Unit 2 – Reproduction and Development Cellular Reproduction

1. Unit 2 – Reproduction and DevelopmentCellular Reproduction

2. Human Karyotype

3. Key features of a chromosome: • centromere (where spindle attaches) • telomeres (special structures at the ends) • arms (the bulk of the DNA).

5. Chromatin: the long fibers that form chromosomes and contain DNA, RNA and various proteins. Found in the nucleus of cells. • Chromosome: condensed chromatin structure formed when cells replicate (divide) (p.578) • Chromatid: one half of a chromosome. Two sister chromatids are joined by a centromere to form a chromosome

6. Chromatin and Chromosomes

8. Chromosomes come in 2 forms depending on the stage of the cell cycle: • The monad form consists of a single chromatid, a single piece of DNA containing a centromere and telomeres at the ends. • The dyad form consists of 2 identical chromatids (sister chromatids) attached together at the centromere. • Chromosomes are in the dyad form before mitosis, and in the monad form after mitosis. • The dyad form is the result of DNA replication: a single piece of DNA (the monad chromosome) replicated to form 2 identical DNA molecules (the 2 chromatids of the dyad chromosome).

9. More Chromosomes • Diploid organisms have 2 copies of each chromosome, one from each parent. The two members of a pair of chromosomes are called homologues. • Each species has a characteristic number of chromosomes, its haploid number n. Humans have n=23, that is, we have 23 pairs of chromosomes. Drosophila have n=4, 4 pairs of chromosomes.

10. Cell cycle

11. Cell cycle • a continuous sequence of cell growth and division • The cell cycle consists of two main stages: • 1. Interphase – growth phase; includes G1, S phase, and G2 • G1 (gap 1): cell carries out metabolic activities and prepares for cell division • S phase: DNA is replicated • G2 (gap 2): centrioles replicate and cell prepares for division

12. Cell cycle (continued) • 2. division stage – includes mitosis and cytokinesis; shortest stage • Different cells have different timing for their cells cycles; some take longer than others to go thorough their cycle, and they also spend different amounts of time in each stage.

13. Cell Cycle • M = mitosis, where the cell divides into 2 daughter cells. The chromosomes go from the dyad (2 chromatid) form to the monad (1 chromatid) form. • G1 = “gap”; where the cell spends most of its time, performing its tasks as a cell. Monad chromosomes • S = DNA synthesis. Chromosomes go from monad to dyad. • G2 = Dyad chromosomes, cell getting ready for mitosis. • G1, S, and G2 are collectively called “interphase”, the time between mitosis

14. Mitosis

15. Parent cell and daughter cells

16. Mitosis • division of the cell’s nucleus where the daughter cells receive the exact number of chromosomes and genetic makeup as the parent cell • In order for an organism to grow, repair, and maintain its function new cells are needed to replace old ones. • Each cell that undergoes mitosis produces 2 new cells. Mitosis allows the regeneration of damaged tissue (like cuts) and to replace worn out cells (like red blood cells) • Mitosis ensures that the same amount of genetic information is in each type of cell.

17. Mitosis • Mitosis is division of Somatic cells (body cells); not germ (sex) cell • Mitosis is ordinary cell division among the cells of the body. • During mitosis the chromosomes are divided evenly, so that each of the two daughter cells ends up with 1 copy of each chromosome. • For humans: start with 46 dyad chromosomes in 1 cell, end with 46 monads in each of 2 cells.

18. Cytokinesis • separation of the cytoplasm and the formation of two new daughter cells; cytokinesis occurs after telophase of mitosis • Parent cell – the original cell that divides during mitosis to form two new daughter cells • Daughter cells – the cells produced during mitosis of a parent cell

19. Stages of Mitosis • Before mitosis begins, DNA is replicated during interphase. • Stages: • Prophase • Metaphase • Anaphase • Telophase

20. Mitosis All Phases Of Mitosis

21. Interphase • The first part of the interphase stage is called gap 1 (G1); cells carry out metabolic activities to prepare for cell division. • S phase; DNA gets replicated. • gap 2 (G2); cells prepare to undergo division. • The division stage includes 2 processes: mitosis – the division of the nucleus, and cytokinesis – the division of the cytoplasm. These two processes are the shortest events in the cell cycle.

23. Prophase • During prophase, the chromatin coils and forms thick condensed chromosomes. • Each chromosome is made up of two sister chromatids (two DNA molecules that are identical to each other). These chromatids are held together by a centromere.

