16. What is crossing over? When does it happen? Why is it important? 17. By whic
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16. What is crossing over? When does it happen? Why is it important? 17. By which mechanism somatic cells duplicate? 18. What is the diploid (2N) number of chromosomes? What cells have this number? 19. What is the haploid (N)number of chromosomes? What cells have this number? 20. What are the 4 stages of mitosis? Describe what happens during each one. 21. What is cytokynesis? How animal and plant cells differ in their cell divisions? 22. What are sister chromatids? What is a centromere? 23. What is a mutation? What causes mutations? 24. What can happen when the control for cell division is lost? When the DNA is damaged? 25. What is an aneuploidy? How does it happen? Give an example of an aneuploidy 26. What is cancer? What is a metastasis? Give examples of some known carcinogens By which mechanism do we make a gamete ? 2 0 2 Mitosis Meiosis here does it happen? Vhat type of cells are roduced? When DNA replicates? How many divisions?Explanation / Answer
16.
The process of homologous chromosomes exchanging segments that result in a re-arrangement of genetic information.
It happens between prophase-1 and metaphase-1.
For every gamete to be completely genetically unique.
17. Mitosis. Meiosis
18. The chromosomes are present in pairs. Somatic cell.
19. Half number of parental chromosomes are present in gametes produced during meiosis. Gametes.
20.
Prophase: The first and longest stage of mitosis. In this stage the chromosomes condensed and become visible. The centrioles separate and move to opposite poles of the cell and start formation of spindle fiber.
Metaphase: In this stage the chromosomes line up across the center of the cell called as metaphase plane. All centromere are connected to the spindle fiber at their centromere.
Anaphase: In this stage the sister chromatids separate into individual chromosomes and are pulled towards opposite poles.
Telophase: During this stage the chromosomes gather at opposite ends of the cell and lose their distinct rod-like shapes. Two new nuclear membranes then form around each of the two regions of DNA and the spindle fibers disappear.
21. With two complete copies of the DNA now in two different regions of one cell, the cell membrane will pinch and divide the cytoplasm in half. The result is two individual cells that are identical to the original cell. Each of the two new cells have a complete copy of the DNA and contain all of the organelles that the original cell had.
In plants: by cell plate formation at the center of the cell
In animals: cell furrow method.
22. When DNA is replicated, identical copies (chromatids) formed by the replication of a chromosome, with both copies joined together by a common centromere.
It is the region of attachment of spindle fiber during cell division. The point of attachment is called kinetochore.
23. A sudden change takes place in nuclear material, which may be changing the structure of chromosome, increasing or decreasing number of chromosome or changing the sequence of DNA.
Cause: mustard gas, chemicals, radiation (UV, X, beta, Gamma ray)
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