Chromosomes

2x chromatid: DNA wrapped around proteins
identical: sisters
Join at centromere
Pairs: homologous chromosomes
Interphase:
DNA replicate, chromatin

Mitotic cell division

Prophase

• Chromosomes shorten and thicken
• nuclear membrane disappears

Metaphase

• Chromosomes line up in the equator
• spindle fibres attach

Anaphase

• Spindle fibres pull sister chromatids to the poles
• Cytoplasm begins to divide

Telophase

• New nuclear membranes formed
• chromosomes uncoil

Cytoplasmic division

animal cells: cell membrane around the centre constricts inwards
polant cells: cell plate formed between nuclei grows

Significances of mitotic cell division

(genetically identical)

1. growth
2. repair
3. asexsual reproduction

Meiotic cell division

Prophase I

• Chromosomes shorten and thicken
• nuclear membrane disappears
• homologous chromosomes pair up

Metaphase I

• homogulous chromosomes line up along the equator
• spindle fibres attach

Anaphase I

• two members of pair separate and move to opposite poles
• cytoplasm begins to divide

Telophase I

• New nuclear membranes formed

Prophase II

• nuclear membrane disappears

Metaphase II

• Chromosomes line up in the equator
• spindle fibres attach

Anaphase II

• Spindle fibres pull sister chromatids to the poles
• Cytoplasm begins to divide

Telophase II

• New nuclear membranes formed
• chromosomes uncoil

Significances of meiotic cell division

1. Haploid (n) gametes

• maintain diploid (const.) number in offspring

2. Genetic variation --> enhance survival

• independent assortment of chromosomes
  • different combinations of chromosomes in gametes
• crossing over
  • Prophase I: non-sister chromatids wrap around forming chiasma, exchange segments of chromatids