of chromosomes in sex cells.

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Chromosome Structure and Cell Division

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  1. Chapter #8 Chromosomes

  2. Chromosomes • A Chromosome is a “rod” shaped structure of DNA and protein. • If a cell were = a basket ball, there would be about 40 miles of DNA in the cell.

  3. Chromosome Structure • During Cell division, the chromatin in the nucleus coils around histones making up a chromosome. • Histones are globular proteins that DNA coils around.

  4. Cont. • Chromosomes are composed of two main parts: • Chromatids: Each half of a chromosome is called a chromatid. (Formed as DNA copies itself) • Centromere: The center of the chromosome, where the chromatids are attached.

  5. Chromosome #s • Each species has a specific set of chromosomes. • Humans: 46 • Cat: 32 • Chimp: 48 • Dog: 78 • GoldFish: 92 • Fern: 1,262

  6. Types • Two Types of Chromosome: • Sex Chromosomes: Determine the sex of an organism. May also carry other genes. Humans = (Male XY, Female XX) • All other chromosomes are called autosomes and they control almost all other aspects of the human make-up.

  7. Huh? • Humans actually have 23 pairs of chromosomes. • 22=Auto 1=Sex (22+1=23 Pairs) • Where do we get the pairs? • 1 comes from your Mom and 1 comes from Dad. 1 + 1 = 2 and 2 makes a pair.

  8. What does it all look like? • We can make a Karyotype which is a micrograph of our chromosomes.

  9. The Cell Cycle & Mitosis

  10. Mitosis • Mitosis (Cell Division): Is the division of a cell into two identical (Daughter) cells. • The Cell Cycle contains Interphase and Mitosis • Mitosis involves • Prophase • Metaphase • Anaphase • Telophase

  11. Interphase • Cells spend most of their “lives” in interphase. • Interphase cells are doing their jobs. • G1: Growth • S: Copy DNA • G2:Preps

  12. Mitosis • Mitosis= Division of Nucleus • Continuous process divided into four phases (prophase, Metaphase, Anaphase, Telophase)

  13. Prophase • Chromatin becomes visable as Chromatin coils around histones making chromosomes. • The Nucleus begins to breakdown • Centrioles (anchors) begin to form outside of nucleus • Spindle fibers (microtubules) stretch from centriole to centriole. • Centrioles begin to move to the poles. • Magic chromosomes?

  14. Metaphase • Meta means Middle in Latin. • Shortest mitotic phase • Karyotypes are generally made from metaphase chromosomes. • Centrioles are at poles • Spindle fibers stretch from pole to pole • Chromosomes align at the equator of the cell.

  15. Anaphase • Anaphase is marked by separation • Spindle fibers shorten • Sister Chromatids are separated at the centromere and begin moving to opposite poles of the cell. • Flexible cell membrane begins to pinch in.

  16. Telophase • Final phase of Mitosis. • Chromosomes reach poles • Spindle fibers break down • Nucleus reforms around chromosomes • Cell completely pinches into two cells called daughter cells • Cell transitions into interphase and the process begins again.

  17. What About Plants • Plants are essentially the same except • They have not centrioles • Due to their inflexible cell wall, plant cells form a cell plate rather than pinch into two cells.

  18. Overview • The main function of mitosis is to produce 2 exact cells from 1. • To put it another way, Mitosis takes a 2n (diploid) cell and ends up with two 2n (diploid) cells

  19. Meiosis • The main goal of Meiosis is to reduce the number of chromosomes in a sex cell to 23 (In the case of humans). • We also call this reducing the n# from 2 to 1. • Recall: 2n = Diploid (two copies of every chromosome), 1n = Haploid (1 copy of each chromosome)

  20. Cont. • Why is it important to reduce the N# from 2 to 1? • If a 2n sperm cell fertilizes a 2n egg, the fertilized egg would then be 4n or have 4 copies of every chromosome. Not going to happen. • By reducing the cell to 1n, the following would happen. • 1n sperm fertilizes 1n egg and the resulting organism is a 2n organism like you.

  21. A little Stp-by-Step • The 1st phase of meiosis is just like mitosis (pro,meta,ana,telo) kind of. • The main difference is something called synapsis. • Synapsis is the pairing of homologous chromosomes. • What this means is Chromosome #1 from Mom lines up next to Chromosome #1 from Dad. This is called a Tetrad.

  22. Cont. • Tetrads are important because they allow for Genetic Recombination in the form of Crossing-Over. • This exchange of information allows for new mixtures of genetic information to be passed from generation to generation.

  23. Cont. • Ok so far we have a 2n cell with Tetrads happening. • Now we metaphase, Everyone (tetrad) to the middle and line up RANDOMLY in regards to the poles you are facing. • The randomness of this is called the Law of Independent assortment and it insures that a cell will have both maternal and paternal DNA.

  24. Cont. • Now we separate (Anaphase) and then we Telophase forming 2 new cells. • What is in these new cells? • 23 chromosomes (yes)……but two copies of each chromosome….why? • We always start with interphase which includes the “S” phase which….. • Copies the DNA.

  25. Where are we now? • We have taken a 2n cell and made 2 2n cells. • Normally in Mitosis we transition from telophase to…….interphase which includes a “S” phase. • Here is the key point to this entire process.

  26. Are you ready…. • Meiosis is made up of two cycles of Mitosis. • After telophase I, the cells transition into prophase II skipping interphase thus not allowing the cells DNA to copy. • Now think….we are now going to take a cell that has 23 pairs of chromosomes through meiosis II (mitosis again) but…. It cannot copy its DNA before we start. If you divide 46 chromosomes into two cells, what do you get? Two cells with 23 chromosomes or 1n cells.

  27. OK Here it is!!

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