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Cellular Reproduction

This article explains why and when cells divide, the role of chemical signals in cell division, and the process of mitosis. It also explores the importance of DNA replication and the formation of chromosomes during cell division.

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Cellular Reproduction

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  1. Cellular Reproduction • How to preserve genetic information.

  2. When and why do cells divide? • Cells divide when there is a chemical signal to do so. • Skin cells may divide in response to crowding. Certain cells send out a chemical signal that tells neighboring cells to divide. • Cells may divide in response to an injury, to mend damaged tissue. • Growth factors can signal cell division in children to lengthen bones and add other tissues.

  3. Specialized cell membrane proteins signal cell division when growth factors are present. growth factor Growth factor binds to receptor and stimulates cyclin synthesis. growth factor receptor (plasma membrane) cyclin Cyclin activates Cdk; active Cdk stimulates DNA replication. cyclin- dependent kinase (cytoplasm) Cdk’s are always present in the cell.

  4. Mitosis • One-celled eukaryotic organisms, and individual cells in a multi-cellular organisms, reproduce by mitosis followed by cytokinesis.

  5. The problem Eukaryotic cells are often diploid: that is, they have two of each kind of chromosome.

  6. W O R K T O G E T H E R • Why do eukaryotic cells have TWO of each chromosome? • If a cell with two of each chromosome divides, how many chromosomes should end up in each of the two daughter cells?

  7. A cell with 14 chromosomes divides by mitosis. How many chromosomes will each daughter cell have? • 28 • 14 • 7 • Each daughter cell will have a different, unpredictable number.

  8. When is DNA replicated? • Just before cell division. • In preparation for transcription and translation. • Any time.

  9. Overview of Mitosis • After DNA is replicated, it is condensed into chromosomes and identical copies are sorted in the process of mitosis. • Mitosis assures that the two daughter cells have exactly the sameDNA.

  10. Warning: Confusing terminology ahead! After cell division, the single strand is a chromosome again. (Again, think of it as a one-chromatid chromosome.) Before cell division, a strand of DNA is a chromosome. (Think of it as a one-chromatid chromosome.) During cell division, two identical copies of a DNA strand link together into a two-chromatid chromosome.

  11. Normal cell processes, including protein synthesis (transcription and translation). telophase and cytokinesis G1: cell growth and differentiation anaphase metaphase prophase mitotic cell division G0: nondividing G2: cell growth Under certain circumstances, cell may return to cell cycle. interphase S: synthesis of DNA; chromosomes are duplicated The cell prepares for division, which includes making specialized proteins (more transcription and translation). DNA is replicated ONLY during S-phase. Once replication has happened, the cell MUST divide. Animated cell cycle at http://cellsalive.com

  12. Prior to Mitosis, DNA is replicated during the S-phase of the cell cycle. Chromosomes appear late in G2 phase, just prior to mitosis. How many cells in this slide of an onion root tip are actually dividing? How can you tell?

  13. INTERPHASE nuclear envelope chromatin nucleolus centriole pairs Late Interphase Can we tell if a cell in Interphase is in G1, S, or G2 of the cell cycle?

  14. DNA (2 nm diameter) histone proteins nucleosome: DNA wrapped around histone proteins (10 nm diameter) coiled nucleosomes (30 nm diameter) chromosome: coils gathered onto protein scaffold (200 nm diameter) protein scaffold DNA coils A strand (double helix) of DNA wraps around histoneproteins to form chromosomes. This protects DNA from damage during cell division.

  15. genes centromere telomeres The structure of a condensed chromosome (before pairing).

  16. W O R K T O G E T H E R • Make a prediction: during chromosome formation, which will pair up? • Homologous chromatids that carry the same genes but different alleles? • Sister chromatids that carry the same genes and the same exact alleles? • (Hint: the end goal is to get identical genetic information into the daughter cells.)

  17. gene 1 gene 2 same alleles different alleles Homologouschromosomes are those that carry the same genes but may have slightly different information (such as dominant or recessive versions of a gene). Homologous chromosomes do not pair together. Chromosomes only pair with their identical sister chromatids.

  18. sister chromatids centromere Identical (sister) chromatids pair up during Prophase, and join at a pinched-in point called the centromere.

  19. duplicated chromosome (2 DNA double helices) sister chromatids The chromosome at the end of Prophase consists of two strands of condensed DNA. Each sisterchromatid carries exactly the same information.

  20. Which of these are genetically identical? • Homologous chromosomes • Sister chromatids • Neither of these

  21. Which of these will pair up to form chromosomes in mitosis? • Homologous chromosomes • Sister chromatids • Chromatids pair at random, so it’s impossible to predict.

  22. MITOSIS: Prophase condensing chromosomes beginning of spindle formation Notice that these cells in prophase have barely visible chromosomes as DNA begins to condense.

  23. MITOSIS: Metaphase spindle microtubules Chromosomes, with their paired identical chromatids, move to the center of the cell.

  24. MITOSIS: Anaphase "free" spindle fibers Identical chromatids separate from one another and migrate to opposite poles of the cell.

