Meiosis and Sexual Life Cycles

Meiosis - A reduction division required to reduce the number of chromosomes to half so sexual reproduction can occur.






Genes code for proteins, including enzymes

Genes are located on chromosomes, which are single long strands of DNA.

A gene's specific location on a chromosome is called its locus (pl: loci).

Reproduction can be asexual or sexual.

Asexual - by mitosis - resultant organisms are clones, genetically identical to the parent and to other offspring.

Includes many single-celled organisms, as well as some invertebrates (Fig 13.1, and movie of hydra budding) and many plants. Aphids, water fleas, and rotifers reproduce by parthenogenesis, a process in which the eggs develop without fertilization and the offspring are haploid. If these haploid offspring reproduce, their eggs are formed by mitosis, not meiosis. Many plants reproduce by fragmentation or runners. This is useful in agriculture.

The sexual life cycle

Involves genetic contribution from both parents. Provides much greater genetic variation within a species. There are many variations of sexual life cycles (Fig 13.5), but we will only concern ourselves with the human life cycle.

The human life cycle (Fig 13.4)

Somatic cells - all the cells of the body except the germ or stem cells which are those that produce gametes (sperm and eggs). Somatic cells all contain paired chromosomes. (Diploid or 2n)

Germ or stem cells are diploid and will undergo meiosis to produce four haploid (1n) gametes.

Humans have 23 pairs, or 46, chromosomes that vary by size, shape, and appearance. 22 pairs are called autosomes. The other pair are the sex chromosomes. These are the X and Y chromosomes. Other species have other arrangements for sex determination.

Arranging these chromosomes, by pairs, according to size and shape, results in a display called a karyotype. (Fig 13.3). Different species show different karyotypes, varying in shape, size and number of chromosomes.

In humans, the gametes will have 22 autosomes and one sex chromosome. At fertilization (syngamy), two gametes will combine to cause the resultant zygote (fertilized egg) to have 44 autosomes (22 pairs) and 2 sex chromosomes (1 pair). The zygote is diploid, as were the parents.

Interphase - (Movie) - chromosomes and centrosomes duplicate as in mitosis.

The Stages of Meiosis - Overview - Fig 13.6

Meiosis I - serves to divide the two versions of each chromosome (fig 13.7, left page).

Prophase I (Movie)

Chromosomes with sister chromatids form and homologous chromosomes pair, forming tetrads. Unlike mitosis, synapsis occurs and the chromatids of homologues cross at chiasmata.
Crossing over occurs when these pieces of homologous chromosomes (fig. 13.10) (nonsister chromatids) exchange places to promote new genetic combinations in the offspring. (movie)
The rest of prohase I is similar to mitosis
    - centrosomes move apart and spindle forms
    - nuclear envelope and nucleoli disperse
    - spindle attaches to kinetochores and begins moving chromosomes

Metaphase I (Movie)

Chromosomes now arranged on the metaphase plate. Independent assortment - the alignment of homologous pairs along the center of the cell is random, with different combinations of parental chromosomes possible for each daughter cell. (Fig. 13.9)
Opposite homologues are attached to microtubules attached to opposite centromeres.

Anaphase I (Movie)

Opposite homologous chromosomes, consisting of paired sister chromatids, are pulled toward opposite ends.

Telophase I (Movie of telophase I and cytokinesis)

Each pole now has a haploid chromosome set, but each chromosome consists of sister chromatids.


Division of cell itself. Often occurs at same time as telophase I.

Meiosis II - separates the two sister chromatids to form four haploid daughter cells or gametes, each with a different genetic make-up. fig 13.7, right page

Prophase II

Spindle forms, microtubules attach to chromosomes and pull them toward the meaphase II plate

Metaphase II

Chromosomes line up on metaphase II plate
Microtubules are attached to kinetochores

Anaphase II

Centromeres of sister chromatids separate and the sister chromatids move toward opposite poles.
They are now consided chromosomes.

Telophase II

Nuclei reform and cytokinesis occurs.
There are now four daughter cells, each haploid, and different genetically from each other.
May be sperm or ova (image).

Meiosis II and cytokinesis movie.

A comparison of mitosis and meiosis. Image Fig.13.8a and summary Fig.13.8b

Origins of genetic variation in sexually reproducing populations.

Sexual reproduction has the capacity to generate new genetic combinations.

Independent Assortment - The random alignment of homologues during meiosis I results in an astounding number of possible kinds of gametes that can be produced.

Crossing over allows for combinations of genes that may never have existed previously.

Random Fertilization - Since the new zygote is the product of two gametes, each with new variations within them, fertilization adds even more genetic diversity.