Single cell: e.g. marine diatoms

Multi-cell: e.g. liver

DNA + proteins = chromosomes --> human 46 --> pine tree 24 --> dog 78 --> maize 20 --> frog 26

Karyote staining: arranged into matching pairs --> 23 homologous pairs of chromosomes --> grouped by length, centromere positions & staining patterns

23rd pair: woman XX homologous pair (longer) --> man XY pair (long, short) --> X,Y are sex chromosomes --> other chromosomes are autosomes

Prokaryotic cell division: Asexual reproduction --> single parent --> binary fission

Eukaryotic cell division: by mitosis of nucleus & cytokinesis of cytoplasm --> growth & division of present cells

Sexual reproduction: two different parents required --> requires gametes --> male: sperm --> female: ovum --> sperm fertilises ovum to form Zygote

Meiosis: variation of normal mitosis cell division --> only during gametes production --> sperm within testis --> ovum within ovary --> each gamete only contain 23 chromosomes --> half of the human chromosomes

Fertilisation: haploids (n) sperm & ovum --> combine to form --> diploid (2n) Zygote --> develops using mitosis as usual

Chromatid: a copy of the chromosome after cell division

Mitosis: cell division into two diploid (complete 46 chromosomes) cells

Meiosis: 1) Meiosis I: two haploid cells --> 2) Meiosis II: 4 haploid cells

Meiosis I:

Prophase: condensed chromosome --> Synapsis: pairing of homologous pair called Tetrad --> Chiasma: criscross region of chromosomes --> single or double cross-overs may occur --> results in genetic variability --> Late prophase: same as mitosis

Metaphase: same

Anaphase: chromosome pairs migrate to opposite poles --> mitosis: chromatid only --> meiosis: in chromosome pairs --> Disjunction

Telophase: spindles draw chromosomes into poles --> only haploid chromosome set

Cytokinesis: same

Meiosis II: from two daughter haploid cells -->

Prophase: same

Metaphase: centromeres line up along equator --> spindles along centromeres --> same

Anaphase: sister chromatids separate --> same as mitosis

Telophase + cytokinesis: into 4 haploid (n) cells produced

Disjunction in meiosis I: one chromosome do not separate --> 2 (n+1) cells & 2 (n-1) cells

Disjunction in meiosis II: one chromosome of one of the two daughter haploid (n) cells do not separate --> 2(n) cells, 1 (n+1) cell & 1 (n-1) cell

Gametogenesis: formation of gametes --> plants (pollin: male gametes, ova: female gametes)

Spermatogenesis: inside testis --> many seminiferous tubule --> spermatogonia develops by mitosis over childhood (2n) --> 1 spermatogonium --> primary spermatocyte by mitosis --> secondary spermatocyte by meiosis I --> spermatids by meiosis II --> sperm cells by maturation

Oogenesis: ovum formation in ovaries --> Oogonia: original diploid cells giving rise to ovum --> primary Oocyte (2n) held in ovary until puberty --> from puberty onwards --> Meiosis I: follicide surrounding primary Oocyte ruptures to form secondary Oocyte (n) & first polar body (n) once every 4 weeks --> Meiosis II: Ootid (n) & 1-2 polar bodies (n) which die --> leaving ovum --> providing plenty of cytoplasmic nutrients for the future zygote once fertilised by sperm

Gametogenesis: Spermatogenesis (4 haploid sperms) + Oogenesis (1 haploid ovum) --> genetic variations through cross-overs of chromatids --> 23 chromosomes each

Meiosis roles:

1) introduce new characteristics into offsprings

2) maintain same no. of chromosomes from one generation to the next