recap

11.5 recap

Meiosis produces four daughter cells in which the chromosome number is reduced from diploid to haploid. Because of the independent assortment of chromosomes and the crossing over of homologous chromatids, the four products of meiosis are not genetically identical. Meiotic errors, such as the failure of a homologous chromosome pair to separate, can lead to abnormal numbers of chromosomes. Several important crop plants, such as wheat, are polyploid.

learning outcomes

You should be able to:

  • Compare and contrast the phases of mitosis with the phases of meiosis.

  • Justify the claim that genetic diversity results from recombination events during meiosis.

  • Use a model to illustrate how independent assortment of homologous chromosomes increases genetic diversity.

  • Explain how errors in meiosis can lead to unusual patterns in chromosomes.

  • Describe polyploidy and explain how it develops.

Question 1

How do crossing over and independent assortment result in unique daughter nuclei?

In crossing over, there is an exchange of some genetic material between non-identical chromosomes of a pair. So the resulting chromosomes carry new combinations of genes, which can be passed on to offspring in a gamete and fertilization. In independent assortment, it is random which chromosome of a homologous pair ends up in a particular gamete. So different gametes will usually have a different set of chromosomes; that is, chromosome 1 from the father, chromosome 2 from the mother, and so on. Fertilization therefore results in diploids, each of which has a different set of chromosomes.

Question 2

Compare prophase I of meiosis with prophase of mitosis. Compare anaphase I of meiosis with anaphase of mitosis.

Prophase I meiosis: Chromosomes have chromatid pairs attached, and the two homologs are lined up gene for gene beside one another.

Prophase mitosis: Chromosomes have attached chromatid pairs but are not lined up beside one another.

Anaphase I meiosis: Homologous chromosomes, each with two attached chromatids, separate and move to the poles.

Anaphase of mitosis: Chromatids separate and become single daughter chromosomes and move to the poles.

Question 3

Rarely, a person is born with an extra X chromosome (XXY). How can this arise by aneuploidy?

In the formation of male gametes, the X and Y chromosomes fail to separate in meiosis I anaphase. At the end of meiosis II, there will be two types of gametes: half without an X or Y, and half with X and Y. If the latter fertilizes a normal egg with a single X chromosome, the offspring will be XXY. However, if there is a similar nondisjunction in the formation of female gametes, there will be eggs with two X chromosomes. If the XX egg is fertilized by a normal, Y-containing sperm, the offspring will be XXY.

Question 4

What is polyploidy, and how does it arise?

Polyploidy refers to an extra set or more of chromosomes (e.g., 3n instead of 2n). It arises because of failure of all chromosomes to disjoin in meiosis in gamete formation, resulting, for example, in a diploid gamete.

An essential role of cell division in complex eukaryotes is to replace dead cells. What causes cells to die?