Ch. 8 Meiosis Essential Standards: Bio.3.2.1 Explain the role of meiosis in sexual reproduction and genetic variation

Meiosis Meiosis is the process of nuclear division that reduces the number of chromosomes in the new cells Produces gametes Meiosis is similar to mitosis except: Meiosis has 2 cell divisions instead of 1 Meiosis results in cells having half the number of chromosomes than the parent cell Meiosis starts in a diploid state and ends in a haploid state

Meiosis I – Prophase I Nucleus and Nucleolus disappear and the mitotic spindle starts to form and homologous chromosomes pair up Homologous chromosomes are chromosomes that are similar in structure that code for the same types of genes. When homologous chromosomes pair up, they sometimes exchange genetic material to increase genetic variation in a process called crossing over.

Meiosis I – Metaphase I Homologous chromosomes line up along the metaphase plate in a random order known as random (independent) assortment

Meiosis I – Anaphase I The mitotic spindle fibers shorten to separate the homologous chromosomes to opposite poles. The previous diploid cell is starting to become haploid during anaphase I.

Meiosis I – Telophase I The nucleus and nucleolus start to reform around each pole and the spindle apparatus disassembles After telophase I, cytokinesis occurs to allow for additional organelles/materials to be made for the second division process.

Meiosis II – Prophase II The nucleus and nucleolus disappear and the mitotic spindle apparatus develops.

Meiosis II – Metaphase II Chromosomes line up singularly on the metaphase plate and the spindle fibers connect to the chromosomes

Meiosis II – Anaphase II The spindle fibers shorten causing the sister chromatids to separate. If the spindle fibers do not attach correctly, it can result in nondisjunction where the chromatids fail to separate.

Meiosis II – Telophase II The nucleus and nucleolus reappears at each pole, chromosomes uncoil, and the spindle apparatus disassembles. Cytokinesis begins resulting in four new cells.

Factors that can increase genetic variation: Chromosomes that are similar in size and shape (homologues) can exchange genetic information in a process called crossing over Homologous chromosomes randomly align on the metaphase plate allowing different combinations of genes when chromosomes separate in process called independent assortment Chromosomes can fail to separate which can lead to genetic disorders such as Down syndrome in a process called nondisjunction

Why do cells go through meiosis? Meiosis is necessary for making gametes (sperm and egg cells) Oogenesis is the process of making female gametes that results in producing 1 egg cell and 3 nonfunctioning polar bodies Spermatogenesis is the process of making male gametes that results in producing 4 sperm cells