Mitosis, Meiosis, Eggs and the Ultimate Meaning of Life…

….well everything except the latter.

Mitosis• The process of cell division that occurs only

in eukaryotic cells. It’s function is growth and repair of tissue in multicellular organisms and it is how single celled eukaryotes (protazoans) reproduce.

Mitosis• It results in the formation of new cells with the

same number of chromosomes as the original cell.• When a cell is about to undergo mitosis changes

occur in the nucleus. It can be seen under a high powered microscope that the nucleus contains worm-like objects called chromosomes (a compact form of DNA as opposed to chromatin which is extended).

• The number of chromosome in a cell differs in different types of organisms but the number is always even. This is because chromosomes exist in match pairs. Each has a matching partner which is similar to itself in size and shape.

• Before mitosis take place each chromosome makes an exact copy of itself so it becomes two identical strands at a point.

Sister chromatids are identical duplicates of a chromosome temporarily joined to together as bivalent chromosome. You can think of them like conjoined twins. DRAW THE DIAGRAM ON THE LEFT ABOVE

Mitosis Stages• Interphase – DNA as extended chromatin.• Prophase – DNA condensed into chromosomes.

Nuclear membrane breaks down.• Metaphase – Chromosomes line up in the middle

(the metaphase plate).• Anaphase – Sister chromatids are separated by

the spindle fibres pulling on the centromere.• Telophase – the chromosomes are divided into

new cells. New cell membranes and nuclear membranes form.

Memory Devices• IPMAT

• I for Invisible DNA (in the from of chromatin). I is also for In-between mitotic stages

• Pro – first stage

• M for chromosomes in the Middle

• A for moving chromosomes Apart

• T for Tearing in Two

Meiosis

• Is the process that produces sex cells. It is like mitosis except it produces cells with half the amount of DNA, one chromosome from each parent.

Remembering the Difference between Mitosis and Meiosis

• Mitosis– My Toes grow because of Mi-toe-sis

• Meiosis– Makes – Eggs – In – Ovaries, – Sperm – In – Seminiferous vesicles

Diploid and Haploid.• Diploid cells(2n): Cells that have pairs of

homologous chromosomes, one from the mother and one from the father. In humans diploid cells have 46 chromosomes (23 pairs).

• Haploid cells (n): Cells that have only one member of each pair of homologous chromosomes. These are sex cells and they are designed this way so that when an sperm fertilises and egg the resulting cell has the full set of chromosomes. Sex cells are also know as gametes. In humans haploid cells have 23 chromosomes

Remember

• Diploid: Di means Two as in 2n

• Haploid: HA-ploid cells have HALF the number of chromosomes normal body cells do.

Interphase – DNA as extended chromatin.Prophase I – DNA condensed into chromosomes. Nuclear membrane breaks down. Bivalent chromosomes pair up and form tetrads and swap DNA in a process called crossing over (or chiasmata formation).Metaphase I – Tetrads line up in the middle (the metaphase plate).Anaphase I– Homologous chromosomes of the tetrads are separated by the spindle fibres pulling on the centromere.Telophase I – the chromosomes are divided into new cells. New cell membranes and nuclear membranes form. These cells are haploid cells (n) with bivalent chromosomes.

END OF MEIOSIS INOTE: In most organisms there is no interphase II or if there is it is brief. No DNA replication occurs at this point since the idea is to halve the amount of DNA.

Meiosis I

Meiosis II• Prophase II – DNA condensed into chromosomes.

Nuclear membrane breaks down.• Metaphase II– Chromosomes line up in the middle (the

metaphase plate).• Anaphase II – Sister chromatids are separated by the

spindle fibres pulling on the centromere. The centrioles pull at right angles to the original division.

• Telophase II– the chromosomes are divided into new cells. New cell membranes and nuclear membranes form. The resulting cells are haploid cells with monovalent chromosomes.

End Result of Meiosis I and II

• Meiosis I: The resulting cells are haploid cells (n) with bivalent chromosomes.

• Meiosis II: The resulting cells are haploid cells with monovalent chromosomes.