Biology – Chapter 8

1. Biology – Chapter 8 Unit 8: The Cellular Basis of Reproduction and Inheritance.

2. The center of the cell not only controls the cells activities but it also initiates and controls the time when your cell’s should reproduce.

3. During cell reproduction, a cell will divide into two identical cells.The original cell is called a “parent” cell while the two new resulting cells are called “daughter” cells.

4. Reproduction of single cell organisms occur through a process known as asexual reproduction. • Asexual reproduction does not require fertilization of an egg by a sperm.Offspring that are produced asexually can allow newly formed daughter cells to inherit their chromosomes from one parent cell.

5. On the other hand, sexual reproduction does involve the fertilization of an egg by a sperm. This process involves a special type of cell division/reproduction called meiosis which only occurs in reproductive organs. .

6. Before we can talk about the cell cycle and cell division let’s talk about chromosomes.

7. Chromosomes are coiled up strands of DNA that contain the genetic material required by the cell. • These chromosomes are found within the nucleus of all plant and animal cells. • These chromosomes will play an important role in the development of genetic characteristics.

8. Chromatin • Chromosomes are long thread like structures that are only visible under the microscope. • In non-dividing cells, chromosomes are seen as a combination of DNA and protein molecules called chromatin. As seen in the diagram to the right.

9. Notice that inside each chromosome is one long strand of DNA super coiled up around special proteins called histones. • The 46 human chromosomes found in each one of our cells contain an estimated 25 thousand genes.

10. The passing on of genetic information from one cell to another occurs when a parent cell divides into two daughter cells. Mitosis is responsible for creating body cells that are “diploid” because they contain the full compliment of chromosomes. Humans have 46 chromosomes per cell. The cell cyclediagram to the right shows the process your body cells continually go through.

11. The Cell Cycle consists of two major phases, interphase and mitosis. • Interphase is the first step of the cell cycle where the chromosomes make identical copies of themselves. This replication process creates an exact set of instructions to be passed on to the new cell. • During the mitotic phase the duplicated chromosomes will now divide evenly among the two daughter cells.

12. The first step in the cell cycle is called Interphase. This is the longest phase of the cell cycle and represents a normal nondividing cell. During interphase the cell grows during the G 1 phase, makes an identical set of chromosomes during the S phase (replication) and forms an additional set of organelles during the G 2 phase.

13. During the process of mitosis 4 additional steps take place. It begins with a stage called Prophase where the nucleus disappears and spindle fibers attach to the chromosomes that are now visible. • Inside the nucleus the chromatin fibers transform from a disorganized group of fibers to thick organized strands called chromosomes viewable with a light microscope.

14. In the final part of prophase the nuclear membrane breaks apart.This will allow the microtubule spindles to reach out and connect to the chromosomes. • These protein fibers will help move and distribute the chromosomes.

15. During Metaphase, the chromosomes are positioned along the middle of the cell. Refer to the diagram to the left for the correct placement of the chromosomes.

16. During Anaphase, the chromosomes are pulled to the opposite ends of the cell. As a result, an equal amount of genetic information/chromosomes will be at each end of the cell or the”poles”.

17. During Telophase, each cell will have received one half of the chromosomes. • Telophase is the direct opposite of prophase. • During Telophase, the nuclear membrane begins to reform as the chromosomes unravel. • The final stage of cell division is called cytokinesis. During this stage the cytoplasm and organelles are divided between the two daughter cells.

18. The process where sexual reproduction of cells occur is called Meiosis. Meiosis is responsible for creating sex cells/gametes that contain only one half the chromosomes. These gametes therefore are “haploid” (contain one half the chromosomes) and are also genetically different from the parent cell. • During the life cycle living multicellular organism move through a stages of life leading from the adult stage of one generation to the adult stage of the next generation. Chromosomes also follow through from one generation to the next however with slight alterations between the generations.

19. Humans are often known as diploid organisms because all of our cells contain two identical chromosomes. • The total number of chromosomes in humans are 46. The exception to his are the egg and sperm cells which are known as gametes.

