CHAPTER 11 MEIOSIS. Meiosis and sexual reproduction Gametes are sex cells of complex organisms. They are formed by meiosis Sex cells are haploid(monoploid)

CHAPTER 11 MEIOSIS

Meiosis and sexual reproduction

• Gametes are sex cells of complex organisms. They are formed by meiosis

• Sex cells are haploid(monoploid) in chromosome number. That is they would be represented by the letter “n”.

• In humans “n” would equal 23. • Remember all other cells in the body are

diploid and are represented by “2n” where 2n = 46 in humans.


• Sperm and egg cells are haploid cells and their nuclei fuse to form a zygote at fertilization.

• A zygote is a diploid cell which is the result of the fusion or joining of the nuclei of 2 gametes or the fusion of the egg and sperm.

• When the nuclei of the two gametes fuse, the process is called fertilization. The single cell formed as a result of fertilization is called the zygote.


• Remember somatic cells are body cells – 2n or diploid chromosome number. They are never the cells involved in sexual reproduction.

• We have 23 pairs of chromosomes in our somatic cells.

• The chromosomes that make up each pair are called homologous chromosomes

46 human chromosomes


• Homologous chromosomes:– Pairs of chromosomes in body or somatic

cells– Similar in size and shape– Carry information for the same traits– Not found in gametes– Same staining patterns


• Diploid refers to cells that have all the homologous chromosomes that are characteristic of the species. “2n”

• Haploid (monoploid) “n” cells with only one chromosome from each pair.

• Meiosis produces gametes in humans and spores in plants.

• Meiosis guarantees half the number of chromosomes that are present in the parent cells.


• Meiosis is a reduction division. – Beginning with a cell that has a 2n

chromosome number – each cell divides twice• There are 2 stages to meiotic cell division

– Meiosis I– Meiosis ll


• First division:– Prophase I– Metaphase I– Anaphase I– Telophase I

• Interkinesis• Second division

– Prophase II– Metaphase II– Anaphase II– Telophase II

» cytokinesis


• First division:– Prophase I:

• Chromosomes already replicated• Each pair of chromatids line up with its homologous pair.• They fasten at their centromeres• Synapsis takes place - Process when homologous pairs of

chromosomes are paired and fastened at their centromeres forming tetrads.

• Four chromatids = one tetrad• Crossing over takes place – when the strands of a tetrad

twist about each other and exchange segments; • Nuclear membrane disappears• Spindle fibers form• End of prophase – tetrads move toward equator of cell


• Metaphase I– Tetrads fastened to spindle microtubules at

their centromeres– Centromeres of tetrads line up on the equator


• Anaphase I– homologous chromosomes separate from

each other and move to opposite ends of the cell.

– Process of separation of homologous chromosomes is called disjunction.

» Failure of the chromosomes to separate correctly is called non disjunction


• Telophase I– – Marks the end of the first meiotic division– Cytoplasm divides (cytokinesis) – daughter

cells have half the number of chromosomes as the parent cells

– Nuclear membrane forms – cells either go through a brief interphase ( interkinesis) or go right to the second division.


• Second meiotic division:• Prophase II –

– Each daughter cells forms a spindle– Double stranded chormosomes move toward the middle

of the spindle

• Metaphase II – – Chromosomes fasten to spindle at centromeres– Centromeres line up on equator– Chromosomes are still 2 strands


– Anaphase II• Centromeres divide and chromatids separate• Single stranded chromosomes move to opposite

ends of spindle

• Telophase II – Daughter cells divide resulting in 4 haploid cells– All chromosomes return to interphase state– Nuclear membrane forms

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CHAPTER 11 MEIOSIS. Meiosis and sexual reproduction Gametes are sex cells of complex organisms. They are formed by meiosis Sex cells are haploid(monoploid)