1. Chapter #6 Cell Reproduction

2. Section 6-1: Chromosomes

3. A. Chromosome Structure 1)DNA (deoxyribonucleic acid) contains the information needed to direct a cell’s activities. 2)A gene is a segment of DNA that codes for a specific piece of information. 3)A chromosome is a rod-shaped structure that forms when a single DNA molecule & its associated proteins coil tightly before cell division.

4. A. Chromosome Structure 4)Chromosomes make a copy of themselves just before cell division. a) The two copies are called chromatids. b) The chromatids are attached by a protein disk at a point called the centromere.

5. A. Chromosome Structure 5)Each body cell contains two sets of each chromosome. a) The two sets are called homologues, or homologous chromosomes. You received one homologue from each parent. b) When a cell contains two homologues of each chromosome, it is termed diploid. This is represented as 2n. Humans have 46 chromosomes, so 2n = 46.

6. A. Chromosome Structure 6)Each sex cell contains just one set of chromosomes. a) Sex cells (eggs & sperm) are also called gametes. b) Gametes are haploid, meaning they only have one homologue of each chromosome. This is represented as n. Humans have 23 chromosomes in their gametes, so n = 23. 7)When an egg cell is fertilized by a sperm cell, a zygote is formed. The zygote contains two homologues of each chromosomes, so it is considered diploid (2n).

7. B. Chromosomes Affect Development 1) Each of the 46 chromosomes play an important part in determining how the body develops and functions. A person must have the correct number of chromosomes in his or her cells. 2) In most cases, humans who are missing even one chromosome do not survive the early stages of embryonic development. a) Monosomy = A diploid cell is missing a chromosome. b) Trisomy = A diploid cell has an extra chromosome. Down syndrome is an example of trisomy in chromosome #21.

8. B. Chromosomes Affect Development 3) A karyotype can be made to examine a person’s chromosome. 4) Trisomy & monosomy occur because of nondisjunction during the formation of the gametes. a) Nondisjunction occurs when the chromosomes fail to separate during cell division. b) As a result, one gamete will receive both homologues of a chromosome, while the other gamete won’t receive one at all.

9. B. Chromosomes Affect Development 5) Prenatal testing can be performed if parents are at risk for genetic diseases or nondisjunction disorders. a) Amniocentesis: A small amount of the amniotic fluid is removed. The fluid contains cells from the fetus that can be analyzed as a karyotype. b) Chorionic Villi Sampling: A sample of the chorionic villi is removed from the placenta. The cells can be grown in a lab and analyzed as a karyotype.

10. B. Chromosomes Affect Development 6) Alterations in chromosome structure a) Mutations: changes in an organism’s genetic material. b) Deletion: a fragment of a chromosome breaks off or is lost c) Duplication: a segment of a chromosome attaches to another chromosome d) Inversion: a fragment of a chromosome reattaches in the reverse orientation e) Translocation: a fragment joins a non-homologous chromosome

11. C. Chromosomes Determine Sex 1) 22 of the 23 pairs of human chromosomes are the same in males & females. They are called autosomes. 2) The chromosomes that are different in males & females are the sex chromosomes. They carry the genes that determine sex. We will later learn that the sex chromosomes also carry some traits not associated with sex.

12. C. Chromosomes Determine Sex 3) There are two sex chromosomes: a) The X chromosome is larger & looks like the letter X b) The Y chromosome is smaller and shorter 4) In humans, the genes that cause a fertilized egg to develop into a male are located on the Y chromosome. a) Any individual with a Y chromosome is male b) Any individual without a Y chromosome is female

13. C. Chromosomes Determine Sex 5) The X chromosome contains traits necessary for life, so males have one X and one Y chromosome and females have two X chromosomes, though there are some exceptions: a) XXY = Klinefelter’s syndrome b) XYY = Increased antisocial behavior c) XO = Turner’s syndrome d) XXX = Triple-X syndrome

14. C. Chromosomes Determine Sex 6) In some insects, such as grasshoppers, there is no Y chromosome. In such cases, females are XX and males are XO. 7) In birds, moths, & butterflies, the male is XX and the female is XO.

