3. j) Each daughter did get a copy of each gene on the chromosomes, if this did not happen then there would be genetic disorders occurring. k) Humans cannot be cloned easily because mitosis and meiosis are not simple processes. In fact, there are literally hundreds of genes that are needed to control the many steps that are involved and to be sure that those steps occur in the correct order. Any mistakes can be deadly. For example, it is necessary to the cell’s survival that the DNA gets replicated and the chromosomes are separated equally so that all daughter cells get the correct number of chromosomes. It is also necessary to control the frequency at which cells divide; if cells divide more often than they should, then cancerous tumours are often the result. l) In mitosis, a single cell divides to form two daughter cells. These cells grow, and then divide to form a total of four cells. Those four cells grow and divide to form eight cells, etc. In meiosis, a single cell divides twice, resulting in four daughter cells that do not grow and divide again. Instead, these cells are modified to become eggs or sperm in humans.
MEIOSIS 4. m) One of the most important requirements of a successful mitosis or meiosis is that the blueprint of life, the DNA, is provided in equal amounts to each of the daughter cells. The DNA in all human cells except eggs and sperm is housed in 46 chromosomes. Eggs and sperm consist of only half that number, 23 chromosomes per cell. Every person inherits one set of chromosomes from their mother (in the egg) and the other set of chromosomes from their father (in the sperm). It is imperative that when mitosis and meiosis are complete, the appropriate number of chromosomes exists in each cell. If there are extra or missing chromosomes, the cells usually do not live. To ensure that there are enough chromosomes to be divided evenly between the two daughter cells during mitosis, an essential step precedes cell division that allows the chromosomes to be copied, a step called DNA replication. In mitosis, the DNA is replicated, the chromosomes are separated into two equal sets, and the cell divides. These steps must occur in this exact order or the resulting cells will have an incorrect number of chromosomes and may not live.
n)
o) Meiosis occurs in, if you are male, the testes. If you are female, these are the ovaries. p) In plants, meiosis occurs in, the male sex organs, the anthers and the female gonad is the ovary.
5.
q) In step 5, during prophase the long threads of chromatin slowly shorten and become thicker. They are now visible as chromosomes. Each chromosome has two identical parts because the DNA is duplicated and both parts are still together. The membrane around the nucleus disappears in the cytoplasm. Also during prophase, a structure called a spindle begins to form. A spindle is shaped like a football and the ends are called poles. There are thin fibers that run between the poles of the spindle.
During metaphase, the chromosomes line up in the middle of the spindle. They line up side-by-side in a straight line. The chromosomes attach to the fibers at their midpoints
At anaphase, the two identical halves of the chromosomes split apart. They move away from each other toward the opposite poles of the spindle. When the chromosome halves reach the poles, anaphase is over. There is the same number of chromosomes at each pole of the spindle. This number is the same number a complete cell is supposed to have.
r) If the pairs of chromosomes did not separate, as they do in anaphase, then the genetic information would not be correct when the cells split.
6. The chromosomes in the cell nucleus are grouped in pairs - 23 pairs in all. One member of each pair has come from the mother of the individual and the other member from the father. Of the 23 pairs, 22 are essentially identical to each other. The 23rd pair is either a pair of X chromosomes, meaning the individual is female, or an X chromosome and a Y chromosome, meaning the individual is male.
When humans produce sperm and eggs, the chromosome pairs separate and the egg or sperm receives only one member of each pair. Each sperm produced by a man has 22 chromosomes plus either an X chromosome or a Y chromosome. Each egg produced by a woman contains 22 chromosomes plus an X chromosome. Before the chromosomes separate in this way the pairs swap pieces of their DNA with each other. In women this process happens with all the chromosome pairs including the double X pair. However, because the X and the Y chromosomes in a man are so different they swap almost no DNA with each other when they separate. When the sperm and the egg join together in fertilization the individual chromosomes pair up again.
New cells for growth and repair are made by mitosis which makes two identical new cells. These contain the same chromosome number. Asexual reproduction also involves mitosis. The other kind of cell division is called meiosis and it makes sex gametes. Each cell divides to make four new cells with half the number of chromosomes.