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Meiosis

Sharni Knight Life Sciences English

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Meiosis

Type of cell division to produce gametes or sex cells
Diploid cell (somatic cell) undergoes two divisions, divides to form four dissimilar haploid cells (sex cells)
Diploid cells - 2 sets of chromosomes, each chromosome has homologous partner
Haploid cells - one set of chromosomes, no homologous partner
First meiotic division
- Number of cells are doubled
- Number of chromosomes stay the same
- Results in half number chromosomes so gametes haploid
- Step one: Prophase 1
  - Chromosomes shorten. Become visible as two chromotids joined by centromere
  - Homologous pairs chromosomes now visible
  - Nuclear membrane and nucleolus dissapear
  - Spindle starts to form
  - Chromotids from every homologous pair touch, at point called chiasma
  - DNA crossed over at chiasma. Ensures genetic variation
  - Spindle continues to form
- Step two: Metaphase 1
  - Spindle extends across whole cell
  - Homologous chromosomes line up along equator of spindle in homologous pairs
  - One chromosome from each pair lies on either side of equator
  - Centromere of each chromosome attaches to spindle fibres
- Step three: Anaphase 1
  - Spindle fibres shorten and pull each chromosome of each pair to opposite poles of cell. Called random assortment
- Step four: Telophase 1
  - Chromosomes reach poles of cell
  - Each pole has half number of chromosomes of original cell
  - Cell membrane constricts, divides cytoplasm in half, forming two cells
Second meiotic division
- Prophase 2
  - Each cell formed during Prophase 1 divides again
  - Spindle forms in each new cell
- Metaphase 2
  - Individual chromosomes line up at equator of each cell, with centromeres attached to spindle fibres
- Anaphase 2
  - Spindle fibres start to contract
  - Centromeres split and daughter chromosomes/chromatids pulled to opposite poles of each cell
- Telophase 2
  - Daughter chromosomes/chromatids reach poles and new nucleus forms
  - Cell membrane of each cell constricts and cytoplasm divides into two cells
  - Four haploid daughter cells are formed
  - Each daughter cell has half number of chromosomes of original cell
  - Daughter cells genetically different from each other
- Duplicates gametes = 4 sperm cells. 1 Egg cell, 3 structures provide nutrition for egg cell
Significance
- Reduces number of chromosomes by half, from diploid to haploid. When fertilization takes place, zygote formed has correct number of chromosomes
- Crossing over and random assortment of chromosomes at equator introduces genetic variation. Results in offspring better adapted to particular environment and ensures better chance of survival
Abnormal meiosis/Non-disjunction
- Failure of homologous chromosomes or chromatids to segregate during meiosis or mitosis
- Can happen in Anaphase 1, homologous chromosomes not separate. Called non-disjunction
- Can happen in Anaphase 2, non-disjunction of sister chromatids
- Down's Syndrome has 47 chromosomes. Mother's egg cell has 24, father sperm has 23. Child has 45 autosomes, three number 21 chromosomes
- Fusion between normal gamete and abnormal gamete with extra copy 21 leads to Down Syndrome
- One daughter cells has both parental chromosomes or chromatids and other has none
Differences between meiosis 1 and meiosis 2
Cell division
- Process by which cells reproduce
Chromosomes
- Cell parts determining what traits a living thing has
Daughter cells
- New cells produced by cell division
Cytokinesis
- Division of cytoplasm of one parent cell into 2 daughter cells
Chromatid
- One half of chromosome consisting of protein core surrounded by DNA, carrying hereditary characteristics. Two chromatids joined by centromere = chromosome
Karyotype
- Set of chromosomes from human cell, shows chromosomes arranged in pairs, according to numbers. Pair one always biggest. Female = XX pair 23, male = XY pair 23
Fertilisation takes place, two haploid gametes fuse forming 1 diploid zygote
Differences between mitosis and meiosis
- Where process occurs
  - Mitosis
    - In every living organism, the somatic cells
  - Meiosis
    - Specialised cells in testes and ovaries of animals
    - Specialised cells in anthers and ovules in plants
- Why process occurs
  - Mitosis
    - Production of genetically identical cells, which specialise and differenciate for growth, repair and replacement of cells
  - Meiosis
    - Reduction division resulting in haploid gametes for sexual reproduction so chromosome number remain same in offspring as in adult
- Similarities
  - Transformation of chromatin network to chromosomes
  - Karyokinesis takes place
  - Cytokinesis takes place
- Differences
  - Mitosis
    - One nuclear division
    - No crossing-over
    - Daughter chromosomes pulled to opposite poles
    - Two identical cells produced with identical chromosomes to original cell
  - Meiosis
    - Two nucleur divisions
    - Crossing-over takes place ensuring different combination of genes when genetic information exchanged
    - Daughter chromatids do not separate, one chromosome from each homologous pair pulled to pole - each chromosome pair separates independently
    - Four cells produced, each containing half of original number of chromosomes in gametes
Biotechnology
- Use of micro-organisms like enzymes to perform industrial/manufacturing processes in agriculture, food science, medicine
- Scientists artificially select characteristics of animals/plants to suit our needs
- Result can be classed as evolutionary mechanism
- Hybrid results in new genetically engineered species
- New breeds, strains and varieties
- Advantages of hybrid
  - Increased size of body
  - Increased production of eggs, offspring, seeds
  - Increased resistance to disease
  - Reach maturity quickly
Polyploidy
- Cells have multiple pairs of chromosomes beyond basic set
- Changed in the gene frequency and chromosome numbers
Interphase
- DNA replication takes place
- Chromosomes which single threads, become double
- Each chromosome consist of two chromatids joined by centromere
- Helps double genetic material so can be shared by new cells arising from cell division
In plants in the special cells in pollen sacs of anthers and specialised cells in ovule