24. Prophase --chromosomes condense --nuclear envelope disappears --centrioles move to opposite ends of the cell --spindle forms

25. Metaphase • During metaphase, the spindle fibers attach to the centromere of the two replicated chromosomes. • The chromatids are then guided by the spindle fibers to the middle of the cell – the cells equator. • A spindle fiber from one pole is then attached to one chromatid and another from the opposite pole is attached to the other chromatids at the centromere.

26. Metaphase • chromosomes are lined up on cell equator, attached to the spindle at the centromeres

27. Anaphase • During anaphase, the centromere splits apart and the chromatids are pulled to the opposite poles of the cell by the spindle fibers. • The pull is caused by the shortening of the microtubules that make up the spindle fibers.

28. Anaphase • Centromeres divide (chromosomes are monads) • The monad chromosomes are pulled to opposite poles by the spindle.

29. Telophase • Telophase begins when the chromatids have all reached the opposite poles within the cell. • At this point each of the chromatids is a “single” – a non-replicated chromosome. • The chromosomes begin to unwind; the spindle fibers break down and disappear because they are no longer needed at this stage. • The nucleus reappears and a nuclear membrane forms around each new set of chromosomes.

30. Telophase • cytokinesis: cytoplasm divided into 2 separate cells • chromosomes de-condense • nuclear envelope re-forms • spindle vanishes

33. Meiosis • Meiosis is a special type of cell division that can occur only in reproductive organs. • Meiosis produces reproductive cells called gametes (either eggs or sperm). • These gametes are haploid cells, which means they contain only one copy of each type of chromosome that the diploid parent cell contains * Haploid – only one * Diploid – contains two

34. Meiosis • Takes 2 cell divisions, Meiosis 1 and Meiosis 2, with no DNA synthesis between. • In humans, start with 46 chromosomes (23 pairs) in dyad state. After M1, there are 2 cells with 23 dyad chromosomes each. After M2 there are 4 cells with 23 monad chromosomes each.

37. Meiosis

38. First Meiotic Division (M1) • Prophase of M1 is very long, with a number of sub-stages. • Main event in prophase of M1 is “crossing over”, also called “recombination”. • In crossing over, homologous chromosomes pair up, and exchange segments by breaking and rejoining at identical locations. • Several crossovers per chromosome, with random positions. This is the basis for linkage mapping.

40. More M1 • In metaphase of M1, pairs of homologous chromosomes line up together. In mitosis and M2, chromosomes line up as single individuals. • Anaphase of M1: the spindle pulls the two homologues to opposite poles. However, the centromeres don’t divide, and the chromosomes remain dyads. • Telophase of M1: cytoplasm divided into 2 cells, each of which has 1 haploid set of dyad chromosomes

41. Prophase I During prophase I, each pair of chromosomes that carry the same gene (homologous chromosomes) become aligned. These homologous pairs are called tetrads. During the pairing process, crossing over of chromosomes can occur, where non-sister chromatids exchange segments of chromosomes.

42. Metaphase I Metaphase I follows prophase I. This is when the spindle fiber attaches to the centromere of each chromosome. A spindle fiber from one pole attaches to one pair of sister chromatids and a spindle fiber from the opposite pole attaches to the other pair.The spindle fibers pull each tetrad to the equator, but they do not line up like in mitosis.

43. Anaphase I During anaphase I, the homologous chromosomes separate and move to the opposite poles of the cell. They are then pulled apart by the shortening of the spindle fibers. However, here they do not split as they did in mitosis and so the sister chromatids stay together. This means that only one chromosome from each pair will move to each pole of the cell.

44. Telophase I When telophase I occurs, cell division goes directly to meiosis II. If telophase does not occur, the homologous chromosomes begin to uncoil and then the spindle fibers disappear. The cytoplasm gets divided, and nuclear membrane forms around each group and two cells are formed. * Telophase I doesn’t occur in all cells

45. Meiosis II The phases of meiosis II are the same as mitosis. The two cells from telophase I go through prophase II , Metaphase II, anaphase II and telophase II. Each of these cells beginning meiosis II is haploid but they do consist of replicated chromosomes. At the end of meiosis II, the daughter cells are still haploid but each cell contains single, un-replicated chromosomes. The daughter cells will develop into gametes in animals and either gametes of spores in plants.

47. Second Meiotic Division (M2) • Meiosis 2 is just like mitosis. • In prophase, the chromosomes condense and the spindle forms. • Metaphase of M2: dyad chromosomes line up singly on the cell equator. • Anaphase of M2: centromeres divide, chromosomes are now monads which get pulled to opposite poles. • Telophase: cytoplasm divided into 2 cells. • After M2: total of 4 cells from the original cell. Each contains one haploid set of monad chromosomes

49. Identify the Stage