Biology Exam 3 - Meiosis

Two important notes of Meiosis II:

1) No DNA replication occurs between Meiosis I and II. 2) Meiosis II proceeds simultaneously in both cells that formed from Meiosis I.

What is the end product of meiosis and how is it relevant for fertilization?

4 haploid gametes are produced. If the chromosome number wasn't reduced in meiosis, the chromosome number would double at fertilization and we would no longer be human.

How is Meiosis II similar to mitosis?

Both are nuclear division processes that separate sister chromatids into new nuclei.

Describe Prophase II:

Centrosomes produce spindle fibers that attach to the chromosomes at the centromere/to the kinetochore.

Describe Prophase I:

Centrosomes produce spindles and begin to move to opposite poles of the cell. Duplicated chromosomes condense. Homologous chromosomes align very close together and swap various segments (cross-over). The points at which segments are crossing over are called chiasmata. The nuclear envelope disintegrates enabling the spindles to attach to the chromosomes at each of their kinetochores.

Describe Metaphase II:

Chromosomes are jerked back and forth by the centrosomes until they line up at the metaphase plate.

Compare and contrast meiosis and mitosis (3 similarities, 7 differences).

Comparisons: • Both processes divide the nucleus after DNA replication. • A spindle forms during both processes. • Each nuclear division has a similar set of phases. Contrasts: • There are two nuclear divisions in meiosis, and one in mitosis. • Nuclei produced by mitosis are genetically identical; those produced by meiosis are not. • Mitosis produces 2 new nuclei; meiosis produces 4 new nuclei. • Mitosis maintains the chromosome number and occurs in somatic cells and, in human diploid cells, gives rise to new diploid cells. By contrast, meiosis goes through an extra set of steps in replication to give half of the original chromosome number. • Mitosis is the basis of tissue repair and growth. By contrast, meiosis is a part of the production of gametes: Each gamete (male and female) has half the chromosome number of somatic cells, so when they fuse into a zygote, the original chromosome number is restored. • Also, it is important to note that crossing over only happens during the very first step in meiosis, and not at all in mitosis. In Meiosis I, the parental chromosomes pair up and swap segments ("cross over") with each other, recombining and rearranging the genetic information. • During anaphase I, homologous chromosomes separate (this does not happen in mitosis). Mitosis is very similar to meiosis II.

What is the function of crossing over and at what phase does crossing over occur?

Crossing over swaps specific DNA segments that create new combinations of alleles in the offspring. The alleles are the basis of differences in traits. This occurs during Prophase I.

Describe homologous chromosomes and sister chromatids.

Homologous chromosomes: two chromosomes of the same type/number, 1 from each parent Sister chromatids: two identical DNA molecules that form after DNA replication and are attached at the centromere

Explain the behavior of chromosome sets during the human life cycle.

Humans have 23 sets of chromosomes that give us the chromosome number of 46. The chromosome number is halved in gamete formation. In reproduction, two gametes come together and fuse, bringing the chromosome number back to 46.

Describe Cytokinesis:

In animal cells, a cleavage furrow forms. In plant cells, a cell plate forms. Either way, this results in two haploid cells forming.

Describe the different ways in which meiosis and the sexual life cycle contributes to genetic variation and evolution.

Meiosis creates new combinations of traits. Sexual life cycle does too because gametes (which differ from one another) combine and cross over segments with other gametes (which differ from one another) at random. So offspring inherit different combinations of alleles and therefore different combinations of traits. These traits can be favorable or unfavorable: In cases of environmental change, the favorable traits will persist over time.

Describe Telophase II:

Nuclei form with each set of chromosomes.

Describe Telophase I:

Nuclei form with each set of duplicated chromosomes.

Describe Cytokinesis:

Occurs again, now giving four haploid daughter cells that are not genetically identical.

Describe Metaphase I:

The homologous chromosomes begin to be pulled back and forth by the microtubules. The chromosomes then line up at the metaphase plate.

Describe Anaphase I:

The homologous chromosomes separate from each other and move toward opposite poles, pulled by the shrinking spindle fibers.

Describe Anaphase II:

The sister chromatids of the chromosomes are pulled apart and pulled toward opposite poles of the cell, becoming individual chromosomes.