Meiosis and Sexual Reproduction

Life Cycle

A life cycle describes the various stages that an organism passes through from conception to reproduction.

Cytokinesis

Formation of four (4) daughter cells Each with half the number of chromosomes as the parent cell

Gametogenesis

Gametogenesis is a general term that refers to the production of gametes such as sex cells, i.e. sperm cells and egg cells. In the sex organs of organisms there are specialized cell masses that undergo meiosis to produce cells that have half the number of chromosomes, i.e. haploid (n), as normal diploid (2n) somatic cells. This is the first stage of gametogenesis. These newly-formed haploid cells pass through a number of subsequent steps that modify their anatomy and physiology to permit them to fully function in the process of fertilization. These subsequent steps comprise the full process of gametogenesis.

Metaphase I

Homologous chromosomes held together by chiasmata Chromosomes move to central plane of cell Random orientation of paternal and maternal chromosome homologues

Anaphase I

Homologues of each chromosome pair separate and move to opposite ends of cell Independent assortment of paternal and maternal homologues Each separated chromosome still consists of two sister chromatids Arms of sister chromatids may be different from each other due to crossing over events during prophase

Gametes

Meiosis is the cell reproduction process that is responsible for the production of very specialized cells, called gametes (e.g. sperm cells and egg cells), that function in the sexual reproduction process.

The Phases of Meiosis

Meiosis, like mitosis, is a process of cell division that involves a number of stages that can be characterized by certain observable cellular events, or phases. Many of the events in meiosis are identical to those of mitosis. There are some very significant differences between mitosis and meiosis, most notable of which is that meiosis consists of two cell division events, as evidenced by the Roman numeral designations after the names of the phases in the list below. This and other significant differences are discussed later in this lesson. The phases of meiosis are as follows: Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

Meiosis and mitosis compared

Mitosis One cell division event Number of chromosomes in daughter cells same as in parent cell Two daughter cells produced Homologous chromosomes do not pair during prophase No crossing over between homologous chromosomes during prophase Meiosis Two cell division events Number of chromosomes in daughter cells one half of the number in parent cell Four daughter cells produced Homologous chromosomes pair during prophase I Possible crossing over between homologous chromosomes during prophase

diploid (2n)

Most of the somatic cells of most organisms have a double set of chromosomes. These cells are said to have the diploid (2n) number of chromosomes. One set of chromosomes had its origin from the female parent of the organism. The other set of chromosomes had its origin from male parent. For the most part, the sets are similar to each other.

Telophase I

Not present in all species; may move directly into prophase II Reformation of the nuclear membrane Uncoiling of the chromosomes Possible cytokinesis (division of the cytoplasm forming two distinct cells)

Puberty (part of the life cycle)

When the new individual becomes sexually mature, i.e. undergoes puberty, the sex organs become activated by hormones that are circulating in the blood. Specialized cells in the sex organs begin (in the case of the male) or continue (in the case of the female) the process of meiosis. This results in the production of sperm cells in the male and egg cells in the female. It is the task of the sperm cell to find its way to the egg cell in order for fertilization to occur.

Gametic Life Cycle

By examining the life cycle of the human you can see that the only stage in the life cycle that is haploid (n), i.e. has half of the number of chromosomes as normal diploid cells, is the gamete stage. All other cells in the bodies at all other stages are diploid (2n). In humans, as in most other animals, this is called the gametic life cycle. As we will see in other lessons, plants have a sporic life cycle, and fungi have a zygotic life cycle.

Somatic Cells

Cells in the organism that are not produced by meiosis and involved in sexual reproduction

Prophase I

Chromatin condenses Nuclear membrane dissolves Homologous chromosomes pair up in the middle of the cell, forming a pair of duplicated chromosomes, i.e. two pair of duplicate sister chromatids Chromosome crossing over may occur between any four sister chromatids Chromosome crossing over site called chiasma (pl. chiasmata)

Anaphase II

Identical to anaphase in mitosis Sister chromatids separate Sister chromatids move to opposite ends of cell

Metaphase II

Identical to metaphase in mitosis Chromosomes migrate to central plane of cell Each chromosome consists of two sister chromatids

Prophase II

Identical to prophase in mitosis Nuclear membrane dissolves, if formed during telophase I Chromatin supercoils to form chromosomes, if uncoiled during telophase I Chromosomes visible

Telophase II

Identical to telophase in mitosis Nuclear membrane reforms Chromosomes uncoil to form chromatin strands

Conception (part of the life cycle)

In humans conception is marked by the union of the sperm cell from the male parent with the egg cell of the female parent. Remember that these sex cells, or gametes, were produced by meiosis, and have half of the number of chromosomes as the normal somatic cells of the body. Sex cells are produced in only specific locations in the body, i.e. the sex organs. Sperm cells are produced in the testes of the male. Egg cells are produced in the ovaries of the female. In the sex organs there are masses of cells that are specialized to generate sex cells by the process of meiosis. Any cell that will eventually give rise to a sex cell is said to be a germ cell, or a cell in the germ line.

