Cell division

• Gap phase 1 (G1)

  • period of growth
  • organelle, RNA and protein synthesis
• Synthesis phase (S)

  • semi-conservative dna replication takes place
  • centrosome (1 pair of centrioles and proteins) replicate
• Gap phase 2 (G2)

  • prepares for mitosis
  • organelle synthesis, spindle protein synthesis, formation of microtubules
• haploid: one complete set of chromosomes (n number of chromosomes)
• diploid: 2 complete sets of chromosomes (2n number of chromosomes)
• homologous chromosomes

  • same size, shape, centromere position and gene loci
  • each member of a pair is a homologue
• Mitosis: produces 2 daughter nuclei containing identical sets of chromosomes
• Function of kinetochore microtubules

  • attach to kinetochore proteins on centromere of chromosomes
  • move chromosomes to metaphase plane (during metaphase) and to opposite poles of the cell (during anaphase)
• Function of non-kinetochore microtubules

  • microtubules elongate, slide in opposite directions to cause poles to move apart, elongates cell also
• Significance of mitosis

  • maintain genetic stability
  • growth
  • regeneration and cell replacement
  • asexual reproduction
• what features of the cell cycle ensures production of genetically identical cells?

  • dna replication
  • arrangement of chromosomes on spindle during metaphase
  • kinetochore must be attached to kinetochore microtubules and pulled to opposite poles for even distribution of daughter chromosomes
• Meiosis

  • produces 4 haploid genetically different daughter nuclei
  • meiosis 1: pairing and homologous chromosomes and separation in 2 daughter cells
  • meiosis 2: separation of 2 sister chromatids
• crossing over

  • exchange of equivalent portions of genetic material between non-sister chromatids of homologous chromosomes (part of chromatid of 1 chromosome with chromatid of another chromosome)
  • chromosomes are seen as tetrads
  • allows for new combinations of alleles
• bivalents: chromosomes pairing up (2 homologous chromosomes tgt)
• independent assortment of homologous chromosomes

  • takes place in metaphase I and anaphase I when chromosomes are at the metaphase plane
  • allows for gametes with different combinations of maternal and paternal chromosomes
  • random arrangement and subsequent separation of chromatids in metaphase II and anaphase II allows for haploid cells to have different combinations of alleles
• significance of meiosis

  • forms haploid gametes in sexual reproduction
  • genetic variation (crossing over and independent assortment of chromosomes, and random arrangement and separation of sister chromatids that allow for gametes with different combinations of alleles)
• Prophase:

  • centrioles move to opposite pole, nucleolus begins to disappear, nuclear envelope begins to disintegrate
  • chromatin condenses into chromosomes
  • mitotic spindles form, kinetochore microtubules attach to kinetochore proteins on centromere of chromosomes
• Metaphase:

  • chromosomes aligned at the equator in a single row, spindle completely formed
• Anaphase:

  • non-kinetochore microtubules elongate and slide in opposite directions, causing poles to move apart
  • centromere of chromosomes divide, sister chromatids are separated, now known as daughter chromosomes, daughter chromosomes pulled to opposite poles of cell via shortening of kinetochore microtubules
• Telophase:

  • cleavage furrow forms, spindle fibres disintegrate
  • daughter chromosomes reach poles, decondense and lengthen into chromatin
  • nuclear membrane reforms around chromatin at each pole, nucleolus reappears
• Prophase I:

  • chromatin coils to form chromosomes, homologues pair up to form bivalents
  • centrioles migrate to opp poles, spindle formation begins
  • crossing over between non-sister chromatids of homologous chromosomes
  • kinetochore microtubules begin to attach to kinetochore proteins of centromeres of each homologue
• metaphase I:

  • homologous pairs align along equator in 2 rows
  • kinetochore microtubules attach to kinetochore proteins on chromosomes, move homologous pairs to equator
  • independent assortment of chromosomes take place
• anaphase I:

  • homologues separate to opposite poles by shortening of kinetochore microtubules
  • non-kinetochore microtubules elongate, slide in opp directions, cause 2 poles to move further apart
• Telophase I:

  • chromosomes with 2 sister chromatids each, each pole of cell has haploid set of chromosomes
  • spindle fibres disintegrate
  • cleavage furrow forms, nuclear envelope starts to reform, nucleolus forms
• Prophase II:

  • chromatin condenses to form chromosomes, spindle fibres begin to form, centrioles start moving
• metaphase II:

  • spindle completely formed
  • kinetochore protein of each centromere attached to kinetochore microtubules
  • kinetochore microtubules align chromosomes at metaphase plane in a single row, plane is perpendicular to the plane in metaphase I
  • independent assortment of chromosomes take place
• anaphase II:

  • centromeres divide, sister chromatids are separated, now called daughter chromosomes
  • daughter chromosomes pulled apart to opposite poles of cell by shortening of kinetochore microtubules, non-kinetochore microtubules lengthen and elongate the cell
• Telophase II:

  • daughter chromosomes reach poles, decondense
  • spindle fibres disintegrate, nuclear envelope reforms, nucleolus reappears
• role of centromeres:

  • non-coding, tandem repeat DNA sequences, allow sister chromatids to adhere to one another
  • allow kinetochores and subsequently kinetochore microtubules to attach
  • allows for alignment of chromosomes at metaphase plate and separation of sister chromatids to opp poles during anaphase