cell division

• haploid


  contains one complete set of chromosomes
• diploid


  contains 2 complete sets of chromosomes
• homo chromo


  Carry the same genes controlling the same inherited characteristics at the same loci, but may not have the same alleles➔ one is of maternal origin and one is of paternal origin similar in size, shape, centromere position and staining pattern
• Sister chromatids are the result of DNA replication and arethus, genetically identical (same allele)
• significance


  Production of genetically identical daughter nuclei with same number and type of chromosomes and the same alleles so that geneticallyidentical daughter cells can be produced for:1.Growth➔ increase number of cells by producing more cells genetically identical to existing ones2. Regeneration and cell replacement➔ damaged cells replaced by cells genetically identical to original ones, retaining the same function.3. Asexual reproduction➔ produces genetically identical offspring
• need for regulation

  
  

  The cell cycle is tightly regulated as it is important for normal growth & development. Regulation is at certain control points known as checkpoints which are at G1, G2 and M phase. These checkpoints are stop and go-ahead signals which determine whether or not the cell cycle can proceed.

  
  

  Cancer occurs when the dysregulation of checkpoints of cell division occur or cells escape the cell cycle control mechanism that normally regulates their growth. This leads to uncontrolled division of cells and possibly cancer
• Reduction division: production of 4 haploid gametes from 1 diploid parent cellChromosome number halved so that the chromosome number can be restored upon fertilisation➔ Chromosome number of species remains the same after many generations
• Independent assortment of homologous chromosomes at the metaphase plate & their subsequent separation during metaphase I& anaphase I respectively &Random orientation of non-identical sister chromatids of each chromosome at the metaphase plate & their subsequent separation during metaphase II and anaphase II respectively➔ results in gametes with different combinations of maternal & paternal chromosomes.
• no. chr for mitosis: double at anaphase, halve after cytokenesis
• Animal cells: Cell membrane invaginates towards the equator of the cell, forming a cleavage furrow. The cleavage furrow deepens and is pinched into 2 ➔ 2 daughter cells produced
• Plant cells: Fluid-filled vesicles appear in the middle of the cell and coalese to form a cell plate, separating the 2 daughter cells
• amt of dna per cell

  during S phase of interphase, DNA replicates and so the amount of DNA increases gradually from x to 2x.The amount of DNA per cell halves only at the end of cytokinesis when 2 cells have formed
• meiosis


  no. of chr: halve aft cytokinesis I, double at anaphase II, halve aft cytokinesis II
• DNA per nucleus doubles during S phase of interphase. Then, during mitosis it halves at telophase when the nuclear envelope reforms. During meiosis, the amount of DNA per nucleus halves during telophase I and halves again during telophase II
• • If DNA is damaged, halted at the G1 checkpoint, preventing cells from entering S phase during which DNA is replicated
  • If DNA is damaged/ all chromosomes are not replicated properly, halted at the G2 checkpoint, preventing cells from entering mitosis, providing an opportunity for repair and stopping the proliferation of damaged cells
  • If all chromosomes are not attached to spindle fibres from both poles, halted at the M check point preventing anaphase from occurring and hence preventing the incorrect separation of sister chromatids
• how to tell cell undergoing mitosis

  HC did not pair up, each chr made of 2 chromatids as DNA has been rep, chromatin undergone condensation to form chromosome which occurs during prophase of mitosis
• why do hc appear as dbl structures

  dna rep takes place at S phase of interphase producing 2 mols of DNA, chr bcom visible as they condense & SHORTEN BFR CELL DIV SO EACH CHR CONSISTS OF 2 SIS CHROMATIDS JOINED AT CENTROMERE