Differences of Meiosis and Mitosis

In the case of meiosis, like mitosis, we also make new cells. However, there are some important differences in the fate of chromosomes and cells. Prior to meiosis we duplicate the number of chromosomes during the S phase of the cell cycle, just as in the case of mitosis. In meiosis this chromosome doubling is followed not by one cell division, as in mitosis, but by two cell divisions. This results in the production of four daughter cells instead of two. This also results in each daughter cell containing only half the number of chromosomes as the parent cell.

Oogenesis

Oogenesis17 is the term that refers to all of the steps in the production of fully functional egg cells. The process begins with meiosis and ends with an egg cell that is receptive to a sperm cell during the fertilization process. The steps in oogenesis are as follows: A specialized diploid (2n) cell in the ovary called a primary oocyte undergoes the first division meiosis (the chromosome reduction division) to produce two haploid (n) cells. One cell is called a secondary oocyte and the other is called the first polar body. The secondary oocyte is the larger of the two cells and will continue in the development process to form an egg cell. The first polar body will eventually shrink and remain non-functional. The secondary oocyte cell undergoes the second division of meiosis to produce an egg cell and a second polar body. The egg cell will continue to develop into a fully functional egg. The second polar body will eventually shrink and remain non-functional. The egg cell undergoes a maturation process resulting in a cell that is fully functional and capable of receiving a sperm cell during the fertilization event. Oogenesis occurs in the ovary of the female. The first division of meiosis and the production of secondary oocytes occur while the human female is yet a fetus in the womb of her mother. Therefore, all of the secondary oocytes that a woman has as an adult she is born with, and are present in her ovaries throughout her reproductive life. While in the ovaries secondary oocytes begin meiosis II but the process is arrested at metaphase II. Upon ovulation (the release of the secondary oocyte from the ovary) the secondary oocyte is passed into the oviduct. Here it has the chance to meet a sperm cell that is traveling toward it after being deposited in the vagina during copulation. If the secondary oocyte is fertilized by a sperm cell, the process of meiosis is allowed to run to completion, and the secondary oocyte becomes a zygote (a fertilized egg cell). If the secondary oocyte is not fertilized by a sperm cell, meiosis will not be completed, and the secondary oocyte will be expelled from the body of the female along with the menstrual flow.

reduction-division

Therefore if a cell contains 12 chromosomes before it undergoes meiosis, each of its four daughter cells will have 6 chromosomes after meiosis. This reduction-division insures that when sperm cells and egg cells unite and add their chromosomes to each other, that the normal chromosome number for the organism will be preserved. If there was no reduction-division, the chromosome number in the cells of the organism would double with each new generation.

Spermatogenesis

Spermatogenesis16 is the term that refers to all of the steps in the production of fully functional sperm cells. The process begins with meiosis and ends with a sperm cell that is capable of fertilizing an egg cell. The steps in spermatogenesis are as follows: A specialized diploid (2n) cell in the testes called a primary spermatocyte undergoes the first division meiosis (the chromosome reduction division) to produce two haploid (n) cells called secondary spermatocytes. Each of the two secondary spermatocyte cells undergo the second division of meiosis to produce two spermatids. There are four spermatids produced for each primary spermatocyte. Each of the four spermatids undergoes a maturation process resulting in cells that are called sperm or spermatozoa. These modifications include the following cellular modifications: The nucleus of the cell becomes condensed. The cytoplasm of the cell is reduced in volume. A long whip-like flagellum is produced at the posterior of the cell. The mitochondria of the cell are packed around the base of the flagellum. The anterior portion of the cell develops a specialized region called the acrosome that will assist in the penetration of the egg cell. Spermatogenesis occurs in the lining of the seminiferous tubules of the testes. After the spermatozoa are fully formed they are stored in the epididymis on the posterior surface of the testes. Here they await the ejaculatory events that will propel them outside of the body through the penis.

Fertilization (part of the life cycle)

The process of fertilization joins the nuclei of the sperm cell with that of the egg cell. This results in the addition of the chromosomes of the two sex cells, thereby restoring the normal diploid number of chromosomes for the species. Once fertilization has taken place the fertilized egg is called a zygote. The zygote quickly divides repeatedly by mitosis to form a small mass of cells. The cells in this mass continue to divide by mitosis to produce a continually enlarging embryo containing all of the tissues and organs that it will need to sustain life. When the embryo has reached full term (about 9 months) it passes from the mother's body in the process of birth. After birth the new individual continues to grow by increasing in size due to the production of new cells by the process of mitosis. Now, and through the remainder of its life, this new individual will continue to grow and replace worn out cells by the process of mitosis. The vast majority of cell division events in the life of an organism are mitotic. Meiosis is a relatively rare event, reserved for the production of sex cells.