Mitosis and Meiosis Aims: Must be able to define the terms mitosis and meiosis. Should be able to describe the stages of each process. Could be able to compare and contrast the two processes.

Cell Cycle Cytokinesis Interphase

Basic Overview

Mitosis - Basics Cytokinesis occurs (division of the cytoplasm) After DNA replication has occurred – mitosis (division of the nucleus) can continue… There are four major stages within the mitosis stage: Prophase Metaphase Anaphase Telophase Cytokinesis occurs (division of the cytoplasm)

Mitosis - In Detail Interphase Cell enters mitosis Early Prophase Nuclear Membrane Interphase Centrosome, which later forms the spindle, is also replicated. DNA is replicated to form 2 chromatids Nucleolus Cell enters mitosis Early Prophase DNA continues condensing into chromosomes and the nuclear membrane begins to dissolve Late Prophase Chromosomes continue to coil up and appear as double-chromatids Metaphase The mitotic spindle is formed to organize the chromosomes. The spindle consists of fibers made of microtubules and proteins. Anaphase The chromosomes segregate, pulling the chromatids apart Two new nuclei form. The cell plate forms across the midline of the parent cell. This is where the new cell wall will form. Telophase Division of the cytoplasm (cytokinesis) is complete. The two daughter cells are now separate cells in their own right. Cytokinesis Late Anaphase

Mitosis - Micrographs 1. Interphase 2. Prophase 3. Metaphase 4. Anaphase 6. Telophase 5. Late Anaphase

Activity Complete the Biozone pages 211 and 212.

Meiosis To produce Haploid cells (gametes). Crossing over may occur at this stage in meiosis Meiosis Intermediate cell First Division (Reduction division) To produce Haploid cells (gametes). Only one copy of each homologous pair of autosomes plus one sex chromosome Second Division ('Mitotic' division) Gametes (eggs or sperm)

Meiosis i Interphase DNA replication Prophase 1 Synapsis and crossing over First division of meiosis is called a ‘reduction’ division because it reduces (halves) the number of chromosomes. One chromosome from each homologous pair is donated to each intermediate cell. Metaphase 1 Bivalents line up on the equator Anaphase 1 Telophase 1 Intermediate cell

NOTE: Half the full chromosome complement shown Intermediate cell NOTE: Half the full chromosome complement shown Meiosis ii Prophase 2 The second division of meiosis is called a ‘mitotic’ division, because it is similar to mitosis. Sister chomatids of each chromosome are pulled apart and are donated to each gamete cell. Metaphase 2 Anaphase 2 Gamete (egg or sperm) Telophase 2

Crossing Over The swapping of sections between homologous pairs of chromosomes. Occurs during Prophase I of meiosis. Results in genes being swapped between maternal and paternal chromosomes. Gametes show greater diversity.

Crossing Over - Process See diagram: Biozone p289/90

Meiosis i - Non-Disjunction The meiotic spindle normally distributes chromosomes to daughter cells without error. Sometimes, homologous chromosomes or sister chromatids fail to separate. One gamete receives two of the same type of chromosome and the other gamete receives no copy. This mishap, called non-disjunction, results in abnormal numbers of chromosomes in the gametes. This example shows non-disjunction in meiosis I; homologous chromosomes fail to separate properly at anaphase during meiosis I. Meiosis II Non-disjunction in Meiosis I n+1 n–1

Meiosis ii – Non-Disjunction Non-disjunction can also occur in meiosis II, when sister chromatids fail to separate during anaphase of meiosis II. In cases of non-disjunction, if either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number (e.g. 2N+1 or 2N–1). This is known as aneuploidy and it accounts for chromosomal defects such as Down syndrome (trisomy 21) and Edward syndrome (trisomy 18). Meiosis II Non-disjunction in Meiosis II n+1 n–1 n

Activity Biozone p288.

Activity Complete the Biozone pages 287 and 211.

Meiosis i - Non-Disjunction The meiotic spindle normally distributes chromosomes to daughter cells without error. Sometimes, homologous chromosomes or sister chromatids fail to separate. In these cases, one gamete receives two of the same type of gamete and the other gamete receives no copy. This mishap, called non-disjunction, results in abnormal numbers of chromosomes in the gametes. This example shows non-disjunction in meiosis I; homologous chromosomes fail to separate properly at anaphase during meiosis I. Meiosis II Non-disjunction in Meiosis I n+1 n–1

Meiosis ii – Non-Disjunction Non-disjunction can also occur in meiosis II, when sister chromatids fail to separate during anaphase of meiosis II. In cases of non-disjunction, if either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number (e.g. 2N+1 or 2N–1). This is known as aneuploidy and it accounts for chromosomal defects such as Down syndrome (trisomy 21) and Edward syndrome (trisomy 18). Meiosis II Non-disjunction in Meiosis II n+1 n–1 n

Activity Complete the Biozone pages 210, 293 and 294.