Mitosis
Mitosis is a form of nuclear division that produces 2 daughter nuclei containing the same number of chromosomes as the parent nucleus. The daughter nuclei are genetically identical
1. Parent cell containing x chromosomes 2. DNA replicates
3. Mitosis
4. 2 genetically identical daughter cells form (with x chromosomes each)
Mitosis is important for organism growth and tissue repair
Since identical cells are produced, new cells can replace worn out cells during repair to perform the functions of the old cells
Mitosis occurs during asexual reproduction Mitosis
Phase Process Interphas
e Not part of mitosis, cells carry out their normal activities
Just before mitosis, chromatin threads replicate producing 2 identical chromatin threads joined at a centre called a centromere. These 2 chromatin threads are called sister chromatids
Prophase Disintegration of nuclear membrane
Asters form around centrioles, and the pair of centrioles move to the opposite poles of the cell
Spindle fibres extend from centrioles
Chromatins condense and DNA replicates, forming X-shaped chromosomes and they migrate towards centre of cell
Metaphas
e Chromosomes are attached by spindle fibres at the centromere to the centrioles
Chromosomes are aligned at the centre plane of the cell
Anaphase Sister chromatids of each chromosome separate and each of them become a distinct chromosome
Spindle fibres holding the chromosomes shorten and pull the daughter chromosomes towards the centrioles
Telophase Spindle fibres break down
Each set of chromosomes unwinds and uncondenses
Nuclear membrane reforms
Now there are 2 diploid daughter cells Cytokines
is Not part of mitosis, cleavage forms in cell and deepens until cell splits into 2
Mitosis in plants
Mitosis in plant cells are similar to animals except that
Centrioles are absent in plant cells
Cleavage of cytoplasm does not occur during cytokinesis. Instead, a cell plate is formed between the 2 daughter nuclei, dividing cell into 2. Cell plate is formed by the fusion of small fluid-filled vesicles produced by the Golgi Apparatus
Chromosomes
Every diploid cell has 2 sets of chromosomes in its nucleus
One set of chromosomes come from the mother, other one comes from the father
Each chromosome in a paternal or maternal set has a corresponding chromosome in the other set that is approximately the same and contains the same genes
The pair of corresponding chromosomes is called a homologous chromosome
22 pairs of homologous chromosomes in a male cell (XY) and 23 pairs of homologous chromosomes in a female cell (XX)
Meiosis
Meiosis is a form of nuclear division that produces 4 daughter nuclei containing half the number of chromosomes as the parent nucleus
Consists 2 sets of divisions
Chromosome number is reduced by half during the first division (from 2n to n)
Results in the formation of 4 haploid cells which will participate in reproduction
All 4 cells produced from the single parent cell are genetically different
Meiosis occurs in the production of gametes
Gametes are haploid reproductive cells
Diploid parent cell contains 2 pairs of chromosomes (2n = 4)
Replication of chromosomes
Meiosis I occurs, each daughter cell contains 2 chromosomes, but each chromosome consists of 2 chromatids
Meiosis II occurs, 4 haploid gametes containing 2 chromosomes (n = 2)
Process in meiosis Phase Process
Interphas Not part of meiosis, cell does its normal activities
e Just before meiosis, chromatin threads replicate, producing 2 identical sister chromatids attached at the centromere
Prophase
I Disintegration of nuclear membrane
Asters form around centrioles and the pair of centrioles move to opposite poles of the cell
Spindle fibres extend from centrioles
Homologous chromosomes pair up (this process is called synapsis)
Crossing over occurs, the homologous chromosomes twist around each other, and may break and exchange parts (this results in genetic
variability between daughter cells) (the point where they cross each other is called chiasma)
Chromatins condense and DNA replicates, forming X-shaped chromosomes and they migrate towards centre of cell
Metaphas
e I Homologous Chromosomes are attached by spindle fibres at the centromere to the centrioles
Homologous Chromosomes are aligned at the centre plane of the cell Anaphase
I Homologous chromosomes separate and each of them become a distinct chromosome
Spindle fibres holding the chromosomes shorten and pull the daughter chromosomes towards the centrioles
Telophase
I Spindle fibres break down
Each set of chromosomes unwinds and uncondenses
Nuclear membrane reforms
Now there are 2 haploid daughter cells Cytokines
is Not part of meiosis, cleavage forms in cell and deepens until cell splits into 2
Prophase
II Disintegration of nuclear membrane
Asters form around centrioles and the pair of centrioles move to opposite poles of the cell
Spindle fibres extend from centrioles
Chromatins condense and DNA replicates, forming X-shaped chromosomes and they migrate towards centre of cell
Metaphas
e II Chromosomes are attached by spindle fibres at the centromere to the centrioles
Chromosomes are aligned at the centre plane of the cell Anaphase
II Chromosomes separate into sister chromatids
Spindle fibres holding the chromosomes shorten and pull the sister chromatids towards the centrioles
Telophase
II Spindle fibres break down
Each set of chromosomes unwinds and uncondenses
Nuclear membrane reforms
Now there are 4 haploid daughter cells Cytokines
is Not part of meiosis, cleavage forms in cell and deepens until cell splits into 2 (Total of 4)
Meiosis produces haploid gametes
Meiosis produces haploid gametes
When haploid male gametes fuses with haploid female gametes, the diploid number of chromosomes is maintained
Meiosis results in genetic variation
Variation occurs due to crossing over and independent assortment
Independent assortment of chromosomes means that one chromosome from each pair can combine wither either chromosome of other pair
Variations increases the chances for the species to survive changes in the environment
Differences between mitosis and meiosis
Mitosis Meiosis
Daughter cell contain the same number of chromosomes as the parent cell
Daughter cells contain half the number of chromosomes as the parent cell
Pairing of homologous chromosomes does not occur
Pairing of homologous chromosomes occurs at prophase I
No crossing over Crossing over may occur
Daughter cells are genetically identical to the parent cell
Daughter cells are not genetically identical to the parent cell
2 daughter cells produced from one parent cell
4 daughter cells produced from one parent cell
Involves only 1 nuclear division Involves 2 nuclear division Occurs in normal body cells during growth
or repair of body parts
Occurs in the gonads during gamete formation