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This diagram represents a parent cell containing a single pair of chromosomes – the pair of chromosomes are the same size and shape and are therefore described as being homologous chromosomes. One member of the pair was donated by the male at reproduction and the other member by the female. Each pair of homologous chromosomes thus has one member that is paternal in origin and another member that is maternal in origin. Humans possess 23 pairs of chromosomes in each body cell. Cells in which the chromosomes occur in homologous pairs are termed DIPLOID (two sets of chromosomes) maternal in origin paternal in origin Each chromosome replicates itself to form a pair of identical chromatids called sister chromatids that remain attached to one another at a region called the centromere centromere The cell divides into two and each daughter cell receives one of each of the sister chromatids The daughter cells are genetically identical to the parent cell The function of mitosis is to construct an exact copy of each chromosome and then to distribute, through division of the parent cell, an identical set of chromosomes to each of the two daughter cells The Process of Mitosis

Mitosis is a continuous process but for convenience of description is divided into FOUR MAIN STAGES: Prophase Metaphase Anaphase Telophase Please Make Another Two The following slides describe the process of mitosis in an animal cell containing two pairs of homologous chromosomes The Process of Mitosis

Interphase – cell prepares for division; DNA replicates, new organelles are manufactured and cell grows; chromosomes present as granular material (chromatin) Early Prophase – chromosomes spiralise and condense and become visible as threads; centrioles move to opposite poles of the cell and spindle fibres begin to form Late Prophase – chromosomes spiralise and condense further and can now be seen to consist of two sister chromatids joined at the centromere Metaphase – nuclear membrane has disintegrated and spindle fibres have grown across the cell; chromosomes line up independently along the equator of the spindle attaching to the fibres via their centromeres Early Anaphase – as the cell enters anaphase, the centromeres divide into two separating the sister chromatids of each chromosome Summary of Mitosis

Anaphase – spindle activity pulls the chromatids apart and the separated chromatids move to opposite poles of the cell Telophase – the chromatids are now described as chromosomes and they begin to uncoil. The spindle fibres disintegrate and the cell begins to constrict along its central axis. A nuclear membrane begins to form around each set of chromosomes Cytokinesis – as the membrane continues to constrict, the cytoplasm becomes divided forming two genetically identical daughter cells. Each cell now possesses an exact copy of each chromosome that was present in the nucleus of the original cell Daughter cells Summary of Mitosis

These photographs are taken from prepared slides of onion root tip cells that were undergoing mitosis: Identify the photograph showing Interphase and the photographs showing the four stages of mitosis – Prophase, Metaphase, Anaphase and Telophase A B C D E

M During interphase, preparations for mitosis take place G 2 - Second Growth Phase The cell grows and prepares for mitosis G 1 - First Growth Phase The cell grows and new organelles and proteins are manufactured S DNA replication takes place The Cell Cycle

Meiosis is another form of cell division that is associated with reproduction in many organisms In humans, meiosis is responsible for the formation of the reproductive cells or gametes SPERM CELL EGG CELL In humans, these are the egg and sperm cells Whereas most body cells have a complement of 23 pairs of chromosomes, human gametes possess only 23 single chromosomes. A gamete’s complement of 23 single chromosomes is constituted by one chromosome taken from each of the 23 pairs of chromosomes Within the human ovaries and testes, gametes are produced by meiosis and this process halves the chromosome number Human body cells are DIPLOID as they possess two sets of chromosomes (23 pairs) Human gametes are described as being HAPLOID as they possess only one set of chromosomes (23 chromosomes) If the gametes were diploid then the number of chromosomes would double at every generation after fertilisation An Introduction to Meiosis

MEIOSIS MITOSIS Diploid body cell Two diploid daughter cells The nucleus divides twice Four haploid, genetically different gametes are produced Meiosis is important as it ensures that, when the gametes fuse at fertilisation, the normal diploid number of chromosomes is maintained; meiosis is also an important source of genetic variation