20. Gametes, made by a process called meiosis, contain a single set of chromosomes. To be exact, 22 autosomes and a single sex chromosome, either X or Y.

21. A cell that has a single chromosome set is known as a haploid cell. • During sexual intercourse a haploid sperm cell fuses with the haploid egg cell during the process called fertilization. The results of this fertilized egg is called a zygote. This zygote now is a diploid organism containing two sets of chromosomes, one set from each parent. • The figure below demonstrates how meiosis creates chromates used in the production of chromosomes for fertilization.

22. The differences between Mitosis and Meiosis can be viewed in the diagram below. As seen in the process of meiosis below the parent cell will first create two haploid cells and then in the Meiosis II the haploid cell divides again to create two more haploid daughter cells.

23. The figure to the left illustrates one way the meiosis process can contribute to a variety in the genetic makeup of a zygote.

24. When a man and a woman produce a diploid zygote (a child) there is roughly 64 trillion combinations of chromosomes that can take into effect. • Chromosomes can also exchange segments between each of them during the prophase of meiosis. This crossing over process is demonstrated in the diagram to the left.

25. Sometimes during the meiosis process an error can occur. Chromosomes can make an extra copy or can forget to make an extra copy. • In the figure below, there are three number 21 chromosomes. This condition is often called trisomy 21.

26. A person who has a trisomy 21 chromosome condition is said to have Down Syndrome. • About one out of every 700 children born in the United States are born with trisomy 21. • Trisomy and Down syndrome can sometimes be the results of women have children later and later in life (see graph to the right).

27. A second type of chromosome accident that can occur during meiosis is called nondisjunction. • During nondisjunction, a chromosome pair fail to separate at anaphase. The results is an abnormal number of chromosomes when gametes are produced.

28. Nondisjunction does not influence the number of autosomes such as chromosomes 21, however, nondisjunction does effect the number of sex chromosomes. • An abnormal number of sex chromosomes (extra X’s and Y’s) can cause a number of different syndromes.

29. Alternation of chromosomes structure can be various. Parakeets, like many living organisms (including humans) inherit a wide variety of characteristics from previous generations.

30. Most often a pregnant women will be asked to have a ultrasound scan of their baby. An ultrasound scanner can produce high frequency sound waves. These sound waves bounce off the fetus and the echoes produce an image on a television monitor (see figure below). • To determine whether certain characteristics or chromosomes have been passed from one generation to another human beings can complete two tests: an Amniocentesis or a Chorionic Villus Sampling (CVS).

31. An amniocentesis process pulls fluid from the women’s uterus. With this fluid doctors can determine whether there are abnormalities in the chromosomes through a process of karyotyping. This amniocentesis karyotyping takes several weeks to produce results.

32. The second type of test to determine abnormalities in the chromosomes is through a chorionic villus sampling. During the CVS process fetal cells are taken from a small piece of fetal issue located on the placenta. By acquiring these cells from the placenta the doctor completing the karyotyping will only need a few hours rather than days to determine the results.

33. Alternative forms of genes that can be produced from a parent are called alleles. • Flowers and green plants that produce offspring like that of the pea plant normally have only two alleles. • Human blood often has multiple alleles. The three alleles for the characteristic of ABO blood type can produce four phenotypes. These four phenotypes are O,A, B, or AB (Also refer to as blood types).

34. Another example of alteration in alleles is in human hypercholesterolema. Here the dominate allele (HH) specifies a cell surface protein called an LDL receptor. Heterozygotes (Hh) have only half the normal number of LDL receptors. Finally, homozygotes (hh) have none. Have little or no LDL receptors can allow for dangers levels of LDL to build up in the blood.

35. People who have homozygous for the sickle-cell allele will have sickle-cell disease. Sickle-cell disease will cause red blood cells to become sickle shaped. Sickle-cell disease can often lead to other health problems (see figure to the left).

36. Finally, color blindness, hemophilia and many other disorders can be determined by sex linked genes passed along by both men and women. Above: A common test for red-green color blindness.