Two types of nuclear division

Meiosis consists of two divisions. The first division, meiosis I, separates the homologoua chromosomes, so that each cell now has only one of each pair. The second division, meiosis II, separates the chromatids of each chromosome. Meiotic division therefore produces four cella, each with one complete set of chromosomes.

Growth

When a diploid zygote, which is one cell, grows into an adult with millions of cells, the new cells must be genetically identical, with the same number of chromosomes as the cells that divided to produce them. The type of nuclear division that achieves this is called mitosis.

Sexual reproduction

If there were no point in the life cycle when the number of chromosomes halved, then the number of chromosomes would double every generation. The type of nuclear division that halves chromosome number is called meiosis. Meiosis produce gametes which are haploid and is decrobed as a reduction devision, because the number of chromosomes is reduced.

Meisosis does more than halve the number of chromosomes in a cell. Meiosis also introduces genetic variation into gametes and therefore the zygotes that afre produced.

Genetic variation may also arise as a result of mutation, which can occur at any stage in a life cycle

Such variation is the raw material on which natural selection has worked to produce the huge range of species that live on Earth

Meiosis

Unlike mitosis, meiosis involves two divisions, meiosis I and meiosis II. Meiosis I is a reduction division, resulting in two daughter neclei with half the number of chromosomes of the parent nucleus. In meiosis II, the chromosomes behave as in mitosis, so that each of the two haploid daughter nuclei divides again. Meiosis therefore results in a total of four haploid nuclei.

Note: it is the behaviour of the chromosomes in meiosis I that is particularly important and contrasts with mitosis

Stages of meiosis:

Meiosis I

1. Prophase

Early prophase I - as mitosis prophase

Middle prophase I - Homologous chromosomes pair up. This process is called synapsis. Each pair is called a bivalent

Centrosomes (contains a pair of centrioles) move to opposite ends of nucleus

Late prophase I

Bivalent crossing over: chromatids may break and may connect to another chromatid

Chiasma = point where crossing over occurs (plural; chiasmata). One or more chiasmata may form, anywhere along the length

At the end of prophase I a spindle is formed

2. Metaphase

Crossing over of long chromatids may occur.

Bivalents line up across equator of spindle, attached by centromeres

Spindle formed as in mitosis

5. Anaphase I

Centromeres do not divide, unlike in mitosis

Whole chromosomes move towards opposite ends of spindle, centromeres first, pulled by microtubules

6. Telophase I

• as mitosis
  • nuclear envelope re-forms
  • nulceolus reforms
  • cytokinases takes place
• chromosomes have reached poles of…