Meiosis and Mitosis

Basis of reproduction - Cell Division Types of cell division 1. Duplicating - Mitosis 2. Reducing - Meiosis Duplicating and reducing refer to the number of chromosomes in the cells after divisiion

Mitosis and Meiosis Mitosis Meiosis

Mitosis - duplication of cells (at the end of the process - produce two cells with the same chromosome number (2N) as the parent cell)

Mitosis Mitosis has 5 phases: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase and Cytokinesis

centrioles chromatin What’s happening? 1. DNA has already been duplicated (chromatin) 2. Centrioles form

Mitotic spindle fibres Prophase Mitotic spindle fibres sister chromatids Condensing chromatin What’s happening? 1. Chromatin condenses and chromosomes become visible 2. Mitotic spindle fibres appear 3. Duplicated chromosomes appear as ‘sister chromatids’

Some terms about chromosome structure Unduplicated chromosome Sister chromatids Duplicated chromosome Centromere

Prometaphase Nuclear envelope Mitotic spindle kinetochore What’s happening? 1. Nuclear envelop breaks down 2. Mitotic spindle invades nuclear area 3. Each sister chromatid develops a kinetochore at the centromere

Metaphase metaphase plate attached kinetochores What’s happening? 1. Chromosomes line up on ‘metaphase plate’ 2. Kinetochores of each sister chromatid are attached to a spindle fibre

Spindle fibres contract Anaphase Spindle fibres contract What’s happening? 1. Proteins holding sister chromatids together break down 2. Spindle fibres contract and pull sister chromatids apart

Telophase and Cytokinesis Daughter nuclei Cleavage furrow Nuclear envelope What’s happening? 1. Two daughter nuclei form 2. Nuclear envelope re-forms 3. Cleavage furrow forms, pinching the cell in two (cytokinesis)

Telophase & Cytokinesis And to summarize ….. G2 interphase Prophase Prometaphase Metaphase Anaphase Telophase & Cytokinesis End up with two identical daughter cells

Meiosis - reduction division of cells (at the end of the process - produce four cells each with the half chromosome number (N) as the parent cell)

Meiosis Meiosis I Meiosis II

(Purpose: To separate pairs of the same chromosome (homologous pairs)) Meiosis I (Purpose: To separate pairs of the same chromosome (homologous pairs)) Meiosis II Meiosis I

(Purpose: To separate pairs of the same chromosome (homologous pairs)) Meiosis I (Purpose: To separate pairs of the same chromosome (homologous pairs)) Prophase 1 1. Chromosomes condense & homologous pairs line up side-by-side (synapsis) 2. Crossing over - homologous pairs exchange genetic material

(Purpose: To separate pairs of the same chromosome (homologous pairs)) Meiosis I (Purpose: To separate pairs of the same chromosome (homologous pairs)) Metaphase 1 1. Chromosomes line up on metaphase plate. One pair on each side of the plate 2. Spindle fibres attach to kinetochores of each homologous chromosome

(Purpose: To separate pairs of the same chromosome (homologous pairs)) Meiosis I (Purpose: To separate pairs of the same chromosome (homologous pairs)) Anaphase 1 1. Homologous chromosomes move to opposite poles of the cell

(Purpose: To separate pairs of the same chromosome (homologous pairs)) Meiosis I (Purpose: To separate pairs of the same chromosome (homologous pairs)) Telophase 1 & cytokinesis 1. Cell divides into two cells each with half the number of chromosomes as the parent cell (haploid)

Meiosis II (- same process as mitosis but with 1/2 the chromosomes) (Purpose: To separate sister chromatids) Telophase & Cytokinesis (Meiosis I) Telophase & Cytokinesis (Meiosis II) Prophase Metaphase Anaphase

Comparison of Mitosis and Meiosis Property Mitosis Meiosis DNA replication During interphase During interphase before mitosis before meiosis I Number of divisions One Two Synapsis of homologous Doesn’t occur During prophase I chromosomes Number of daughter cells 2 4 Number of chromosomes in each 2N (diploid) N (haploid) daughter cell Function of cells produced growth, maintenance gametes