15. Section 6-2: Mitosis & Cell Division

16. A. Bacteria 1) Cell division in single-celled bacteria takes place in two stages: a) The DNA is copied b) The cell splits into two equal halves by a process called binary fission 2) Binary fission is a form of asexual reproduction that produces identical offspring. a) A new plasma membrane is added at a point on the membrane between the two DNA copies. b) The growing plasma membrane pushes inward and the cell is constricted in two. c) A new cell wall forms around the new membrane and eventually the dividing bacterium is pinched into two independent cells.

17. B. Eukaryotic cells undergo nuclear division. The life of a eukaryotic cell is traditionally diagramed as a cell cycle that consists of 5 phases shown in Figure 6-8 on page #127.

18. The Cell Cycle G 1 Phase 1) G 1 Phase --> This is the growth phase of the cell. It is where the cell grows rapidly and carries out its routine functions. For most organisms, this phase occupies the major portion of the cell’s life between divisions. S Phase 2) S Phase --> This is when the DNA is copied. At the end of this phase, and individual chromosome consists of two chromatids attached at the centromere.

19. The Cell Cycle G 2 Phase 3) G 2 Phase --> Preparations for nuclear division are made during this phase. Mitochondria and other organelles replicate. Microtubules are reassembled which will be used to form the spindle fibers during mitosis. M Phase 4) M Phase --> This is the phase in which mitosis occurs. Mitosis is the process by which the nucleus of a cell is divided into two nuclei, each with the same number and kinds of chromosomes. C Phase 5) C Phase --> This is when the cytoplasm divides during a process called cytokinesis.

20. C. What happens during mitosis and cell division? 1) A eukaryotic cell spends most of its life in the G 1, S, and G 2 phases, which are collectively called interphase. 2) As interphase ends & mitosis begins, the chromosomes begin to condense. Enzymes break down the nuclear envelope.

21. 3) Centrioles (in animal cells) start to separate and move to opposite ends of the cell. a) As the centrioles move apart, a network of protein cables, called the spindle, forms between them. b) Each cable is called a spindle fiber. 4) A second group of microtubules extends out from a region of the centromere of each chromosome called the kinetochore. It is a disk of protein that serves as a platform for assembling microtubules. a) The two sets of microtubules extend out toward opposite poles of the cell. b) Each set continues to grow until it makes contact with the pole of the spindle, with one chromatid attached to each pole. 3) Centrioles (in animal cells) start to separate and move to opposite ends of the cell. a) As the centrioles move apart, a network of protein cables, called the spindle, forms between them. b) Each cable is called a spindle fiber. 4) A second group of microtubules extends out from a region of the centromere of each chromosome called the kinetochore. It is a disk of protein that serves as a platform for assembling microtubules. a) The two sets of microtubules extend out toward opposite poles of the cell. b) Each set continues to grow until it makes contact with the pole of the spindle, with one chromatid attached to each pole.

22. 5) Once the microtubules are attached to the centromeres, the centromeres split, freeing the chromatids from each other. 6) Mitosis is now simply a matter of reeling in the chromatids, now considered chromosomes, to the poles. When they arrive, each pole has one complete set of chromosomes. 7) In the final step of cell division, a new nuclear envelope forms around each pole, forming two nuclei. Then cytokinesis takes place, and the cytoplasm divides in half. Two new cells have now formed. 5) Once the microtubules are attached to the centromeres, the centromeres split, freeing the chromatids from each other. 6) Mitosis is now simply a matter of reeling in the chromatids, now considered chromosomes, to the poles. When they arrive, each pole has one complete set of chromosomes. 7) In the final step of cell division, a new nuclear envelope forms around each pole, forming two nuclei. Then cytokinesis takes place, and the cytoplasm divides in half. Two new cells have now formed.

