Meiosis

 Meiosis is a special type of cell division that occurs only in reproductive organs. Meiosis makes reproductive cells called gametes (egg or sperm).

Chromosome Number  All cells in the human body, except the gametes contain the same number of chromosomes (46).  This is called the diploid (2n) number of chromosomes.  In gametes that number need to be cut in half in order to produce viable offspring.  This is called the haploid (n) number of chromosomes.

This means that gametes contain only one copy of each type of chromosome that the diploid parent cell containsThis means that gametes contain only one copy of each type of chromosome that the diploid parent cell contains The two copies of each chromosome in human cells are homologousThe two copies of each chromosome in human cells are homologous Curly hair allele Straight hair allele

Families: similar yet different. Why?  Asexual Reproduction  Single parent  Offspring identical to each other and parent  Sexual Reproduction  Two parents  Offspring are unique  Offspring are similar to each other and parents  Combine DNA from two individuals  Combines characteristics of both individuals

Phases of Meiosis  While mitosis conserves chromosome number, meiosis reduces the chromosome number by half  Meiosis involves two sequences of the mitosis phases: 1.Meiosis I - reduces the chromosomes from diploid to haploid, referred to as reduction division 2.Meiosis II – further divides the cell

Interphase  Just as in mitosis, during interphase the cell grows and replicates DNA.  After replication, each chromosome is made up of a pair of identical sister chromatids held together by a centromere  Centrosomes also replicate

Prophase I  Homologous Chromosomes begin to condense and pair up.  These pairs of chromosomes are called tetrads (cluster of four chromatids).  Nuclear membrane and nucleoli disappear  Spindle fibres begin to form  Centrosomes move away from each other

Prophase I continued  Microtubules form between the centrosomes  Homologous chromosomes can criss- cross, allowing the chromatids to switch segments, called crossing-over  Prophase I can last for days or longer, and takes up 90% of the time required for meiosis

Metaphase I  Spindle fibres attach to the centromere of each chromosome  Spindle fibres pull each tetrad to the centre of the cell (metaphase plate or equator)

Anaphase I  The homologous chromosomes separate and move to opposite poles of the cell.  Sister chromatids remain attached at their centromeres, moving one chromosome to each of the poles.

Telophase I  Does not occur in all cells  The homologous chromosome pairs continue to move apart to each pole  Each pole contains a haploid (n) number of a chromosome set  Cleavage furrows form in animal cells, and cell plates appear in plant cells

Telophase I continued  Cytokinesis usually occurs simultaneously with telophase I, forming two daughter cells  There is no further replication of genetic material  Depending on the species, the chromosomes de-condense and the nuclear membranes and nucleoli re-form.

Prophase II  Nuclear membrane disappears and spindle fibers are formed.  Chromatin coil and become visible.  Chromosomes progress towards the metaphase II plate

Metaphase II  Spindle fibers attach to the centromere of the sister chromatids  Spindle fibers move the chromosomes to the middle of the cell

Anaphase II  The centromeres of the sister chromatids finally separate  The sister chromatids of each pair, now individual chromosomes, are pulled by spindle fibres to opposite poles of the cell

Telophase II  The nuclear membrane reappears and spindle fibres disappear  Cytoplasm divides and the new cells are formed (cytokinesis)  The result of telophase two are 4 non- identical haploid cells (daughter cells)