  25. MITOSIS: Telophase chromosomes extending nuclear envelope re-forming Telophase completes Mitosis. Both poles of the cell now have identical DNA, and the cell can divide in half.

  26. MITOSIS: Cytokinesis After Mitosis has finished sorting the chromosomes, cytokinesis takes place, dividing the cell into two new cells.

  27. INTERPHASE Before S phase, the cell was diploid (two copies of each chromosome). After cytokinesis, are the cells diploid or haploid?

  28. The process of cytokinesis 1 Microfilaments form a ring around the cell's equator. 2 The microfilament ring contracts, pinching in the cell's “waist.” 3 The waist completely pinches off, forming two daughter cells.

  29. Cytokinesis in plant cells Golgi apparatus cell wall plasma membrane carbohydrate- filled vesicles 1 Carbohydrate-filled vesicles bud off the Golgi apparatus and move to the equator of the cell. 2 Vesicles fuse to form a new cell wall (red) and plasma membrane (yellow) between daughter cells. 3 Complete separation of daughter cells.

  30. In which phase do chromosomes condense? • Prophase • Metaphase • Anaphase • Telophase

  31. In which phase do sister chromatids separate from one another? • Prophase • Metaphase • Anaphase • Telophase

  32. In which phase do chromosomes line up in the middle of the cell? • Prophase • Metaphase • Anaphase • Telophase

  33. Meiosis • Meiosis is cell division that involves the reduction of chromosomes in a cell.

  34. The problem: • When diploid organisms reproduce sexually, two cells must fuse and share genetic information. • The end result of sexual reproduction is a new diploid organism that has genetic information from both parents.

  35. 2n n meiotic cell division 2n 2n n fertilization diploid parental cells haploid gametes diploid fertilized egg The cells from the parents must be haploid if their offspring is to be diploid. While diploid cells hold two copies of each chromosome (one from each parent), haploid sex cells hold one copy of each chromosome.

  36. homologous chromosomes sister chromatids Meiosis is reduction division. It begins with a diploid cell and produces haploid cells. Why does it produce four haploid cells?

  37. Meiosis also involves the cell cycle, and takes place after S phase of the cell cycle. DNA is replicated before meiosis. G1: cell growth and differentiation mitotic cell division G0: nondividing G2: cell growth Under certain circumstances, cell may return to cell cycle. interphase S: synthesis of DNA; chromosomes are duplicated

  38. MEIOSIS I Homologous chromosomes pair and cross over. Homologous chromosomes line up in pairs. Homologous chromosomes move to opposite poles. paired homologous chromosomes recombined chromosomes chiasma spindle microtubule (a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I First half of meiosis: separation of homologous chromosomes.

  39. Prophase I Homologous chromosomes pair and cross over. paired homologous chromosomes spindle microtubule chiasma Notice that four strands — maternal and paternal chromosomes and their identical sister chromatids — join into a single unit, called a tetrad.

  40. protein strands joining duplicated chromosomes direction of “zipper” formation Protein strands “zip” the homologous chromosomes together.

  41. While in tetrads, homologous chromosomes often swap ends, further mixing up genetic information. recombination enzymes chiasma chiasma Recombination enzymes bind to the joined chromosomes. Recombination enzymes snip chromatids apart and reattach the free ends. Chiasmata (the sites of crossing over) form when one end of the paternal chromatid (yellow) attaches to the other end of a maternal chromatid (purple). Recombination enzymes and protein zippers leave. chiasmata remain, helping to hold homologous chromosomes together.

  42. Metaphase I Homologous chromosomes line up in pairs. Tetrads line up in the center of the cell. recombined chromosomes

  43. Anaphase I Homologous chromosomes move to opposite poles. Because homologous chromosomes separate (instead of identical sister chromatids), each pole of the cell gets a full set of chromosomes but different genetic information.

  44. MEIOSIS II (e) Prophase II (f) Metaphase II (g) Anaphase II (h) Telophase II (i) Four haploid cells Meiosis II begins immediately after Meiosis I, with a short rest in between (no interphase in between). In Meiosis II, sister chromatids separate from one another.

  45. Anaphase II Metaphase II In both cells, chromosomes line up in Metaphase II so that sister chromatids can separate in Anaphase II.

  46. Telophase II End The result of meiosis is four haploid cells. Each has one copy of each chromosome, which may carry different versions of the same genes. Each gamete (sex cell) can have different genetic information.

  47. In prophase I of meiosis: • Sister chromatids pair up. • Homologous chromatids pair up. • Homologous chromosomes pair up. • Nothing pairs up.

  48. In anaphase I of meiosis: • Tetrads split, homologous chromosomes migrate to opposite poles. • All four chromatids migrate to four quarters of the cell. • Chromatids are randomly distributed.

  49. The end product of meiosis is: • Two identical cells. • Two non-identical cells. • Four identical cells. • Four non-identical cells.

  50. Each cell produced by meiosis is: • Diploid • Haploid • Tetraploid • A random ploidy

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