BY1: Cell Division

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-Made up of DNA, proteins and small amounts of RNA.
-Only at cell division do they condense and become visible.
-A single chromsome doubles at cell division, and become parallel chromatids; joined at the centromere.
-Number of chromosomes vary with species (i.e. 46 in humans).
-Humans have 23 homologous pairs, the total amount is diploid, half is haploid.
-Human gametes have 23 chromosomes.

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-Interphase: Longest part of the cell cycle, where cell increases in size, replicates DNA and organelles, synthesises ATP and proteins, and considerable metabollic activity.

-Prophase: Longest stage of mitosis, chromosomes condense as chromatids, centrioles divide and move to opposite poles, spindle fibre develops, nuclear membrane disintergrates.

-Metaphase: Chromosomes arrange on the equator of spindle attatched by centromere, contraction causes chromatids slightly apart.

-Anaphase: Very fast phase, centromere splits, and spindle fibre contracts, moving chromatids to poles centromere first.

-Telophase: Final stage, chromsomes are at poles, uncoil and lengthen, the spindle breaks down, centromere replicate, the nuclear membrane reforms. Animal cell = cytokinesis, where the cell splits. Plant cell = A cell plate forms across. 

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Mitosis (Cont.)

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Significance Of Mitosis

-Produces two cells that have the same number of chromosomes as the parent cell and each chromosome is an exact replica, so induces genetic stability.
-Leads to growth of an organism, and allows for repair of tissues. (e.g. in plants of the root tip).
-Asexual reproduction results in identical offspring, this means bacteria (and some plants, tubers/bulbs) can produce large number very quickly, with no variation.

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-In sexual reproduction two gameres fuse to form a zygote, so chromosomes must be halved first.
-Meiosis involves two divisions, first haploid cells are formed, the second is like mitosis.

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Meiosis And Variation

Variation is necessary for evolution, caused by meiosis:
-During sexual reproduction, the genotype of one parent is mixed with that of the other.
-Independant assortment results in gametes containing different chromosome combinations.
-During crossing over at chiasmata, genetic information is swapped; causing new combinations of genes. 

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This table highlights: Mitosis has only two daughter cells and one split, with no crossing over and genetically identical replications.
-Furthermore at metaphase in mitosis the chromosomes line up on the equator, but in meiosis I, homologous chromosomes line align on equator. 

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