mitosis & meiosis

• cell division jobs:

  • replace worn out cells
  • reproduction in every organism
  • for multi-cellular organisms to grow
• Mitosis - cloning/copies
• each chromosome consists of 2 chromatids joined together by a centromere at the centre.

  
• The cell cycle:

  interphase (G1, S & G2)

  • first growth stage
  • synthesis
  • gap 2 (second growth phase)

  mitosis

  • prophase
  • metaphase
  • anaphase
  • telophase

  cytokinesis
• interphase - makes up the largest part of the cell cycle

  • cell is said to be at rest
  • carries out all the functions the cell need to perform
  • DNA replicated (S - synthesis)
  • Check for error (G2 - gap 2)
  • protein synthesis
  • mitochondria & chloroplasts grow and divide (g1 - growth stage 1)
  • normal metabolic functions occur (g1)
• G0

  • name given where the cell moves out of the cell cycle
  • can be permanent or temporary
• Reasons for G0:

  differentiation - cell becomes specialised to carry out a particular function. This can make it unable to divide

  DNA has become damaged - state of permanent arrest. most cells do this as they can only divide a set number of times
• Mitosis:

  prophase (podgy) - chromatin condenses and becomes visible, at the end nuclear envelope disintegrates

  metaphase (middling) - chromosomes line up at the equator of the cell

  anaphase (away) - chromosomes move towards the poles away from each other

  telophase (tuck in) - nuclear envelope reforms

  cytokinesis - cell division

  
• DNA + histones = chromatin
• Early prophase:

  • chromosomes become more distinct

  they coil up

  shorten

  thicken

  take up stain more intensely

  • centrioles divide
  • nucleolus becomes more prominent

  
• Late prophase:

  • chromosomes have become more distinct and are seen to consist of two chromatids joined by a centromere
  • the centrioles migrate to opposite poles of the cells
  • nucleolus continuous to shrink and disappears
  • the nuclear envelope disintegrates

  
• Metaphase:

  • each centriole is at a pole
  • centriole spindle fibres formed
  • spindle fibres attach to the centromere of the chromosome
  • each centromere is attracted to both poles
  • chromosomes pulled to the metaphase plate/equator

  
• Anaphase:

  • spindle fibres contract
  • centromere divides
  • chromatids (daughter chromosomes) are pulled to opposite poles of the cell
  • pulled centromere first
  • each half of the cell receives one chromatid from each chromosomes

  
• Telophase:

  • chromatids reach the poles of the spindle
  • they begin to uncoil
  • they become less distinct
  • nuclear envelope starts to reform
  • nucleolus reformed

  image should have membrane around both chromatids

  
• cytokinesis:

  • cell divides

  animals cells:

  • starts by constriction from the edges of the cells

  plant cells:

  • a cell wall is laid down
  • daughter cells have the same chromosome number and genetic makeup as each other and the parent cell - DNA replication precedes mitosis

  
• Asexual reproduction (mitosis)

  • single organisms divide by mitosis
  • new organisms are genetically identical to the parent
• Sexual reproduction (meiosis)

  • meiosis produces haploid gametes
  • which fuse at fertilisation to form a diploid zygote (first cell)
  • this produces genetic variation amongst offspring
• Meiosis

  • reduction division, resulting in daughter cells having half the original number of chromosomes
  • daughter cells are haploid
  • can be used for sexual reproduction
  • source of genetic variation
  • meiosis has two divisions
• Meiosis 1 - first division is the reduction division when the pairs of homologous chromosomes are separated into two cells.
• Meiosis II - pairs of chromatids present in each daughter cell are separated, forming two more cells. 4 haploid cell are produced in total
• Prophase I (podgy and pairing)

  • chromosomes condense
  • Nucleolus disappears
  • spindle fibres form
  • Homologous pairs form a bivalent
• Metaphase I (middling)

  • bivalents line up on the equator of the cell
• Anaphase (away)

  • homologous chromosome in each bivalent are pulled to opposite poles
• Telophase I (tuck in)

  • two new nuclear envelopes form
  • chromosomes uncoil
  • cell divides by cytokinesis
• prophase II - podgy

  • nucleolus + nuclear envelope disappears
  • chromosomes condense
  • spindle fibres forms
• Metaphase II - middling

  • chromosomes arrange themselves on equator
  • attach by centromere to spindle fibres
• Anaphase II - away

  • centromeres divide
  • chromatids pulled apart to opposite poles
• Telophase II (tuck in)

  • chromosomes uncoil
  • nuclear envelope reforms around haploid nuclei
  • cell divide by cytokinesis
• sources of variation

  • crossing over - prophase 1
  • independent assortment of chromosomes - metaphase 1 & 2 - up to chance the side of the spindle the maternal + paternal ends up at
  • random fertilisation
  • chromosomes mutations - structure of chromosomes and number of chromosomes
• allele - different version of the same gene
• locus - location on gene of chromosome
• Crossing over:

  1. During prophase I, homologous chromosomes condense & pair up.
  2. Chromatids of each chromosome twist around one another, forming a chiasmata.
  3. When the chromosomes are separated during anaphase I, the chromatids break at the chiasmata and then reconnect to the chromatid from the homologous chromosome.
  4. This swaps alleles between the homologous chromosomes to produce different combinations on each chromosome.