23. D. Stages of Mitosis Prophase 1) Prophase � The chromosomes begin condensing and become visible. The nuclear envelope begins to break down, and the network of spindle fibers becomes visible. Metaphase 2) Metaphase � The chromosomes move to the center of the cell and line up along the “equator”. Once in place at the equator, each chromosome is held in place by the microtubules attached to the kinetochore.

24. D. Stages of Mitosis Anaphase 3) Anaphase � The two chromatids physically separate when the centromere divides. The chromatids, which may now be called a chromosome, move toward opposite poles of the cell as the fibers attached to them shorten. Telophase 4) Telophase � The chromosomes uncoil and a new nuclear envelope forms. The spindle fibers break down and disappear. Mitosis is complete.

25. E.During cytokinesis, the cytoplasm is cleaved in half, and the cell membrane grows to enclose both cells. 1) Animal cells and other cells that lack cell walls are pinched in half by a belt of protein threads. 2) Plant cells have a rigid cell wall and a different strategy of cell division. a) Vesicles formed by the Golgi bodies fuse at the equator of the cell and form the cell plate, which is a membrane across the middle of the cell. b) A new cell wall then forms on both sides of the cell plate. E.During cytokinesis, the cytoplasm is cleaved in half, and the cell membrane grows to enclose both cells. 1) Animal cells and other cells that lack cell walls are pinched in half by a belt of protein threads. 2) Plant cells have a rigid cell wall and a different strategy of cell division. a) Vesicles formed by the Golgi bodies fuse at the equator of the cell and form the cell plate, which is a membrane across the middle of the cell. b) A new cell wall then forms on both sides of the cell plate.

26. Section 6-3: Meiosis

27. A. What Happens During Meiosis? 1) Meiosis is the mechanism that halves the number of chromosomes in cells. The diploid number is reduced to the haploid number. 2) Before meiosis, the DNA is copied, like in mitosis. 3) Meiosis consists of two divisions: a) Meiosis I � Homologous chromosomes separate into two cells b) Meiosis II � The two chromatids of each chromosome separate into two haploid cells

28. 4) One diploid cell that undergoes meiosis produces four haploid cells. a) In animals, meiosis often results in haploid gametes (see figure 6-11) b) In plants, meiosis often leads to spores, haploid cells that later lead to the production of gametes. 5) During meiosis, two unique events occur: a) In the beginning of meiosis I, homologous chromosomes pair up next to each other. While paired, the arms of the chromosomes exchange reciprocal segments of DNA in a process called crossing-over. b) Because there is only one replication of DNA but two divisions, meiosis halves the number of chromosomes, called skipping replication.

29. B. Meiosis I 1) Prophase I � The chromosomes condense, the nuclear envelope breaks down, the homologous chromosomes pair up & crossing-over occurs. 2) Metaphase I � The pairs of homologous chromosomes are moved by spindle fibers to the equator of the cell.

30. B. Meiosis I 3) Anaphase I � The homologues separate. The chromatids do not separate, but remained joined at the centromere. This is the stage in which nondisjunction can occur. 4) Telophase I � The individual chromosomes gather at each of the two poles. The cytoplasm divides, forming two new cells with half the number of chromosomes of the original cell. This is why meiosis I is often called “reduction division”.

31. C. Meiosis II 1) Prophase II � The centrioles in the newly formed cells split again and move to opposite poles. The spindle reforms. 2) Metaphase II � The chromosomes line up in the middle of the cell. The fibers attach to the centromeres.

32. C. Meiosis II 3) Anaphase II � The centromeres divide and the chromatids are pulled apart. Each one is now called a chromosome. 4) Telophase II � The chromosomes are at opposite poles of the cell. Each new cell has the haploid number of chromosomes. 5) Cytokinesis follows telophase II.

33. D. Crossing-Over 1) Crossing-over is an efficient way to produce genetic recombination. 2) As a result, the two chromatids of a chromosome no longer contain identical genetic material. 3) Crossing-over provides a source of genetic variation, which impacts the speed at which evolution occurs. (See Fig. 6-13)