MEIOSIS

• Meiosis produces four daughter cells.

• Daughter cells have fewer chromosomes than parent cells. (haploid)

• Daughter cells have chromosomes with different combinations of genes compared to parent cells. (not identical)

Stages

• Meiosis involves the same stages of cell division as mitosis, but each stage occurs twice.

  

• Meiosis begins with interphase and DNA replication.

Prophase I involves the condensation of chromatin, formation of spindle fibres, and alignment of homologous chromosomes.

• Crossing over or "mixing" of genes occurs during prophase I of meiosis.

  

Metaphase I involves the attachment of spindle fibres to the centromere of each chromosome.

• Homologous chromosomes line up as pairs, and the orientation of each pair is independent.

  

Anaphase I involves the shortening of spindle fibres and the separation of homologous chromosomes.

Telophase I involves the uncoiling of homologous chromosomes, the formation of a nuclear membrane, and the division of cytoplasm.

• Each new cell is haploid.

  

Meiosis II does not involve interphase or DNA replication.

• The result is four haploid daughter cells.

• Independent assortment occurs during meiosis, leading to a greater diversity of gametes.

  

• Each haploid cell formed after telophase II proceeds through cytokinesis to form gametes.

• In males, this process is called spermatogenesis and results in the formation of four viable sperm cells.

• In females, the cytoplasm is not equally divided among gametes, resulting in one viable ovum and three polar bodies. Female gametogenesis involves unequal division of the cytoplasm

Nondisjunction during meiosis I and meiosis II produces gametes with too few or too many chromosomes

Monosomy X (Turner Syndrome) is caused by the fertilization of a normal egg by a sperm lacking an X chromosome due to nondisjunction in meiosis II

Trisomy 21 (Down Syndrome) is caused by the fertilization of an egg with an extra chromosome 21 due to nondisjunction in meiosis I

• The incidence of Down Syndrome increases with maternal age due to sticky centromeres and nondisjunction

• Trisomy 21 can also be caused by nondisjunction in sperm cells

16.4

Reproductive strategies involve sporophyte and gametophyte stages

• The human life cycle relies on regular patterns of meiosis and mitosis, while other organisms rely on alternative reproductive strategies

  

Asexual reproduction - the reproductive process in which a parent organism produces genetically identical offspring by itself.

Sexual reproduction - involves the production of gametes by meiosis followed by fertilization between genetically distinct parental gametes to produce genetically distinct offspring.

Fragmentation - a form of asexual reproduction or to a step in certain cellular activities, such as apoptosis and DNA cloning

sporophyte - spore-making body, produces multiple haploid spores, which spread and develop without fertilization.

Gametophyte - gamete-making body, produces male and female gametes. Upon fertilization, gametes develop into another sporophyte, and the cycle repeats.

Binary fission is a method of reproduction utilized by bacteria, allowing for rapid growth through asexual reproduction.

Conjugation is a process in bacteria where plasmids are transferred between cells, creating new genetic combinations

Budding is a form of asexual reproduction where a miniature version of the parent grows out and separates once mature.

• Commonly utilized by metazoans such as corals, sponges, and hydra

Vegetative reproduction is a form of budding that takes place in plants, involving nodes, crown runners, and clones. (without the use of spores and seeds)

Parthenogenesis is a form of asexual reproduction where an unfertilized egg develops into an adult

• Example: Honeybees have fertilized eggs developing into female worker bees and unfertilized eggs developing into male drones

Spores are a form of asexual reproduction that allows parents to disperse their offspring over long distances. (may be haploid or diploid)

“Alteration of Generations” - The life cycle of some plants consists of two generations.

1. A diploid generation or sporophyte.

2. A haploid generation or gametophyte.

• The life cycle of some plants consists of two generations: a diploid sporophyte and a haploid gametophyte

• Sporophyte produces haploid spores through meiosis, which develop into gametophytes that produce male and female gametes

• Upon fertilization, gametes develop into another sporophyte, repeating the cycle

• Many plants spend most of their lives in either the diploid or haploid generation

• Conifers spend most of their lives in the diploid generation, with the tree being the sporophyte and the cones containing the gametophytes

• Microsporangia in cones produce microspores that develop into pollen grains (male gametophytes)

• Pollen grains are transferred to female cones for fertilization

• Megasporangia in cones produce ovules (megasporangia) that contain megaspores

• Fertilization results in the development of a diploid sporophyte

• Some organisms alternate between sexual and asexual reproduction instead of alternating between haploid and diploid