MeiosisMeiosis

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

Chromosome NumberChromosome Number

All cells in the human body, except the All cells in the human body, except the gametes contain the same number of gametes contain the same number of chromosomes (46). chromosomes (46).

This is called the diploid (2n) number of This is called the diploid (2n) number of chromosomes.chromosomes.

In gametes that number need to be cut in In gametes that number need to be cut in half in order to produce viable offspring. half in order to produce viable offspring.

This is called the This is called the haploidhaploid (n) number of (n) number of chromosomes.chromosomes.

• This means that gametes This means that gametes contain only one copy of contain only one copy of each type of chromosome each type of chromosome that the diploid parent cell that the diploid parent cell containscontains

• The two copies of each The two copies of each chromosome in human cells chromosome in human cells are homologousare homologous

Curly hair allele

Straight hair allele

Families: similar yet Families: similar yet different. Why?different. Why?

Asexual ReproductionAsexual Reproduction Single parentSingle parent Offspring identical to each other and parentOffspring identical to each other and parent

Sexual ReproductionSexual Reproduction Two parentsTwo parents Offspring are uniqueOffspring are unique Offspring are Offspring are similarsimilar to each other and to each other and

parentsparents Combine DNA from two individualsCombine DNA from two individuals Combines characteristics of both individualsCombines characteristics of both individuals

Phases of MeiosisPhases of Meiosis

While mitosis conserves While mitosis conserves chromosome number, meiosis chromosome number, meiosis reduces the chromosome number by reduces the chromosome number by halfhalf

Meiosis involves two sequences of Meiosis involves two sequences of the mitosis phases:the mitosis phases:

1.1. Meiosis I - reduces the chromosomes Meiosis I - reduces the chromosomes from diploid to haploid, referred to as from diploid to haploid, referred to as reduction divisionreduction division

2.2. Meiosis II – further divides the cellMeiosis II – further divides the cell

InterphaseInterphase

Just as in mitosis, during Just as in mitosis, during interphase the cell grows interphase the cell grows and replicates DNA.and replicates DNA.

After replication, each After replication, each chromosome is made up of chromosome is made up of a pair of identical a pair of identical sister sister chromatidschromatids held together held together by a by a centromerecentromere

Centrosomes also replicateCentrosomes also replicate

Prophase I Prophase I

Homologous ChromosomesHomologous Chromosomes begin to begin to condense and pair up. condense and pair up.

These pairs of chromosomes are called These pairs of chromosomes are called tetrads (cluster of four chromatids).tetrads (cluster of four chromatids).

Nuclear membrane and nucleoli Nuclear membrane and nucleoli disappeardisappear

Spindle fibres begin to formSpindle fibres begin to form Centrosomes move away from each Centrosomes move away from each

otherother

Prophase I continuedProphase I continued

Microtubules form between the Microtubules form between the centrosomescentrosomes

Homologous chromosomes can criss-Homologous chromosomes can criss-cross, allowing the chromatids to switch cross, allowing the chromatids to switch segments, called crossing-oversegments, called crossing-over

Prophase I can last for days or longer, Prophase I can last for days or longer, and takes up 90% of the time required for and takes up 90% of the time required for meiosismeiosis

Metaphase IMetaphase I

Spindle fibres attach to the centromere of Spindle fibres attach to the centromere of each chromosomeeach chromosome

Spindle fibres pull each tetrad to the Spindle fibres pull each tetrad to the centre of the cell (metaphase plate or centre of the cell (metaphase plate or equator)equator)

Anaphase IAnaphase I

The homologous chromosomes separate The homologous chromosomes separate and move to opposite poles of the cell.and move to opposite poles of the cell.

Sister chromatids remain attached at Sister chromatids remain attached at their their centromeres, moving one centromeres, moving one chromosome to each of the poles.chromosome to each of the poles.

Telophase I Telophase I

Does not occur in all cellsDoes not occur in all cells The homologous chromosome pairs The homologous chromosome pairs

continue to move apart to each polecontinue to move apart to each pole Each pole contains a haploid (n) number Each pole contains a haploid (n) number

of a chromosome setof a chromosome set Cleavage furrows form in animal cells, Cleavage furrows form in animal cells,

and cell plates appear in plant cellsand cell plates appear in plant cells

Telophase I continuedTelophase I continued

Cytokinesis usually occurs Cytokinesis usually occurs simultaneously with telophase I, forming simultaneously with telophase I, forming two daughter cellstwo daughter cells

There is no further replication of genetic There is no further replication of genetic materialmaterial

Depending on the species, the Depending on the species, the chromosomes de-condense and the chromosomes de-condense and the nuclear membranes and nucleoli re-form.nuclear membranes and nucleoli re-form.

Prophase IIProphase II

Nuclear membrane disappears and Nuclear membrane disappears and spindle fibers are formed.spindle fibers are formed.

Chromatin coil and become visible.Chromatin coil and become visible. Chromosomes progress towards the Chromosomes progress towards the

metaphase II platemetaphase II plate

Metaphase IIMetaphase II

Spindle fibers attach to the centromere of Spindle fibers attach to the centromere of the sister chromatids the sister chromatids

Spindle fibers move the chromosomes to Spindle fibers move the chromosomes to the middle of the cellthe middle of the cell

Anaphase IIAnaphase II

The centromeres of the sister chromatids The centromeres of the sister chromatids finally separatefinally separate

The sister chromatids of each pair, now The sister chromatids of each pair, now individual chromosomes, are pulled by individual chromosomes, are pulled by spindle fibres to opposite poles of the cellspindle fibres to opposite poles of the cell

Telophase IITelophase II

The nuclear membrane reappears and The nuclear membrane reappears and spindle fibres disappearspindle fibres disappear

Cytoplasm divides and the new cells are Cytoplasm divides and the new cells are formed (cytokinesis)formed (cytokinesis)

The result of telophase two are The result of telophase two are 4 non-4 non-identical haploid cells identical haploid cells (daughter cells)(daughter cells)